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GEOGRAPHICAL APPROACHES TO CROP CONSERVATION: THE PARTITIONING OF GENETIC DIVERSITY IN ANDEAN POTATOES1 K A R L S. ZIMMERER AND D A V I D S. D O U C H E S

Zimmerer, Karl S. (Department of Geography, 384 Science Hall, University of Wisconsin, Madison, W15 37 06) and David S. Douches (Department of Crop and Soil Science, Michigan State University, East Lansing, M I 48824-1325). G~OGRAPHICALAPPROAOt~TO CROPCONSERVATION:ThE PARTmO~G OFGENETICDIVERSITYINA~q~Ea,N POTATOES.Economic Botany 45(2): 176-189. 1991. The geographical concepts of spatial scale and the human-geographic region offer significant contributions to the conservation of crop genetic resources. They are used in the present study to examine the partitioning of genetic diversity along two axes: geographical location and landrace population. Locations in the study consist of three micro-regions within the highland Paucartambo region of southern Peru. Six widely distributed landraces of the potato species Solanum stenotomum Juz. et Buk. and S. tuberosum subsp, andigena (Juz. et Buk.) Hawkes are evaluated. Electrophoretic analysis of isozyme loci demonstrates that the majority of allelic variation is contained within the geographical and landrace populations. Geographically, greater than 99% of total variation is found within single micro-regions. Taxonomically, approximately 75 % of variation occurs within individual landraces. The weak geographical partitioning of allelic variation is due in part to formerly high rates of seed-tuber exchange. The weak-moderate taxonomic partitioning of variation is attributed to common parentage and shared introgression. Unique genotypes are microgeographically concentrated. Findings recommend that conservation strategies focus on intensive sampling or preservation in micro-regional areas due to the concentration of unique genotypes. Evaluation of the spatial patterning of diversity and recognition of the taxonomic specificity of results (not necessarily applicable even to related potato landraces) rely on biogeographical and human-geographic concepts.

justice along with the maintenance of biological diversity (Altieri and Merrick 1987; Gade 1981; Ingrain and Williams 1984). Systematic field research indicates a geographically uneven persistence of diverse native crops in the wake of rural development (Brush and Vaupel nd.). To evaluate the biological significance of the patchy persistence of native crops, studies must address the spatial patterns inscribed by the genetic diversity therein. This article examines the usefulness of the geographical concepts of spatial scale and the human-geographic region for examining the partitioning of genetic diversity and thus aiding in the Received 25 June 1990;accepted6 December 1990. planning of crop conservation. The first section Presented at the Symposium on New Directions in ("Geographical Concepts and Crop ConservaCrop Genetic Resource Conservation, Thirtieth Antion Research") reviews the two concepts and nual Meeting, Society for Economic Botany, University of Tennessee, Knoxville, TN, 12-13 June 1989; their application to cultivated plants and agricultural ecosystems. Subsequent sections desymposium organized by Stephen B. Brush. First proposed during the early 1970s, the in situ conservation of crop genetic resources merits implementation because of agroecological, economic, and cultural advantages (Altieri and Merrick, 1987; Brush 1989; Iltis 1974; Nabhan 1985; Oldfield and Alcorn 1987; Wilkes and Wilkes 1972; Zimmerer n.d.,b). Early proposals that recommended the economic isolation of peasant and indigenous farmers have given way to discussion of in situ crop conservation that asserts the necessity of fostering rural development and social

Economic Botany 45(2) pp. 176-189. 1991 9 1991, by The New York Botanical Garden, Bronx, NY 10458 U.S.A.

1991]

ZIMMERER & DOUCHES: ANDEAN POTATOES

177

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Scale 1:733,000 10 20 30

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topographic

maP. uNC.CHGEOGDEPTrc~=hnm)

Fig. I. Study region of highland Paucartambo centered on an easterly valley (the Mapacho River) in the southern Peruvian sierra. Micro-regions of Colquepata, Maqhopata, and Mollomarca are indicated.

scribe a research project combining field study and laboratory analysis focused on the partitioning of genetic diversity in the native potatoes of Peru's southern sierra (see Fig. l). Design of the investigation was stimulated by the observations of conservation biologists Frankel and Soul6 (I 98 I), who highlight the absence of research on the most basic partitioning of biological variation in native crops: " . . . genetic diversity . . . has thus two dimensions: between sites and populations, and within sites and populations" (182). Two specific subject areas laid out by Frankel and Soul6, microgeographic settings and local landraces (also referred to as "native cultivars" and "primitive varieties"), are especially crucial

for the development of crop-conservation strategies. GEOGRAPHICAL CONCEPTS AND CROP CONSERVATION RESEARCH BIOGEOGRAPHY AND SPATIAL SCALE Biogeographical concepts are notably lacking from most research on in situ crop conservation. Notwithstanding the expansion of biogeographical knowledge stemming from the increased collection of native crops (e.g., Lyman 1984) and pertaining to the initiation of crop domestication (e.g., Harlan 1975a; Hawkes 1983), biogeography has scarcely entered into the discussion of

178

ECONOMIC BOTANY

in situ programs. In marked contrast, biogeography's focus on the distribution of diversity and its formation has served as a foundation for the design of other conservation strategies. Conserving non-crop biological diversity is broadly seen as requiring that biogeographical approaches be cast in a major role (e.g., Schonewald-Cox et al. 1983; Simberloffand Abele 1982; Soul6 and Wilcox 1980). The la~.~ging recognition of biogeography in crop conservation research has recently begun to be redressed by a handful of studies that emphasize the relations of spatial scale to crop diversity. Areal frameworks based on regional and island biogcography have been hi~hllghted in recent discussions of in situ conservation (Brush 1989; Nabhan 1985). Underscoring the relationships between crop diversity and spatial scale, the attention to areal frameworks is able to draw on a well-developed tradition within biogeographical research. Although dealing principally with wild (uncultivated) plants, many biogeographers have focused on the import of spatial scale in the distribution and diversity of taxa as well as ecological communities (e.g., Cain 1944; MacArthur and Wilson 1967; Polunin 1960; Rapoport 1982; J. D. Saner 1988). In addition to the regional and island concepts utilized in the discussions of Nabhan (1985) and Brush (1989), respectively, crop conservation efforts can benefit by building on yet other biogeographical models. In particular, conceptual frameworks assessing the multiple spatial levels in the distribution of taxa and ecological communities would appear to offer a cornerstone for crop conservation (reviewed by Udvardy 1969; see also Kolasa 1989). Cultivated plants have not been examined in detail with respect to the various interlinked scales of spatial patterning. The preponderance of research locating clusters or "centers" of diversity does not specify or differentiate among spatial scales (Zeven and de Wet 1982). Yet all crop species, even those whose diversity is distributed diffusely in so-called non-centers (Harlan 1971), display an uneven patterning of intra-specific variation. Notwithstanding the lack of an integrated or comprehensive conceptual framework, existing studies can be used to identify three primary spatial scales--referred to here as the macrogeographic, mesogeographic, and microgeographic--at which the biological diversity of crops is clustered. N. I. Vavilov's (1926) landmark phytogeographical research first circumscribed

[VOL. 45

the macrogeographic clustering ("macro-centers") of crop diversity, eight areas of subcontinental proportions that were deduced by examining intra-specific morphological variation and mapping the region or regions of greatest diversity. Subsequent revisions of Vavilov's initial centers have redrawn boundaries, identified new centers, and divided existing centers, although continuing to describe similar subeontinental areas of 1,000,000 km 2 or more (Vavilov 1950; Zhukovsky 1975). The uneven distribution of crop diversity within macro-centers was emphasized by Harlan (1951) in his observations on the spatial patterning of morphological diversity in Turkish wheat. Mesogeographic clusters ("meso-centers") of diversity within the macrocenter were seen as covering at least 10,000 km 2. A differentiation of diversity at the mesogeographic scale was also found to be manifest by wild crop relatives. Using allozyme techniques to examine genetic variation within populations, Rick et al. (1977) showed that a close relative (Lycopersicon pimpenellifolium [Jusl.] Mill.) of cultivated tomatoes exhibits a meso-center ofallelic diversity in northern Peru and southern Ecuador. Although the sub-centers of Turkish wheat diversity were initially labelled "micro-centers" (Harlan 1951), they occupy areas much larger than the microgeographic scale identified in later studies. Microgeographic concentrations ("micro-centers") of diversity have been demonstrated in wild plant species but not cultivated ones. Slender oats (Arena barbara[Port] Link), for instance, is characterized by both regional and local patterns of morphological variation that coalesce in small-scale micro-centers (Hamrick and Allard 1972; Jain 1969, 1975). At the regional level, highly polymorphic populations of the species mark certain coastal areas of central and northern California (10-1000 kin2). At the local level, the most polymorphic populations were found to be concentrated in specific sites within the boundaries of a field station (< 10 km2). Jain (1969:95) concludes that "the degree of polymorphism . . . varies highly among individual plants. Site characteristics as well as microclimatic variation and other sources of heterogeneous environments might be involved." Environmental factors also have been posited as the basis for microgeographic patterns in the distribution of allelic variation within wild bar-

1991]


179

human-geographic territories, although such concurrence does not necessarily take place. Human-geographic criteria have underlain a different array of areal units, including the following: the field or the several fields of a single household (e.g., Brush et al. 1981); the ethnic group (e.g., Alcorn 1984); and continental assemblages of peoples (e.g., C. Sauer 1950, 1952). Absent from the list of the primary human-geographic scales employed in ethnobotanical research is the region. Regions defined on the basis of human-geographic criteria potentially provide a useful basis for crop-conservation research. Generally ranking between the field and ethnic group, they form an intermediate-scale unit that represents areas where environmental, subeultural, economic, or other differences cross-cut the territory of a people or political state (Gilbert 1988; Pudup 1988). Because the formation of regions largely depends HUMAN-GEOGRAPHIC REGION on social relations dealing with economic proIf crop-conservation research and planning duction, their spatial manifestation corresponds were to adopt spatial-scaleconcepts solely from closely to certain aspects of agricultural and landbiogeography, it would overlook the impact on use organization. Sub-divisions of these social crop diversity of agricultural practices and the regions, referred to here as micro-regions, fregeographical organization of agriculturalsociet- quently approximate a microgeographic scale in ies. Spatial differentiationof diverse ~xa at the the biogeography of cultivated plants. Microremacro- and mesogeographic levelsduc to human gions therefore offer a human-geographic temactivities is illustrated masterfully by major plate for examining the agricultural activities most scholarly works on the origin and evolution of relevant to the local partitioning of genetic dicrop plants (Harlan 1975a; C. O. Sauer 1950, versity. 1952; Vavilov 1926, 1950). Even at the level of T H E PARTITIONING OF CROP individual fields, an evolutionary-ecological D I W R S I ~ : POTATOr,S IN THE viewpoint reveals multiple links between the CENTRALANDES contemporary distributionsof cultivated plants and the spatial organization of agriculture by A complex of seven domesticated potato spepresent-day farmers (e.g.,Anderson 1952). Eth- cies exhibits a macro-center of diversity that exnobotanical research has furtherunderscored the tends from central Bolivia to central Peru at elimportance of relationsbetween the distribution evations between 3000 and 4200 m above sea of crop diversity and the geographical dimen- level (Hawkes 1978, 1990). In the absence of sions of human agriculturalactivities(e.g.,Jo- mesoscale studies, it is unknown whether the dihannessen ctal. 1970). versity of S. stenotomum Juz. et Buk. and S. Areal frameworks employed in most ethno- tuberosum subsp, andigena (Juz. et Buk.) botanical research have been defined on the basis Hawkes--the two most commonly cultivated of either physical-geographic or human-geo- species--is spread evenly throughout the latitugraphic criteria.Use of physical criteriahas led dinal and ecological extent of the Central Andean to the selectionof physiographic regions,such as macro-center. Genetic diversity within most if the Vilcanota Valley in the southern Peruvian not all cultivated potatoes is presumed to have Andes (Gade 1975) and, in other Central Andean arisen as the result of hybridization and inareas, physiographic "micro-regions," spatial trogression among domesticates and wild species units "defined by small river valleys" (Brush et (Hawkes 1978; Ugent 1970). Hawkes (1978:66) al. 1981:75). Regions demarcated on the basis conjectures that the entire complex of domestiof physiographic features might coincide with cates evolved from a single wild ancestor, S. canley (Hordeum spontaneum C. Koch) in the eastem Mediterranean (Nevo et al. 1979). A clinal gradient in genetic differences was found to correspond to mean annual temperature, evaporation, and rainfall. Moreover, Nevo and his colleagues indicate that genetic diversity is clustered in areas smaller than 1000 km 2. The allelic variation of wild rice (Oryza perennis Moench) also has been found to correlate positively with environmental heterogeneity (Morishima et al. 1984). In sum, studies of the microgeographic patterning of allelic diversity within three wild plant species (slender oats, wild barley, wild rice) indicate micro-centers o f diversity that are thought to be molded by environmental factors. The analogous question of whether environmental heterogeneity shapes similar micro-centers of diversity in cultivated plants has not previously been investigated.

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ECONOMIC BOTANY

[VOL. 45

asense Hawkes. He reasons that the diploid S. soil type, time of seeding, date of maturity, height, stenotomum represents "the most 'wild-looking', nutritive value, use, and other properties." To explain the geographical and taxonomic and therefore presumably primitive, species of cultivated potato." According to Hawkes the partitioning of genetic diversity in landraces of other common potato of Andean agriculture (S. Andean potatoes, the study examines a pair of tuberosum subsp, andigena) originated as an am- evolutionary forces: physical-environmental sephiploid hybrid ofS. stenotomum and the weedy lection and gene flow (Hedrick 1986). Although genetic drift structures the distribution of diverspecies S. sparsipilum (Bitt.) Juz. et Buk. The location of regions and the identification sity in many organisms, it probably is not imoftaxa that ought to be the focus of conservation portant in crop species because of the general strategies incorporating Andean potatoes require practice of selecting seed for planting from hunassessments of the geographical and taxonomic dreds of plants (Bradshaw 1975:47). In exampartitioning of genetic diversity in the crop. Gen- ining the partitioning of genetic diversity, the eralizing from the genetic structure of wild plant present study was motivated by findings on the populations, Brown (1978) recommends that lo- microgeographic variation of wild plant species cally distributed alleles occurring at high fre- described above. Accordingly, we investigated quencies need to serve as the objective of crop the degree of physical-environmental heterogeconservation efforts. His suggestions, however, neity among sites as a potential cause of geoare derived exclusively from evaluations of twelve graphical differences in allelic diversity. Our study wild species and therefore might not be com- also focused on the potential genetic imprint of pletely applicable to cultivated plants. Moreover, social networks that local farmers use to obtain for conservation purposes, "locally distributed seed potatoes. Shown to shape the occurrence of alleles" must be identified with respect to con- locally endemic landraces and thereby mold areal crete geographical and taxonomic units. In ad- "cultivar regions" (Zimmerer nd.,a), such netdition to independently distributed alleles, com- works might be expected to configure the distribinations contained in so-called coadapted gene bution of allelic variation as well. complexes need to be considered in formulating MATERIALS AND PROCEDURE crop-conservation strategies (Brown 1978; FranSix landraces of potatoes--referred to by kel and Soul6 1981). Like single alleles, unique genotypes are distributed distinctly in geograph- Quechua agriculturalists as kusi, pitiki~a, puka ical space and within taxonomic categories. mama, qompis, suyt'u, and wakoth'u (see Fig. Micro-scale partitioning of allelic variation in 2)--were selected for study. Morphologically dislandraces of the Andean potato crop comprise tinct and given a separate name by local inhabone axis of the present study. The study defines itants, each taxon meets the standard definition the microgeographic scale to encompass the fields ofa landrace (Harlan 1975b). Three human-geoof a micro-region, an area whose inhabitants share graphic micro-regions within highland Paucara local cultural identity and ties to the same outlet t a m b o were chosen for study. Colquepata, for the marketing of agricultural production. In Maqhopata (adjacent to the village of Challathe case of highland Peru, micro-regions often bamba), and Mollomarca comprise ecologically correspond to political districts, administrative distinct segments of the topographically diverse territories that reflect past and present patterns eastern Andes (Fig. 1). Human-geographic coof socio-cultural and economic organization. The hesiveness of each micro-region is due primarily taxonomic focus of the study, and its second axis, to differences in ethnic identity that originated are landraces. Landraces are defined by Harlan during the Inca period (14th-16th centuries) or (1975b:618) to " . . . have a certain genetic in- earlier (Zimmerer nd.,a). Ethnic contrasts have tegrity. They are recognizable morphologically; been reinforced by subsequent designation as farmers have names for them and different land- separate political districts. In addition, agriculraces are understood to differ in adaptation to tural production in each micro-region relies on ---4

Fig. 3. (Bottom) An agriculturalist of the Mollomarca micro-region in her potato field at 3820 m above sea level (Paucartambo Province, Cuzco Department). Her dress, Quechua dialect, marketing locale, and local "subcultural identity" differ from the rural inhabitants of other micro-regions in highland Paucartambo.

1991]


1$1

Fig. 2. (Top) Twenty of the seventy-nine landraces of native potatoes collectedin the region of highland Paucartambo. The six landracc types analyzed in the present study arc: kusi, bottom row, 5th from the left; qompis, top row, 6th from the left;pitikifta,middle row, 3rd from the left;puka mama, top row, 3rd from the left;suyt'u,bottom row, 2nd from the left;and wakoth'u, top row, 8th from the left.

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ECONOMIC BOTANY

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a separate local marketplace, thus further TABLE 1. TUBERS TESTED FOR ALLOZYME VARIATON (LANDRACES OF HIGHLAND PAUCAR= strengthening cultural-historical contrasts. Along with one hundred or more other potato TAMSO). landraces, the six selected landraces form the Number per micro-~-gion mainstay of agriculture between 3700 and 4100 Colqur Maqho-. Mollom above seal level in each micro-region. They Land,race Total pata pata maraca belong to the tetraploid species S. tuberosum kusi 23 4 12 7 subsp, andigena except pitikifTa which is a mempitikifza 12 1 4 7 ber of the diploid S. stenotomum. Unlike the puka mama 16 3 5 8 majority of the other potato landraces, which qompis 30 10 8 12 evidence locally endemic distributions (Zim- suyt'u 37 13 11 13 meter nd.,a), the six selected taxa are widely dis- wakoth "u 21 4 9 8 tributed. Their commonness permitted the par- Total 139 35 49 55 titioning of allelic variation to be examined as comprehensively as possible. For each of the study's six landraces, tubers were collected from extracts for electrophoresis were obtained by six fields in each of the study's three micro-regions macerating a freshly expanded leaf (150 mg) in (see Fig. 3). Tubers were identified by local ag- a tray over ice. One hundred twenty ~1 of fresh riculturalists and conformed to the morpholog- 0.1 Tris-HC1 buffer, pH 7.5, containing 2% gluical definition of the landrace given above. In tathione was added to the tissue sample prior to each micro-region, five tubers were sampled ran- crushing. The extracts were then absorbed onto domly from each of six fields, yielding a total two 3 x 8 m m Whatmann 3MM wicks and stored sample of 540 tubers (6[landraces] x 3[micro- overnight at - 20~ Electrophoretic assays were performed in two regions/landrace] x 6[fields/micro-region] • buffer systems: Tris-Borate pH 8.3 and I-Iistidine3[tubers/field] = 540 tubers). Ecological and ethnobotanical studies com- Citrate, pH 5.7 (Stuber et al. 1988). The gel slabs plemented the samplihg of tubers for later genetic consisted of 10% potato starch. Phosphogiucoanalysis. Ecological research was designed to es- mutase (PGM), glutamate oxaloacetate transtimate the extent of environmental heterogeneity aminase (GOT) and diaphorase (DIA) were asin each micro-region. The regular variation of sayed in the first system. Malate dehydrogenase climate characteristics (temperature, precipita- (MDH), phosphoglucoisomerase (PGI), 6-phostion, frost frequency) and soil properties (soil type, phogluconate dehydrogenase (6-PDGH), and texture, fertility) as a function of elevation jus- isocitric acid dehydrogenase (IDI-I) were assayed tified the use of the latter value as a rough esti- in the second system. Enzyme activity stains were mate of environmental conditions (Fr~re et al. prepared according to Vallejos (1983). General 1975; Troll 1966). Thus the environmental het- techniques and procedures were described by erogeneity of each micro-region could be ap- Quiros (1981). A total of 139 tubers were assayed proximated by considering the range of eleva- electrophoretically. Although a significant portions spanned in the sample o f fields. tion of the original sample of 540 tubers did not Ethnobotanical inquiry in the study concentrated survive transportation and storage, the 139-tuon the present-day and historical exchange of ber sample represented the geographical scales seed tubers by agriculturalists. The ethnobotan- and taxonomic units originally considered of inical emphasis sought specifically to clarify the terest to the study (Table 1). temporal and spatial coordinates of exchange Three estimates of genetic diversity (total practices. number of alleles, the number of alleles mainStarch-gel electrophoresis was used to measure rained at each polymorphic locus [Ap], and Nei's allozyme variation in the sampled tubers. Allelic measure of total allelic diversity [HT]; see Hamvariation was examined at 10 polymorphic iso- rick 1983 for a review ofgene diversity statistics) zyme loci (Mdh- 1, Mdh- 2, Idh- 1, 6-Pgdh- 3, Pgi- were calculated for tubers grouped on the basis 1, Got-l, Pgm-1, Pgm-2, Dia-1). Nomenclature, of geographical micro-region (3 groups) and inheritance, and lineage relationships for these landrace (6 groups). Each group was treated as a loci are described in Douches and Quiros (1988) genetic subpopulation in order to coincide with and Quiros and McHale (1985). Crude protein the definitions commonly employed in statistical

1991]


TABLE 2. PARTrnONn,~OOF GENETICDIVERSITY IN SIX LANDRACESOF ANDEAN POTATOES FROM mGHLANDPAUCARTAMBO(LANDRACESLISTEDIN TABLE 1; LANDRACE= SUBPOPULATION). Landrace

Genetic measure

Total gene diversity (I-IT) Within subpopulation diversity (Hs) Among subpopulation diversity (D~) Among subpopulafion component of diversity (Dsr/HT)

Microregion

.328 .322 .256 .320 . 0 7 2 .002 .300 .008

treatments of population genetics. For each geographical and landrace subpopulation, the total gene diversity (HT) was partitioned into the diversity within subpopulations (Hs) and the diversity among subpopulations (Dsr). The amongsubpopulation component of total gene diversity was calculated as the ratio DsT:HT. Finally, to assess the distribution and extent of unique genetic combinations, tubers were grouped on the basis of genotype.

RESULTS AND DISCUSSION: DIWRSrrY IN LANDRACES AND MICRO-REGIONS Allozyme results indicate that landraees ofnafive potatoes contain substantial amounts of genetic diversity. The value of total gene diversity (Hr) within landraces varies between. 194 (pitikifuO and .304 (kus0, a range similar to moderately diverse populations of wild plant species (Hamrick 1983: 501-508). (HT values for other landraees are as follows: puka mama, .242; qompis, .295; suyt'u, .266; wakoth'u, .268.) Findings demonstrate a significant association between the average heterozygosity of landrace populations and ploidy level. The five tetraploid landraces belonging to S. tuberosum subsp, tuberosum display greater variation than the single diploid type (pitikifta). The greater amount of genetic diverTABLE 3.

GENE

DIVEI~ITY

183

sity in tetraploid than in diploid potatoes is opposite the pattern characteristic of wild species (Hamrick 1983). Vegetative propagation and corresponding limitations on the entry of sexual recombinants into the gene pool presumably constrain the level of genetic diversity more in the outcrossing diploid potatoes than in the largely self-pollinated tetraploids. Analysis of the partitioning of allelic variation within and among landrace subpopulations shows that greater than three-fourths of the total genetic diversity present in the sample of 139 tubers was partitioned within landraces (I-Is : DST; Table 2). Results confirm previous findings of minor genetic differences among more )ban 30 distinct landraces from the Paucartambo region (Quiros et al. 1990). The allelic variation in landrace populations analyzed here also is found to differ from wild species (Douches, unpublished data). Genetic diversity statistics displayed in Table 2 indicate that the partitioning of allelic variation among landrace subpopulations (Ds = .072) is significantly less than among populations of four wild species of Solanum for which isozyme studies estimated that the among-population contribution to total diversity ranged from .200 to .312 (I-Iamrick 1983: 337). The relatively minor genetic divergence of landrace subpopulations shown in the present study, as noted in Table 3, reveals a similarity of allele type and frequency in diverse landraces. Despite their unambiguous status as separate taxa on the basis of morphological and folk-taxonomic criteria, the six landraces studied presumably share alleles as a consequence of common ancestral populations or interbreeding with broadly similar groups of introgressing wild taxa. Subpopulations defined geographically on the basis of micro-region also differ insignificantly in terms of the magnitude of genetic diversity. Grouping each micro-regional sample of the six landraces into a single subpopulation, similar

BETWEEN LANDRACE SUBPOPULATIONS ( O F F - D I A G O N A L ) A N D AVERAGE

HETEROZYGOSITY(ON-DiAGONAL).

1. 2. 3. 4. 5. 6.

kusi pitikff~a puka mama qompis suyt'u wakoth'u

l

2

3

4

5

6

.303

.024 .188

.084 .092 .297

.022 .027 .060 .294

.031 .017 .085 .042 .267

.023 .022 .046 .017 .026 .268

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TABLE 4. PERCENT POLYMORPHISM PER LOCUS AND GEOGRAPHICAL MICRO-REGION (POTATO LANDRACES OF HIGHLAND PAUCARTAMBO). Locus

Mdh- 1

Mdh-2

Idh- 1

6-Pgdh- 1

Pgi. 1

Got- 1

Got-2

Pgrn-1

Pgm-2

Dia- 1

Mean

Colquepata Maqhopata Mollomarca

100 95.9 96.4

14.3 28.6 10.9

51.4 44.9 49.1

100 93.9 85.5

11.4 30.6 43.6

25.7 14.9 30.2

14.3 24.5 18.9

88.6 71.4 67.3

0 2.0 9.1

97.1 93.9 86.8

50.3 50.1 49.8

amounts of genetic diversity were found in each micro-region (Hr = .322, .311, and .326 for Colquepata, Maqhopata, and Mollomarca, respectively). Other indices of gene variation, ineluding the percent of polymorphic loci (Table 4) and total number of alleles (Table 5), are also alike across the study's three micro-regions. Combined findings confound the expected relationship between allelic variation and the extent of environmental heterogeneity. Based on elevation differences, environmental heterogeneity varies considerably among micro-regions. (The elevations of fields in the study ranged across 380 m in Colquepata, 40 m in Maqhopata, and 230 m in Mollomarca.) The absence of correlation between the extent of physical-environmental variation and genetic diversity differs notably from the findings on wild plant species by studies mentioned above (Jain 1969, 1975; Morishima et al. 1984; Nevo et al. 1979). It appears that the potential divergence of allelic variation in the six landraces studied due to physical-environmental selection is counteracted. Characteristics of agricultural management that distinguish cultivated from non-agricultural environments probably counter the otherwise strong selection pressures of physicalenvironmental factors. The similarity of selection and cultivation practices carded out by agriculturalists in each micro-region, for instance, would contribute to annulling the possible differences arising from the unequal range of physical-environmental conditions (Zimmerer n.d.,b). Allelic variation is partitioned mainly within micro-regions rather than among them (Table 2). An extremely small fraction of total genetic di-

versity can be attributed to differences among micro-regional subpopulations (DsT = .0079). Instead, the majority (99 percent) of total diversity is contained within such geographically defined units. The minor effectiveness of microregions in dividing allelic variation suggests that gene flow or common ancestry overrides selection pressures stemming from physical-environmental differences and isolation. The preponderance of gene flow in vegetatively propagated crops occurs via the exchange of seed tubers. Moreover, the seed tubers of native potatoes are exchanged frequently in order to reduce their load of viral pathogens. Seed tubers of most landraces produced for subsistence are exchanged between intermediate-elevation (3500-3900 m above sea level) and nearby high-elevation fields (39004100 m above sea level) (Zimmerer n.d.,a). In contrast, the six landraces studied here were found to have been traded more widely. Prior to the introduction of improved highyielding varieties roughly two decades ago, the six selected landraces had been produced commercially for sale in local markets as well as in the department's capital city (Cuzco). While used in commerce, the landraces were exchanged frequently and widely throughout the region and beyond (e.g., through the purchase of seed potatoes in the market of Cuzco), leading to widespread distributions in several provinces and departments of Peru's southern highlands that were documented by Vargas (I 949, 1956). (Taking in the Departments of Puno, Cuzco, Apurlmac, the biogeographical range of each of the six landraces covered a minimum of several thousand km 2. Present-day distributions are probably similar.)

TABLE 5. NUMBER OF ALLELES PER LOCUs AND GEOGRAPHICAL MICRO-REGION (POTATO LANDRACES OF HIGHLAND PAUCARTAMBO). Locus

Mdh-I

Mdh-2

Idh-I

6.Pgdh-I

Pgi-I

Got-1

Got-2

Pgm-1

Colquepata Maqhopata Mollomarca

3

2

4

2

2

2

2

3

3

2

4

2

2

2

2

3

3

3

4

2

2

3

2

3

1

2

Pgm-2

Dia-1

Total

1

2

2

2

23 24 25

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TAaI.~ 6. F~Qu~,~cY oF On,~OTYPESPER POTATO to correlate significantly with gene diversity, a LANDRACE(HIGHLANDPAUCARTAMBO). result that is expected given differences in their average heterozygosity (Table 3, on diagonal). Number Finally, although the frequency of unique geofgenoLandrace types Number of plants per genotype notypes among different landraces was not investigated in the present study, it is noteworthy kusi 5 19, 1, 1, 1, 1 pitiki~a 2 11, 1 that the landrace wakoth'u contains a dispropuka mama 2 15, 1 portionately large number, due perhaps to the qompis 5 24, 3, 1, 1, 1 nominal extent of its former commercialization. suyt'u 4 29, 4, 2, 1 Marketed less than the other taxa, wakoth'u prewakoth'u 12 5,2,2,2,2,2 1, 1, 1,1, 1, 1 sumably was exchanged infrequently, thus facilitating intra-landrace divergence. Extra-regional dispersal, whether historical or The distribution ofgenotypes within the samcontemporary, shapes the spatial patterning of ple does not occur evenly in the three microdiversity in a distinct fashion. Many native po- regions of highland Pancartambo. Among locally tato types, however, are not distributed widely endemic genotypes (i.e., those occurring in only one micro-region), which comprise 22 of the 30 at the regional scale. Widespread exchange of the seed potatoes of distinct allelic combinations in the sample, highcommercial landraces differs from the more lim- ly skewed spatial patterns are evident. Fourteen ited exchange network characterizing potato types (14) endemic genotypes originated in Maqhopata that are used solely for subsistence. The latter while significantly smaller numbers were colsystem links adjacent intermediate and high-el- lected in Mollomarca (7) and Colquepata (1). To evation areas, thus molding spatially discrete explain the unequal geographical occurrence of "cultivar regions" (mentioned above) that gen- genotypes, factors influencing rates of sexual reerally do not reach more than 20-30 km 2 (Zim- combination and migration (seed-tuber exmeter n.d.,a). Thus at least two distinct networks change) need to be examined. Contrasting levels channeling the exchange of seed tubers are seen of sexual recombination potentially arise from to shape the spatial patterning of diversity in the differences in the elevational or areal distribution native potatoes of highland Paucartambo. The of two major determinants of recombination likelihood that distinct groups of landraces are rate--the availability of bee pollinators and the distinguished by different scales of distribution interfertile weed S. sparsipilum. Distinct districoncurs with a recent assertion that the spatial butions might be due to the different ranges of patterning of sets of related taxa often cluster field elevations in each micro-region. Yet, notaccording to a nested hierarchy (Kolasa 1989). withstanding a lack of detailed ecological studies, Although clearly controlled by several factors that bumble bees and S. sparsipilum are not reported differ from wild organisms, the distribution pat- to duster markedly within the elevational range terns of cultivated plants appear to evince a sim- of fields present in the study (Hawkes 1978; White ilar set of relations to spatial scale. 1983). A total of 30 genotypes was identified within Local variation in two attributes of potato agthe 139-tuber sample (Table 6). The moderate- riculture also offer a possible explanation for the large number of genotypes within single potato unequal frequencies of unique genotypes in each landraces probably arose through sexual recom- micro-region. The area and spatial patterning of bination and subsequent cultivation of botanical landrace cultivation differ among micro-regions. seed rather than by somatic mutation. This find- Micro-regions with a greater number of distinct ing suggests that each potato landrace does not genotypes-- Maqhopata and Mollomarca -- com necessarily constitute a single clone. It contro- prise areas where fields of landraces are planted verts the common assumption that equates the contiguously in a unit referred to as "suerte" or landrace of potatoes (as well as other vegetatively "mafiay." (This pattern of planting is associated propagated crops managed in peasant and indig- with the practice of sectoral fallow, common in enous agriculture) with the clone (e.g., Brush et the Central Andes [see Orlove and Godoy 1986].) al. 1981:73, 78; Brush 1986:156; Grim 1990). In contrast, fields containing landraces in The number of unique genotypes within each Colquepata are spatially scattered, reflecting the landrace subpopulation (Table 6) was found not absence of sectoral fallow. Moreover, the micro-

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region with the greatest number of distinct genotypes (Maqhopata) also subsumes the largest area oflandrace-containing fields (> 5 ha). Smaller extensions of such parcels cover Mollomarca (3-4 ha) and Colquepata ( < 2 ha). Both the contiguous pattern of fields and their relatively large area would be expected to favor gene flow among landraces and between landraces and wild potato species, thus differentiating the rate of recombination and leading to a spatial concentration of unique genotypes. As importantly, the lower rate of seed-tuber exchange in Maqhopata than in the other micro-regions probably promotes a greater concentration of unique genotypes. The unequal geographical distribution of distinct genotFpes cannot be explained without accounting for the incorporation of genetic recombinants into the gene pool of potato landraces. Ethnographic research on this aspect of Andean agriculture has periodically discussed the possibility that cultivators plant the botanical seed of potato plants (Brush et al. 1981; Jackson et al. 1980). Evidence for the planting of botanic~d potato seed by peasant farmers, however, has been scant. In highland Paucartambo, fieldwork revealed that the botanical seed of native potatoes entered the gene pool of cultivated types via two avenues. First, Paucartambo agriculturalists often plant either a second crop of potatoes or other tuber species such as oca (Oxalis tuberosum Mol.) or a~u (Tropaelum tuberosum R&P) in fields that have been cultivated with native potatoes during the previous year. This rotation practice, especially in fields (known as kutirpa) occupied by potatoes for two consecutive years, permits the coincidental harvesting of volunteer tubers developed from botanical seed. Secondly, field study revealed that at least two Paucartambo cultivators had purposefully propagated tubers that seeded in barley fields during the year following production of potato landraces. Both methods of sowing botanical seed would permit the introduction of sexual recombinants into landrace gene pools. Comprehensive explanations of the spatial patterning of distinct genotypes is critical because of the importance of novel allellic combinations in evolution and hence to conservation programs as well. CONCLUSION This article reviews the usefulness of bingeographical concepts of spatial scale and human-

[VOL. 45

geographic ideas of region for planning the conservation of crop genetic diversity. The twin geographical concepts are applied in examining the genetic diversity of six common landraces belonging to the potato species S. tuberosum subsp, andigena and S. stenotomum, each widely distributed in the study area of highland Paucartambo (southern Peru). Research results indicate that genetic variation is partitioned similarly along both taxonomic and geographical axes. The majority of variation occurs within landraces (75%) and within microgeographic regions (99%). The latter finding confounds the expectation, based on analyses ofallelic variation in wild plants, that contrasting levels of environmental heterogeneity in the study's microregions (Colquepata, Maqhopata, Mollomarca) would cause genetic variation to be distributed in a geographically uneven fashion. Gene flow channeled through the exchange of seed tubers probably accounts for the absence of micrngeographic differentiation in the partitioning of allelic variation. During a several-decade period prior to the introduction of improved potato varieties (circa 1970), the landraces studied were important in local commerce and, in association with this commercial importance, were exchanged widely among agriculturalists both in highland Paucartambo and between the region and outside areas. Exchange of propagules not only produced characteristic region-wide distributions, but it also joined the gene pools of landraces in separate micro-regions and thereby precluded local differentiation. Dispersal of commercial landraces across a relatively large area contrasts the more restricted networks formed by the transfers of potato types that are utilized primarily for household subsistence. The latter are mostly confined to so-called cultivar regions, reticula of high-elevation and adjacent intermediate-elevation areas that are associated loosely with micro-regions. Significant differences in the distribution of distinct genotypes among geographical microregions indicates that unique allelic combinations are distributed unevenly. The spatial patterning and area of potato fields containing landraces appear to shape unequal rates of sexual recombination and hence the uneven distribution of distinct genotypes. It is worthy of note that a much larger number of unique genotypes was found in the Maqhopata micro-region than in Colquepata (14 versus 1); fields of landraces

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located in the former locale are more contiguous and occupy a larger area than those in the latter. The latter micro-region is characterized by a smaller area of native-potato production and one in which fields are scattered rather than contiguous. The entry of diverse genotypes into the gene pool of potato landraces depends on the documented activities, both intentional and unintentional, of Paucartambo's agriculturalists. Through agriculturalists' adoption of genetic recombinants, each morphologically defined potato landrace actually takes in several separate genotypes. The presumed higher rates of sexual recombination in Maqhopata are reinforced by the low degree of commercialization and near absence at present of widespread seed-tuber exchange involving the landraces studied. Findings on the partitioning ofallelic diversity and unique genotypes along both taxonomic and geographical axes offer several suggestions for the design of strategies to conserve native crop diversity in situ. Such suggestions, however, must be prefaced by their reference to a specific combination of biogeographical and human-geographic conditions characteristic of the crop type and its socio-cultural setting. Results of the present study refer to only two of the eight domesticated Andean potatoes (S. tuberosum subsp. andigena and S. stenotomum) and, even more specifically, to a group of six widely distributed landraces. Potato species and landraces possessing more restricted ranges probably evidence a different spatial patterning of genetic diversity. Nonetheless, the existence of a large number of native potatoes possessing regional and extraregional distributions (Brush 1986; Vargas 1949, 1956) similar to the those examined in the present study indicates the need for planning their conservation. One major recommendation for designing conservation strategies sensitive to population ecology counsels the in-depth collection of crop samples in a small number of micro-regions rather than making cursory collections from many sites (Brown 1978: 149). The recommendation assumes that the prominence of localized common alleles would warrant thorough site-specific collections. Results of the present study, on the other hand, indicate that few rare alleles are conrained in regionally widespread landraces of the Andean potato crop. Notwithstanding this significant contrast, they corroborate the argument for a local in-depth design, although for different

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reasons than previously proposed. The present study emphasizes that a microgeographic focus will be necessary in conservation programs directed towards potato landraces with region-wide ranges due to the uneven distribution ofgenotype diversity, which in turn reflects the local concentration of unique genetic combinations. The probable importance of the area and spatial arrangement of fields in producing the concentration of diverse genotypes further underscores the several potential contributions of geographical concepts in conserving crop resources. ACKNOWLEDGMENTS Karl S. Zimmerer's fieldwork for this study was completed between March of 1986 and August of 1987 with the support o f t b e Fulbright Foundation, the National Science Foundation, and the Joint Committee on Latin American Studies of the Social Science Research Council and American Council of Learned Societies with funds provided by the Andrew W. Mellon Foundation. The initial research (March-June 1986) was undertaken while he was employed as Field Supervisor for the "Changes in Andean Agriculture" project of Stephen B. Brush and Enrique Mayer. He is appreciative of their support and encouragement for this project and also acknowledges Cornelio Cusi, Ing. Leonidas Concha, lug. Edsar Gudiel, and Claudio Palamino for their work as field assistants. The helpful cooperation of many peasant farmers in highland Paucartambo is also gratefully acknowledged. Electrophoresis research was supported by a grant from the National Science Foundation to Karl Zimmerer. Dr. K. Rifland (DepurUnent of Botany, University of Toronto, Toronto, Ontario, Canada) kindly provided the sot~ware programs for the calculation of population-genetic measures. The authors are indebted to Stephen B. Brush, Daniel W. Gade, and Lawrence Kaplan for providing helpful comments on the manuscript.

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