LANIER. G.N. and KIRKENDALL. L.R. 1986. Karyology of pseudogamous Ips bark beetles. - Hereditas .... USA, Colorado, Larimer Co., Red Feather Lake.
Hrrediias 105. 87-96 (1986)
Karyology of pseudogarnous Ips bark beetles GERALD N. LANIER and LAWRENCE R . KIRKENDALL
College of Environmental Science and Forestry, State University of New York, Syracuse, N Y 13210, USA, and Department of Biology, Division of Zoology, University of Oslo, Norway
LANIER.G.N. and KIRKENDALL. L.R. 1986. Karyology of pseudogamous Ips bark beetles. - Hereditas 105: 87-96. Lund, Sweden. ISSN 0018-0661. Received November 28.1985 Females in unisexual strains of North American Ips tridens, I. borealis, I . pilifrons, and I . perturbatus and I . acuminatu from Norway and Bulgaria are triploid (3n = 48) and are associated with males and diploid (2n = 32) females that produce bisexual broods. Triploid females require insemination for brood production, hut male-contributed genes do not appear in their offspring: inheritance of morphological markers (frons shape and setation) in North American spp. is strictly mother-to-daughter, as has been shown for isozymes in I. acuminatus. These unisexual Ips strains are therefore pseudogamous (=gynogenetic). For the North American spp.. morphological markers and the prevalence of all-female strains in natural populations indicate that pseudogamy has arisen sporadically. Triploid lines may arise by fertilization of diploid ova by normal sperm. The occurrence of diploid ova was inferred from observations of putatively diploid and tetraploid spermatozoa associated with distortions in meiosis of hybrid males and of nonhybrid males affected by apparently genetically-controlled asynapsis at first meiosis. Lawrence R . Kirkendall, Department of Biology, Division of Zoology, Universify of Oslo, P . 0. Box 1050, Blindern, N-0316 Oslo 3, Norway
Pseudogamy (or gynogenesis: see footnote p. 294 in S T E N S E Tal.H 19853 ~ ~ is one of the most interesting forms of parthenogenesis. Pseudogamous females must mate with a male in order to produce viable broods, but sperm make no genetic contribution to the offspring; only daughters are produced, and they are genetically identical to their mothers (OLIVER 1971). The phenomenon is known from a wide variety of animals and plants (e.g., FOCKE1881; GUSTAFSSON 1946-47; NYGREN 1954; CONNOR 1979; KIESTER et al. 1981). Since pseudogamous females are dependent upon sperm (pseudogamous flowers upon pollen), gonochoristic (dioecious) pseudogamous taxa must exist as “sperm parasites” of one or more sexual species. In both plants and animals, pseudogamous strains have often proven to be interspecific hybrids. In many instances pseudogamous lines are morphologically distinct enough from their sexual ancestors to have been described as one or more separate species. Pseudogamous plants and animals are also usually polyploid, most often triploid. Among the few zoological exceptions of which we are aware are the predominately diploid Poecilia formosa (Girard) (PREHN and RASCH1969), the pseudogamous form of the psychid moth Luffia lapidella
Goeze (NARBEL-HOFSTETTER 1963) and parasitic nematodes of the genus Strongyloides (BOLLA and and MONCOL 1977). ROBERTS 1968: TRIANTAPHYLLOU Diploid pseudogamous plants include Ranunculus auricomus L. and Potentilla argentea L. (GUSTAFSSON 1946 - 47; ASKER 1970). HOPPING (1961; 1962; 1964) discovered unisexual strains of four morphotypes (then considered species: see LANIER 1967; WOOD1977) of the bark beetle Ips tridens (Mannerheim). Daughters in unisexual broods were identical with their mothers, for conspicuous characteristics of the frons. LANIER and OLIVER (1966) reported that broods produced by pairs of sexual individuals from different morphotypes were highly variable; the morphotypes of unisexual strains were within the range of this variability. Pseudogamy was shown to be the reproductive mechanism in all-female broods of I. tridens when crosses of unisexual females with males from a different morphotype, or from a different species, produced all-female broods which reflected only maternal parentage. Pseudogamy was also uncovered in I. borealis Swaine and I . perturbatus (Eichhoff); females of all three species, both sexual and pseudogamous, failed to oviposit unless mated. Pseudogamy was also hypothesized for Ips acu-
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Hereditas 105 (1986)
G. N . LANIER AND L. R. KIRKENDALL
minatus (Gyllenhal) (BAKKE1968). Norwegian populations were found to be female-biased, some only slightly so, but others consisting of 30 or more females per male (see also KIRKENDALL and STENSETH,in prep.). Breeding isolated pairs revealed unisexual strains that were morphologically indistinguishable from normal females; Bakke also stated that unmated females did not produce offspring. In this paper we report the presence of pseudogamy in an additional North American sprucefeeding Ips species, describe the karyology of bisexual and all-female strains of all five Ips species in which pseudogamy is now known to occur, and present evidence for meiotic anomalies that might result in the spontaneous origin of pseudogamy . Treatments of demographic aspects of pseudogamy in Ips and the biotaxonomy of the spruce-feeding species will be presented elsewhere.
Materials and methods 1 . General
North American beetles. - Laboratory colonies were initiated from beetles collected from the bark of naturally infested host material at the localities listed in Table 1. Pedigreed lines were initiated with teneral (young) adults collected from pupal chambers in the bark. To insure virginity of females used in controlled mating studies, the sexes were always held in separate corked shell vials that contained strips of phloem on which the insects fed until they reached reproductive maturity. insects in these vials could be refrigerated (1-3°C) for several months without apparent effect on their vitality or fecundity. The sexes of Ips tridens, I. borealis and I . pilifrons Swaine were separated by conspicuous differences on the frons (LANIER and CAMERON 1969). The sexes of I. perturbatus and I . hunteri Swaine could not be readily separated by external characters (LANIER and CAMERON 1969) so they were either dissected or sexed by the type of excavations individual beetles made when they were forced to bore into logs (males constructed broad “nuptial” chambers while females made narrow tunnels). European beetles. -Collection, storage, and breeding of beetles from Norway and Bulgaria differed in only insignificant details. I. acuminatus was sexed by the shape of the third spine on each lateral margin of the elytral declivity; those of the male are differently shaped and larger (EICHHOFF 1881).
2. Verification ofpseudogamy For North American beetles, the prerequisite of mating for brood production by females of both unisexual and bisexual lines was verified by inducing virgin females to bore into small spruce logs with or without males. Insemination of ovipositing and brood-producing females was checked by examination of spermathecae crushed in physiological saline between a slide and a coverslip. I . acuminatus from north (Moen), west (Austestad, near Eikelandsosen) and east (Saggrenda, near Kongsberg) Norway were bred in the laboratory as described in (1) above. In one set of experiments with Saggrenda beetles, females were removed from under the bark in spring of 1983, before swarming and pairing, and two groups of females each placed with pieces of fresh Pinus sylvestris. The pieces were dissected after 3 weeks at 25°C and females removed from gallery arms and examined for the presence of sperm in their spermathecae. The gallery arms were recorded as with or without eggs and larvae, open or sealed; sealed arms were plugged solidly with frass, while open arms were either not plugged or had one or more beetle-diameter holes leading from the egg arm to the surface. Only females in sealed arms can safely be considered mothers of the eggs with which they are found. In summer 1983 Saggrenda females found starting to construct arms alone (see BAKKE 1968), or with other females but without males being present, were removed and brought back to the laboratory where they were treated as the pre-emergence females discussed above.
3 . Cytology Karyotype determinations were usually made from aceto-orcein squashes (LANIER 1967) of various tissues taken directly from live insects. Some of the I. acuminatus studied were fixed in carnoy (1 part glacial acetic acid:3 parts absolute ethanol) before tissues were squashed and stained with aceto-carmine. Chromosome numbers of males were first ascertained by counting bivalents in meiotic metaphase I cells; these were generally abundant in testes of light brown adults. Meiosis in females proceeds only to pachynema while eggs are in the ovaries, and is completed after the sperm penetrates the egg. Therefore, chromosome numbers in females were determined from mitotic cells in the ovary tips of female prepupae or pupae, in the brains of prepupae or in whole embryos about
Hereditas I05 (1986)
KARYOLOGY OF PSEUDOGAMOUS I P S BARK B E k l L E S
48 h post oviposition. The best preparations of females were from prepupae and pupae that had been injected 6-18 h earlier with about 0.5 pl of 0.01 % colchicine. Cytological preparations were studied by light microscopy at 1200x and photomicrographs were made on Kodak Panatomic-X film. Freehand drawings aided in interpretation of photographs.
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Results Verification Of pseudogamy
Ips females introduced alone into breeding material failed to reproduce if not first inseminated. Females with empty spermathecae constructed irregular, winding tunnels, while egg tunnels of inseminated females were relatively straight.
Table I . Collection localities of Ips species examined cytologically Karyotyped individuals ~
Collection locality
Host species
Ips borealis CAN, Alberta, Banff CAN. Alberta, Cypress Hills CAN. BritishColumbia, Enderhy CAN, Nova Scotia. Margaree USA. South Dakota, Black Hills Interpopulational hybrids
Picea glaucu x, Picea glauca Picea glauca Picea glauca Picea glauca
Ips tridens CAN, Alberta, Banff National Park CAN, British Columbia, Kootenay National Park CAN, British Columbia, Donald Station CAN, British Columbia, 8 mi E of Golden CAN, British Columbia, Rogers Pass USA, California, Crescent City USA, Montana. West Yellowstone Interpopulational hybrids
9
1
0
2 1
0
0
0
0
1
n
0
14
0
0
Subtotal
59
1
3
Picea engelmanni x Picea engelmanni x Picea engelmanni x Picea glauca x Picea engelmanni Picea sitchensis Picea engelmanni
11
0 1 4 0 0 0
0
Picea engelmanni Picea engeimanni Picea engelmanni
Ips hunteri USA. Colorado, Charris USA, New Mexico, Taos Ski Area Ips acuminatus BULGARIA, Govedarci (near Samokov) NORWAY, Hordaland, Austestad NORWAY. Buskerud, Saggrenda NORWAY,Troms, Moen
5
0 17 47 8
0 0
n 4 4 0
2 2
0 0 0
9
8
4 0
10 2 1
0
0
0
4
13
6
0 1
4
1
7
1
4
Piceapungens Picea pungens Subtotal
2
0
0
3
0
0
Pinus sylvestris Pinus sylvestris Pinus sylvestris Pinus sylvestris Subtotal
4 3 0
I 0
7 0
Picea glauca x Piceaglauca Subtotal
GrandTotal a.
5 5 0 4
10 18
Subtotal Ips perturbatus CAN, Alberta, Banff CAN, Alberta, Cypress Hills
~
SR females
0 7 28
Subtotal Ips pilifrons USA, Colorado, Larimer Co., Red Feather Lake USA, Wyoming, GrandTeaton National Park USA. Colorado, Huerfano County, Greenhorn Mt Interpopulation hybrids
Males
~~
Normal females
0
1 1
5
1 8
3
12
142
18
40
0
Picea glauca and P. engelmanni introgress in the Canadian Rocky Mountains, but populations in poorly-drained flats are characteristic of P . glauca and those of an upland site are characteristic of P. engelmanni
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Hereditas 105 (1986)
G N . LANIER A N D L. R . KIRKENDALL
Table2 Association between insemination status and arm status: pre-swarming females, and post-swarrning females which had bored under the bark without males. for Ips ncuminatus from eastern Norway Number of females taken from arms with Females A . Pre-swarming
N o eggs
Eggs only
-
14 0
6
9 2
0
+
B. Boring without males ~
Sperm
-
+
~
~
-
Total
+ larvae
1 0
____
23 2
+
Eggs
6
3
1
10
Table 3. Relationship between insemination and egg hatch; females could have moved into or out of "open" arms, while females in "sealed" arms are presumed to be mothers to the eggs in those arms (see text). Same I . acuminafus females as in Table 2 Number of females taken from: Sperm
-
+
Open arms with
Sealed arms with
Eggs only
Eggs
5 0
3 0
+ larvae
Eggs only
Eggs + larvae
1 1
0 10
At least some Ips females from most localities produced all-female broods, with very high (occasionally 100 %) egg to adult survivorship. Daughters from all-female broods also produced unisexual broods. We therefore consider the unisexual lines in I . acurninatus, I . tridens, I . pilifrons and I. perturbatus to be parthenogenetic. In over three years of breeding 1. acuminatus from various Norwegian and Bulgarian localities and more than 2,000 pairings of the North American species, there has been no evidence that parthenogenetic females can breed without sperm (KIRKENDALL, unpubl. obs.; LANIER,unpubl. obs.; LANIERand OLIVER1966). On the contrary, attempts to breed females of either type without exception failed if the male died or escaped. The inability to breed without sperm was noted in parthenogenetic Norwegian I . acuminatus from Moen, Austestad and Saggrenda, but has been more closely investigated only in beetles from the latter locality (Tables 2,3). No attempt was made to breed Bulgarian parthenogenetic females without males, due to the limited number available. Uninseminated I. acuminatus females occasionally laid eggs, usually just a few but up to 18 in laboratory rearings; ovipositing but uninseminated females have also been found in the field (in Saggrenda;
KIRKENDALL, unpubl.). These eggs did not hatch. Most of the pre-swarming I . acuminatus had not been inseminated (Table 2 ) ; the females with sperm were clearly darker and therefore were overunpubl.). wintering parent females (KIRKENDALL, Most of the uninseminated females produced no eggs, while all of the inseminated females ovipositcd. In one of the two containers with preswarming females, 019 females had sperm; 10 eggs had been laid (maximum 4 in one arm), and 4 females had an egg in the oviduct when dissected. Roughly equal proportions of inseminated and uninseminated females were found under the bark after swarming and colonisation (Table 2). Nine of 10 uninseminated females did not oviposit, but one uninseminated female was found in the unsealed portion of an egg arm which in the distal portion contained a second female, with sperm in the spermatheca. This arm had both eggs and larvae. Most inseminated females were found in arms with both eggs and larvae, but two were in eggless arms when the pieces were dissected. In sealed arms, larvae were produced only by females with sperm in their spermathecae (Table 3). It was not known whether the females withoui sperm were .parthenogenetic or sexual, though it can safely be assumed that almost all were parthe-
Hereditas 105 (1986)
KARYOLOGY OF PSEUDOGAMOUS IPS BARK BEETLES
Fig. 1-6. Normal and irregular cell division in Ips species. Fig. 1. Mitosis in female Ips borealis, bisexual strain (2n = 32). Fig. 2. Mitosis in female I . borealis, unisexual strain (3n = 48). Fig. 3. First meiotic prometaphase (PMI) in male F, I . perturbatus x hunieri showing synapsis of all 15 autosomal bivalents and the parachute-shaped (Xyp) sex bivalent. Fig. 4. PMI in male I . borealis “asynaptic line” showing 8 bivalents, 14 univalents, and an Xy, configuration. Fig. 5. PMI in F, male I . perturbatus x tridens with univalents, bivalents and a quadrivalent (arrow). Fig. 6. Bridged telophase I1 and putative diploid spermatocytes in F, I. borealis x hunteri. Bar in figures indicates 5-micron actual scale.
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Hereditas 105 (1986)
nogenetic: beetles emerging from the same mate- ploidy of individual mitotic cells was easily determinrial were 97 YO female (sex ratio 35:1; KIRKENDALL ed by the number of times autosome no. 1 was repreand STENSETH, unpubl.), and 24 of 26 that were bred sented. Thus, even if exact counts of 48 chromoproduced all-female broods (=92 YO partheno- somes could not be made, the 3n nature of a given cell unpubl.). Since the data in was usually obvious because there were three large genetic; KIRKENDALL, Table 3 indicate that all females require sperm for autosomes present (Fig. 2). Nevertheless, an indisuccessful reproduction, we consider the partheno- vidual was not scored for ploidy and included in genetic females to be pseudogamous. Table 1 unless an unequivocal count was made in at Although sperm is necessary for development of least one cell. eggs in unisexual strains, it need not be sperm from Centromeric position for autosomes nos. 1 and 2 conspecific males. Interspecific crosses between varied among species (Table 4). A difference the tridens-borealis-pilifrons group and the pertur- between the I . pilifrons no. 1 and the I. hunteri no. batus-hunteri group resulted in successful repro- 1 was clearly evident in mitosis of the hybrid. duction of unisexual strains but partial or complete Except for such size and centromeric position diffesterility for sexual females. rences, mitotic divisions in hybrids were normal. Morpho1,ogical differences among North AmeAt first meiotic metaphase males of all species rican species and intraspecific markers (shape and had 15 autosomal bivalents and a parachute-shaped setation of the frons) in I. tridens, I. borealis and I. sex bivalent to give the formula 15AA + Xy, (terpilifrons showed mixed inheritance when the moth- minology of SMITH 1950) (Fig. 3). In females, meier was from a bisexual brood but strictly matrocli- osis proceeds to pachynema, where it rests until nous transmission when the mother was from a uni- oviposition. Attempts to study meiosis in recently sexual brood. laid eggs were frustrated by inability to find the nucleus among masses of yolk and other materials. At second meiotic metaphase (MII) centromeric 2. Karyology positions are revealed by the free chromatid arms; Preparations from 200 individuals contained thus metacentric chromosomes form symmetrical meiotic or mitotic cell divisions with chromosomes crosses, submetacentric chromosomes form asymsufficiently spread to accurately count them (Table metrical crosses, and acrocentric chromosomes 1). Of these, 142 were males, 18 were females from form “V”s. MI1 cells in males corroborated the bisexual broods, and 40 were females from unisex- judgments on centromeric position in mitotic cells ual broods (“Sex Ratio”, or SR females). in both sexes, but one must use caution in compaIn all of the species and populations studied, indi- ring mitotic and second meiotic chromosomes viduals from bisexual strains had 16 pairs of chro- because most of the smaller autosomes are diphasic mosomes (2n = 32) (Fig. 1) while females from uni- and the heterochromatic arm appears to contract to sexual lines were invariably triploid (3n = 48) (Fig. a greater degree at MI1 than it does at mitosis. 2). In all species, one autosome (no. 1 in size seMeiosis was usually normal in hybrid males proquence) was clearly larger than all of the other chro- duced in reciprocal crosses of I. tridens with either mosomes in the haploid complement. Autosome I . pilifrons or I. borealis. In the tridens-borealis no. 2 was also distinguishable in most preparations hybrid at least one of the smaller bivalents apwhile other autosomes were not generally individu- peared to be heteromorphic, but this is believed to ally recognizable by size or other features. The result principally from differences in the sizes of heterochromatic portions of the homologues. Occasionally, one or more bivalents disassociated Table 4. Arm ratios for the two largest autosornes (1 and 2) measprematurely. As could be predicted from male meiured in mitotic ceills osis, the hybrids of tridens with borealis orpilifrons were highly fertile. A considerable size difference Autosome no. in I . borealis and I. pilifrons seemed to preclude 1 2 mating between these species; 10 pairings of each reciprocal of this combination were made and no 1. acuminatus 1:l 1:9 egg galleries were constructed. 1. borealis 4:5 4:5 Crosses of I . perturbatus and I . hunteri produced I . tridens 4:s 4:s 1.pilifrons 4:5 4:s semifertile F1 which had many normal meiosis I I . perturbatus 3:s 1:3 prometaphase cells and occasional (ca 20 “/o) figuI . hunteri 3:4 1:3 res with univalent or multivalent associations. Bndg-
Hereditas 105 (1986)
ed telophase figures suggest that homologues of the two species may have inverted segments. Crosses between members of the I. tridens-pilifrons-borealis group and the I . perturbatus-hunteri group rarely produced offspring; offspring were sterile and showed massive distortion of male meiosis (Fig. 5,6). The parachute-shaped Xyp was the only consistently normal chromosome association. Multivalent associations suggest chromosome segment translocations in the respective genomes (Fig. 5 ) . First anaphase was either bridged or failed completely because no bivalents had formed. These unreduced cells apparently divided equationally to produce diploid telophase I1 nuclei (spermatocytes) (Fig. 6) that evolved into spermatozoa and were presumably 2n (Fig. 9). Still larger spermatozoa (Fig. 8,lO) were produced by individuals in which first anaphase was severely bridged. We suggest that the bridged cells form “pseudo-telophase 11” restitution nuclei (Fig. 6) that form tetraploid spermatozoa. None of the sperm produced in these hybrid males appeared to be normal. A male 1. borealis from Margaree, Nova Scotia, had very few autosomal bivalents at metaphase I (MI) (Fig. 4). These chromosomes formed restitution nuclei that went into MI1 with 32 diploid chromosomes that divided equationally and eventually formed abnormal spermatozoa (Fig. 9) similar to those of sterile hybrids. Pairings of siblings of this male, inter se, and with I . borealis from other populations revealed that some of the siblings were sterile while others were highly fertile. Broods that were produced were bisexual. Further crosses and karyological study involving descendants of the original brood revealed sporadic sterility and asynapsis. The breeding patterns suggested that asynapsis was a product of homozygous recessive genes, but the line died out before this hypothesis could be definitively tested. An asynaptic cell that showed 32 unpaired chromosomes that were typical in shape but larger than true MI1 chromosomes, was seen in an I. perturbarus male. Other MI cells in the same individual were normal. Unfortunately, the sperm in this individual were not scrutinized, but those incidentally captured in photographs appear to be normal.
Discussion Although gametogenesis in pseudogamous females has not been investigated, the uniform transmission of morphological markers in I. borealis, I . pilifrons and I . tridens suggests that no genetic reassortment
KARYOLOGY OF PSEUDOFAMOUS IPS BARK BEETLES
93
Fig. 7-10. Normal and abnormal spermatozoa in testes of males. Fig. 7. I . borealis, normal. Fig. 8. I. borealis asynaptic line, putative tetraploid-bearing. Fig. 9. I . borealis asynaptic line, putative diploid-bearing and one (arrow) of normal size. Fig. 10. F, 1. perturbatus x hunteri putative tetraploid-bearing.Bar in Fig. 8 indicates 5-micron actual scale for all figures.
occurs (see also HOPPING 1964; LANIER and OLIVER 1966; LANIER 1967). In Norwegian I . acuminatus, daughters from unisexual broods have isozyme phenotypes that are the same as their mothers but different from their fathers, when the two parents differ at loci exhibiting Mendelian inheritance patterns in sexuals (KIRKENDAL.L, KAUKEINEN, SAURA and LOKKI, unpubl.). We did not find unisexual lines in 1. hunteri, nor have such been reported from other Ips species. Pre-swarming sex ratios for most Ips populations are 1:l (THOMAS 1961; SEITNER 1923; COOKet al. 1983), though such reports do not exclude the possibility that pseudogamous lines exist in other populations of the same species. For example, GOBEIL(1936) found roughly equal numbers of male and female progeny (48 % female; n=500) of I. perturbatus under the bark of one tree in Quebec,
94
G . N . LANIER A N 0 L. R. KIRKENDALL
Hereditas 105 (1986)
whereas we found pseudogamy and female-biased site -diploid pseudogamy giving rise to triploid pseupopulation sex ratios (LANIER, unpubl. obs.) in dogamous lines - seems to be true for at least Poecilia Alberta but have not examined specimens from formosa and related triploids, and diploid pseuother provinces. dogamous clones are regularly created in fishNatural hybridization has been implicated in the breeding programs by hybridization and temperature origin of pseudogamy in a variety of plants (GUSTAFS-shock (CHERFAS 1981). Pseudogamous lines arising SON 1946-47; NYGREN 1954) and animals (e.g., PALA from experimental manipulations of newt (Pleuroet al. 1982; RUTTNER-KOLISKO 1969; DROSOPOULOS deles waltlii, P. poireti) eggs also are mostly diploid 1976, 1977; MOORE et a1 1970; CIMIN01972; TURNER(JAYLET 1978). On the other hand, the and FERRIER et al. 1980; SCHULTZ 1980; CHERFAS 1981; MINTON expression of pseudogamy is dependent on ploidy 1954; UZZELL 1964; SESSIONS 1982). In many instan- level in the freshwater planarian Dugesia lugubris: ces, hybridization is believed to be the primary - or reduction occurs in normal oocytes but not in the only - stimulus producing pseudogamous lines. polyploid oocytes (BENAZZI LENTATI 1975, 1979). Pseudogamous Ips borealis from Banff, Alberta (as We have not found diploid genotypes in any of the well as from certain other populations not studied chromosome preparations we have made from cytologically) are very similar in size and mor- known unisexual lines. We cannot exclude the posphology to I . borealis-tridens hybrids produced in sibility that diploid pseudogamous clones exist, but the laboratory (LANIER, unpubl.). Gonochoristic they cannot be common in the populations we have females in the same population are typical I . studied. borealis. We believe that the pseudogamous The existence of pseudogamy in various morphs females in this case present a vignette of hybridiza- and species of Ips as well as its broad geographic tion that has been protected from dilution by gene- occurrence argue for multiple origins of unisexual rations of pseudogamous reproduction. lines rather than a unique origin followed by recomHowever, the origin of pseudogamy by hybridi- bination and/or mutation. It is likely that the North zation is improbable for Norwegian I . acuminatus, American spruce-feeding Ips and I. acurninatus I . borealis from the Cypress Hills area, and I. pili- have the ability to repeatedly "spin off" triploid frons from southeastern Colorado because these pseudogamous lines. Future morphological, cytoareas are remote from ranges of other Ips with logical and electrophoretic investigations should which hybridization might occur. More important- therefore reveal considerable variation within and ly, we were not able to find morphological differen- among pseudogamous populations, as has been ces between pseudogamous and gonochoristic found in many other pseudogamous organisms females from most localities (see also BAKKE 1968). (see, e.g., reviews or summaries in GUSTAFSSON A mechanism for the production of triploids other 1946-47; NYGREN 1954; BENAZZI 1982;TURNER 1982; than by hybridization seems necessary. VRIJENHOEK 1984a,b; FumYMA et al. 1984; BOOIJ The asynaptic strain of I. borealis we discovered 1981). If this hypothesis of multiple origins for pseuappears to produce diploid spermatozoa which are dogamy in Ips is correct, then the wide distribuprobably nonfunctional. If analogous meiotic or tion of triploidy suggests that either pseudogamy premeiotic events take place in females with the normally arises in triploid individuals, or that tripsame mutant gene(s), diploid eggs would result, loidy has arisen and displaced diploid pseudogamy and successful fertilization would then produce in ernh of the instances we have studied. triploid zygotes. Stabilization of the triploid genoIt is noteworthy that pseudogamy has arisen types would then depend on (1)successful develop- independently in a variety of Ips taxa. Thelytoky is ment into reproductively competent, female indivi- found in only two other bark beetle species (one duals; (2) expression of their parents' ability to pro- population of Scolytus rugulosus in Israel, GUREduce eggs with a somatic ploidy; and (3) the ability VITZ 1975; Pityophthorus puberulus in eastern of the egg nucleus to exclude the chromosome North America, DEYRUP and KIRKENDALL 1983). Ips complement of the sperm while retaining a require- bark beetles must share an unusual vulnerability to ment for sperm stimulation of zygote development. the accumulation and expression of mutations Triploidy might also arise without participation of a affecting the meiotic process in such a way that male, as has been experimentally demonstrated for sperm-dependent parthenogenesis and triploidy certain weevils exposed as eggs to temperature readily evolve in this genus. shock (see review by VIRKKI 1983). In the above scenario we have assumed that AcknowledgemenLy. - W e thank .I. Lokki and A Saura (Departpolyploidization precedes pseudogamy. The oppo- ment of Genetics, University of Helsinki), N.C. Stenseth (Depart-
Hereditas 105 (1986) ment of Biology, Division of Zoology, University of Oslo), and N. Virkki (Agricultural Experiment Station, University of Puerto Rico) for comments on the manuscript. We are also grateful to A. Bakke (Norwegian Forest Research Institute) for supplying some of the Ips acuminatus used in the breeding experiments. The results are based on research supported by Forestry Canada and NATO, the Nansen Foundation, the Norwegian Agricultural Science Foundation (NLVF), the Norwegian National Science Foundation (NAVF), and the Department of Biology, Division of Zoology of the University of Oslo.
Literature cited ASKER, S . 1970. Apornixis and sexuality in the Porentilla argentea complex. - Herediras 66: 127-144 BAKKE, A. 1068. Field and laboratory studies on sex ratio in Ips acuminarus (Coleoptera: Scolytidae) in Norway. - Can. Entomol. 100: 640-648 BENAZZI, M. 1982. Speciation events evidenced in Turbellaria. - In Mechanisms of Speciation (Ed. C. BARIGOZZI),Alan R. Liss, Inc., New York, p. 307-344 BENAZZI LENrATI, G . 1975. Different relationship between egg and sperm in the pseudogamy of planarians according to the type of oogenesis. synaptic or asynaptic. - Rend. Acad. Nuz. Lincei, Ser. V l l l , 49: 287-289 BENAZZI LENTATI. G . 1979. Polyploidy and types Of development in the triclad Dugesia lugubris s.1.: interaction between the ploidy level and the origin of amphimixis or pseudogamy. Cnryologiu 32: 247-263 BOLLA, R.I. and ROBERTS, L.S. 1968. Gametogenesis and chromosomal complement in Strongyloides ratti (Nematoda: Rhabdiasoidea). - J . Purasirol. 54: 849-855 BOOIJ. C.J.H. 1981, Biosystematics of the Muellerianellu complex (Hornoptera, Delphacidae). taxonomy, Morphology and distribution. - h’eth. J . Zoo/. 31: 572-595 CHERFAS.N.B. 1981. Gynogenesis in fishes. - In Genetic Bases of Fi.4 Selection (Ed. V.S. KIRPICHNIKOV). Springer-Verlag, Berlin, p. 255-273 CIMINO, M.C. 1972. Egg-production, polyploidization and evolution in a diploid all-female fish of the genus Poeciliopsis. Evolution 20: 294-306 CONNOR. H.E. 1979. Breeding systems in the grasses, a survey. -New 2. J . Rot. 17: 547-574 COOK, S.P.. WAGNER, T.L., FLAMM, R.O., DICKENS, J.C. and COULSON.R. N . 1983. Examination of sex ratios and mating habits of Ips avulsus and I. calligraphus (Coleoptera, Scolytidae). -Ann. Entomol. SOC.Am. 76: 56-60 DEYRUP. M. and KIRKENDALL, L.R. 1983. Apparent parthenogenesis in Pityophthorus pirberulus (Coleoptera: Scolytidae). At7n. Entomol. SOC.A m . 76: 400-402 DROSOPOULLIS, S . 1976. Triploid pseudogamous biotype of the leafhopper Muellerianella fairmairei. - Nature 263: 499-500 DROSOPOuLUS, S . 1977. Biosystematic studies on the MuelIcrioncllu complex (Delphacidae, Hornoptera, Auchenorrhyncha). - Meded. Lundbouwhogeschool, Wageningen 77: 1-133 EICHHOFF, H’. 1881. Die europaischen Borkenkafer. - Julius Springer, Berlin FOCKE,W.O. 1881, Die Pflanzenmischlinge.- Bornrraeger, Berlin FUTUYMA, D.J., CORT, R.P. and VAN NOORDWIJK. 1. 1984. Adaption tri host plants in the fall cankerworm (Alsophila pometaria) and its bearing on the evolution of host affiliation in phytophagous insects. - A m . Nat. 123: 287-296 GOBEIL, A.R. 1936. The biology of Ips perturbntirs Eichhoff. Can. J . Res. 14: 181-204
KARYOLOGY OF PSEUDOGAMOUSIPS BARK BEETLES
95
GUREVITZ,E. 1975. Contribution a I’itude des Scolytidae I. Comportment de diffkrents stades du scolyte mbditerranden. Scolytus (Ruguscolyrus) mediterraneus Eggers en Israel. - Ann. 2001. - ikol. Anim. 7: 477-489 GUSTAFSSON,A. 1946-47. Apornixis in the Higher Plants. Part 1-111. -Lunds Univ. A m k r . N. F. Avd. 11, 42(3), 43(2), 43(12). 370 PP HOPPING, G.R. 1961. Techniques for rearing Ips DeGeer (Coleoptera: Scolytidae). - Can. Entomol. 93: 1050-1053 HOPPING, G.R. 1962. The sex-ratios in Ips rridens Mannerheim (Coleoptera: Scolytidae). - Can. Enfomol. 94: 506 HOPPING, G.R. 1964. The breeding evidence indicating two genetic types of females of Ips tridens (Mannerheim). - Can. Entomol. 96: 117-118 JAYLET, A. and FERRIER,V. 1978. Experimental gynogenesis in the newt species Pleurodeles walrlii and P. poireti. - Chromosoma 69: 65-80 KIESTER, A.R.. NAGYLAKI,T . and SHAFFER,B. 1981. Population dynamics of species with gynogenetic sibling species. Theor. Pop. Biol. 19: 358-369 LANIER,G.N. 1967. Biosystematic, cytological and sex ratio studies of closely related bark beetles (Coleoptera: Scolytidae). - Ph.D. thesis, Univ. Calif., Berkeley LANIER, G.N. and CAMERON, E.A. 1969. Secondary sexual characters in the North American species of the genus Ips (Coleoptera: Scolytidae). - Can. Enromol. 101: 862-870 LANIER, G.N. and OLIVER,J.H., Jr. 1966. “Sex-ratio” condition: unusual mechanisms in bark beetles. -Science 153: 208-209 MINTON, S.A., JR. 1954. Salamanders of the Ambysfoma jeffersonianum complex in Indiana. - Herpetologica 10: 173-179 MOORE, W.S., MILLER,R.R. ~ ~ ~ S C H U L T Z . 1970. R . J .Distribution, adaption and probable origin of an all-female form of Poeciliopsis (Pisces: Poeciliidae) in northwestern Mexico. Evolution 24: 789-795 NARBEL-HOFSTETTER, M. 1963. Cytologie de la pseudogamie chez Luffiu lapidella Goeze (Lepidoptera Psychidaef. - Chromosoma 13: 623-645 NYGREN, A . 1954. Apomixis in the angiosperms. 11. - Bot. Rev. 20: 577-649 OLIVER, J.H., JR. 1971. Introduction to the byrnposium on parthenogenesis. - A m . 2001.11: 241-243 PALA, M . , CASU, S. and LEPORI,N.G. 1982. Stabilized natural interspecific hybrid population of the fresh water planarians Dugesia gonocephala s.l. (Turbellaria, Tricladida). - Curyologia 35: 247-256 PREHN, L.M. and RASCH, E.M. 1969. Cytogenetic studies of Poecilia (Pisces). I . Chromosome numbers of naturally occurring poecilid species and their hybrids from eastern Mexico. Can. J . Genet. Cyrol. 11: 880-895 RUTTNER-KOLISKO, A. 1969. Kreuzungsexperimente zwischen Brachionus urceolaris und Brachionits quadridentatus, ein Beitrag zur Fortpflanzungsbiologie der heterogonen Rotatoria. -Arch. Hydrobiol. 65: 397-412 SCHULTZ, R.J. 1980. Role of polyploidy in the evolution of fishes. - In Polyploidy: Biological Relevance, Basic Life Sciences, pol. 13, (Ed. W.H. LEWIS),Plenum Press, New York, p. 313-340 SEITNER. M . 1923. Beobachtungen und Erfahrungen aus dem Auftreten des achtzahnigen Fichtenborkenkafers Ips typographus L. in Oberosterreich und Steiermark in den Jahren 1921 und 1922.-Zentralbl. Ges. Forstwesen 49: 1-1 I , 149-162.270-277 SESSIONS, S.K. 1982. Cytogenetics of diploid and triploid salamanders of the Ambystoma jeffersonianum complex. - Chromosoma 84: 599-622 SMITH, S.G. 1950. The cyto-taxonomy of Coleoptera. - Can. Entomol. 82: 58-68 STENSETH,N.C., KIRKENDALL, L.R. and MORAN. N. 1985. On the evolution of pseudogamy. - Evolution 39: 294-307
96
G N L A N I E R AND L. R . KIRKENDALL
THOMAS,J.B. 1961. 'The life history of Ips pini (Say) (Coleoptera, Scolytidae). - Can. Entomol. 93: 384-390 TRIANTAPHYLLOU, A.C. and MONCOL,D.J. 1977. Cytology, reproduction, and sex determination of Strongyloides ransomi and S. papillosus. -J . Parasitol. 63: 961-913 TURNER, B.J. 1982. The evolutionary genetics of a unisexual fish, Poecilia formosa. - In Mechanisms of Speciation (Ed. C. BARIGOZZI),A.R. Liss, New York, p. 265-305 TURNER, B.J., BREIT, B. -L. H . , RASCH,E M . and BALSANO, J.S. 1980. Evolutionary genetics of a gynogenetic fish, Poecilia formosa, the Amazon molly. -Evolution 34: 246-258 UZZEL, T.M., JR. 1964. Relations of the diploid and triploid species of the Ambystoma jeffersonianum complex (Amphibia, Caudata). - Copeia 1964: 257-300
Hereditas 105 ( I 986) VIRKKI, N. 1983 Chromosomes in evolution of Coleoptera. - In Chromosomes in Evolution of Eukaryotic Groups. Vol. II (Eds. A.K. and A. SHARMA),CRC Press, Boca Raton, Florida, p. 42-68 VRIJENHOEK, R.C. 1984a. The evolution of clonal diversity in Poeciliopsis. - In Evolutionary Genetics of Fishes (Ed. B.J. TURNER),Plenum Press, New York, p. 399-429 VRIJENHOEK, R.C. 1984b. Ecological differentiation among clones: the frozen niche variation model. - In Population Biology and Evolution (Eds. K. WOHRMANNand V. LOESCHKE),Springer-Verlag, Berlin, p. 21 7-231 WOOD, S.L. 1977. New synonymy and new species of American bark beetles (Coleoptera: Scolytidae). Part V. - Grear Basin Nut. 37: 383-394
