Homeostasis, enzymatic heterozygosity and inbreeding depression in natural populations of Drosophila melanogaster. C. Bi6mont. Department of general and ...
Homeostasis, enzymatic heterozygosity and inbreeding depression in natural populations of Drosophila melanogaster C. Bi6mont Department of general and applied Biology, University 1.yon 1, 69622 Villeurbanne, France
Abstract Brother-sister mating effects on offspring viability in natural populations of Drosophila melanogaster were analysed in relation to developmental homeostasis and enzymatic heterozygosity of the individuals constituting the sib pairs. High developmental homeostasis and high degree of heterozygosity are associated with a low inbreeding depression in offspring and, therefore, with a low frequency of major deleterious factors. These resuits are discussed in relation to maintenance of polymorphism in natural populations and the role of genetic load and founder events in speciation and adaptation. We conclude that an unusual environmental condition followed by a strong reduction in effective population size due to random elimination of individuals is a better candidate to speciation than an environmental shift leading to selection for homeostatic, highly heterozygous lethal-free individuals.
Introduction The discovery of certain forces which operate to maintain genetic variability in natural populations is one of the essential problems within the study of evolutionary biology. Many works have suggested that highly heterozygous individuals have enhanced developmental homeostasis, which enables them to adjust their development and physiological mechanisms to the circumstances of the environment (Lerner, 1954). Such a relationship has been postulated in an attempt to explain the relationship between the degree of heterozygosity measured at the enzymatic level and the variance for morphological characters. Indeed, recent surveys of enzymatic polymorphism in natural populations indicate that individuals heterozygous for an enzyme locus are likely to have a lower level of morphological variation than individuals homozygous for that locus. Though found in different species (Mitton, 1978; Eanes, 1978; Singh & Zouros, 1978; Soul6, 1979; Zouros, Singh & Miles, 1980), this relationship was Genetica61, 179 189 (1983). © Dr W. Junk Publishers. The Hague. Printed in The Netherlands.
not corroborated in other studies, thus suggesting that the existence of this particular phenomenon is not to be taken as universal (Handford, 1980; McAndrew, Ward & Beardmore, 1982). Working on the population rather than the individual level, it has been shown recently (Kat, 1982) that peripheral populations of freshwater bivalve species exhibit an increase in deviation from bilateral symmetry (a measure of developmental stability) with a decrease in heterozygosity. This author stressed the importance of founder events and inbreeding for species-level differentiation. It is still a matter of speculation and of personal preference as to whether the effects of loss of heterozygosity of the genome are due mainly to loss of homeostatic capacity of the more homozygous individual or to the effects of recessive deleterious factors present in all wild chromosomes and exposed by the i.ncreasing homozygosity of the genome. Another aspect of the research is the attempt to establish a possible relationship between the fre-
180 quencies of deleterious genes and genome heterozygosity, varying environmental conditions, seasonal fluctuations and selective values of deleterious genes in heterozygous conditions. Though many experiments dealing with such questions have already been carried out, no clear conclusion can be drawn for natural populations. Recently, we have shown that in natural populations of Drosophila melanogaster it was possible to distinguish two groups ofsib pairs that differ in the level of morphological variation of the individuals constituting them, in relation with the inbreeding depression in offspring viability (Bi6mont & Bouclier, 1982). Since such a relationship is of great importance in understanding natural selection and maintenance of genome variability, we have studied inbreeding effects on viability in relation to developmental homeostasis and enzymatic polymorphism in the parental sib pairs. We show, here, that a high degree of heterozygosity is associated with a high developmental homeostasis and low frequency of deleterious factors. This raises the problem of founder events and maintenance of polymorphism in natural populations.
Material and methods
Four American and three French populations of
Drosophila melanogaster were used in this study. The American populations were established by posting a set of isofemale lines, each of which was derived from a single fertilized female collected in the wild (6 isofemale lines for St Paul Q, 11 for St Paul P, 20 for Sonoma, 25 for Pellstom). The French populations were established from flies recently caught in cellars in Lentilly (near Lyon) and Valence. The populations were maintained by mass culture at 24 o C for the American and 25 ° C for the French ones, and for at least four generations before the experiments were started. 160 to 200 F 0 males and females were chosen at r a n d o m from each posted population and crossed in pairs. The pairs so formed were set up and allowed to lay eggs on a cornmeal molasses medium seeded with live yeast (for the American flies) or a standard axenic maize-dried yeast-agar medium (for the French populations). In order to avoid crowding and obtain F t flies from standardized conditions, 20 eggs from each F o pair were trans-
ferred to vials with fresh medium to allow F~ progeny to develop. At hatching time, one brother-sister F t pair was set up for each progeny and allowed to lay eggs on a spoon. Two replicates of 50 eggs laid by each sib pair were transferred to new vials to allow F 2 progeny to dcvelop. The F2 adults produced from the eggs were counted until all had cmerged. Egg viability was then estimated for each pair of F~ sibs as the percentage of fertilized eggs that produced adults. The data from replicates, found to be homogeneous by chi-square tests, were pooled. Length of the right and left wings was measured on each F~ fly. The fluctuating asymmetry (FA), a measure of developmental homeostasis (Thoday, 1953; l.erner, 1954; Van Valen, 1962; Wright, 1977), was expressed in terms of variance of the difference in score between left and right sides. The flies were then electrophoresed.
Electrophoresis techniques Only the loci polymorphic (loci where the most c o m m o n allele had a frequency of less than 95%) in at least one of the populations have been considered in this study. In the American populations four enzymatic loci were examined: Alcohol dehydrogenase (Adh), c~glycerophosphate dehydrogenase (cl-Gpdh), esterase-6 (Est-6) and esterase-C (Est-C). Each sample was submitted to standard acrylamide gel electrophoresis, all four enzymes being stained in the same gel. In the French populations, in addition to the four above enzymatic loci, I have studied phosphoglucomutase (PGM) and superoxide dismutase (SOD) or tetrazolium oxidasc (To). Each sample was submitted to standard starch gel electrophoresis. It should be pointed out that the male and female measured in the F~ pairs are related (brother and sister) but in no way inbred. In addition, at the end of the experiment, each F~ sib pair is characterized by values of fluctuating asymmetry, degree of heterozygosity of both the male and the female and by the viability of their inbred eggs.
181 males were analyzed scparately. A pattern which is the same for each case except for the males of the St Paul Q population is seen; the flies of the low-viability class show higher fluctuating asymmetry and lower degree of heterozygosity than the flies of the high-offspring-viability class. [ h e differences between the two classes are significant when tested globally by variance analyses done on the mean degree of heterozygosity (F = 92.7, P < 0.05) or on the logarithm of the variances (Scheffe, 1959; F = 24.3, P < 0.05). Thus, on the basis of sib-mating effects on viability, a population can be subdivided into two groups that differ in their level of developmental homeo-
i / Pellstom n='/6
'~
50 St Paul P 1n:62
Lentllly 1 11:89
¢
o
Table 1. Values of fluctuating asymmetry for wing length (V-': 0
0,5
I
variance of the difference in .,,core bctwccn left and right gides) and of obscrvcd enzymatic heterozygosity (flu) for t:~ males and females grouped into two classes of viability values based on inbrcd offspring resulting from sib matings. N: sample size. For the meaning of tbc viability classes, sec lext and Figure I. 0
0,5
1
Populations
viability
Fig. I. Distribution ofegg-to-aduh viability, in proportion to the fertilized F 2inbred eggs developing to the adult stage, in progenies from sib pairs, for the 7 populations studied. N: number of pairs. For rapid graphical comparison, the distributions are based on 100 pairs.
Viability N lnferior to 0.90
Male.~ Pellstom St Paul P St Paul Q
47
Results
Sonoma Valley
39
Viability distributions
Lentilly 1
50
l.entilly 2
63
Valence
17
Females I'ellstom
45
St Paul P
47
St Paul Q
49
Sonoma
39
I.cntilly I
5(1
Lentilly 2
63
Valence
17
tlomeostasis and heterozygosity The values of" fluctuating asymmetry and observed heterozygosity of the F~ flies for the two viability classes appear in Table I. Males and fe-
Superior or equal
to 0.90 46 \:2 I~i,~ 47
In agreement with previous studies (Bi6mont, 1978; Bi6mont & Gautier, 1982), the distribution of viability values is not uniform. There is one group of viability values above around 0.90 and another group for values ranging from 0.50 to 0.90 depending on the population (see Fig. 1). We can thus separate sib pairs into two classes on the basis of the viability value of their inbred offspring: a class for viability greater than or equal to 0.90 and the other for values lower than 0.90.
N
2 100 0.26 I 830 0.27 940 0.42 1 700 0.23 2491 0.16 1 292 0.15 691 0.09 1 367 0.19 2045 0.20 1 400 0.30 I 000 0.12 3 986 0.16 1 553 0.15 524 0.09
30 15 23 50 39 22 17
29 15 22 50 39 22 22
I 820 0.32 1 050 I).37 880 0.41 900 0.25 1 304 0.20 788 0.21 561 0.11 1 012 0.22 975 0.33 1 230 0.32 900 0.19 2 111 0.21 401 0.21 133 0.11
182 stasis and degree of enzymatic hcterozygosity. This dissection of a population is repeatable in the seven populations studicd independently in time. Moreover, neither of the two groups is negligible in its proportion in the population. As shown in Figure 1, the high-offspring-viability group ranges from about 20% to 55% of the population. This phenomenon is thus of great importance for populations.
Discussion
An obvious interpretation of the viability distributions represented in Figure 1 is that the lowoffspring viability observed in some sib pairs is merely the result of the presence of recessive lethal heterozygosity in both sibs; the shape of the distributions being due to the binomial variability inherent in the observed frequencies of adult emergency or to variable expressivity of the genes involved. These assumptions have been called into question by previous results showing a 80 to 100% mortality rate a m o n g offspring of particular sib pairs (Bi6mont, 1978). This high mortality rate could, of course, be the result of crosses between either two homozygous recessive lethals or one homozygous and one heterozygous. However, it is difficult to understand how a lethal factor can be maintained in a homozygous state, and a spontaneous dominant mutant can hardly explain the effects observed in m a n y pairs of a same family (Bi6mont, 1978). It has also been noted, by recrossing experiments, that the shape of the viability distribution is not only due to sampling variability but also, and in the main, to characteristics of the females that determine the value of viability. Moreover, this viability can be much lower than that expected in case of crosses between two lethal-heterozygotes. 1 thus have postulated the existence in populations of individuals homozygous or heterozygous for a particular genetic structure that is responsible for the mortality rate observed throughout development of inbred organisms (Bi6mont, 1978, 1980). The effects of inbreeding are thus explained in terms of pleiotropy or specific combinations of genes that are deleterious only in a particular genomic constitution and, therefore, reproductive system. Under this hypothesis, the crossing between individuals (sibs) heterozygous or dominant homozygous for this system shows no inbreeding depression in the offspring
(Bi6mont, 1978; Bi6mont & Bouletreau-Merle, 1978; Bi6mont & Lemaitrc, 1978; Bouclier & Bi+mont, 1982). Everything takes place as if these individuals were free or nearly free of major deletcrious factors. This agrees well with the fact that the highoffspring-viability sibs are more heterozygous and show higher developmental homeostasis than the low-offspring-viability sibs, as seen in Table 1. These results raise the problems of the genetic load in relation to developmental homeostasis and natural selection, the nature of lethality, the expression of mutator activity in natural populations, and finally the role of inbreeding in founder events and the 'genetic revolutions' that can follow a drop in effective size of a population. Thesc problems are discussed in the following sections. Genetic load, developmental homeostasis and natural selection A very wide r a n g e o f information is now available concerning the occurrence and evolutionary significance of recessive deleterious and visible mutants in natural populations (I)obzhansky, 1955). This refers to the concept of genetic load and its associated inbreeding depression reported to a different extent in various species (Wright, 1977). The inbreeding depression results in a low viability due to numerous causes of mortality throughout the dcvelopment of the organism (see Lewontin, 1974). Viability has thus been postulated to be controlled by polygenes with an extremely high spontaneous mutation rate (Simmons & Crow, 1977). Some population geneticists assume that natural populations should not be able to tolerate the huge gene"tic load involved, especially if polymorphism is maintained by heterosis operating independently at each locus (Lewontin & Hubby, 1966). An attempt to resolve this dilemma has been to postulate that there is a limit to m a x i m u m fitness (Sved, Reed & Bodmer, 1967)or global elimination of the worst combination of genes by natural selection (King, 1967). What is observed under inbreeding could then be elimination of individuals of poor homeostasis as inferred by Lerner (1954). In some experiments, there is, however, evidence against greater buffering capacity (measured by lower asymmetry) due to heterozygosity (Bradley, 1980). Moreover, heterozygosity per se does not necessarily promote homeostasis, and coadaptation of gene allelcs or
183 gene complexes may be necessary to bring homeostasis about (Tebb & Thoday, 1954). Spurious heterosis can also bc attributed to neutral loci due to association with detrimentals (Ohta & Kimura, 1971). Such hypotheses are strengthened by the recent observation that it is not too uncommon to observe, in organisms frec of inversion, the presence of linkage disequilibrium between isozyme markers and lethality (Malpica & Briscoe, 1982); and, for some authors, electrophoretic alleles are considered as integral parts of coadapted supergenes not due to random events (Powell & Wistrand, 1978). This and the fact that there is no significant diffcrencc in the proportion of deleterious gencs between inversion-free and inversioncarrying chromosomes (Watanabe, Yamaguchi & Mukai, 1976) lead to the conclusion that inversion can not be invoked to explain our results of an association between high heterozygosity and low frequency of dclcterious factors. It has bccn postulated that occasional catastrophes may provide a mechanism reducing the fitness depression brought about by inbreeding, in sifting out the less homeostatic homozygotes and retaining the heterozygotes (Wool & Sverdlov, 1976). A similar process can be clearly shown from our study" but with the additional element that the hcterozygotcs retained by a selection process, due for example to a shift in the environment, are free or nearly free of lcthal factors. Band (1963) also reported that lethal-free heterozygotes exhibit higher fitness than lethal-carrying chromosomes. Band and Ires (1961, 1968) and Band (1972) also found that the frequency of lethal factors was correlated with climatic factors as rainfall. However, in other studies, no effect of such climatic factors was observed on the frequency of lethals and scmi-lcthals (Minamori et al., 1973). Several studies of Drosophila have also failed to show any' dominance of lethal heterozygotes or any evidence that heterozygotes for one or two lethals differed in viability from any, other heterozygotes; but, nevertheless, some lethals were found to be slightly hctcrotic (see Lewontin, 1974). Seasonal fluctuations of lethal mutations were reported in natural populations of Drosophila metanogaster. Some data lend support to the suggestion that the frequency of lethals increases simultaneously in different populations during the wintcr period and then decreases, which is due to selection
of heterozygotes (Golubovsky, 1970). The author then concluded that the adaptive value of some lethal heterozygotes is high during winter but then in the period from summer to autumn it decreases. However, such differences in frequency of lethals between seasons do not seem to be a general phenomenon (Sperlich, Jaksh & Karlik, 1963; Minamori et al., 1973) and Minamori and Saito (1964) observed that the lethal plus semi-lethal frequency increased and allclism frequency decreased from early summer to fall in each },ear. According to our results, a shift in environment such as may happen between wintering and spring and summer, may be followed at first by a decrease in lethal gene frequency, as the result of a selection process for deleterious factor-free heterozygotes. But, as the population size increases, the frequency of lethal factors increases and simultaneously the allelic frequency decreases. However. what happens in the middle of the season depends largely on uncontrolled environmental factors that may affect population size. ]'hen, if the population is reduced to a dcnsity so low that close inbreeding of the remaining individuals becomes unavoidable, many lethals will be eliminated and the frequency of lethal factors will be reduced (see Dubinin, 1946; Pavan et al., 1951). Lethality and m u t a t o r activity in natural populations Strangely enough, high frequencies of genetic abnormalities have been found to be c o m m o n in natural populations (Dobzhansky & Boesiger, 1968; Ochando, 1978). However, the work involved has been almost exclusively orientated towards a few species of Drosophila for which genetical techniques are highly developed. A fact that should be underlined is that the general method used to obtain such homozygous chromosomes is an inbred mating system (generally between brothers and sisters; see Lewontin, 1974), so that the entire genome is rendered more homozygous. Furthermore, recent investigations postulate that most of the high rate of mutations reported in natural populations could be the result of interactions between the wild strain studied and the marker strain used to determine frequency of deleterious genes (Bregliano et al., 1980; Kidwell, 1981). This could explain such results as for example those of
184 Reeve and Robertson (1953). Thesc authors found in one strain of Drosophila melanogaster that a certain gene combination behaved as a lethal when made homozygous by standard genetic techniques using a second genetically marked strain, but that the lethal potential of this gene remained totally inactive within the background of the original strain. Synthetic lethality is sometimes observed when non-lethal chromosomes are allowed to rccombine (Dobzhansky, 1946). It is also known that the frequency of lethals and semi-tethals depends on the background in which they were tested (Watanabe, 1969). Sometimes, however, techniques rcvealing deleterious genes on X chromosomes do not use brother-sister inbreeding, but rather attachedX chromosomes or backcrosses without inbreeding. Interactions between strains may again be a perturbing factor as regards accurate estimations of deleterious-gene frequencies. For example, high frequencies of X-recessive lethal mutations in the female germ line of Drosophila, are shown to be quantitatively correlated with the IR and the PM systems of hybrid dysgenesis (Brcgliano et al., 1980; Berg, Engel & Kreber, 1980; Kidwell, 1981). Thus, great care is necessary in the interpretation of estimates of deleterious-gene frequencies when marker strains and inbred matings are used for these estimates. Furthermore, the mutations revealed by these systems do not seem to be distributed at rand o m but are at only a few localized positions on the chromosomes and are associated with lethals and hence with short deletions (Bregliano et al., 1980; Simmons & Lira, 1980). It is of interest to note that mutator factors which induce recessive lethal mutations or c h r o m o s o m e aberrations also induce male recombination and deletions, probably due to c h r o m o s o m e or chromatid breaks in the homologous chromosomes which are mutator-free (Cardellino & Mukai, 1975): Though some high mutation rates and deleterious effects on viability due to inbreeding have been reported to be associated with a particular chromosome (for example the third chromosome: Green & Lefevre, 1972; Green, 1976; Bi6mont, 1978, 1980, or the second chromosomc: Cardellino & Mukai, 1975), many results suggest secondary elements or transposable elements (Voelkcr, 1974; Slatko & Hiraizumi, 1975). The mutator or controlling element system involved can exert its effects on a variety of structural genes (McClintock, 1951 ; Green, 1967; Nevers & Saedler,
1977); it is then associated with genetic instability of a particular locus (Green, 1967). Nomadic DNA sequences appear now to be of great importance in Drosophila genetics (Davidson & Posakoni, 1982); they are reported to be polymorphic within a stock of Drosophila but show only small divergence between two different stocks. Inbred lines of Drosophila melanogaster are characterized by a fixed distribution of some mobile dispersed geneticelements (Georgiev et al., 1980). However, selection for high viability, in these inbred lines, leads to change in sequence locations (Belyaeva et al., 1982). This suggests a possible influence of D N A clements on gene expression in inbred lines. Such middle-repetitive DNA sequences, which frequently transpose to new chromosomal locations, have been postulated to be capable, theoretically, of mediating responses to fluctuating levels of inbreeding (Seger, 1980). Excision and insertion of such genetic elements could then be a primary process in inbreeding mechanisms. This is particularly relevant since deletions and duplications of small DNA segments are postulated to be the most general processes by which most evolutionary changes in the genome have taken place (Bachmann, (Join & (Join, 1974). Searching for an association between inbreeding and the regulation of gene expression should be useful as illustrated b y the finding of Kirk and Jones (1974), who showcd that nuclear histones increase in inbred lines, suggesting thus an increasing repression of genetic activity. Allelic tests usually used to assess the genetic history or structure of a population can be reinterpreted using the above information. Indeed, a population characterized by a steady specific configuration of controlling elements should undergo a specific pattern of mutational events as revealed under inbreeding. External factors or a shift in the reproductive structure, for example from outbrceding to inbreeding, could then modify the expression of the genetic system, leading to fluctuation in thc rate of revealed mutations, as observed in some populations (Spencer, 1935). This model is consonant with the recent proposal ot" T h o m p s o n and Woodruff( 1980, 1981) that regulation of mutations could be accomplished by stabilizing transposable elements within a strain or population, fixing both their number and their positions. The existence of some specific sites also accounts for the observation
185 of high rates of particular mutations reported by many authors (Petit, 1952; Kidwell, Kidwell & Nei, 1973; Cardellino & Mukai, 1975) and for the persistence of the same total concentration of lethals (15 20%), their al]elic state being permanently renewed (Berg, 1974; Golubovsky, 1978). Inbreeding and f o u n d e r events
Some authors (Thompson & Woodruff, 1980) predict that the mixing of the co-adapted gene pool by migrants will lead to a higher mutation rate in newly founded colonies than in well established ones. Moreover, if the population is expanding rapidly, the new variants formed by inbrecding might quickly attain polymorphic frequencies (Thompson & Neel, 1978), allowing the maintenance of variability within the population. So, a specific inbreeding cffect, promoting new genetic novcltics, arises only in populations undergoing a radical change in genetic environment. Such a change can occur in populations subjected to cyclical bottlenecks followed by rapid expansion of population size, or in new colonies recently formed from a few founders and whosc effective population size is expanding rapidly. This is in agreement with the recent concept of'genetic transilience" of Templeton (1980). This author postulatcs that genetic transiliencc does not shake up the whole genomc, unlike Mayr's concept of genetic revolution (Mayr, 1954) which assumes that a shift in mating system may affect all genes. Transiliencc is confined principally to polygenic systems effecting fitness and characterized by possessing a handful of major genes with strong epistatic interactions with sevcral minor genes (Tcmpleton, 1980). Thesc disturbances in the genetic material may then lead to new outcomes in population phcnotypes (Palenzona et aL, 1974) and the reorganization of gcnomes (Mayr, 1954). A mechanism of inbreeding based on destabilization of the dcvelopmental control system of the organism, thus strongly effecting fitness, through the perturbation of highly coadapted systems (Bi6mont, 1974, 1978, 1980) is thus relevant to such concepts of evolutionary changes. Substantial changes in quantitative characters may also result from destabilization of development through inbreeding, Examples are: fecundity, thermogenesis and some morphological characters (Bibmont & Bouletrcau-Merlc, 1978; Bi6mont &
Lemaitre, 1978; Bouclier & Bi6mont, 1982). We showed that adult offspring from brother-sister matings exhibit reduced fecundity (egg-laying capacity) associated with gonadal distrophy and ovariole abnormalities (Bi6mont & Bouletreau-Merle, 1978). They also show lowered overall thermogenesis, which is a measure of the amount of heat emitted by a fly per time unit (Bi6mont & Lcmaitre, 1978). This thcrmogenesis depends on the metabolic capacities of the flies and also on their activity; it can be considered as a measure of developmental homeostasis. More recently, it has been shown that high between- and within-organ variabilities for wing and thorax length, together with lack of developmental homeostasis, are associated with a high rate of embryonic and larvo-pupal mortalities (Bouclier & Bi6mont, 1982). There is indeed strong correlation between effects on viability and other components of fitness, as recently inferred (Simmons, Preston & Engels, 1980). Thus, a wide variety of morphological novelties can be the result of changes in processes controlling morphogenesis. This emphasizes, as well, the importance of pleiotropy in inbreeding as suggested by Vetta (personal communication) (see also Rose, 1982); this is then in opposition with Lande's concepts (1980) that a founder effect plus continued inbreeding is not likely to create very substantial changes in many quantitative characters. Observations in several organisms agree with the potential importance of inbreeding. For example, Ctarkia species that undcrgo inbreeding in populations of periodically extremely small size, are known for their chromosomal novelties (Lewis, 1973). Considerable chromosomal evolution is also reported in rodent populations which have both small population size and a high level of inbreeding (Bushetal., 1977; Wilson, 1976). In Hawaiian Drosophila, 300 endemic spccies are known. They have evolvcd explosively in the few million years since the islands were formed. Speciation in these species has been attributed eithcr to single gravid females colonizing newly formed open habitats (Carson, 1970, 1971) or to repeated cycles of population flush under favorable environmental conditions (Hartl, 1980). Thcse hypotheses are made plausible by the fact that founder-flush cycles can lead to incipient reproductive isolation (Powell, 1978). Subsequent to these genetic events, these populations show a genetic variability which is severely
186 reduced or even absent (Selander & Hudson, 1976; Saura et al., 1973). This suggests that there must be a very small initial effective population size to substantially reduce the number of alleles per locus and so have a very strong effect on average hcterozygosity (Nei et al., 1975). However, extensive work on Drosophila has failed to demonstrate reduced genetic variability in populations that are geographically or ecologically marginal (Soul6, 1973; I.oukas & Krimbas, 1980). This could be duc to low rate of gene flow from other populations, which can quickly restore adaptive alleles to an impoverished gene pool. This hypothesis is enforced by recent observations of great potential migration rates in Drosophila (Jones et al., 1981; Coyne et al., 1982). Temporal instability of marginal environments may also bring about a mechanism restoring heterozygosity (Lewontin, 1974). Such unstable and unpre-' dictable environments play a role in maintaining high levels of genetic variability in heavily inbred plants (Wool & Sverdlov, 1976; Jain, 1969). So, as inferred by Lewontin (1974), chromosomal variability and reorganization cannot be equated to genic hcterozygosity or homozygosity. This is particularly relevant since inbreeding per se can lead to new c h r o m o s o m a l rearrangements and chromosomal modifications as has been reported in some plants in which meiosis is perturbed (Blanco, 1949; Zecevic, 1960; Strzyzewska, 1976; Dayal, 1977); in rye, radish, red clover, one of the reasons for low fertility and sterility under inbreeding is the disturbance in the development of reproductive cells. In Trifolium (Strzyzewska, 1976), sterility in the brother-sister mating system is determined by asynapsis. Inbreeding p e r se is then a good candidate for being responsible for chromosomal speciation (Bush et al., 1977). Note that recent observations fail to show an effect of inbreeding on recombination rate (Luning, 1982). However, only the offspring of two females were analyzed in the experiment. So, obviously, no definite conclusion could be drawn from such data especially in view of our results o f a bimodality of the distribution of viability (see Fig. I) which indicates a high proportion of sib pairs whose offspring develop normally and thus do not show any inbreeding depression. A 'genetic r e v o l u t i o n '
The basis of a genetic revolution is that novelties arise from populations, through a founder event
which delimits a small isolated population (Mayr, 1954). The probability of fixation of new variants is generally very small. Thus, many authors have suggested that both speciation and chromosomal evolution may be encouraged by small effective population size (Wright, 1940; Mayr, 1954, 1978; Templeton, 1980). Meiotic drive associated with genetic drift and inbreeding (Hedrick, 1981) are then involved in fixing the new genetic configuration or single chromosomal rearrangement appearing at first in an individual. This event might be sufficient to allow the beginning of a wedge of evolutionary distance between two groups of individuals. Underlying these concepts are the following events: population size reduction, appearance of new developmental configurations in a small group, fixation of this new arrangement through inbreeding and then its distribution among a large number of offspring. The necessity of some kind of inbreeding pressure is associated with appearance and fixation of the new genetic configuration. Hence, the occurrence of genetic novelties should be increased if these two events were directly associated. As seen above, inbreeding p e r se is capable of not only revealing and fixing preexisting genetic change, but of promoting new genetic configurations and variability and perturbing developmental processes. Hence, a reduction in effective population size has two main results: First, by promoting new genetic changes due to inbreeding in recently founded populations, it may show up new evolutionary potentialities. We do not have a complete idea of the effects of inbreeding in most of the species we know, though inbreeding depression seems a general effect in diploid species (Wright, 1977). However, though Drosophila species show inbreeding depression, it is easy to maintain a constant inbreeding pressure for a hundred generations (Petit, 1963). On the contrary, man and birds, for example, are known to be very sensitive to inbred matings (Schull & Neel, 1972; Boesiger, 1969) and only three generations of inbreeding are sufficient to obtain complete extinction of a quail population (Bocsiger, 1969). It should then be of interest to study the relationships between the magnitude of inbreeding depression and the evolutionary path of the species. Numerous species of Drosophila exist for many centuries without showing new evolutionary trends (Throckmorton, 1975), though this group is still able to
187 s h o w v a r i a b i l i t y a n d , as s e e n in t h e H a w a i i a n s p e cics, r a p i d s p e c i a t i o n . S c c o n d , if t h e r e d u c t i o n in p o p u l a t i o n size is d u e t o a s h i f t in t h e e n v i r o n m e n t , it m a y h e l p n a t u r a l s e l e c t i o n p r e v e n t a h i g h level o f h o m o z y g o s i t y b y s e l e c t i n g h o m e o s t a t i c a n d h i g h l y h e t e r o z y g o u s individuals. Wc have shown that this group of indiv i d u a l s s h o w s n o i n b r e e d i n g d e p r e s s i o n in its offs p r i n g . T h u s , t h o u g h a r e d u c t i o n in e f f e c t i v e p o p u l a t i o n size m a y f o l l o w t h e s e l c c t i o n p r o c e s s , n o i n b r e e d i n g d e p r e s s i o n w o u l d b e e x p c c t e d in t h e i m m e d i a t e l y f o l l o w i n g g e n e r a t i o n s . In n a t u r a l p o p u l a t i o n s u n d e r g o i n g s u c h a p h e n o m e n o n , it w o u l d n o t be c x p e c t e d t h a t i n b r e e d i n g be a v o i d e d . A radical modification of the environment that, in a r a n d o m w a y , kills a h i g h p r o p o r t i o n o f i n d i v i d u a l s is t h u s a b e t t e r c a n d i d a t e f o r e v o l u t i o n a r y p r o gress than a minor environmental shift that leads to selection for homeostatic and highly hetcrozygous lcthal-free individuals.
Acknowledgements W e t h a n k R. C. L e w o n t i n , in w h o s e l a b o r a t o r y t h e A m e r i c a n p o p u l a t i o n s w e r e s t u d i e d ; M . G. K i d w e l l f o r p r o v i d i n g s o m e i s o f e m a l e lines; R. W . M a r k s a n d P. G i r a r d f o r a d v i c e w i t h e l e c t r o p h o r e sis a n d J. D a v i d , 1). D e b o u z i e , R. C. L e w o n t i n a n d anonymous reviewers for their comments. Support w a s p r o v i d e d in p a r t b y a N A T O g r a n t a n d t h e C e n t r e N a t i o n a l d e la R e c h e r c h e S c i e n t i f i q u e ( a s s o c i a t e d L a b o r a t o r y n ° 243).
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Receivcd 8.11.1982
Acceptcd 18.2.1983.
