The nucleotide changes governing cuticular hydrocarbon variation and their evolution in Drosophila melanogaster

The cuticular hydrocarbon (CH) pheromones in Drosophila melanogaster exhibit strong geographic variation. African and Caribbean populations have a high ratio of 5,9 heptacosadiene/7,11 heptacosadiene (the "High" CH type), whereas populations from all other areas have a low ratio ("Low" CH type). Based on previous genetic mapping, DNA markers were developed that localized the genetic basis of this CH polymorphism to within a 13-kb region. We then carried out a hierarchical search for diagnostic nucleotide sites starting with four lines, and increasing to 24 and 43 lines from a worldwide collection. Within the 13-kb region, only one variable site shows a complete concordance with the CH phenotype. This is a 16-bp deletion in the 5' region of a desaturase gene (desat2) that was recently suggested to be responsible for the CH polymorphism on the basis of its expression [Dallerac, R., Labeur, C., Jallon, J.-M., Knipple, D. C., Roelofs, W. L. & Wicker-Thomas, C. (2000) Proc. Natl. Acad. Sci. 97, 9449--9454]. The cosmopolitan Low type is derived from the ancestral High type, and DNA sequence variations suggest that the former spread worldwide with the aid of positive selection. Whether this CH variation could be a component of the sexual isolation between Zimbabwe and other cosmopolitan populations remains an interesting and unresolved question.

Cryptic reproductive isolation in the Drosophila simulans species complex

Forms of reproductive isolation that act after copulation but before fertilization are potentially important components of speciation, but are studied only infrequently. We examined postmating, prezygotic reproductive isolation in three hybridizations within the Drosophila simulans species complex. We allowed females to mate only once, observed and timed all copulations, dissected a subset of the females to track the storage and retention of sperm, examined the number and hatchability of eggs laid after insemination, counted all progeny produced, and measured the longevity of mated females. Each of the three hybridizations is characterized by a different set of cryptic barriers to heterospecific fertilization. When D. simulans females mate with D. sechellia males, few heterospecific sperm are transferred, even during long copulations. In contrast, copulations of D. simulans females with D. mauritiana males are often too short to allow sperm transfer. Those that are long enough to allow insemination, however, involve the transfer of many sperm, but only a fraction of these heterospecific sperm are stored by females, who also lay fewer eggs than do D. simulans females mated with conspecific males. Finally, when D. mauritiana females mate with D. simulans males, sperm are transferred and stored in abundance, but are lost rapidly from the reproductive tract and are therefore used inefficiently. These results add considerably to the list of reproductive isolating mechanisms in this well-studied clade and possibly to the list of evolutionary processes that could contribute to their reproductive isolation.

The population genetics of the origin and divergence of the Drosophila simulans complex species

The origins and divergence of Drosophila simulans and close relatives D. mauritiana and D. sechellia were examined using the patterns of DNA sequence variation found within and between species at 14 different genes. D. sechellia consistently revealed low levels of polymorphism, and genes from D. sechellia have accumulated mutations at a rate that is approximately 50% higher than the same genes from D. simulans. At synonymous sites, D. sechellia has experienced a significant excess of unpreferred codon substitutions. Together these observations suggest that D. sechellia has had a reduced effective population size for some time, and that it is accumulating slightly deleterious mutations as a result. D. simulans and D. mauritiana are both highly polymorphic and the two species share many polymorphisms, probably since the time of common ancestry. A simple isolation speciation model, with zero gene flow following incipient species separation, was fitted to both the simulans/mauritiana divergence and the simulans/sechellia divergence. In both cases the model fit the data quite well, and the analyses revealed little evidence of gene flow between the species. The exception is one gene copy at one locus in D. sechellia, which closely resembled other D. simulans sequences. The overall picture is of two allopatric speciation events that occurred quite near one another in time.

Mechanisms of conspecific sperm precedence in Drosophila

The postmating, prezygotic isolating mechanism known as conspecific sperm precedence (CSP) may play an important role in speciation, and understanding the mechanism of CSP is important in reconstructing its evolution. When a Drosophila simulans female mates with both a D. simulans male and a D. mauritiana male, the vast majority of her progeny are fathered by D. simulans, regardless of the order of mating. The dearth of hybrid progeny does not result from inviability of eggs fertilized by heterospecific sperm or from the relative inviability of heterospecific larvae. Instead, CSP apparently results from a prefertilization obstacle to heterospecific sperm. We identified two independent barriers to heterospecific fertilization, sperm displacement and incapacitation, whose action depends on the order of mating. When a D. simulans female mates first with a conspecific male, the seminal fluid from this mating incapacitates heterospecific sperm transferred two days later. This sperm incapacitation occurs with no change in the retention of stored sperm over time, but does not occur when the conspecific mating lasts for only 5 min. When the order of matings is reversed, the seminal fluid from the second mating physically displaces heterospecific sperm from storage, even if the conspecific copulation lasts only 5 min. Conspecific sperm are not susceptible to displacement by a second conspecific copulation, but are susceptible to interference by heterospecific sperm if the conspecific copulation is interrupted after 12 min. Curing the D. mauritiana males of their infection with the endosymbiont Wolbachia had no effect on CSP. Sperm displacement and incapacitation involve the same basic mechanisms seen in second-male sperm precedence within species, supporting the hypothesis that CSP is an evolutionary by-product of adaptations affecting sperm competition within species.

Little evidence for sympatric speciation in island birds

It has been suggested that the presence of sister species in small circumscribed areas, such as isolated lakes or islands, might imply that these species originated sympatrically. To investigate this possibility in birds, we searched for endemic, congeneric species on isolated islands in the ocean. Among 46 islands and small archipelagos chosen because they contain at least one species of endemic land bird, we identified seven pairs of endemic congeners (excluding flightless rails). Of these seven, only four pairs are potentially sister species and thus possible candidates for sympatric speciation. However, three of these four pairs have always been considered the results of double invasion from a mainland source (in two of these cases, molecular-phylogenetic work has either confirmed a double invasion or is ambiguous). The one remaining pair may have speciated allopatrically on a small archipelago. Additional phylogenetic studies are required to understand these cases, and our results should also be considered in light of the large number of island-bird extinctions in historic time. We conclude that, at present, there is little evidence for sympatric speciation in island birds.

Sperm competition between Drosophila males involves both displacement and incapacitation

Females in almost all animal groups copulate with multiple males. This behaviour allows different males to compete for fertilization and gives females the opportunity to mediate this competition. In many animals and most insects, the second male to copulate with a female typically sires most of her offspring. In Drosophila melanogaster, this second-male sperm precedence has long been studied but, as in most species, its mechanism has remained unknown. Here we show, using labelled sperm in doubly mated females, that males can both physically displace and incapacitate stored sperm from earlier-mating males. Displacement occurs only if the second male transfers sperm to the female, and in only one of her three sperm-storage organs. Incapacitation can be caused by either fertile or spermless second males, but requires extended intervals between matings. Sperm from different males are not 'stratified' in the storage organs but mix freely. Many animal species may have multiple mechanisms of sperm competition like those observed here, and revealing these mechanisms is necessary to understand the genetic and evolutionary basis of second-male sperm precedence in animals.

A gene responsible for a cuticular hydrocarbon polymorphism in Drosophila melanogaster

Drosophila melanogaster is polymorphic for the major cuticular hydrocarbon of females. In most populations this hydrocarbon is 7,11-heptacosadiene, but females from Africa and the Caribbean usually possess low levels of 7,11-heptacosadiene and high quantities of its position isomer 5,9-heptacosadiene. Genetic analysis shows that the difference between these two morphs is due to variation at a single segregating factor located on the right arm of chromosome 3 near map position 51.5 and cytological position 87C-D. This is precisely the position of a desaturase gene previously sequenced using primers derived from yeast and mouse, and localized by in situ hybridization to the polytene chromosomes of D. melanogaster. Alleles of this desaturase gene may therefore be responsible for producing the two hydrocarbon morphs. Mating tests following the transfer of these isomers between females of the two morphs show that, in contrast to previous studies, the hydrocarbon profiles have no detectable effect on mating behaviour or sexual isolation.

Relative paucity of genes causing inviability in hybrids between Drosophila melanogaster and D. simulans

Using deficiencies from Drosophila melanogaster, we looked for genomic regions in the sister species D. simulans that could cause lethality when hemizygous on a hybrid genetic background. Such genotypes allow hemizygous genes from one species to interact with heterozygous genes from other species and may correspond to the kinds of genotypes causing Haldane's rule, the observation that if only one gender is sterile or inviable in species hybrids, it is nearly always the heterogametic sex. A survey of roughly 50% of the D. simulans genome (114 chromosome regions) revealed only four regions causing hybrid lethality and five causing severe reductions in hybrid viability. However, the viability of all of these genotypes was at least partially restored by rearing hybrids at lower temperature or using different genetic backgrounds from D. simulans. We therefore detected no D. simulans chromosome regions causing unconditional hybrid lethality, although several regions were shown to be deleterious under most tested temperatures and genetic backgrounds. The relative paucity of "inviability genes" supports the idea, suggested by work on other species, that hybrid inviability between closely related species might be caused by interactions among relatively few genes, while hybrid sterility may involve many more loci.

The evolutionary genetics of speciation

The last decade has brought renewed interest in the genetics of speciation, yielding a number of new models and empirical results. Defining speciation as 'the origin of reproductive isolation between two taxa', we review recent theoretical studies and relevant data, emphasizing the regular patterns seen among genetic analyses. Finally, we point out some important and tractable questions about speciation that have been neglected.

Genetics of a pheromonal difference affecting sexual isolation between Drosophila mauritiana and D. sechellia

Females of the sibling species Drosophila sechellia and D. mauritiana differ in their cuticular hydrocarbons: the predominant compound in D. sechellia is 7,11-heptacosadiene (7,11-HD), while that in D. mauritiana is 7-tricosene (7-T). We investigate the genetic basis of this difference and its involvement in reproductive isolation between the species. Behavioral studies involving hydrocarbon transfer suggest that these compounds play a large role in the sexual isolation between D. mauritiana males and D. sechellia females, while sexual isolation in the reciprocal hybridization results more from differences in female behavior than hydrocarbons. This interspecific difference in hydrocarbon profile is due to evolutionary change at a minimum of six loci, all on the third chromosome. The localization of evolutionary change to the third chromosome has been seen in very other genetic analysis of female hydrocarbon differences in the D. melanogaster group. We suggest that the high 7,11-HD phenotype seen in two species evolved twice independently from ancestors having the high 7-T phenotype and the recurrent third-chromosome effects are evolutionary convergences that may be due to a concentration of "hydrocarbon genes" on that chromosome.

The developmental genetics of hybrid inviability: a mitotic defect in Drosophila hybrids

We report studies of the developmental basis of hybrid inviability in the Drosophila melanogaster complex. The pathology of these hybrids closely resembles that of mitotic mutants in D. melanogaster. We use mosaic and cytological analyses to show that hybrid male inviability is associated with, and probably caused by, a defect in mitotic cell division. In the mosaic study, we find that male clones produced in otherwise female hybrids are not cell lethal but are very small, probably reflecting defects in mitotic proliferation. Cytological inspection of larval neuroblasts reveals a profound mitotic defect in hybrids: chromosomes show a near-complete failure to condense even after 2 hr of incubation in colchicine. Both the defect in clonal proliferation and in chromatin condensation are rescued by mutations known to rescue normally inviable hybrid males. We present a simple model in which hybrid inviability is partly or entirely caused by a mitotic defect; this defect is, in turn, caused by an interaction between the Hybrid male rescue (Hmr) locus of D. melanogaster and autosomal gene(s) from D. melanogaster's sister species.

Evolution of a lesser fitness trait: egg production in the specialist Drosophila sechellia

In the evolutionary process during which Drosophila sechellia became specialized on a toxic fruit (morinda), a spectacular decrease in female reproductive capacity took place when compared with the species' generalist relatives D. mauritiana and D. simulans. Comparisons of species and interspecific crosses showed that two different traits were modified: number of ovarioles and rate of egg production. During the conservation of a D. sechellia strain on usual food, adaptation to laboratory conditions led to an increase in the rate of oogenesis but not in ovariole number. Comparison of F1 and backcross progeny also suggests that the two traits are determined by different genes (ovariole number has already been shown to be polygenic). When morinda is available as a resource, the low rate of egg production in D. sechellia is partly compensated by a stimulating effect, while an inhibition occurs in D. simulans. It is assumed that D. sechellia progressively adapted itself from rotten, non-toxic morinda to a fresher and more toxic resource. During this process the rate of oogenesis evolved from an inhibition to a stimulation by morinda. Simultaneously a spectacular decrease in ovariole number took place, either as a consequence of stochastic events related to the small population size of D. sechellia and a metapopulation dynamics, or as an adaptive process favouring dispersal capacities of the female.

Genetics of sexual isolation in male hybrids of Drosophila simulans and D. mauritiana

Sexual isolation between the sibling species D. simulans and D. mauritiana is due largely to the rejection of D. simulans males by D. mauritiana females. Genetic analysis shows that genes on the X and third chromosomes contribute to the differences between males causing sexual isolation, while the Y chromosome, second chromosome and cytoplasm have no effect. These chromosome effects differ from those observed in a previous analysis of sexual isolation in hybrid females, implying that different genes cause sexual isolation in the two sexes.

Genetics of a difference in cuticular hydrocarbons between Drosophila pseudoobscura and D. persimilis

We identify a fixed species difference in the relative concentrations of the cuticular hydrocarbons 2-methyl hexacosane and 5,9-pentacosadiene in Drosophila pseudoobscura and D. persimilis, and determine its genetic basis. In backcross males, this difference is due to genes on both the X and second chromosomes, while the other two major chromosomes have no effect. In backcross females, only the second chromosome has a significant effect on hydrocarbon phenotype, but dominant genes on the X chromosome could also be involved. These results differ in two respects from previous studies of Drosophila cuticular hydrocarbons: strong epistasis is observed between the chromosomes that produce the hydrocarbon difference in males, and the difference is apparently unrelated to the strong sexual isolation observed between these species.

Genetics of a difference in male cuticular hydrocarbons between two sibling species, Drosophila simulans and D. sechellia

In seven of the eight species of the Drosophila melanogaster group, the predominant cuticular hydrocarbon of males is 7-tricosene, but in the island endemic species D. sechellia it is 6-tricosene. The phylogeny of the group implies that the novel hydrocarbon profile of D. sechellia is a derived character. Genetic analysis of hybrids between D. sechellia and its close relative D. simulans show that each of the five major chromosome arms carries at least one gene affecting the ratio of the two tricosene isomers, with the right arm of the third chromosome having the largest effect. The species difference in this character is therefore polygenic with the effects of the different chromosome arms generally additive, although there is some epistasis among third-chromosome genes. Observations of courtship by males who have been coated with foreign hydrocarbons suggest that a male's hydrocarbon profile may slightly affect the degree of sexual isolation in one of the reciprocal hybridizations between these species, but that this role is small compared to that played by hydrocarbon differences between females.

Genetics of differences in pheromonal hydrocarbons between Drosophila melanogaster and D. simulans

Females of Drosophila melanogaster and its sibling species D. simulans have very different cuticular hydrocarbons, with the former bearing predominantly 7,11-heptacosadiene and the latter 7-tricosene. This difference contributes to reproductive isolation between the species. Genetic analysis shows that this difference maps to only the third chromosome, with the other three chromosomes having no apparent effect. The D. simulans alleles on the left arm of chromosome 3 are largely recessive, allowing us to search for the relevant regions using D. melanogaster deficiencies. At least four nonoverlapping regions of this arm have large effects on the hydrocarbon profile, implying that several genes on this arm are responsible for the species difference. Because the right arm of chromosome 3 also affects the hydrocarbon profile, a minimum of five genes appear to be involved. The large effect of the third chromosome on hydrocarbons has also been reported in the hybridization between D. simulans and its closer relative D. sechellia, implying either an evolutionary convergence or the retention in D. sechellia of an ancestral sexual dimorphism.

Localization of pheromonal sexual dimorphism in Drosophila melanogaster and its effect on sexual isolation

Drosophila melanogaster is sexually dimorphic for cuticular hydrocarbons, with males and females having strikingly different profiles of the long-chain compounds that act as contact pheromones. Gas-chromatographic analysis of sexual mosaics reveals that the sex specificity of hydrocarbons is located in the abdomen. This explains previous observations that D. melanogaster males display the strongest courtship toward mosaics with female abdomens. We also show that males of the sibling species Drosophila simulans preferentially court D. melanogaster mosaics with male abdomens. Because the primary male hydrocarbon in D. melanogaster is also the primary female hydrocarbon in D. simulans, this supports the idea that interspecific differences in cuticular hydrocarbons contribute to sexual isolation.

Genetics of a pheromonal difference contributing to reproductive isolation in Drosophila

Although sexual isolation is one of the most important causes of speciation, its genetic basis is largely unknown. Here evidence is presented that suggests that sexual isolation between two closely related species of Drosophila is largely caused by differences in female cuticular hydrocarbons. This difference maps to only one of the three major chromosomes, implying that reproductive isolation might have a fairly simple genetic basis. The effect of the hydrocarbons on courtship may help explain the ubiquitous asymmetry of sexual isolation between many pairs of Drosophila species.

Effects of the fourth chromosome on the sterility of hybrids between Drosophila simulans and its relatives

Genetic analysis of the fourth chromosome of Drosophila simulans shows that it has no effect on either male or female fertility in hybrids with two sibling species, D. mauritiana and D. sechellia. The lack of effect holds even when the D. simulans chromosome is homozygous in a foreign genetic background. The absence of major fourth chromosome "sterility genes" in these hybridizations contrasts with the presence of a fourth-chromosome gene causing complete male sterility in hybrids of the more distantly related species D. melanogaster and D. simulans.

The fertility effects of pericentric inversions in Drosophila melanogaster

Heterozygotes for pericentric inversions are expected to be semisterile because recombination in the inverted region produces aneuploid gametes. Newly arising pericentric inversions should therefore be quickly eliminated from populations by natural selection. The occasional polymorphism for such inversions and their fixation among closely related species have supported the idea that genetic drift in very small populations can overcome natural selection in the wild. We studied the effect of 7 second-chromosome and 30 third-chromosome pericentric inversions on the fertility of heterokaryotypic Drosophila melanogaster females. Surprisingly, fertility was not significantly reduced in many cases, even when the inversion was quite large. This lack of underdominance is almost certainly due to suppressed recombination in inversion heterozygotes, a phenomenon previously observed in Drosophila. In the large sample of third-chromosome inversions, the degree of underdominance depends far more on the position of breakpoints than on the inversion's length. Analysis of these positions shows that this chromosome has a pair of "sensitive sites" near cytological divisions 68 and 92: these sites appear to reduce recombination in a heterozygous inversion whose breakpoints are nearby. There may also be "sensitive sites" near divisions 31 and 49 on the second chromosome. Such sites may be important in initiating synapsis. Because many pericentric inversions do not reduce the fertility of heterozygotes, we conclude that the observed fixation or polymorphism of such rearrangements in nature does not imply genetic drift in very small populations.

Genetics of sexual isolation in females of the Drosophila simulans species complex

Genetic analysis of hybrids between Drosophila simulans and D. sechellia shows that sexual isolation in females is caused by at least two genes, one on each major autosome, while the X chromosome has no effect. These results are similar to those of a previous study of hybrids between D. simulans and another sibling species, D. mauritiana. In this latter hybridization, each arm of the second chromosome carries genes causing sexual isolation in females, implying a total divergence of at least three loci. The genetic similarity between the D. simulans/D. mauritiana and D. simulans/D. sechellia hybridizations probably results from independent evolution and not phylogenetic artifacts, because the dominance relationships and behavioural interactions differ between the two hybridizations. The lack of an X-chromosome effect on sexual isolation contrasts with genetic studies of post-zygotic reproductive isolation, which invariably show strong effects of this chromosome.

Genetics and speciation

Called the "mystery of mysteries" by Darwin, speciation is still a little-understood area of evolution. Genetic analysis, however, has yielded new generalizations about speciation and suggests promising avenues of research.

Four decades of inversion polymorphism in Drosophila pseudoobscura

We report data that continue the studies of Dobzhansky and others on the frequencies of third-chromosome inversions in natural populations of Drosophila pseudoobscura in North America. The common gene arrangements continue to be present in frequencies similar to those described four decades ago, and the broad geographic patterns also remain unchanged. There is only one pronounced trend over time: the increase in frequency of the Tree Line inversion in Pacific coast populations.

Lack of underdominance in a naturally occurring pericentric inversion in Drosophila melanogaster and its implications for chromosome evolution

In(2LR)PL is a large pericentric inversion polymorphic in populations of Drosophila melanogaster on two Indian Ocean islands. This polymorphism is puzzling: because crossing over in female heterokaryotypes produces inviable zygotes, such inversions are thought to be underdominant and should be quickly eliminated from populations. The observed fixation for such inversions among related species has led to the idea that genetic drift can cause chromosome evolution in opposition to natural selection. We found, however, that In(2LR)PL is not underdominant for fertility, as heterokaryotypic females produce perfectly viable eggs. Genetic analysis shows that the lack of underdominance results from the nearly complete absence of crossing over in the inverted region. This phenomenon is probably caused by mechanical and not genetic factors, because crossing over is not suppressed in In(2LR)PL homokaryotypes. Our observations do not support the idea that the fixation of pericentric inversions among closely related species implies the action of genetic drift overcoming strong natural selection in very small populations. If chromosome arrangements vary in their underdominance, it is those with the least disadvantage as heterozygotes, like In(2LR)PL, that will be polymorphic or fixed in natural populations.

Genetics of morphological differences and hybrid sterility between Drosophila sechellia and its relatives

We conducted classical genetic analysis of the difference in male genitalia and hybrid sterility between the island-dwelling sibling species Drosophila sechellia and D. mauritiana. At least two loci (one on each autosome) are responsible for the genital difference, with the X chromosome having no significant effect. In contrast, male hybrid sterility is caused by at least four gene loci distributed among all major chromosomes, with those on the X chromosome having the largest effect. We also show that the large difference in ovariole number between D. sechellia and its mainland relative D. simulans is due to at least two gene substitutions, one on each major autosome. The X and the left arm of the second chromosome, however, have no significant effect on the character. This implies that the evolution of reduced ovariole number involved relatively few gene substitutions. These results extend previous findings that morphological differences between Drosophila species are caused by genes distributed among all chromosomes, while hybrid sterility and inviability are due primarily to X-linked genes. Because strong X-effects on male sterility have been found in all three pairwise hybridizations among D. simulans, D. sechellia and D. mauritiana, these effects must have evolved at least twice independently.

Mutation rates in hybrids between sibling species of Drosophila

It has recently been suggested that sterility or inviability in species hybrids might result from the movement of transposable elements. Because such movement is often detectable by an increased mutation rate, I studied the effect of interspecific hybridization in the Drosophila melanogaster group on the mutation rate of X-linked visibles. This rate did not differ between hybrids and intraspecific controls. This was also true for the germ-line excision rate of a transposable element, although the rate of somatic excision was two to sixfold higher in hybrids than in pure species. Combined with previous work, these results do not support a role for transposable elements in speciation.

Genetics of sexual isolation between two sibling species, Drosophila simulans and Drosophila mauritiana

Drosophila simulans and Drosophila mauritiana are sibling species that show substantial sexual isolation in one of their two reciprocal hybridizations. Genetic analysis reveals that in females this isolation is caused by at least one recessive gene on each autosome, while the X chromosome has little or no effect. Our results, combined with those of previous studies, show that in Drosophila the genetics of sexual isolation differs from that of postzygotic reproductive isolation, which invariably involves large effects of the X chromosome.

The genetics of postzygotic isolation in the Drosophila virilis group

In a genetic study of postzygotic reproductive isolation among species of the Drosophila virilis group, we find that the X chromosome has the largest effect on male and female hybrid sterility and inviability. The X alone has a discernible effect on postzygotic isolation between closely related species. Hybridizations involving more distantly related species also show large X-effects, although the autosomes may also play a role. In the only hybridization yet subjected to such analysis, we show that hybrid male and female sterility result from the action of different X-linked loci. Our results accord with genetic studies of other taxa, and support the view that both Haldane's rule (heterogametic F1 sterility or inviability) and the large effect of the X chromosome on reproductive isolation result from the accumulation by natural selection of partially recessive or underdominant mutations. We also describe a method that allows genetic analysis of reproductive isolation between species that produce completely sterile or inviable hybrids. Such species pairs, which represent the final stage of speciation, cannot be analyzed by traditional methods. The X chromosome also plays an important role in postzygotic isolation between these species.

Genetic analysis of X-linked sterility in hybrids between three sibling species of Drosophila

Three morphological markers (yellow, miniature, and forked) are used to map the location of X-chromosome segments causing male sterility in Drosophila simulans/D. mauritiana and D. simulans/D. sechellia hybrids. In both hybridizations at least three sections of the chromosome contain genes with substantial effects on sterility. This represents the maximum genetic divergence detectable with the three markers, suggesting that the X chromosome contains many loci affecting postzygotic reproductive isolation. The tight linkage between some markers and "sterility loci" may be useful in localizing and later cloning genes important in speciation.

Heritability of two morphological characters within and among natural populations of Drosophila melanogaster

Heritabilities of wing length and abdominal bristle number, as well as genetic correlations between these characters, were determined within and among populations of Drosophila melanogaster in nature. Substantial "natural" heritabilities were found when wild-caught flies from one population were compared to their laboratory-reared offspring. Natural heritabilities of bristle number approximated those derived from laboratory-raised parents and offspring, but wing length heritability was significantly lower in nature than in the laboratory. Among-population heritabilities, estimated by regressing population means of wild-caught flies against those of their laboratory-reared descendants, were close to 0.5. The genetic differentiation of populations was clinal with latitude, and was accompanied by significant geographic differences in the norms of reaction to temperature. These clines are similar to those reported on other continents and in other Drosophila species, and are almost certainly caused by natural selection. Genetic regressions between the characters reveal that the cline in bristle number may be a correlated response to geographic selection on wing length, but not vice versa. Our results indicate that there is a sizable genetic component to phenotypic variation within and among populations of D. melanogaster in nature.

Lack of response to selection for directional asymmetry in Drosophila melanogaster

Thirty generations of selection for directional asymmetry of eye size was practiced on a stock of D. melanogaster homozygous for the mutant eyeless-recessive. There was no obvious response, supporting previous findings that it is difficult to select for right- and left-handed expression of traits in Drosophila.

Meiotic segregation and male recombination in interspecific hybrids of Drosophila

Male hybrids between three pairs of Drosophila species show no substantial distortion of Mendelian segregation and no appreciable male recombination. These results do not support the theories that meiotic drive alleles of large effect are often fixed within species and that transposable genetic elements cause speciation.

Location of an X-linked factor causing sterility in male hybrids of Drosophila simulans and D. mauritiana

We report the first mapping of a genetic factor responsible for reproductive isolation: a small segment of genome strongly affecting sperm motility in hybrids between the sibling species Drosophila simulans and D. mauritiana. Maximum-likelihood analysis of data from ten generations of backcrossing places this factor at 1.1 +/- 0.2 map units from the forked locus, at position 54.9 +/- 0.2 or 57.1 +/- 0.2 on the X chromosome.

Genetic studies of three sibling species of Drosophila with relationship to theories of speciation

Drosophila melanogaster,D. simulansandD. mauritianaare closely related species, the first two cosmopolitan and the last restricted to the oceanic island of Mauritius.D. simulansandD. mauritianaare the most closely related pair, with the latter species probably resulting from a founder event. The relatedness of the three species and their ability to hybridize allow tests of recent theories of speciation. Genetic analysis of two characters differing betweenD. simulansandD. mauritiana(sex comb tooth number and testis colour) show that the differences are due to at least five and three loci respectively. Behavioural tests further demonstrate that sex combs are probably used by males at a crucial step in mating, and that the differences between the two species may be related to differences in their mating ability. These genetic studies and previous work indicate that differences among these species are polygenic and not (as proposed by recent theories) attributable to only one or two loci of large effect. Further studies of interspecific hybrids show that genetic divergence leading to developmental anomalies is more advanced in the older species pairD. simulans/D. melanogaeterthan in the younger pairD. simulans/D. mauritiana. This supports the neo-Darwinian contention that reproductive isolation is one step in a continuous process of genetic change among isolated populations, and does not support current theories that such change occurs only during the evolution of reproductive isolation. Finally, investigations of the degree of gonadal atrophy and its sensitivity to temperature inD. simulans/D. mauritianahybrids fail to support recent speculations that phenomena similar to hybrid dysgenesis (which causes such atrophy inD. melanogaster) play a role in speciation.

The genetic basis of Haldane's rule

'Haldane's rule', formulated by J. B. S. Haldane in 1922, states that: "When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex". His rule is now known to apply in mammals, lepidopterans, birds, orthopterans and dipterans. In Drosophila, for example, Bock cites 142 cases of interspecific hybridizations that produce one sterile and one fertile sex in the offspring, all but one of these crosses yielding sterile XY males and fertile XX females. Despite much speculation, however, the genetic basis of Haldane's rule remains unknown. Haldane himself rejected the simple explanation that males are innately more sensitive than females to the effects of hybridization because groups with heterogametic females (such as birds and butterflies) usually show female sterility in hybrids, so that heterogamety itself is the critical feature. He and others suggested that heterogametic infertility or inviability in hybrids arises by a genetic imbalance between X chromosomes and autosomes. An alternative explanation is that this syndrome is caused by a mismatch of X and Y chromosomes. Here I show that in the Drosophila melanogaster subgroup, Haldane's rule for fertility apparently arises from a genetic interaction between X and Y chromosomes and not from an imbalance between sex chromosomes and autosomes. This finding has important implications for understanding the evolution of interspecific reproductive isolation.

Genetic basis of male sterility in hybrids between two closely related species of Drosophila

Drosophila simulans and Drosophila mauritiana are closely related sibling species, the former cosmopolitan and the latter restricted to the small oceanic island of Mauritius. Genetic analysis of male sterility in hybrids between these species shows that at least five loci (one on each chromosome arm) are responsible for this reproductive isolation. This is the most loci that could have been detected with the techniques used and implies that the true genetic divergence for sterility is even greater. The effects of chromosome segments on the character are roughly additive, with the X-linked segment making the largest contribution to sterility. The large effect of X chromosomes on male-limited reproductive isolation and the frequent limitation of hybrid sterility to males may be attributable to fertility interactions between X and Y chromosomes. These results parallel what has been found in other Drosophila species and relate to recent theories of how reproductive isolation evolves in small founder populations.

Restoration of mutationally suppressed characters in Drosophila melanogaster

We examined three strains of Drosophila melanogaster made wingless for periods of up to 40 years by homozygosity for the recessive mutant, vestigial. The intent was to quantify the degeneration of genes for wing structure and function that were not expressed during these periods. No increase in abnormality of wing morphology or use was found in the oldest strains despite up to 1000 generations of winglessness. We propose that these genes have remained intact because of important pleiotropic effects not connected with wing formation.

The sensitivity of gel electrophoresis as a detector of genetic variation

Three experiments based on an idea of YOUDERIAN have been performed to determine the proportions and kinds of amino acid substitutions that are detected by gel electrophoresis when applied to surveys of protein variation in populations. The experiments involved applying the sequential method of electrophoresis under several conditions of pH and gel concentration to a large sample of human hemoglobins with known amino acid substitutions. In the first experiment, a random sample of 20 different hemoglobin variants was studied, and these were separated into 17 distinct electrophoretic classes by three sequential gel conditions, thus giving a detectability of 85%. A single pass under standard conditions detected eight classes. The second experiment compared groups of substitutions that were chemically identical, but in different positions in the alpha and beta chains, while the third experiment compared pairs of substitutions that were charge equivalent, but chemically different at the same chain position. The sequential method distinguished 90% of all chemically identical substitutions when they were at different chain locations, and four out of five charge equivalent but chemically different substitutions at the same site. Examination of the location of each substitution in the three-dimensional structure of hemoglobins showed that interior substitutions usually are less different from Hb A than are surface substitutions and that local interactions with chain and spatial neighbors are sufficient to distinguish substitutions in very similar positions on the outside of the molecule. The "charge ladder" model of electrophoretic classes is clearly incorrect, and it appears that sequential gel electrophoresis as practiced in our Drosophila surveys has detected a substantial fraction of amino acid substitutions if hemoglobin is regarded as a model. This estimate may be modified as other molecules beside hemoglobin are subjected to similar calibration experiments.