Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females X D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

Genetics of Hybrid Inviability and Sterility in Drosophila: Dissection of Introgression of D. simulans Genes in D. melanogaster Genome

Interspecific crosses between Drosophila melanogaster and Drosophila simulans usually produce sterile unisexual hybrids. The barrier preventing genetic analysis of hybrid inviability and sterility has been taken away by the discovery of a D. simulans strain which produces fertile female hybrids. D. simulans genes in the cytological locations of 21A1 to 22C1-23B1 and 30F3-31C5 to 36A2-7 have been introgressed into the D. melanogaster genetic background by consecutive backcrosses. Flies heterozygous for the introgression are fertile, while homozygotes are sterile both in females and males. The genes responsible for the sterility have been mapped in the introgression. The male sterility is caused by the synergistic effect of multiple genes, while the female sterility genes have been localized to a 170 kb region (32D2 to 32E4) containing 20 open reading frames. Thus, the female sterility might be attributed to a single gene with a large effect. We have also found that the Lethal hybrid rescue mutation which prevents the inviability of male hybrids from the cross of D. melanogaster females and D. simulans males cannot rescue those carrying the introgression, suggesting that D. simulans genes maybe non-functional in this hybrid genotype. The genes responsible for the inviability have not been separated from the female sterility genes by recombination.     

Hybrid Lethal Systems in the Drosophila Melanogaster Species Complex. II. the Zygotic Hybrid Rescue (Zhr) Gene of D. Melanogaster

Hybrid females from Drosophila simulans females X Drosophila melanogaster males die as embryos while hybrid males from the reciprocal cross die as larvae. We have recovered a mutation in melanogaster that rescues the former hybrid females. It was located on the X chromosome at a position close to the centromere, and it was a zygotically acting gene, in contrast with mhr (maternal hybrid rescue) in simulans that rescues the same hybrids maternally. We named it Zhr (Zygotic hybrid rescue). The gene also rescues hybrid females from embryonic lethals in crosses of Drosophila mauritiana females X D. melanogaster males and of Drosophila sechellia females X D. melanogaster males. Independence of the hybrid embryonic lethality and the hybrid larval lethality suggested in a companion study was confirmed by employing two rescue genes, Zhr and Hmr (Hybrid male rescue), in doubly lethal hybrids. A model is proposed to explain the genetic mechanisms of hybrid lethalities as well as the evolutionary pathways.     

Misregulation of sex-lethal and disruption of male-specific lethal complex localization in Drosophila species hybrids

A major model system for the study of evolutionary divergence between closely related species has been the unisexual lethality resulting from reciprocal crosses of Drosophila melanogaster and D. simulans. Sex-lethal (Sxl), a critical gene for sex determination, is misregulated in these hybrids. In hybrid males from D. melanogaster mothers, there is an abnormal expression of Sxl and a failure of localization of the male-specific lethal (MSL) complex to the X chromosome, which causes changes in gene expression. Introduction of a Sxl mutation into this hybrid genotype will allow expression of the MSL complex but there is no sequestration to the X chromosome. Lethal hybrid rescue (Lhr), which allows hybrid males from this cross to survive, corrects the SXL and MSL defects. The reciprocal cross of D. simulans mothers by D. melanogaster males exhibits underexpression of Sxl in embryos.     

A Genetic Basis for the Inviability of Hybrids between Sibling Species of Drosophila

A mutation of Drosophila melanogaster whose only known effect is the rescue of otherwise lethal interspecific hybrids has been characterized. This mutation, Hmr, maps to 1-31.84 (9D1-9E4). Hmr may be the consequence of a P element insertion. It rescues hybrid males from the cross of D. melanogaster females to males of its three sibling species, D. simulans, D. mauritiana and D. sechellia. This rescue is recessive, since hybrid males that carry both Hmr and a duplication expected to be Hmr+ are not rescued. Hmr also rescues the otherwise inviable female hybrids from the cross of compound-X D. melanogaster females to males of its sibling species. This rescue is also recessive, since a compound-X heterozygous for Hmr does not rescue. Another mutation, discovered on the In(1)AB chromosome of D. melanogaster, is also found to rescue normally inviable species hybrids: unlike Hmr, however, In(1)AB rescues hybrid females from the cross of In(1)AB/Y males to sibling females, as well as hybrid males from the cross of In(1)AB females to sibling males. These data are interpreted on the basis of a model for the genetic basis of hybrid inviability of complementary genes.     

Genetic studies of two sister species in the Drosophila melanogaster subgroup, D. yakuba and D. santomea

We performed genetic analysis of hybrid sterility and of one morphological difference (sex-comb tooth number) on D. yakuba and D. santomea, the former species widespread in Africa and the latter endemic to the oceanic island of S?o Tomé, on which there is a hybrid zone. The sterility of hybrid males is due to at least three genes on the X chromosome and at least one on the Y, with the cytoplasm and large sections of the autosomes having no effect. F1 hybrid females carrying two X chromosomes from either species are perfectly fertile despite their genetic similarity to completely sterile F1 hybrid males. This implies that the appearance of Haldane's rule in this cross is at least partially due to the faster accumulation of genes causing male than female sterility. The larger effects of the X and Y chromosomes than of the autosomes, however, also suggest that the genes causing male sterility are recessive in hybrids. Some female sterility is also seen in interspecific crosses, but this does not occur between all strains. This is seen in pure-species females inseminated by heterospecific males (probably reflecting incompatibility between the sperm of one species and the female reproductive tract of the other) as well as in inseminated F1 and backcross females, probably reflecting genetically based incompatibilities in hybrids that affect the reproductive system. The latter 'innate' sterility appears to involve deleterious interactions between D. santomea chromosomes and D. yakuba cytoplasm. The difference in male sex-comb tooth number appears to involve fairly large effects of the X chromosome. We discuss the striking evolutionary parallels in the genetic basis of sterility, in the nature of sexual isolation, and in morphological differences between the D. santomea/D. yakuba divergence and two other speciation events in the D. melanogaster subgroup involving island colonization.     

The impact of shared ancestral variation on hybrid male lethality--a 16 codon indel in the Drosophila simulans Lhr gene

The Lethal hybrid rescue (Lhr) gene causes hybrid male lethality in crosses between Drosophila simulans and D. melanogaster. Lhr(2) is a D. simulans allele, which rescues hybrid males. It has been recently proposed that a 16 codon insertion, which distinguishes the D. melanogaster and the canonical D. simulans allele, and is lacking in Lhr(2), may be responsible for the functional divergence of D. melanogaster and D. simulans Lhr alleles. Here, we show that the Lhr(2) allele lacking the insertion represents an ancestral polymorphism segregating at a moderate frequency in D. simulans. Crosses of D. melanogaster females to males from two D. simulans strains carrying this deletion showed a severe deficiency of viable hybrid males. Our results suggest that the absence of this insertion alone is not sufficient to explain functional differences between D. melanogaster and D. simulans Lhr alleles.     

A fine-scale genetic analysis of hybrid incompatibilities in Drosophila

The sterility and inviability of species hybrids is thought to evolve by the accumulation of genes that cause generally recessive, incompatible epistatic interactions between species. Most analyses of the loci involved in such hybrid incompatibilities have suffered from low genetic resolution. Here I present a fine-resolution genetic screen that allows systematic counting, mapping, and characterizing of a large number of hybrid incompatibility loci in a model genetic system. Using small autosomal deletions from D. melanogaster and a hybrid rescue mutation from D. simulans, I measured the viability of hybrid males that are simultaneously hemizygous for a small region of the D. simulans autosomal genome and hemizygous for the D. melanogaster X chromosome. These hybrid males are exposed to the full effects of any recessive-recessive epistatic incompatibilities present in these regions. A screen of approximately 70% of the D. simulans autosomal genome reveals 20 hybrid-lethal and 20 hybrid-semilethal regions that are incompatible with the D. melanogaster X. In further crosses, I confirm the epistatic nature of hybrid lethality by showing that all of the incompatibilities are rescued when the D. melanogaster X is replaced with a D. simulans X. Combined with information from previous studies, these results show that the number of recessive incompatibilities is approximately eightfold larger than the number of dominant ones. Finally, I estimate that a total of approximately 191 hybrid-lethal incompatibilities separate D. melanogaster and D. simulans, indicating extensive functional divergence between these species' genomes.     

Characterization of defects in adult germline development and oogenesis of sterile and rescued female hybrids in crosses between Drosophila simulans and Drosophila melanogaster

Crosses between Drosophila melanogaster and D. simulans normally result in progeny that are either inviable or sterile. Recent discovery of strains that rescue these inviability and sterility phenotypes has made it possible to study the developmental basis of reproductive isolation between these two species in greater detail. By producing both rescued and unrescued hybrids and examining the protein product staining patterns of genes known to be involved in early germline development and gametogenesis, we have found that in crosses between D. simulans and D. melanogaster, hybrid female sterility results from the improper control of primordial germline proliferation, germline stem cell maintenance, and cystoblast formation and differentiation during early oogenesis. Rescued hybrid females are fertile, yet they generally have lower amounts of adult germline from the outset and show a premature degeneration of adult germline cells with age. In addition, older rescued hybrid females also exhibit mutant egg phenotypes associated with defects in dorso-ventral patterning which may result from the improper partitioning of cytoplasmic factors during early oogenesis that could stem from the early defect. Although a variety of germline and oogenic defects are described for the hybrid females, all of them can potentially result from the same underlying primary defect. Hybrid males from these same crosses, on the other hand, have no detectable germline in adult reproductive tissues, even when hybrid sterility rescue strains are used, indicating that male sterility and female sterility stem from distinctly different developmental defects.     

Quantitative trait loci for sexual isolation between Drosophila simulans and D. mauritiana

Sexual isolating mechanisms that act before fertilization are often considered the most important genetic barriers leading to speciation in animals. While recent progress has been made toward understanding the genetic basis of the postzygotic isolating mechanisms of hybrid sterility and inviability, little is known about the genetic basis of prezygotic sexual isolation. Here, we map quantitative trait loci (QTL) contributing to prezygotic reproductive isolation between the sibling species Drosophila simulans and D. mauritiana. We mapped at least seven QTL affecting discrimination of D. mauritiana females against D. simulans males, three QTL affecting D. simulans male traits against which D. mauritiana females discriminate, and six QTL affecting D. mauritiana male traits against which D. simulans females discriminate. QTL affecting sexual isolation act additively, are largely different in males and females, and are not disproportionately concentrated on the X chromosome: The QTL of greatest effect are located on chromosome 3. Unlike the genetic components of postzygotic isolation, the loci for prezygotic isolation do not interact epistatically. The observation of a few QTL with moderate to large effects will facilitate positional cloning of genes underlying sexual isolation.     

Loss of notum macrochaetae as an interspecific hybrid anomaly between Drosophila melanogaster and D. simulans.

With the aim of revealing genetic variation accumulated among closely related species during the course of evolution, this study focuses on loss of macrochaetae on the notum as one of the developmental anomalies seen in interspecific hybrids between Drosophila melanogaster and its closely related species. Interspecific hybrids between a line of D. melanogaster and D. simulans isofemale lines exhibited a wide range in the number of missing bristles. By contrast, D. mauritiana and D. sechellia lines showed almost no reduction in bristle number in hybrids with D. melanogaster. Genetic analysis showed that the D. simulans X chromosome confers a large effect on hybrid bristle loss, although X-autosome interaction may be involved. This suggests that at least one genetic factor contributing to hybrid anomalies arose recently on a D. simulans X chromosome. Moreover, the results indicate sex dependency: the male hybrids were more susceptible to bristle loss than the female hybrids were. Use of cell type markers suggests that the defect does not lie in cell fate decisions during bristle development, but in the maintenance of neural fate and/or differentiation of the descendants of sensory mother cells.     

Nup96-dependent hybrid lethality occurs in a subset of species from the simulans clade of Drosophila

The cross of Drosophila melanogaster females to D. simulans males typically produces lethal F(1) hybrid males. F(1) male lethality is suppressed when the D. simulans Lhr(1) hybrid rescue strain is used. Viability of these F(1) males carrying Lhr(1) is in turn substantially reduced when the hybrids are heterozygous for some mutant alleles of the D. melanogaster Nup96 gene. I show here that similar patterns of Nup96-dependent lethality occur when other hybrid rescue mutations are used to create F(1) males, demonstrating that Nup96 does not reduce hybrid viability by suppressing the Lhr(1) rescue effect. The penetrance of this Nup96-dependent lethality does not correlate with the penetrance of the F(1) hybrid rescue, arguing that these two phenomena reflect genetically independent processes. D. simulans, together with two additional sister species, forms a clade that speciated after the divergence of their common ancestor from D. melanogaster. I report here that Nup96(-) reduces F(1) viability in D. melanogaster hybrids with one of these sister species, D. sechellia, but not with the other, D. mauritiana. These results suggest that Nup96-dependent lethality evolved after the speciation of D. melanogaster from the common ancestor of the simulans clade and is caused by an interaction among Nup96, unknown gene(s) on the D. melanogaster X chromosome, and unknown autosomal gene(s), at least some of which have diverged in D. simulans and D. sechellia but not in D. mauritiana. The genetic properties of Nup96 are also discussed relative to other hybrid lethal genes.     

Incompatibility between X chromosome factor and pericentric heterochromatic region causes lethality in hybrids between Drosophila melanogaster and its sibling species

The Dobzhansky-Muller model posits that postzygotic reproductive isolation results from the evolution of incompatible epistatic interactions between species: alleles that function in the genetic background of one species can cause sterility or lethality in the genetic background of another species. Progress in identifying and characterizing factors involved in postzygotic isolation in Drosophila has remained slow, mainly because Drosophila melanogaster, with all of its genetic tools, forms dead or sterile hybrids when crossed to its sister species, D. simulans, D. sechellia, and D. mauritiana. To circumvent this problem, we used chromosome deletions and duplications from D. melanogaster to map two hybrid incompatibility loci in F(1) hybrids with its sister species. We mapped a recessive factor to the pericentromeric heterochromatin of the X chromosome in D. simulans and D. mauritiana, which we call heterochromatin hybrid lethal (hhl), which causes lethality in F(1) hybrid females with D. melanogaster. As F(1) hybrid males hemizygous for a D. mauritiana (or D. simulans) X chromosome are viable, the lethality of deficiency hybrid females implies that a dominant incompatible partner locus exists on the D. melanogaster X. Using small segments of the D. melanogaster X chromosome duplicated onto the Y chromosome, we mapped a dominant factor that causes hybrid lethality to a small 24-gene region of the D. melanogaster X. We provide evidence suggesting that it interacts with hhl(mau). The location of hhl is consistent with the emerging theme that hybrid incompatibilities in Drosophila involve heterochromatic regions and factors that interact with the heterochromatin.     

Genetics of Drosophila simulans male mating discrimination in crosses with D. melanogaster

The genetic bases of sexual isolation between Drosophila melanogaster and D. simulans have been mainly studied in females, and there is little information about the role of the males in interspecific mating discrimination. Using D. simulans synthetic lines with compound chromosomes from a population of the Seychelles Islands (high frequency of interspecific mating) and a multimarker strain (low frequency), we show that D. simulans males play an important role in discriminating D. melanogaster females. The genetics of male discrimination fits well with the inheritance mode of a single locus, dominant for sexual isolation, located in chromosome II near the net mutation (2L-0.0). The heterospecific mating success of the male was not related to his sexual vigor. The specific load of male cuticular hydrocarbons was counted as a possible source of discrimination used by the D. melanogaster female.     

'Exceptional sons' from Drosophila melanogaster mothers carrying a balancer X chromosome

This study reports on exceptional males which are obtained by using Drosophila melanogaster mothers carrying the balancers In(1)FM6 or In(1)FM7 as one of their X chromosomes. The phenomenon was first observed in interspecific crosses between D. melanogaster females and males of its closest relatives which normally produce unisexual female hybrid progeny. Whereas hybrid sons from these crosses die as third instar larvae, the presence of the particular X balancers in the mother allows a low percentage of sons to survive. Similar sterile males are also observed among non-hybrid flies. Data are presented which suggest that the males thus generated could be hyperploid for part of their X chromosome as a result of a meiotic event in their mothers or else they could start life as female zygotes and change sex through a mitotic event at an early stage.     

Genetic studies on premating isolation in Drosophila simulans. II. A D. simulans line highly crossable to males of D. yakuba and D. teissieri

Females of the S1 line of Drosophila simulans, an isofemale line from a natural population at Mishima, Japan, were found to show high crossability to males of D. yakuba and D. teissieri. With D. yakuba males, 24.0-58.5% of the S1 females were inseminated, while the rate was only 0.0-6.3% for the control lines. The high crossability was ascribed to the genes on the X and third chromosomes. With D. teissieri males, 38.0-51.5% of the S1 females were inseminated, while the rate was only 0.0-6.2% for the control lines. The crossability was ascribed to the genes on the third chromosome, and possibly the second chromosome genes acted additively. The crossabilities of hybrid females between the S1 and the control lines were intermediate between the parental lines both to D. yakuba males and to D. teissieri males.