CYTOLOGICAL CHARACTERIZATION OF PREMEIOTIC VERSUS POSTMEIOTIC DEFECTS PRODUCING HYBRID MALE STERILITY AMONG SIBLING SPECIES OF THE DROSOPHILA MELANOGASTER COMPLEX

Rapid Divergence of Key Spermatogenesis Genes in nasuta-Subgroup of Drosophila

The crosses between closely related Drosophila species usually produce sterile hybrid males with spermatogenesis disrupted at post-meiotic phase, especially in sperm individualization stage than the pre-meiotic stage. This is possibly due to the rapid interspecies divergence of male sex and reproduction-related genes. Here we annotated 11 key spermatogenesis genes in 35 strains of species belonging to nasuta-subgroup of Drosophila, where many interspecies crosses produce sterile males. We characterized the divergence and polymorphism in the protein coding regions by employing gene-wide, codon-wide, and lineage-specific selection analysis to test the mode and strength of selection acting on these genes. Our analysis showed signature of positive selection at bag of marbles (bam) and benign gonial cell neoplasma (bgcn) despite the selection constrains and the absence of endosymbiont infection which could potentially drive rapid divergence due to an arms race while roughex (rux) showed lineage-specific rapid divergence in frontal sheen complex of nasuta-subgroup. cookie monster (comr) showed rapid divergence consistent with the possibility of meiotic arrest observed in sterile hybrids of Drosophila species. Rapid divergence observed at don juan (dj) and Mst98Ca-like was consistent with fused sperm-tail abnormality observed in the hybrids of Drosophila nasuta and Drosophila albomicans. These findings highlight the potential role of rapid nucleotide divergence in bringing about hybrid incompatibility in the form of male sterility; however, additional genetic manipulation studies can widen our understanding of hybrid incompatibilities. Furthermore, our study emphasizes the importance of young species belonging to nasuta-subgroup of Drosophila in studying post-zygotic reproductive isolation mechanisms.

Defective Satellite DNA Clustering into Chromocenters Underlies Hybrid Incompatibility in Drosophila

Although rapid evolution of pericentromeric satellite DNA repeats is theorized to promote hybrid incompatibility (HI) (Yunis and Yasmineh 1971; Henikoff et al. 2001; Ferree and Barbash 2009; Sawamura 2012; Jagannathan and Yamashita 2017), how divergent repeats affect hybrid cells remains poorly understood. Recently, we demonstrated that sequence-specific DNA-binding proteins cluster satellite DNA from multiple chromosomes into "chromocenters," thereby bundling chromosomes to maintain the entire genome in a single nucleus (Jagannathan et al. 2018, 2019). Here, we show that ineffective clustering of divergent satellite DNA in the cells of Drosophila hybrids results in chromocenter disruption, associated micronuclei formation, and tissue atrophy. We further demonstrate that previously identified HI factors trigger chromocenter disruption and micronuclei in hybrids, linking their function to a conserved cellular process. Together, we propose a unifying framework that explains how the widely observed satellite DNA divergence between closely related species can cause reproductive isolation.

Distinct spermiogenic phenotypes underlie sperm elimination in the Segregation Distorter meiotic drive system

Segregation Distorter (SD) is a male meiotic drive system in Drosophila melanogaster. Males heterozygous for a selfish SD chromosome rarely transmit the homologous SD+ chromosome. It is well established that distortion results from an interaction between Sd, the primary distorting locus on the SD chromosome and its target, a satellite DNA called Rsp, on the SD+ chromosome. However, the molecular and cellular mechanisms leading to post-meiotic SD+ sperm elimination remain unclear. Here we show that SD/SD+ males of different genotypes but with similarly strong degrees of distortion have distinct spermiogenic phenotypes. In some genotypes, SD+ spermatids fail to fully incorporate protamines after the removal of histones, and degenerate during the individualization stage of spermiogenesis. In contrast, in other SD/SD+ genotypes, protamine incorporation appears less disturbed, yet spermatid nuclei are abnormally compacted, and mature sperm nuclei are eventually released in the seminal vesicle. Our analyses of different SD+ chromosomes suggest that the severity of the spermiogenic defects associates with the copy number of the Rsp satellite. We propose that when Rsp copy number is very high (> 2000), spermatid nuclear compaction defects reach a threshold that triggers a checkpoint controlling sperm chromatin quality to eliminate abnormal spermatids during individualization.

Hybrid Sterility, Genetic Conflict and Complex Speciation: Lessons From the Drosophila simulans Clade Species

The three fruitfly species of the Drosophila simulans clade- D. simulans, D. mauritiana, and D. sechellia- have served as important models in speciation genetics for over 40 years. These species are reproductively isolated by geography, ecology, sexual signals, postmating-prezygotic interactions, and postzygotic genetic incompatibilities. All pairwise crosses between these species conform to Haldane's rule, producing fertile F1 hybrid females and sterile F1 hybrid males. The close phylogenetic proximity of the D. simulans clade species to the model organism, D. melanogaster, has empowered genetic analyses of their species differences, including reproductive incompatibilities. But perhaps no phenotype has been subject to more continuous and intensive genetic scrutiny than hybrid male sterility. Here we review the history, progress, and current state of our understanding of hybrid male sterility among the D. simulans clade species. Our aim is to integrate the available information from experimental and population genetics analyses bearing on the causes and consequences of hybrid male sterility. We highlight numerous conclusions that have emerged as well as issues that remain unresolved. We focus on the special role of sex chromosomes, the fine-scale genetic architecture of hybrid male sterility, and the history of gene flow between species. The biggest surprises to emerge from this work are that (i) genetic conflicts may be an important general force in the evolution of hybrid incompatibility, (ii) hybrid male sterility is polygenic with contributions of complex epistasis, and (iii) speciation, even among these geographically allopatric taxa, has involved the interplay of gene flow, negative selection, and positive selection. These three conclusions are marked departures from the classical views of speciation that emerged from the modern evolutionary synthesis.

A common suite of cellular abnormalities and spermatogenetic errors in sterile hybrid males in Drosophila

When two species interbreed, the resulting hybrid offspring are often sterile, with the heterogametic (e.g. XY) hybrid usually being more severely affected. The prevailing theory for this pattern of sterility evokes divergent changes in separate lineages having maladaptive interactions when placed together in a hybrid individual, with recessive factors on the sex chromosome interacting with dominant factors on the autosomes. The effect of these interactions on gametogenesis should not be uniform across species pairs unless genetic divergence follows the same paths in different lineages or if a specific stage of gametogenesis is more susceptible to detrimental genetic interactions. Here, we perform a detailed cellular characterization of hybrid male sterility across three recently diverged species pairs of Drosophila. Across all three pairs, sterile hybrid sperm are alive but exhibit rapid nuclear de-condensation with age, with active, but non-differentiated, mitochondria. Surprisingly, all three sets of interspecies hybrids produce half of the number of sperm per round of spermatogenesis, with each sperm cell containing two tails. We identify non-disjunction failures during meiosis I as the likely cause. Thus, errors during meiosis I may be a general phenomenon underlying Drosophila male sterility, indicating either a heightened sensitivity of this spermatogenic stage to failure, or a basis to sterility other than the prevailing model.

Premeiotic and meiotic failures lead to hybrid male sterility in the Anopheles gambiae complex

Hybrid male sterility (HMS) contributes to speciation by restricting gene flow between related taxa. Detailed cytological characterization of reproductive organs in hybrid males is important for identifying phenotypes that can help guide searches of speciation genes. To investigate possible cellular causes of HMS, we performed crosses between closely related species of the Anopheles gambiae complex: An. merus with An. gambiae or An. coluzzii. We demonstrate that HMS in African malaria mosquitoes involves two defects in the reciprocal crosses: a premeiotic arrest of germline stem cells in degenerate testes and a failure of the reductional meiotic division of primary spermatocytes in normal-like testes. The premeiotic arrest in degenerate testes of hybrids is accompanied by a strong suppression of meiotic and postmeiotic genes. Unlike pure species, sex chromosomes in normal-like testes of F1 hybrids are largely unpaired during meiotic prophase I and all chromosomes show various degrees of insufficient condensation. Instead of entering reductional division in meiosis I, primary spermatocytes prematurely undergo an equational mitotic division producing non-motile diploid sperm. Thus, our study identified cytogenetic errors in interspecies hybrids that arise during the early stages of postzygotic isolation.

The fate of W chromosomes in hybrids between wild silkmoths, Samia cynthia ssp.: no role in sex determination and reproduction

Moths and butterflies (Lepidoptera) have sex chromosome systems with female heterogamety (WZ/ZZ or derived variants). The maternally inherited W chromosome is known to determine female sex in the silkworm, Bombyx mori. However, little is known about the role of W chromosome in other lepidopteran species. Here we describe two forms of the W chromosome, W and neo-W, that are transmitted to both sexes in offspring of hybrids from reciprocal crosses between subspecies of wild silkmoths, Samia cynthia. We performed crosses between S. c. pryeri (2n=28, WZ/ZZ) and S. c. walkeri (2n=26, neo-Wneo-Z/neo-Zneo-Z) and examined fitness and sex chromosome constitution in their hybrids. The F1 hybrids of both reciprocal crosses had reduced fertility. Fluorescence in situ hybridization revealed not only the expected sex chromosome constitutions in the backcross and F2 hybrids of both sexes but also females without the W (or neo-W) chromosome and males carrying the W (or neo-W) chromosome. Furthermore, crosses between the F2 hybrids revealed no association between the presence or absence of W (or neo-W) chromosome and variations in the hatchability of their eggs. Our results clearly suggest that the W (or neo-W) chromosome of S. cynthia ssp. plays no role in sex determination and reproduction, and thus does not contribute to the formation of reproductive barriers between different subspecies.

Hybrid male sterility and genome-wide misexpression of male reproductive proteases

Hybrid male sterility is a common barrier to gene flow between species. Previous studies have posited a link between misregulation of spermatogenesis genes in interspecies hybrids and sterility. However, in the absence of fully fertile control hybrids, it is impossible to differentiate between misregulation associated with sterility vs. fast male gene regulatory evolution. Here, we differentiate between these two possibilities using a D. pseudoobscura species pair that experiences unidirectional hybrid sterility. We identify genes uniquely misexpressed in sterile hybrid male reproductive tracts via RNA-seq. The sterile male hybrids had more misregulated and more over or under expressed genes relative to parental species than the fertile male hybrids. Proteases were the only gene ontology class overrepresented among uniquely misexpressed genes, with four located within a previously identified hybrid male sterility locus. This result highlights the potential role of a previously unexplored class of genes in interspecific hybrid male sterility and speciation.

Hybrid male sterility between Drosophila willistoni species is caused by male failure to transfer sperm during copulation

BACKGROUND

The biological concept of species stresses the importance of understanding what mechanisms maintain species reproductively isolated from each other. Often such mechanisms are divided into premating and postmating, with the latest being the result of either prezygotic or postzygotic isolation barriers. Drosophila willistoni quechua and Drosophila willistoni willistoni are two subspecies that experience reproductive isolation. When a D. w. quechua female is crossed with a D. w. willistoni male, the hybrid males (F1QW) are unable to father progeny; however, the reciprocal cross produces fertile hybrids. Thus, the mechanism of isolation is unidirectional hybrid male sterility. However, the sterile F1QW males contain large amounts of motile sperm. Here we explore whether pre-copulatory or post-copulatory pre-zygotic mechanisms serve as major deterrents in the ability of F1QW males to father progeny.

RESULTS

Comparisons of parental and hybrid males copulation durations showed no significant reduction in copulation duration of F1QW males. Interrupted copulations of the parental species confirmed that sperm transfer occurs before the minimum copulation duration registered for F1QW males. However, we found that when females mate with F1QW males, sperm is not present inside the female storage organs and that the lack of sperm in storage is due to failure to transfer sperm rather than spillage or active sperm dumping by females.

CONCLUSIONS

Sterility of F1QW hybrid males is primarily driven by their inability to transfer sperm during copulation.

The roles of cis- and trans-regulation in the evolution of regulatory incompatibilities and sexually dimorphic gene expression

Evolutionary changes in gene expression underlie many aspects of phenotypic diversity within and among species. Understanding the genetic basis for evolved changes in gene expression is therefore an important component of a comprehensive understanding of the genetic basis of phenotypic evolution. Using interspecific introgression hybrids, we examined the genetic basis for divergence in genome-wide patterns of gene expression between Drosophila simulans and Drosophila mauritiana. We find that cis-regulatory and trans-regulatory divergences differ significantly in patterns of genetic architecture and evolution. The effects of cis-regulatory divergence are approximately additive in heterozygotes, quantitatively different between males and females, and well predicted by expression differences between the two parental species. In contrast, the effects of trans-regulatory divergence are associated with largely dominant introgressed alleles, have similar effects in the two sexes, and generate expression levels in hybrids outside the range of expression in both parental species. Although the effects of introgressed trans-regulatory alleles are similar in males and females, expression levels of the genes they regulate are sexually dimorphic between the parental D. simulans and D. mauritiana strains, suggesting that pure-species genotypes carry unlinked modifier alleles that increase sexual dimorphism in expression. Our results suggest that independent effects of cis-regulatory substitutions in males and females may favor their role in the evolution of sexually dimorphic phenotypes, and that trans-regulatory divergence is an important source of regulatory incompatibilities.

Rapid male-specific regulatory divergence and down regulation of spermatogenesis genes in Drosophila species hybrids

In most crosses between closely related species of Drosophila, the male hybrids are sterile and show postmeiotic abnormalities. A series of gene expression studies using genomic approaches have found significant down regulation of postmeiotic spermatogenesis genes in sterile male hybrids. These results have led some to suggest a direct relationship between down regulation in gene expression and hybrid sterility. An alternative explanation to a cause-and-effect relationship between misregulation of gene expression and male sterility is rapid divergence of male sex regulatory elements leading to incompatible interactions in an interspecies hybrid genome. To test the effect of regulatory divergence in spermatogenesis gene expression, we isolated 35 fertile D. simulans strains with D. mauritiana introgressions in either the X, second or third chromosome. We analyzed gene expression in these fertile hybrid strains for a subset of spermatogenesis genes previously reported as significantly under expressed in sterile hybrids relative to D. simulans. We found that fertile autosomal introgressions can cause levels of gene down regulation similar to that of sterile hybrids. We also found that X chromosome heterospecific introgressions cause significantly less gene down regulation than autosomal introgressions. Our results provide evidence that rapid male sex gene regulatory divergence can explain misexpression of spermatogenesis genes in hybrids.

Interspecific Y chromosome introgressions disrupt testis-specific gene expression and male reproductive phenotypes in Drosophila

The Drosophila Y chromosome is a degenerated, heterochromatic chromosome with few functional genes. Nonetheless, natural variation on the Y chromosome in Drosophila melanogaster has substantial trans-acting effects on the regulation of X-linked and autosomal genes. However, the contribution of Y chromosome divergence to gene expression divergence between species is unknown. In this study, we constructed a series of Y chromosome introgression lines, in which Y chromosomes from either Drosophila sechellia or Drosophila simulans are introgressed into a common D. simulans genetic background. Using these lines, we compared genome-wide gene expression and male reproductive phenotypes between heterospecific and conspecific Y chromosomes. We find significant differences in expression for 122 genes, or 2.84% of all genes analyzed. Genes down-regulated in males with heterospecific Y chromosomes are significantly biased toward testis-specific expression patterns. These same lines show reduced fecundity and sperm competitive ability. Taken together, these results imply a significant role for Y/X and Y/autosome interactions in maintaining proper expression of male-specific genes, either directly or via indirect effects on male reproductive tissue development or function.

Elevated Evolutionary Rates among Functionally Diverged Reproductive Genes across Deep Vertebrate Lineages

Among closely related taxa, proteins involved in reproduction generally evolve more rapidly than other proteins. Here, we apply a functional and comparative genomics approach to compare functional divergence across a deep phylogenetic array of egg-laying and live-bearing vertebrate taxa. We aligned and annotated a set of 4,986 1 : 1 : 1 : 1 : 1 orthologs in Anolis carolinensis (green lizard), Danio rerio (zebrafish), Xenopus tropicalis (frog), Gallus gallus (chicken), and Mus musculus (mouse) according to function using ESTs from available reproductive (including testis and ovary) and non-reproductive tissues as well as Gene Ontology. For each species lineage, genes were further classified as tissue-specific (found in a single tissue) or tissue-expressed (found in multiple tissues). Within independent vertebrate lineages, we generally find that gonadal-specific genes evolve at a faster rate than gonadal-expressed genes and significantly faster than non-reproductive genes. Among the gonadal set, testis genes are generally more diverged than ovary genes. Surprisingly, an opposite but nonsignificant pattern is found among the subset of orthologs that remained functionally conserved across all five lineages. These contrasting evolutionary patterns found between functionally diverged and functionally conserved reproductive orthologs provide evidence for pervasive and potentially cryptic lineage-specific selective processes on ancestral reproductive systems in vertebrates.

Male sex interspecies divergence and down regulation of expression of spermatogenesis genes in Drosophila sterile hybrids

Male sex genes have shown a pattern of rapid interspecies divergence at both the coding and gene expression level. A common outcome from crosses between closely-related species is hybrid male sterility. Phenotypic and genetic studies in Drosophila sterile hybrid males have shown that spermatogenesis arrest is postmeiotic with few exceptions, and that most misregulated genes are involved in late stages of spermatogenesis. Comparative studies of gene regulation in sterile hybrids and parental species have mainly used microarrays providing a whole genome representation of regulatory problems in sterile hybrids. Real-time PCR studies can reject or reveal differences not observed in microarray assays. Moreover, differences in gene expression between samples can be dependant on the source of RNA (e.g., whole body vs. tissue). Here we survey expression in D. simulans, D. mauritiana and both intra and interspecies hybrids using a real-time PCR approach for eight genes expressed at the four main stages of sperm development. We find that all genes show a trend toward under expression in the testes of sterile hybrids relative to parental species with only the two proliferation genes (bam and bgcn) and the two meiotic class genes (can and sa) showing significant down regulation. The observed pattern of down regulation for the genes tested can not fully explain hybrid male sterility. We discuss the down regulation of spermatogenesis genes in hybrids between closely-related species within the contest of rapid divergence experienced by the male genome, hybrid sterility and possible allometric changes due to subtle testes-specific developmental abnormalities.

Evolution in the fast lane: rapidly evolving sex-related genes in Drosophila

A large portion of the annotated genes in Drosophila melanogaster show sex-biased expression, indicating that sex and reproduction-related genes (SRR genes) represent an appreciable component of the genome. Previous studies, in which subsets of genes were compared among few Drosophila species, have found that SRR genes exhibit unusual evolutionary patterns. Here, we have used the newly released genome sequences from 12 Drosophila species, coupled to a larger set of SRR genes, to comprehensively test the generality of these patterns. Among 2505 SRR genes examined, including ESTs with biased expression in reproductive tissues and genes characterized as involved in gametogenesis, we find that a relatively high proportion of SRR genes have experienced accelerated divergence throughout the genus Drosophila. Several testis-specific genes, male seminal fluid proteins (SFPs), and spermatogenesis genes show lineage-specific bursts of accelerated evolution and positive selection. SFP genes also show evidence of lineage-specific gene loss and/or gain. These results bring us closer to understanding the details of the evolutionary dynamics of SRR genes with respect to species divergence.

Two alternate mechanisms contribute to the persistence of interdependent lineages in Pogonomyrmex harvester ants

Some populations of Pogonomyrmex harvester ants comprise pairs of highly differentiated lineages with queens mating at random with several males of their own and of the alternate lineage. These queens produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into new queens and those fertilized by males of the other lineage which mostly develop into functionally sterile workers. This unusual mode of genetic caste determination has been found in 26 populations and a total of four lineage pairs (F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2)) have been described in these populations. Despite the fact that a few interlineage queens are produced, previous studies revealed that there is a complete lack of genetic introgression between lineages. Here we quantify the proportion of interlineage queens produced in each of the four lineage pairs and determine the fate of these queens. In the F(1)-F(2), G(1)-G(2) and H(1)-H(2) lineage pairs, interlineage queens were produced by a minority of colonies. These colonies exclusively produced interlineage queens and workers, suggesting that interlineage eggs can develop into queens in these three pairs of lineages in the absence of competition with pure-lineage brood. An analysis of three key stages of the colony life cycle revealed that colonies headed by interlineage queens failed to grow sufficiently to produce reproductive individuals. In laboratory comparisons, interlineage queens produced fewer viable eggs, with the effect that they raised fewer workers and lost more weight per worker produced than pure-lineage queens. In the J(1)-J(2) lineage pair, we did not find a single interlineage queen, raising the possibility that interlineage eggs have completely lost the ability to develop into queens in this lineage pair. Hence, two distinct mechanisms seem to account for the complete lack of between-lineage gene flow in the F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2) lineage pairs.

Gene regulation divergence is a major contributor to the evolution of Dobzhansky-Muller incompatibilities between species of Drosophila

The Dobzhansky-Muller model denotes incompatible gene interactions between diverging populations/species and is recognized as the basis of postzygotic reproductive isolation. Little is known about the molecular nature of such gene interactions. We have carried out comparative gene expression analyses in the testes of 3 closely related species of the Drosophila melanogaster subgroup and their hybrids (all of which are sterile). We show that in hybrids 1) a higher proportion of male-biased genes (i.e., genes with a higher level of expression in males) are underexpressed (or not expressed) compared with non-sex-biased genes, 2) the majority of the underexpressed genes appear to be under stabilizing selection by virtue of showing similar levels of expression in the parental species, and only a small proportion of genes show signs of directional selection, 3) very few of the misexpressed genes are shared between species pairs, suggesting that there may not be a "common" set of "speciation genes," and 4) expression of non-testes-specific genes is observed in the testes of interspecific hybrids, and the number of such genes is positively correlated with divergence time. These results suggest that gene regulation divergence of sex- and reproduction-related genes is a major contributor to the evolution of Dobzhansky-Muller incompatibilities between species of Drosophila.

Rapid evolution and gene-specific patterns of selection for three genes of spermatogenesis in Drosophila

Hybrid males resulting from crosses between closely related species of Drosophila are sterile. The F1 hybrid sterility phenotype is mainly due to defects occurring during late stages of development that relate to sperm individualization, and so genes controlling sperm development may have been subjected to selective diversification between species. It is also possible that genes of spermatogenesis experience selective constraints given their role in a developmental pathway. We analyzed the molecular evolution of three genes playing a role during the sperm developmental pathway in Drosophila at an early (bam), a mid (aly), and a late (dj) stage. The complete coding region of these genes was sequenced in different strains of Drosophila melanogaster and Drosophila simulans. All three genes showed rapid divergence between species, with larger numbers of nonsynonymous to synonymous differences between species than polymorphisms. Although this could be interpreted as evidence for positive selection at all three genes, formal tests of selection do not support such a conclusion. Departures from neutrality were detected only for dj and bam but not aly. The role played by selection is unique and determined by gene-specific characteristics rather than site of expression. In dj, the departure was due to a high proportion of neutral synonymous polymorphisms in D. simulans, and there was evidence of purifying selection maintaining a high lysine amino acid protein content that is characteristic of other DNA-binding proteins. The earliest spermatogenesis gene surveyed, which plays a role in both male and female gametogenesis, was bam, and its significant departure from neutrality was due to an excess of nonsynonymous substitutions between species. Bam is degraded at the end of mitosis, and rapid evolutionary changes among species might be a characteristic shared with other degradable transient proteins. However, the large number of nonsynonymous changes between D. melanogaster and D. simulans and a phylogenetic comparative analysis among species confirms evidence of positive selection driving the evolution of Bam and suggests an yet unknown germ cell line developmental adaptive change between these two species.

Speciation in Progress? A Continuum of Reproductive Isolation in Drosophila Bipectinata

Incipient species in the early stages of divergence can provide crucial information about the genetic basis of reproductive isolation and the evolutionary forces that promote speciation. In this report, we describe two subspecies of Drosophila bipectinata that show a continuum of reproductive isolation. Crosses between strains of the same subspecies produce fully fertile offspring. At the same time, each subspecies harbors extensive variation for the degree of reproductive isolation from the other subspecies. The percentage of fertile hybrid males varies from 0 to 90%, depending on the origin of parental strains, indicating that the genes responsible for hybrid sterility are not fixed within either subspecies, or even within local populations. Reproductive isolation is non-transitive, so that the extent of hybrid sterility depends on the particular combination of strains. The two subspecies show little or no evidence of genetic differentiation at three nuclear loci, suggesting that they diverged very recently or continue to experience significant levels of gene flow. A hybrid zone between the two subspecies may exist in New Guinea and Northeastern Australia.

Male sex drive and the masculinization of the genome

Charles Darwin remarked that "males, with their superior strength, pugnacity, armaments, unwieldly passion and love songs, are almost always the more active and most often, the initiators of sexual interactions". Here, we propose that such male sex drive directly impacts the genome by leading to its progressive masculinization--genes that possess sex-specific effects on male fitness accumulate to a much greater extent and are generally more diverged. The larger proportion of male versus female fitness modifiers in combination with stronger sexual selection may generate evolutionary signatures such as a greater sensitivity to male sterility and a paucity of X-linked male-specific genes. Male sex-drive theory complements the female-choice theory of sexual selection and allows for the genetic variation of costly sexual traits to be continuously replenished.

Evolutionary novelties in islands: Drosophila santomea, a new melanogaster sister species from São Tomé

The finding of new melanogaster sister species may help us in understanding more about how the emergence of genetic novelties, particularly in insular habitats, can result in speciation. Here we report on the discovery of Drosophila santomea, which is the first melanogaster sibling found off West-equatorial Africa, on São Tomé, one of the Gulf of Guinea islands. Although the eight other melanogaster sister species are remarkably conservative in their morphology except for their terminalia, the new find has a morphological trait distinguishing it from all of these: a pure yellow body coloration of both sexes without the normal black abdominal banding. Evidence from the terminalia, polytene and mitotic chromosomes, period gene and allozymes are provided indicating that it is nonetheless the nearest relative of Drosophila yakuba with which it coexists on the island. The new find is a clear-cut taxon as shown by the production of sterile male hybrids, eventually with developmental defects, in both directions of cross with yakuba and by the existence of an altitudinal divide accompanied by a hybrid zone at mid-elevation on the island. Molecular and karyotypic data further support this conclusion. In contrast to the significant divergence of their nuclear DNAs, an intriguing similarity in their cytochrome b sequences was observed indicating a recent coalescence common to santomea, yakuba and also teissieri cytoplasms. These were shown to harbour the same Wolbachia endosymbiotic bacteria which could possibly be responsible for mitochondrial DNA hitchhiking across the species barrier.