Rapid Evolution of Assortative Fertilization between Recently Allopatric Species of Drosophila

The virilis group of Drosophila represents a relatively unexplored but potentially useful model to investigate the genetics of speciation. Good resolution of phylogenetic relationships and the ability to obtain fertile hybrid offspring make the group especially promising for analysis of genetic changes underlying reproductive isolation separate from hybrid sterility and inviability. Phylogenetic analyses reveal a close relationship between the sister species, Drosophila americana and D. novamexicana, yet excepting their contemporary allopatric distributions, factors that contribute to reproductive isolation between this species pair remain uncharacterized. A previous report has shown reduced progeny numbers in laboratory crosses between the two species, especially when female D. novamexicana are crossed with male D. americana. We show that the hatch rate of eggs produced from heterospecific matings is reduced relative to conspecific matings. Failure of eggs to hatch, and consequent reduction in hybrid progeny number, is caused by low fertilization success of heterospecific sperm, thus representing a postmating, prezygotic incompatibility. Following insemination, storage and motility of heterospecific sperm is visibly compromised in female D. novamexicana. Our results provide evidence for a mechanism of reproductive isolation that is seldom reported for Drosophila species, and indicate the rapid evolution of postmating, prezygotic reproductive barriers in allopatry

Multiple Genes Cause Postmating Prezygotic Reproductive Isolation in the Drosophila virilis Group

Understanding the genetic basis of speciation is a central problem in evolutionary biology. Studies of reproductive isolation have provided several insights into the genetic causes of speciation, especially in taxa that lend themselves to detailed genetic scrutiny. Reproductive barriers have usually been divided into those that occur before zygote formation (prezygotic) and after (postzygotic), with the latter receiving a great deal of attention over several decades. Reproductive barriers that occur after mating but before zygote formation [postmating prezygotic (PMPZ)] are especially understudied at the genetic level. Here, I present a phenotypic and genetic analysis of a PMPZ reproductive barrier between two species of the Drosophila virilis group: D. americana and D. virilis. This species pair shows strong PMPZ isolation, especially when D. americana males mate with D. virilis females: ∼99% of eggs laid after these heterospecific copulations are not fertilized. Previous work has shown that the paternal loci contributing to this incompatibility reside on two chromosomes, one of which (chromosome 5) likely carries multiple factors. The other (chromosome 2) is fixed for a paracentric inversion that encompasses nearly half the chromosome. Here, I present two results. First, I show that PMPZ in this species cross is largely due to defective sperm storage in heterospecific copulations. Second, using advanced intercross and backcross mapping approaches, I identify genomic regions that carry genes capable of rescuing heterospecific fertilization. I conclude that paternal incompatibility between D. americana males and D. virilis females is underlain by four or more genes on chromosomes 2 and 5

Evolutionary Dynamics of Male Reproductive Genes in the Drosophila virilis Subgroup

Postcopulatory sexual selection (PCSS) is a potent evolutionary force that can drive rapid changes of reproductive genes within species, and thus has the potential to generate reproductive incompatibilities between species. Male seminal fluid proteins (SFPs) are major players in postmating interactions, and are important targets of PCSS in males. The virilis subgroup of Drosophila exhibits strong interspecific gametic incompatibilities, and can serve as a model to study the genetic basis of PCSS and gametic isolation. However, reproductive genes in this group have not been characterized. Here we utilize short-read RNA sequencing of male reproductive organs to examine the evolutionary dynamics of reproductive genes in members of the virilis subgroup: D. americana, D. lummei, D. novamexicana, and D. virilis. We find that the majority of male reproductive transcripts are testes-biased, accounting for ∼15% of all annotated genes. Ejaculatory bulb (EB)-biased transcripts largely code for lipid metabolic enzymes, and contain orthologs of the D. melanogaster EB protein, Peb-me, which is involved in mating-plug formation. In addition, we identify 71 candidate SFPs, and show that this gene set has the highest rate of nonsynonymous codon substitution relative to testes- and EB-biased genes. Furthermore, we identify orthologs of 35 D. melanogaster SFPs that have conserved accessory gland expression in the virilis group. Finally, we show that several of the SFPs that have the highest rate of nonsynonymous codon substitution reside on chromosomal regions, which contributes to paternal gametic incompatibility between species. Our results show that SFPs rapidly diversify in the virilis group, and suggest that they likely play a role in PCSS and/or gametic isolation

Mating-Induced Transcriptome Changes in the Reproductive Tract of Female Aedes aegypti.

The Aedes aegypti mosquito is a significant public health threat, as it is the main vector of dengue and chikungunya viruses. Disease control efforts could be enhanced through reproductive manipulation of these vectors. Previous work has revealed a relationship between male seminal fluid proteins transferred to females during mating and female post-mating physiology and behavior. To better understand this interplay, we used short-read RNA sequencing to identify gene expression changes in the lower reproductive tract of females in response to mating. We characterized mRNA expression in virgin and mated females at 0, 6 and 24 hours post-mating (hpm) and identified 364 differentially abundant transcripts between mating status groups. Surprisingly, 60 transcripts were more abundant at 0 hpm compared to virgin females, suggesting transfer from males. Twenty of these encode known Ae. aegypti seminal fluid proteins. Transfer and detection of male accessory gland-derived mRNA in females at 0 hpm was confirmed by measurement of eGFP mRNA in females mated to eGFP-expressing males. In addition, 150 transcripts were up-regulated at 6 hpm and 24 hpm, while 130 transcripts were down-regulated at 6 hpm and 24 hpm. Gene Ontology (GO) enrichment analysis revealed that proteases, a protein class broadly known to play important roles in reproduction, were among the most enriched protein classes. RNAs associated with immune system and antimicrobial function were also up-regulated at 24 hpm. Collectively, our results suggest that copulation initiates broad transcriptome changes across the mosquito female reproductive tract, "priming" her for important subsequent processes of blood feeding, egg development and immune defense. Our transcriptome analysis provides a vital foundation for future studies of the consequences of mating on female biology and will aid studies seeking to identify specific gene families, molecules and pathways that support key reproductive processes in the female mosquito

Transfer of eGFP mRNA during mating.

<p>GFP mRNA content in wild type females was measure by qRT-PCR. Each sample was obtained from either wild type Thai females (4 or 5 individuals) mated to AAEL010824-GFP transgenic males or wild type Thai females (4 or 5 individuals) mated to non-transgenic males. Relative expression values were calculated by normalizing the expression with <i>RpS17</i>. This graph represents three technical replicates, with the error bars representing the standard deviation between those three replicates.</p

Expression patterns of seven genes using quantitative RT-PCR.

<p>Each sample was obtained from the female reproductive tract minus the ovaries at different time points after mating. Each sample represents three different biological replicates, two of them using the expression of the gene <i>RpS17</i> for normalization and a third using <i>actin</i> expression. Black bars show the results of the quantitative PCR and gray bars show the results of the RNAseq data. Error bars represent standard deviation. A Pearson correlation coefficient shows a positive correlation between qRT-PCR and RNAseq results (<i>R</i>² = 0.912, <i>p</i>-value = 4.88E-6).</p

Transcripts that are significantly differentially expressed after mating.

<p>(A) Venn diagram showing the number of down-regulated (left) and up-regulated (right) transcripts across the three post-mating time points. The up- and down-regulated gene counts are not mutually exclusive, such that transcripts that are both significantly up- and down-regulated are counted in both sets. (B) Histogram of log2 fold-change for transcripts with 2-fold or higher difference in abundance between virgin and post-mating samples (divided into 0.5 log2 bins). Each bin is partitioned into the time-point in which differential expression is detected (DE stage). The histograms are cumulative, such that transcripts that are differentially expressed in multiple time-points are represented multiple times.</p

Gene ontology (GO) enrichment analysis for up- and down-regulated genes.

<p>Gene ontology (GO) enrichment analysis for up- and down-regulated genes.</p

Expression profile of transcripts that are differentially expressed between the virgin and 0hpm sample, including their behaviors at 6 and 24hpm.

<p>The color scale represents the median-centered log2 RPKM values. Each row is a transcript and the samples are indicated at the bottom (ordered chronologically from left to right). The top 16 transcripts represent the down-regulated set, while the remaining 60 are those with higher abundances at 0hpm relative to virgin. Transcripts found to be up-regulated in male reproductive organs (MRO)[<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004451#pntd.0004451.ref029" target="_blank">29</a>], as well as known Sfp genes (Sfp)[<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004451#pntd.0004451.ref039" target="_blank">39</a>], are indicated on the left by blue (n = 20) and purple markers (n = 33), respectively.</p

Transcripts that are significantly up- or down-regulated at 6 and 24hpm.

<p>(A) Merged clusters from the <i>K</i>-means clustering analysis depicting the four mean expression profiles (red line) among transcripts differentially expressed between virgin and later time-points (6hpm and 24hpm). (B) Pie charts of GO terms associated with up-regulated transcripts (C1 and C2) and down-regulated transcripts (C3 and C4). Only ancestral GO terms are shown for the three ontology classes.</p