The genetic basis of gametic incompatibilities
In species where females store sperm from multiple males, the opportunity for postcopulatory sexual selection arises, resulting in cryptic female choice and/or sperm competition. Over evolutionary time and across diverging lineages, these forces have the potential to cause rapid differentiation of reproductive genes such that these lineages become reproductively isolated. This phenomenon has been observed across a diverse array of taxa, and is thought to be a major engine of speciation. The virilis group of Drosophila is an ideal model to study the genetic basis of gametic incompatiblities because interspecies crosses between all its members produce strong gametic incompatibilities with varying severity in reciprocal crosses. In addition, it is an increasingly robust genetic model system where hybrids are mostly viable/fertile and can be utilized for fine-scale genetic mapping. Our lab uses this system to dissect the genetic basis of these gametic incompatibilities (postmating prezygotic barters; PMPZ).
Phylogeny of virilis subgroup members (top) and a measure of gametic incompatibility between its members (bottom). From Ahmed-Braimah et al. (2017)
Molecular evolution of reproductive genes
One of the most pervasive patterns in molecular evolution is the rapid divergence of reproductive genes, especially those coding for seminal fluid proteins (SFPs). While this pattern is observed across many taxa, the phenotypic consequences of this rapid evolution remain a mystery. For example, what functional benefit do these rapidly divergent SFPs confer between species? Do new amino acid variants function in epistatic interactions? What's the selective advantage gained by these rapidly diverging amino acid substitutions? To address this problem, we are studying several rapidly diverging SFPs between members of the virilis group, some of which reside on a major paternal PMPZ quantitative trait loci (QTL). Our goal is to understand the role of these SFPs in reproduction and the evolutionary significance of their rapid divergence.
Candidate SFPs (green = derived from accessory glands, purple = derived from ejaculatory bulb) plotted by chromosomal coordinates against their Ka/Ks value. Paternal PMPZ QTL are highlighted in pink. From Ahmed-Braimah et al. (2017)
Mating-induced changes in gene expression in the female reproductive tract
Females undergo a variety of behavioral and physiological changes after mating, such as facilitate the uptake and storage of sperm, reduce her receptivity to remate, and induce ovulation. These changes are necessary for reproductive fitness, and are also subject to intense postcopulatory selective pressures. Several studies have identified transcriptome changes after mating, but we not yet know how these changes facilitate the physiological and behavioral changes that occur after mating, and which genes are essential to initiate this response. We are studying this process in several species to understand the consequences of postmating transcript abundance changes.
Changes in transcript abundance in the female reproductive tract after con- (purple) or heterospecific (green) mating (unpublished).
Quantitative proteomic analysis of female and male reproductive proteins
The goal of this project is to use shotgun/discovery proteomics to characterize the reproductive "interactome" of male and female proteins. Advances in mass spectrometry-based proteomics allow for increasingly higher sensitivity in detecting even the lowest abundance proteins. Another goal of this project is toapply targeted quantification of protein analytes using parallel/selected reaction monitoring (PRM/SRM) to quantify the relative abundances of each sex's reproductive proteins—an often neglected feature of reproductive protein interaction. Leveraging the virilis group for this study allows identification of aberrant protein interactions in heterospecific crosses to understand features of reproductive interactions in general and how evolutionary forces drive divergence in these interactions.
By isotopically labelling an amino acid in one sex, we are able to differentiate the between male and female protein in a mixed pool, i.e. the reproductive tract of recently mated female. This approach was successfully used in other taxa.
Strategy for selective reaction monitoring of a known set of peptide ions (above). Strategy for SILAC labeling and identification of male and female protein in a reproductive tract sample that is extracted from a recently-mated female (right).
Dynamics of in vivo sperm behavior in the Drosophila virilis group
Sperm storage and utilization dynamics are poorly understood in the Drosophila virilis group. Sperm and seminal receptacle length are far greater than species in the Sophophora subgenus, and the mechanism of storage and fertilization is disrupted in heterospecific crosses. We have developed fluorescently tagged sperm head/tail strains for virilis group members to facilitate the analysis of sperm behavior in vivo.
Female reproductive tract of D. novamexicana 3 hours after mating to a conspecific male (left): sperm heads are visible in the paired spermathecae and seminal receptacle. Zoom in of sperm head bundles inside the female seminal receptacle.
The genetic basis of hybrid male sterility in D. virilis/D. novamexicana hybrids
Hybrids in nearly all interspecific crosses between virilis group members result in largely fertile offspring. The one exception are male hybrids between D. virilis females and D. novamexicana males; Heikkinen and Lumme (1998) showed that this sterility is strongly driven by an incompatibility between the Y chromosome of D. novamexicana and the X chromosome of D. virilis, but other autosomal factors are also involved. We have shown that the sterility of these hybrids is caused by a defect late in spermatogenesis, whereby mature spermatids fail to individualize. Using long-read sequencing we are identifying Y chromosome sequences to identify factors on the Y chromosome that differ between species, and also mapping regions of the genome that contribute to this sterility.
Testes of D. novamexicana (top left) and hybrid (bottom left) males. Insets show individualized sperm for D. novamexicana and aberrant sperm bundles in extruded from hybrid testes.