IAP-24-010

Do genes that evolve under sexual selection cause speciation in Drosophila pseudoobscura?

Speciation occurs due to the evolution of reproductive isolation between diverging species. Sexual isolation is when divergence occurs prior to gamete fertilisation and one of the developing fields in recent speciation biology is the detection and analysis of post-mating but pre-zygotic (PMPZ) reproductive isolation [1]. This occurs when gametes have the opportunity to meet (typically, following copulation in animals or pollination in plants) but gametes from diverging populations suffer a fertilisation barrier. It is increasingly being discovered that this is an important and widespread cause of reproductive isolation [1]. The ways in which it can occur are not well understood but could include gametic incompatibilities, sperm-reproductive tract interactions and cryptic female choice.
Conspecific sperm precedence (CSP) is a form of PMPZ isolation and is revealed when females mated to two or more males produce offspring more likely to be fathered by males of the conspecific sperm type [2]. CSP could potentially arise as a by-product of sexual selection and sperm competition if sperm and female effects coevolve. However in some systems it seems to be stronger where different species interact, so has probably been strengthened to prevent deleterious hybridisation. In Drosophila pseudoobscura it has been demonstrated that CSP is stronger in populations which co-occur with D. persimilis, a sibling species with which D. pseudoobscura females produces sterile hybrids when cross-mating [3].
Recently, a suite of candidate genes evolving under sexual selection have been identified in D. pseudoobscura following a long experimental evolution study [4]. We have used CRISPR gene knockout approaches to test these genes for influences on mating success, fertilisation, and aspects of sperm competition in D. pseudoobscura.
The student will ask whether and how these new mutant genes influence post-mating but pre-zygotic isolation with D. persimilis. Are mutants in males less successful at conspecific sperm precedence? Are mutants in females less likely to bias paternity? The student will conduct a series of studies of competitive mating success in crosses between the species and analyse fertility and hybridisation rates. There is only a single study [5] which has previously asked this question, and it concentrated on genes identified in D. melanogaster, which are potentially likely to differ if such genes evolve rapidly, which is expected.
Results could be developed in numerous ways. For example, analysing the nature of post-mating effects (sperm storage, gene expression) in mutants, and analyses of gene sequence or expression evolution in natural populations depending on sympatry with D. persimilis will be of potential interest. Both evolutionary and mechanistic (gene function, neurogenetic analyses) are possible as follow-up work as it the potential to make new mutants.
This project should be attractive to students with interests in behaviour, sexual selection, gene function, evolutionary genetics and speciation biology.[1] Garlovsky, M. D., et al. (2023). “Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation.” Cold Spring Harb. Perspect. Biol. 10.1101/cshperspect.a041429[2] Howard, D. J. (1999). “Conspecific sperm and pollen precedence and speciation.” Ann. Rev. Ecol. Syst. 30: 109-132.[3] Castillo, D. M. and L. C. Moyle (2019). “Conspecific sperm precedence is reinforced, but postcopulatory sexual selection weakened, in sympatric populations of Drosophila.” Proc. Roy. Soc. B 286(1899).[4] Wiberg, R. A. W., P. Veltsos, R. R. Snook and M. G. Ritchie (2021). “Experimental evolution supports signatures of sexual selection in genomic divergence.” Evol. Lett. : 214-229.[5] Castillo, D. M. and L. C. Moyle (2014). “Intraspecific sperm competition genes enforce post-mating species barriers in Drosophila.” Proc. Roy. Soc. B 281(1797).

Fly behaviour genetic analysis; behavioural experiments with novel genetic mutants. Bioinformatics and basic molecular ecology lab methods (some DNA sequencing followed by population genetic analyses to detect natural selection on sequence and expression variation. Possiby generation of new CRISPR mutants. Visits to collaborators to help with CRISPR and expression analysis, and in particular analyses of molecular interactions in reproductive tracts.

Year 1

Behavioural analyses of mutants, particularly of mating success and sperm competition success in crosses

Year 2

Further behavioural analysis and DNA sequencing and bioinformatics of promising mutants. Neurogenetic analyses of where and when mutants expressed in relevant tissues. Possibly generation and screening of new mutants

Year 3

Analyses of new mutants and expression analyses.

Year 3.5

Tidying up; finishing bioinformatic and expression analyses and reproductive physiology in Stockholm lab.

Evolutionary genetics, behaviour genetics, Bioinformatics, gene sequencing and expression analyses, insect reproductive physiology