Research

Hybridization is a pervasive feature of organic evolution. However, some groups of organisms show extensive histories of hybridization while others do not and the causes of this variation are not yet well-known. In the simplest terms, hybridization has an inverse relationship with genetic and physical distance; closely related species are more likely to have experienced hybridization events than distantly related ones, and hybridization can only occur when species’ ranges overlap (i.e. there is opportunity).

In speciation, reproductive barriers between species are often found to be loci or regulatory sequences that exhibit high rates of evolution. One cause of an accelerated rate of evolution is sexual conflict, where the ideal genotypes differ between sexes; if genetic sexual conflict is strong enough, rapid sequence divergence can occur, effectively isolating one population from another in an evolutionary arms race (Sexually Antagonistic Co-evolution or SAC). Though sexual conflict occurs in both pre-fertilization (pre-zygotic) or post-fertilization (post-zygotic) contexts, it is most likely to affect lineage isolation in post-zygotic contexts.

Viviparity is particularly susceptible to post-zygotic sexual conflict. Mediated by the paternal genetic contribution to the fetal genome, offspring manipulate maternal physiology to confer the most nutrients possible via close association with female tissues. In turn, mothers provision offspring with enough to survive while maintaining her own fitness (the viviparity-driven conflict hypothesis or VDCH). Although it has been well-studied in microevolutionary contexts, macroevolutionary studies are lacking, especially in vertebrate taxa.

To do this, I am building a whole-genome phylogeny of the livebearing fishes of the family Poeciliinae. This family includes species commonly found in the pet trade (swordtails, mollies, and guppies) alongside some of the less well-known genera such as Phalloceros and Gambusia. The Poeciliinae are particularly interested because they present an opportunity to examine how traits shaped by sexual conflict may preclude hybridization and facilitate speciation. Distributed widely across the Americas, the Poeciliinae exhibit a continuum of live-bearing strategies, from fishes that carry fully yolked eggs (lecithotrophic) to those with a placenta-like follicle (matrotrophic) that nourishes young through development. However, despite the universal state of live-bearing, fishes within this taxon hybridize widely and there are at least two species of putative hybrid origin. Further, work on the lecithotrophic genus Xiphophorus shows that hybridization is not only recent but ancient as well. Poeciliinae also show sexual characters that correspond to reproductive mode: a high degree of matrotrophy corresponds with sexual monochromatism, size increases in male genitalia (gonopodium), and coercive mating behaviors, while lecithotrophic species are dichromatic, have comparatively small male genitalia, and exhibit female mate choice. This relationship is hypothesized to indicate increasing post-zygotic sexual conflict with increasing degrees of matrotrophy.