Pervasive gene flow despite strong and varied reproductive barriers in swordtails

Abstract

One of the mechanisms that can lead to the formation of new species occurs through the evolution of reproductive barriers. However, recent research has demonstrated that hybridization has been pervasive across the tree of life even in the presence of strong barriers. Swordtail fishes (genusXiphophorus) are an emerging model system for studying the interface between these barriers and hybridization. We document overlapping mechanisms that act as barriers between closely related species,X. birchmanniandX. cortezi, by combining genomic sequencing from natural hybrid populations, artificial crosses, behavioral assays, sperm performance, and developmental studies. We show that strong assortative mating plays a key role in maintaining subpopulations with distinct ancestry in natural hybrid populations. Lab experiments demonstrate that artificial F1 crosses experience dysfunction: crosses withX. birchmannifemales were largely inviable and crosses withX. cortezifemales had a heavily skewed sex ratio. Using F2 hybrids we identify several genomic regions that strongly impact hybrid viability. Strikingly, two of these regions underlie genetic incompatibilities in hybrids betweenX. birchmanniand its sister speciesX. malinche. Our results demonstrate that ancient hybridization has played a role in the origin of this shared genetic incompatibility. Moreover, ancestry mismatch at these incompatible regions has remarkably similar consequences for phenotypes and hybrid survival inX. cortezi×X. birchmannihybrids as inX. malinche×X. birchmannihybrids. Our findings identify varied reproductive barriers that shape genetic exchange between naturally hybridizing species and highlight the complex evolutionary outcomes of hybridization.

Complex structural variation and behavioral interactions underpin a balanced sexual mimicry polymorphism

“…We reasoned that if coding changes were involved in phenotypic differences between individuals, we would see this reflected in this analysis of amino acid sequences. We extracted the predicted exons from previously generated pseudoreference sequences for 25 X. birchmanni individuals 120 and five X. malinche individuals 121 . We found no segregating variation in the amino acid sequence at kitlga within these X. birchmanni individuals, nor within X. malinche .…”