Genomics of Introgression

Hybridization in plants is common and impacts evolutionary trajectories. Yet, few empirical studies ask: How do selection, recombination, and genetic architecture interact to dictate the outcomes of hybridization?

Population genomics of introgression

To first characterize the extent of hybridization, I conducted low-coverage whole-genome sequencing of population genetic samples from Costus villosissimus, C. allenii, and putative hybrids. I found that hybrids are limited to intermediate habitat that connects parental populations, and that they are mostly first-generation (F1s) or backcrosses (F1xP). These geographic and genetic hybridization limits could indicate that the hybrid zone is recent and/or that strong selection limits further introgression. To determine whether hybridization is only recent, I tested for ancient introgression with ABBA/BABA analysis and found clear evidence of ancient introgression. The presence of ancient introgression and recent hybridization indicate ongoing gene flow. Yet, these species remain distinct, displaying strong ecogeographic isolation.

Genomic architecture of adaptation

Given repeated introgression, how are species distinctions maintained? Theory predicts that genomic regions where recombination rates are low or selection is strong should be resistant to introgression. Introgression is also less likely to impact quantitative traits controlled by linkage or pleiotropy than those controlled by disparate loci, which are more easily broken up by recombination. Therefore, the genetic architecture (location, number, and effect size) of genomic regions controlling adaptive traits mediates the impact of selection on introgression and species maintenance.

To identify the genetic architecture of traits identified by my and others' research as key to divergent adaptation (and thus ecogeographic isolation) between C. villosissimus and C. allenii, I am conducting quantitative trait mapping (QTL). I am also using the hybrid mapping population to create a recombination map. If the theory holds in this rapidly diversified tropical clade, selection will prevent introgression into key differentiating regions, and areas with low recombination will have less introgression. I will compare the recombination map and QTL map to genomic patterns of ancient introgression to understand the role genetic architecture of key traits plays in mediating introgression and enabling species maintenance.

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