Variable hybridization between two Lake Tanganyikan cichlid species in recent secondary contact

Abstract

Closely related taxa frequently exist in sympatry before the evolution of robust reproductive barriers, which can lead to substantial gene flow. Post‐divergence gene flow can promote several disparate trajectories of divergence ranging from the erosion of distinctiveness and eventual collapse of the taxa to the strengthening of reproductive isolation. Among many relevant factors, understanding the demographic history of divergence (e.g. divergence time and extent of historical gene flow) can be particularly informative when examining contemporary gene flow between closely related taxa because this history can influence gene flow's prevalence and consequences. Here, we used genotyping‐by‐sequencing data to investigate speciation and contemporary hybridization in two closely related and sympatrically distributed Lake Tanganyikan cichlid species in the genus Petrochromis. Demographic modelling supported a speciation scenario involving divergence in isolation followed by secondary contact with bidirectional gene flow. Further investigation of this recent gene flow found evidence of ongoing hybridization between the species that varied in extent between different co‐occurring populations. Relationships between abundance and the degree of admixture across populations suggest that the availability of conspecific mates may influence patterns of hybridization. These results, together with the observation that sets of recently diverged cichlid taxa are generally geographically separated in the lake, suggest that ongoing speciation in Lake Tanganyikan cichlids relies on initial spatial isolation. Additionally, the spatial heterogeneity of admixture between the Petrochromis species illustrates the complexities of hybridization when species are in recent secondary contact.

Mating system and environment predict the direction and extent of introgression between incipientClarkiaspecies

“…Secondary contact provides the ultimate test of whether incipient species have diverged sufficiently to remain distinct. Population genomic analyses have revealed that introgression is quite common for taxa in secondary contact [1][2][3][4] and that introgression varies among contact zones [5][6][7] and parts of genomes 1 . However, the ecological factors and phenotypic traits that modulate the direction and magnitude of admixture, and its effects on species boundaries, remain unresolved.…”

“…Variation in introgression among independent contact zones may be caused by multiple mechanisms, e.g., differences in relative abundance 6 , the local genetic architecture of reproductive isolation, or the environmental context 7 . The environment can mediate both the expression of traits impacting hybridization 8,9 and the fitness consequences of hybridization and introgression 10 , making reproductive isolation context dependent.…”