Admixture of Holothurian Species in the Hellenic Seas (Eastern Mediterranean) as Revealed by RADseq

Abstract

Admixture and hybridization may play a key role in population dynamics and speciation with respect to habitat, demographic history, and adaptive selection. The present study examines the genetic diversity of two congeneric—but in different subgenera—holothurians that live in sympatry in mixed populations. Strong evidence of admixture was provided by analyzing RAD sequencing data from 90 and 67 individuals of Holothuria (Holothuria) tubulosa and Holothuria (Roweothuria) poli, respectively, from various areas of the Hellenic Seas (eastern Mediterranean). Coalescent demographic analysis revealed a recent directional gene flow from H. poli to H. tubulosa. The two species populations diverged around 13.5 thousand years ago, just after the Last Glacial Maximum. According to the pairwise sequential Markovian coalescent approach, the historical population effective sizes for both species declined during the last Pleistocene glaciations, probably due to population decline, followed by a relative rapid recovery as it is calculated using LD methods. The presented results imply a role for admixture upon secondary contact and are consistent with the recent suggestion that the genomic underpinning of ecological speciation often has an older, allopatric origin.