Introgression in two species of broadcast spawning marine invertebrate

Abstract

Because broadcast spawning marine invertebrates have fewer premating reproductive barriers than other taxa, gene flow in this group of organisms might be possible at greater genetic divergences than in other taxa. We examined introgression between two ascidian species, Ciona intestinalis and Ciona robusta, in a zone of sympatry in the English Channel. A total of 97.2% of the Ciona individuals were C. intestinalis, 0.84% were C. robusta, and 1.96% had alleles from both species. We combine these data with data collected from this sympatric zone in 2007 and 2009 to calculate an introgression rate of 3%. We describe a second sympatric zone in the southern Bay of Biscay where C. intestinalis and C. robusta show evidence of gene flow. About 24% of the Ciona were C. intestinalis, 71.2% were C. robusta, and 4.8% were introgressed. Although this gene flow is likely to be historical rather than current, the existence of recent gene flow between these two genetically divergent species reinforces the importance of premating barriers in the process of speciation.