Evidence for gene flow and trait reversal during radiation of Mexican Goodeid fish.

Understanding the phylogeographic history of a group and identifying the factors contributing to speciation is an important challenge in evolutionary biology. The Goodeinae are a group of live-bearing fishes endemic to Mexico. Here, we develop genomic resources for species within the Goodeinae and use phylogenomic approaches to characterise their evolutionary history. We sequenced, assembled and annotated the genomes of four Goodeinae species, including Ataeniobius toweri, the only matrotrophic live-bearing fish without a trophotaenia in the group. We estimated timings of species divergence and examined the extent and timing of introgression between the species to assess if this may have occurred during an early radiation, or in more recent episodes of secondary contact. We used branch-site models to detect genome-wide positive selection across Goodeinae, and we specifically asked whether this differs in A. toweri, where loss of placental viviparity has recently occurred. We found evidence of gene flow between geographically isolated species, suggesting vicariant speciation was supplemented by limited post-speciation gene flow, and gene flow may explain previous uncertainties about Goodeid phylogeny. Genes under positive selection in the group are likely to be associated with the switch to live-bearing. Overall, our studies suggest that both volcanism-driven vicariance and changes in reproductive mode influenced radiation in the Goodeinae.