The cybrid invasion: widespread postglacial dispersal byPhoxinus(Pisces: Cyprinidae) cytoplasmic hybrids

Abstract

Peculiar reproductive processes that arise during sperm-dependent asexual reproduction may have important roles in allowing coexistence of sperm-dependent asexuals and their sexually reproducing hosts. Though typically clonal, nonclonal reproduction in the hybrid gynogenetic species Phoxinus eos–neogaeus can reestablish sexual northern redbelly dace ( Phoxinus eos (Cope, 1861)) nuclear genomes. These hybrid-derived P. eos biotypes, known as cybrids, have finescale dace ( Phoxinus neogaeus Cope, 1867) mitochondria but are otherwise indistinguishable from “pure” P. eos. Our goal was to determine the extent of the cybrids’ distribution, and hence to evaluate their role in mechanisms of asexual–sexual coexistence. We also set out to determine the geographic origins of P. eos–neogaeus and cybrids. Our sampling revealed that all Phoxinus species and biotypes are widely distributed in North America, but that cybrids have replaced pure P. eos, or preempted postglacial colonization by pure P. eos, in the northern portion of their distribution. Also, all P. eos–neogaeus and cybrids had mitochondrial DNA (mtDNA) sequences closely related to a lineage originating from a Mississippi glacial refugium. No P. eos–neogaeus or cybrids were derived from a divergent P. neogaeus lineage discovered in Atlantic Canada. It is therefore unlikely that P. eos–neogaeus or cybrids originated in any Atlantic glacial refugia. If P. neogaeus mitochondria are better adapted to northern environments, widespread mtDNA introgression may facilitate coexistence in northern areas by reducing the disparity in fitness between P. eos–neogaeus and its sexual hosts.