Late Pleistocene Range Expansion of North American Topminnows Accompanied by Admixture and Introgression

Aim: We used genome-scale sampling to assess the phylogeography of a group of topminnows in the Fundulus notatus species complex. Two of the species have undergone extensive range expansions resulting in broadly overlapping distributions, and sympatry within drainages has provided opportunities for hybridization and introgression. We assessed the timing and pattern of range expansion in the context of late Pleistocene-Holocene drainage events and evaluated the evidence for introgressive hybridization between species. Location: Central and southern United States including drainages of the Gulf of Mexico Coastal Plain and portions of the Mississippi River drainage in and around the Central Highlands. Taxon: Topminnows, Genus Fundulus, subgenus Zygonectes-Fundulus notatus, Fundulus olivaceus, Fundulus euryzonus. Methods: We sampled members of the F. notatus species complex throughout their respective ranges, including numerous drainage systems where species co-occur. We collected genome-wide single nucleotide polymorphisms (SNPs) using the genotype-by-sequencing (GBS) method and subjected data to population genetic analyses to infer the population histories of both species, including explicit tests for admixture and introgression. The methods employed included STRUCTURE, principal coordinates analysis, TreeMix and approximate Bayesian computation. Results: Genetic data are presented for 749 individuals sampled from 14 F. notatus, 20 F. olivaceus and 2 F. euryzonus populations. Members of the species complex differed in phylogeographic structure, with F. notatus exhibiting geographic clusters corresponding to Pleistocene coastal drainages and F. olivaceus comparatively lacking in phylogeographic structure. Evidence for interspecific introgression varied by drainage. Main conclusions: Populations of F. notatus and F. olivaceus exhibited contrasting patterns of lineage diversity among coastal drainages, indicating interspecific differences in their Pleistocene southern refugia. Phylogeographic patterns in both species indicated that range expansions into the northern limits of contemporary distributions coincided with and continued subsequent to the Last Glacial Maximum. There was evidence of introgression between species in some, but not all drainages where the species co-occur, in a pattern that is correlated with previous estimates of hybridization rates

Hybridization and Reproductive Isolation among Syntopic Populations of the Topminnows Fundulus Notatus and F. Olivaceus

Fundulus notatus and Fundulus olivaceus are two closely related topminnow species that exhibit similar ecological niches and broad, largely overlapping, North American ranges extending throughout much of the Mississippi River drainage as well as the coastal drainages of the Gulf of Mexico. Previous studies have suggested that these two species are reproductively compatible despite cytogenetic differences and will hybridize when syntopic. We used nuclear and mtDNA loci to assess levels of hybridization and test for introgression in syntopic populations of these two species in four drainages in southern Illinois. Although hybridization was detected in all syntopic populations, an assessment of the proportion of hybrid individuals indicated a deficiency of hybrids relative to expectations under random mating. We determined that, although mtDNA introgression was prevalent and extended beyond the zones of contact, evidence of nuclear introgression was limited to the zone of sympatry