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  1. Abstract

    Pairs of species that exhibit broadly overlapping distributions, and multiple geographically isolated contact zones, provide opportunities to investigate the mechanisms of reproductive isolation. Such naturally replicated systems have demonstrated that hybridization rates can vary substantially among populations, raising important questions about the genetic basis of reproductive isolation. The topminnows,Fundulus notatusandF. olivaceus, are reciprocally monophyletic, and co‐occur in drainages throughout much of the central and southern United States. Hybridization rates vary substantially among populations in isolated drainage systems. We employed genome‐wide sampling to investigate geographic variation in hybridization, and to assess the possible importance of chromosome fusions to reproductive isolation among nine separate contact zones. The species differ by chromosomal rearrangements resulting from Robertsonian (Rb) fusions, so we hypothesized that Rb fusion chromosomes would serve as reproductive barriers, exhibiting steeper genomic clines than the rest of the genome. We observed variation in hybridization dynamics among drainages that ranged from nearly random mating to complete absence of hybridization. Contrary to predictions, our use of genomic cline analyses on mapped species‐diagnostic SNP markers did not indicate consistent patterns of variable introgression across linkage groups, or an association between Rb fusions and genomic clines that would be indicative of reproductive isolation. We did observe a relationship between hybridization rates and population phylogeography, with the lowest rates of hybridization tending to be found in populations inferred to have had the longest histories of drainage sympatry. Our results, combined with previous studies of contact zones between the species, support population history as an important factor in explaining variation in hybridization rates.

     
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  2. Abstract Aim

    We used genome‐scale sampling to assess the phylogeography of a group of topminnows in theFundulus notatusspecies 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, GenusFundulus, subgenusZygonectesFundulus notatus, Fundulus olivaceus, Fundulus euryzonus.

    Methods

    We sampled members of theF. notatusspecies 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 includedSTRUCTURE, principal coordinates analysis, TreeMix and approximate Bayesian computation.

    Results

    Genetic data are presented for 749 individuals sampled from 14F. notatus, 20F. olivaceusand 2F. euryzonuspopulations. Members of the species complex differed in phylogeographic structure, withF. notatusexhibiting geographic clusters corresponding to Pleistocene coastal drainages andF. olivaceuscomparatively lacking in phylogeographic structure. Evidence for interspecific introgression varied by drainage.

    Main conclusions

    Populations ofF. notatusandF. olivaceusexhibited 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.

     
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