Buffalofishes (Genus Ictiobus) are large, robust-bodied suckers adapted to large rivers and lakes of North America. Currently recognized species are readily diagnosed by morphological characters, and the group is known from fossils dating back to the Miocene. However, sympatrically occurring species in the Mississippi River Basin are known to hybridize in nature and in the laboratory. Here we describe patterns of morphological (morphometric) and DNA sequence variation (mitochondrial and nuclear genes) across the geographic ranges of extant species of genus Ictiobus. We show that Ictiobus species form more of less discrete entities based on body morphometry, consistent with current taxonomy. However, except for I. labiosus, there is extensive sharing of alleles of nuclear and mitochondrial genes among species, and the species do not form reciprocally monophyletic groups in nuclear or mitochondrial gene trees. Moreover, the pattern is not confined to the broad area of sympatry in the Mississippi River Basin. We attribute this to a long history of introgressive hybridization and gene flow among species inhabiting the present-day Mississippi River Basin, and recent colonization of the Great Lakes, Hudson Bay drainage and gulf coastal rivers east and west of the Mississippi River by introgressed Mississippi River Basin stocks.
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