Timing of clade divergence and discordant estimates of genetic and morphological diversity in the Slender Madtom, Noturus exilis (Ictaluridae)

Phylogeographic relationships, the timing of clade diversification, and the potential for cryptic diversity in the Slender Madtom, Noturus exilis, was investigated using mitochondrial Cyt b, nuclear RAG2, shape analysis, and meristic and pigmentation data. Three well-supported and deeply divergent clades were recovered from analyses of genetic data: Little Red River (White River drainage) clade, Arkansas + Red River (Mississippi River) clade, and a large clade of populations from the rest of the range of the species. Recovered clades showed little to no diagnostic morphological differences, supporting previous hypotheses of morphological conservatism in catfishes, and indicating morphology may commonly underestimate diversity in this group of fishes. The Little Red River clade is the most distinct lineage of N. exilis with 11 POM pores (vs. 10 in other populations) and unique Cyt b haplotypes and RAG 2 alleles. However, treating it as a species separate from N. exilis would imply that the other major clades of N. exilis are more closely related to one another than they are to the Little Red River clade, which was not supported.The UCLN age estimate for Noturus was 23.9 mya (95% HPD: 13.49, 35.43), indicating a late Oligocene to early Miocene origin. The age of N. exilis was estimated as late Miocene at 9.7 mya (95% HPD: 5.32, 14.93). Diversification within the species spanned the late Miocene to mid-Pleistocene. The largest clade of N. exilis, which dates to the late Miocene, includes populations from the unglaciated Eastern and Interior Highlands as well as the previously glaciated Central Lowlands. Diversification of this clade coincides with a drastic drop in sea-level and diversification of other groups of Central Highlands fishes (Centrarchidae and Cyprinidae). Sub-clades dating to the Pleistocene show that northern populations occurring in previously glaciated regions resulted from dispersal from populations in the Ozarks up the Mississippi River following retreat of the Pleistocene glaciers. Pre-Pleistocene vicariance, such as drainage pattern changes of the Mississippi River, also played a prominent role in the history of the species. The incorporation of a temporal estimate of clade diversification revealed that in some instances, phylogeographic breaks shared with other aquatic species were best explained by different or persistent vicariant events through time, rather than a single shared event.

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► Phylogeography, clade ages, and cryptic diversity in the Slender Madtom were examined.
► Genes recovered Miocene to Pleistocene age clades, but clades showed little to no diagnostic phenotypic variation.
► Morphology alone may commonly underestimate diversity in catfishes.
► Pleistocene and pre-Pleistocene vicariance and dispersal have contributed to phylogeographic patterns.
► Certain phylogeographic breaks shared with other taxa reflect pseudocongruent histories.