Southwestern endemic Macrhybopsis: legacies of peripheral isolation

Authors: Christopher Hoagstrom, Anthony Echelle

Year: 2021

Abstract

The Macrhybopsis aestivalis complex (nine recognized species of small-bodied, riverine cyprinids) includes four or five distinct taxa endemic to the Great Plains-Chihuahuan Desert region. We detail the origins of these taxa following a mitochondrial (ND2) chronogram interpreted against the background of a published nuclear gene phylogeny (S7 intron 1). In addition, we consider present taxonomic distributions and paleo-geomorphology. The complex arose in the Early Miocene Red River drainage. Macrhybopsis marconis diverged in the Middle Miocene as the Brazos-Colorado River system captured tributaries from the Red River. Elevated aridity in the Late Miocene potentially confined M. marconis to springfed rivers within the Edwards Plateau portion of the Colorado River drainage. However, it may have reached the lower R?o Grande in the Late Miocene, giving rise to M. aestivalis sensu stricto. The Late Miocene lower R?o Grande also maintained springfed climatic refugia and harbors other endemic species of similar ages, each also having closest relatives on the Edwards Plateau. Although presently synonymized with M. sterletus of the upper R?o Grande drainage, M. aestivalis sensu stricto is so-far missing from DNA studies and previous morphological evaluation appears insufficient to support synonymization. In any case, the origin of M. cf. sterletus corresponds with Pliocene expansion of the middle Pecos River, which captured a major Red River tributary. Its sister lineage, left behind in the Pliocene Red River, became the common ancestor of M. australis and M. tetranema, which most likely separated from each other in the Pleistocene when the Arkansas River captured the northern portion of the Red River drainage (upper Arkansas, Cimarron, Canadian). Ancestral M. tetranema appears to have introgressed with Arkansas River M. hyostoma, while M. australis introgressed with Red River M. hyostoma. Nevertheless, both western endemics remained distinct from M. hyostoma. Meanwhile, head cutting across the Balcones Fault allowed the Blanco, Guadalupe, and Medina rivers to capture Colorado River tributaries. One or more of these captures transferred M. marconis into the San Antonio Bay drainage. Finally, integration of the upper R?o Grande, R?o Conchos, and Pecos River with the lower R?o Grande allowed M. cf. sterletus to colonize the R?o Grande mainstem, where it expanded far upstream. It evidently expanded downstream as well, hypothetically making secondary contact with M. aestivalis in the lower R?o Grande. Thus, all western endemics appear to be peripheral isolates, with ages determined by geomorphic events (i.e., river captures). They are imperiled because their need for fluvial, riverine habitat conflicts with human water-resource development (e.g., damming, dewatering). All require conservation action and the synonymy of M. sterletus with M. aestivalis needs detailed reevaluation (including DNA sampling) with emphasis on populations in the lower R?o Grande, R?o Salado, R?o San Juan, and adjacent R?o San Fernando.