Comparative riverscape genomics of the rainbow darter (Etheostoma caeruleum) in glaciated and unglaciated environments

Periodic glaciation during the Quaternary period shaped the contemporary
riverscape and distribution of freshwater fishes in the Mississippi River
drainage of central North America. The rainbow darter (Etheostoma
caeruleum) is a member of this ichthyofauna and has a disjunct
distribution in glaciated and unglaciated environments west of the
Mississippi River. Based on glacial history of the region there are
different expectations on the observed spatial genetic structure of
populations in these environments. The aim of this study was to utilize
genome-wide SNP data to compare the population genomic structure of the
rainbow darter in river networks with disparate glacial histories; the
Volga River in the glaciated upper Mississippi River basin and the Meramec
River in the unglaciated Ozark Plateau. Individuals were sampled from
localities within each river system at distances dictated by the
organismal life history and habitat preferences. Riverscape analyses were
performed on three datasets: total combined localities of both rivers and
one for each river independently. The results revealed a lasting influence
of historic glaciation on the population genomic structure of rainbow
darter populations. There was evidence of population expansion into the
glaciated northern region following glacial retreat. The population
genetic signature within the Volga River did not fit expectations of the
stream hierarchy model, but revealed a pattern of repeated colonization
and extirpation due to cyclic glaciation. The population within the
unglaciated Meramec River adhered to the stream hierarchy model, with a
directional order of genetic diversity based on the life history and
habitat preferences of the species. These results demonstrate the
importance of considering the geologic and climatic history of a region as
well as the life history of an organism when interpreting spatial genetic
patterns.