4.1 | Colonization of the upper Mississippi River basin
Analyses of the combined river dataset revealed a signature of expansion of populations into the glaciated riverscape of the upper Mississippi River basin. Even though the study sites were chosen for comparison due to their similarity in habitat and lack of obvious barriers to dispersal, they revealed different patterns of spatial genetic structure. The northern population in the Volga River (Figure 2) exhibited low genetic diversity (Table 1A) and little population substructure (Figures 3A, 4A). In contrast, the southern Ozark Plateau population in the Meramec River exhibited greater genetic diversity and increased population substructure. The predominant genetic cluster observed in the North is also present in the more diverse and admixed population in the South (Figure 3A). Both the AMOVA (Table 2A) and FST (Table S2) results indicated the greatest sources of genetic variation are between the northern and southern populations.
The conclusion of expansion to the North from a southern refugium in the Ozark Plateau following glacial retreat is consistent with a range-wide phylogeographic study of E. caeruleum based on mtDNA sequence data (Ray et al., 2006). This pattern has also been repeatedly observed in other fishes within the region that also exhibit a similar disjunct distribution west of the Mississippi River. Phylogeographic studies of the Ozark minnow (Notropis nubilus ; Berendzen et al., 2010), gilt darter (Percina evides ; Near et al., 2001), Carmine shiner (Notropis percobromus ; Berendzen et al., 2008) and northern hogsucker (Hypentelium nigricans ; Berendzen et al., 2003) revealed shallow genetic divergences and lack of geographic structure between the northern and southern populations. It is hypothesized that populations of fishes expanded northward either during the Sangamon interglacial period between ~ 125,000 – 75,000 years ago or following the last glacial maximum ~ 19,000 to 10,500 years ago (Berendzen et al., 2010; Near et al., 2001).
This repeated pattern supports the influence of periodic glaciation during the Quaternary Period on shaping the distribution of the contemporary fish assemblage in the upper Mississippi River basin. As ice sheets retreated, populations of freshwater fishes expanded northward out of an Ozarkian refugium into suitable habitat in the upper Mississippi River basin (Berendzen et al., 2010; Burr & Page, 1986). Following colonization of the region, contemporary populations in the North became isolated from Ozark Plateau due to the loss of suitable habitat in the intervening region. The aquatic habitat and flow regime of river systems flowing through southern Iowa and northern Missouri were altered during the mid-Pleistocene interglacial periods between the Pre-Illinoian and Illinoian Glacial Stages, ~ 500,000 – 300,000 years ago (Anderson, 1998; Bettis, 1989; Elfrink & Siemens, 1998). As the ice sheet retreated, immense volumes of glacial melt carried large quantities of fine-grained sediments to the alluvial plains adjacent to the Mississippi River which led to extensive aeolian loess accumulation throughout the Southern Iowa and Northern Missouri Drift Plain (Forman et al., 1992; Anderson, 1998; Young & Hammer, 2000; Muhs et al., 2018; Pflieger, 1975). This resulted in changes to the clarity and siltation of rivers in this region and eliminated habitat for E. caeruleum and other species with similar preferences.