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.