4.2 | Historical range contraction and expansion
Analyses of the Volga River only dataset revealed a spatial distribution
of genetic diversity that does not fit expectations under the stream
hierarchy model. Local populations exhibited low diversity (Table 1B)
and lack any discernable pattern of population substructure that
reflects the distribution (Figures 3B, 4B). In contrast to the combined
river dataset, the Volga only dataset revealed a greater number of
genetic clusters with the most upstream (VO1) and downstream (VO5)
localities representing unique clusters with no admixed individuals
(Figure 3B). However, there was no evidence of isolation by distance
(Figure 5A). The FST values (Table S3A) indicated
locality VO1 was the most genetically distinct; it contained an
unusually high number of private alleles (Table 1B) as compared to the
other localities. In addition, localities VO2 and VO3 are more similar
to the most downstream locality, VO5, than the intervening locality VO4.
The greatest sources of variation are differences among individuals
within populations (Table 2B).
There are two possible explanations for the genetic signature observed
in the Volga River: fragmentation of local populations due to isolation
or historical range contractions and expansion. It is possible that the
local populations are fragmented. The Volga River flows through a highly
impacted landscape that has been dramatically altered by intensive
agricultural activities and urbanization. Rivers and streams have been
heavily impacted, causing extensive modification to habitat structure,
water quality, and flow regime (Menzel, 1983; Knox, 2007). However, the
spatial pattern of genetic diversity is random and does not fit
expectations for fragmentation and isolation. In addition, the pattern
does not make sense given the life history of the fish and the
distribution of localities within the system.
The best explanation is this pattern results from the influence of
historical range contractions and expansions during periodic glacial
cycles. There is low overall genetic diversity in the Volga River only
dataset, which is reflected in the lower number of variant loci and
private alleles as compared to the population in the South (Table 1B).
In addition, the observed heterozygosities are relatively high with low
FIS values, which are consistent across sampled
localities within the system. These observations suggest that the
population in the Volga River is not in equilibrium; the observed
genetic signature is not reflective of the contemporary demographic and
evolutionary processes (Whitlock & McCauley,1990; Epps & Keyghobodi,
2015). Rather, the pattern observed is a function of the founding events
that established populations in the North. The bottleneck caused by the
founder effect reduces variation, but can have little effect on the
observed heterozygosity (Allendorf & Luikart, 2007). This is especially
true when a population returns to a large size relatively quickly, which
is consistent with the life history of E. caeruleum . Even though
populations can be isolated, the rainbow darter tends to be very
abundant and one the most common fish in a stream (Page, 2000).
The disconnect between the contemporary landscape and the observed
genetic pattern is the result of a time-lag (Epps & Keyghobadi, 2015).
Even though it has been 10,500 years since the retreat of the last
glacial maximum, a genetic signature of expansion persists. Following a
major population disturbance, it takes time for genetic variation to
reach equilibrium, which is determined by a number of factors including
generation time, dispersal rates, effective population size and
population growth rates (Epps & Keyghobadi, 2015). Based on the complex
interactions of these factors, it can take tens of thousands of
generations for a population to reach equilibrium (Varvio et al., 1986;
Zellmer & Knowles, 2009). This conclusion is further supported by the
results of the analyses of the Meramec River only dataset; see
discussion below.
The interpretation of the observed genetic signature in the Volga River
only dataset is consistent with other landscape genetic studies ofE. caeruleum . A recent study utilized microsatellite data to
understand the spatial distribution of genetic diversity of the rainbow
darter in tributaries of the upper Mississippi River basin in northeast
Iowa (Davis et al., 2015). The analyses revealed a single genetic
population and no evidence of significant population subdivision despite
the large geographic distance separating local populations. The
population of rainbow darters in this region were genetically diverse,
but the diversity was evenly distributed across the landscape suggesting
extensive gene flow and connectivity among population across the
landscape. After considering the life history and habitat requirements
of the rainbow darter and the glacial history of the region, they
concluded that the best explanation was historical events overwhelmed
the observed genetic signature and the data were unable to detect the
influence of contemporary processes (Davis et al., 2015). Although this
study came to a similar conclusion, the use of microsatellite DNA
markers presumably limited the resolution of the genetic data. The
genome-wide SNP data utilized in this study were able to reveal
fine-scale population substructure that was not observed in Davis et al.
(2015). Haponski et al. (2009) observed similar patterns of genetic
variation in E. caeruleum populations in glaciated regions of the
Lake Erie and the Ohio River basins east of the Mississippi River.