Population Structure in the Roundtail Chub (Gila robusta Complex) of the Gila River Basin as Determined by Microsatellites: Evolutionary and Conservation Implications

Abstract

Ten microsatellite loci were characterized for 34 locations from roundtail chub (Gila robusta complex) to better resolve patterns of genetic variation among local populations in the lower Colorado River basin. This group has had a complex taxonomic history and previous molecular analyses failed to identify species diagnostic molecular markers. Our results supported previous molecular studies based on allozymes and DNA sequences, which found that most genetic variance was explained by differences among local populations. Samples from most localities were so divergent species-level diagnostic markers were not found. Some geographic samples were discordant with current taxonomy due to admixture or misidentification; therefore, additional morphological studies are necessary. Differences in spatial genetic structure were consistent with differences in connectivity of stream habitats, with the typically mainstem species, G. robusta, exhibiting greater genetic connectedness within the Gila River drainage. No species exhibited strong isolation by distance over the entire stream network, but the two species typically found in headwaters, G. nigra and G. intermedia, exhibited greater than expected genetic similarity between geographically proximate populations, and usually clustered with individuals from the same geographic location and/or sub-basin. These results highlight the significance of microevolutionary processes and importance of maintaining local populations to maximize evolutionary potential for this complex. Augmentation stocking as a conservation management strategy should only occur under extreme circumstances, and potential source populations should be geographically proximate stocks of the same species, especially for the headwater forms.

Can members of the south‐western Gila robusta species complex be distinguished by morphological features?

“…intermedia . Mitochondrial and nuclear markers (Schwemm, ) and microsatellite markers (Dowling et al ., ) demonstrate that genetic variation is partitioned by population, rather than by species and population‐level variation may obscure species‐level distinctions (Dowling et al ., ). Schönhuth et al .…”

“…A physical connection between waterways allows for gene flow and the opportunity for hybridization between previously isolated species. The microsatellites that have been used to genetically differentiate these three species (and geographic populations; Dowling et al, 2015) are rapidly evolving markers; thus, populations may become genetically distinct (through drift or selection) during dry periods when populations are physically disjunct, but genetic differences may deteriorate via interbreeding during wet periods as previously isolated populations become sympatric. Indeed Douglas et al (1999) speculated that phenotypic variation in G. robusta, G. intermedia and G. nigra is the result of vicariance events that occurred in the late Miocene.…”

“…among the individuals. A second possibility is that microsatellite markers do not provide accurate information about relationships among species because the rapid mutation rate of microsatellite markers creates homoplasy, which obscures true genetic relationships (Dowling et al, 2015).…”

Molecular Genetics Informs Spatial Segregation of Two Desert Stream Gila Species

“…Samples of Roundtail Chub were also taken from the Verde River at Perkinsville (Yavapai County [N = 20]) and West Clear Creek near its confluence with the Verde River (Yavapai County [N = 29]); Headwater Chub were obtained from Fossil Creek at Fossil Springs (N = 26). The latter three samples were characterized in previous analyses of the entire Gila complex (Schwemm 2006;Dowling et al 2008Dowling et al , 2015 and allowed comparison of unknown temporal samples collected throughout Fossil Creek with the remainder of the complex.…”

“…Schwemm (2006) also failed to identify species-specific markers with mtDNA and nuclear DNA; however, he was able to diagnose many local populations of Headwater Chub and Gila Chub with these markers. Characterization of microsatellite variation (Dowling et al 2008(Dowling et al , 2015 also allowed for diagnosis of most local populations (especially for Headwater Chub and Gila Chub), indicating that fragmentation and relatively rapid local evolution are significant in these species. While the processes leading to differentiation within Headwater Chub and Gila Chub have the benefit of making it easy to discriminate among local populations by means of molecular methods, they make it more difficult to identify species-specific markers.…”