The Role of Introgressive Hybridization in the Evolution of the Gila Robusta Complex (Teleostei: Cyprinidae)

Abstract

The extent and impact of introgressive hybridization was examined in the Gila robusta complex of cyprinid fishes using mitochondrial DNA (mtDNA) sequence variation. Lower Colorado River basin populations of G. robusta, G. elegans, and G. cypha exhibited distinct mtDNAs, with only limited introgression of G. elegans into G. cypha. The impact of hybridization was significant in upper Colorado River basin populations; most upper basin fishes sampled exhibited only G. cypha mtDNA haplotypes, with some individuals exhibiting mtDNA from G. elegans. The complete absence of G. robusta mtDNA, even in populations of morphologically pure G. robusta, indicates extensive introgression that predates human influence. Analysis of the geographic distribution of variation identified two distinctive G. elegans lineages; however, the small number of individuals and localities sampled precluded a comprehensive analysis. Analysis of haplotype and population networks for G. cypha mtDNAs from 15 localities revealed low divergence among haplotypes; however, significant frequency differences among populations within and among drainages were found, largely attributable to samples in the Little Colorado River region. This structure was not associated with G. cypha and G. robusta, as morphotypes from the same location are more similar than conspecific forms in other locations. This indicates that morphological and mtDNA variation are affected by different evolutionary forces in Colorado River Gila and illustrates how both hybridization and local adaptation can play important roles in evolution.

Phylogenetic utility of two existing and four novel nuclear gene loci in reconstructing Tree of Life of ray-finned fishes: The order Cypriniformes (Ostariophysi) as a case study

“…This can occur between species that are closely related and between species that are not closely related. Interestingly, introgressive hybridization has long been hypothesized to play an important role in the evolutionary diversification of living organisms whereby lineages have been hypothesized to benefit through the incorporation of new genetic variations (Anderson, 1949;Dowling and Secor, 1997;Gerber et al, 2001). This naturally-occurring process may also have a significant impact in the conservation of species when a rare or endangered species is genetically assimilated by a more common species (DeMarais et al, 1992;Rhymer and Simberloff, 1996;Rieseberg, 1998;Costedoat et al, 2007).…”

Genetic structure within and among populations of the endangered razorback sucker (Xyrauchen texanus) as determined by analysis of microsatellites

“…Studies of sequence variation in humpback chub and roundtail chub (Gila cypha and G. robusta, respectively) from the Colorado River within and above the Grand Canyon indicated that levels of genetic diversity were evenly distributed throughout much of the Colorado River basin (Gerber et al 2001), and analysis of long-term female effective population size of humpback chub from the Grand Canyon was consistent with a stable population (Garrigan et al 2002). In contrast, Douglas et al (2003) found that most of the allelic diversity in populations of flannelmouth sucker was also evenly distributed among populations but levels of nucleotide diversity increased upstream from southwest to northeast.…”

Current and historical hybridization with differential introgression among three species of cyprinid fishes (genus Cyprinella)

“…The nature of reproductive barriers appears to be dynamic, as reflected by spatial and temporal variation in the degree of reproductive (e.g., Aboim et al 2010). While hybridization appears to be an important component of freshwater fish evolution (Smith 1992;Dowling and Secor 1997;Gerber et al 2001), our understanding of the factors that promote or allow frequent interspecific mating remains limited.…”