Genetic Characterization of an Admixed Species Complex as Adaptive Management Tool

Authors: Marlis R. Douglas, Michael E. Douglas

Year: 2010 (xlii)

Abstract

The Colorado River Compact (1922) allocated water among 7 western states and Mexico , yet created conservation issue for a depauperate big-river fish community through its water management projects. Four species (57%) are now endangered [2 are Humpback (Gila cypha), and Bonytail (G. elegans)] while 3 (43%) are ‘of concern’ [one is Roundtail Chub (G. robusta)]. Propagation and/or translocation of these fishes from high- to low-density areas of the basin can be a tool for adaptive management, but can simultaneously be deleterious if genetically-distinct lineages are intermingled. To clarify this situation, we evaluated 4 mitochondrial DNA regions (1,869 base pairs/ 336 specimens) as a means to identify ESUs (evolutionarily significant units) among upper and lower basin G. cypha and G. robusta. Neither could be discriminated due to extensive introgressive hybridization, yet both separate from G. elegans at 4.8% sequence divergence. This broad admixture of two species across populations and basins is unusual given their fossil histories and distinct morphologies. To identify potential MUs (management units) in contemporaneous time, 643 specimens were genotyped across 16 microsatellite DNA loci. Here, each species was discriminated according to phenotype in all but one locale, with distinct gene pools within different regions of the basin. Six basin-wide MUs were identified for G. cypha. Grand Canyon sites were linked via downstream gene flow from its largest tributary, yet all were clearly separated from Upper Basin populations. Translocation/ augmentation is a risky management tool for it could confound historic admixture between Humpback and Roundtail chubs while concomitantly homogenizing those populations that remain genetically distinct. Adaptive management should instead sustain in-situ populations, with focus on (a) understanding habitat requirements, (b) maintaining flows conducive to all life history stages and (c) controlling predatory non-natives.