Thirty Years of Hybridization between Toads along the Agua Fria River in Arizona: Part II: Fine-Scale Assessment of Genetic Changes over Time Using Microsatellites
The ecological consequences of impoundment construction on riparian systems throughout the U.S. Southwest has profoundly affected a variety of organisms, including many amphibians. To better understand the current extent of hybridization and changes in genetic composition over time in Bufo (Anaxyrus) woodhousii and Bufo microscaphus, we used microsatellite loci to evaluate 260 individuals representing 10 total populations constituting B. woodhousii, B. microscaphus, and putative hybrids along the Agua Fria River in Arizona during two time periods (1992–97 and 2009–10). Consistent with prior work with these two anurans documenting unidirectional replacement or genetic introgression, we predicted that microsatellites would provide evidence of directional introgression of B. woodhousii into B. microscaphus. The putative hybrid populations exhibited the highest number of alleles, and B. microscaphus exhibited the lowest number of alleles. Structure analysis indicated K = 2 as the best-fit population number for both time periods. All pairwise F-statistics were highly significant, corroborating differentiation among populations as inferred by structure analysis. The principal coordinates analysis demonstrated three distinct clusters of individuals that corresponded strongly with prior morphological and mitochondrial assignments within this region over both sampling periods. Our findings indicate that the genetic identity of B. microscaphus remains distinct from B. woodhousii and the hybrids, suggesting that the genetic structure of the corresponding populations has remained intact. Bufo woodhousii has not replaced B. microscaphus along the Agua Fria River beyond those habitats directly associated with impoundment construction.Abstract
Survey sites along the Agua Fria River in central Arizona. Sites to the south (1–3) represent putatively pure populations of Bufo woodhousii; those above Waddell Dam (4–7) represent sites of hybridization; those well upstream of Black Canyon City (8–10) represent putatively pure Bufo microscaphus (modified from Sullivan et al., 2015).
Results from the Structure analysis for K = 2 and K = 3 for 1992–97 and 2009–10. For 1992–97, K = 2 had a mean −lnP(K) ± SE = −3226.48 ± 0.290 (panel A) and K = 3 had a mean −lnP(K) ± SE = −3219.18 ± 0.175 (panel B). For 2009–10, K = 2 had a mean −lnP(K) ± SE = −33.78.64 ± 0.151 (panel C) and K = 3 had a mean −lnP(K) = −3341.26 ± 0.327 (panel D).
Results from the principal coordinates analysis by population identified three distinct groups, including Bufo woodhousii, hybrids, and Bufo microscaphus, consistent with prior morphological and mitochondrial assignments of Sullivan et al. (2015). The combined axes explained 32.12% of the variation in 1992–97 (see plot A.) and 26.09% of the variation in 2009–10 (see plot B.); note that over time, there was less separation between hybrids and B. microscaphus and the loss of influence of B. woodhousii at the hybrid sites.
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