Evidence for Introgression in the Endangered Sonora Tiger Salamander, Ambystoma tigrinum stebbinsi (Lowe)
Introduction of nonnative species and consequent genetic introgression of native taxa is a primary conservation concern, particularly for endangered species. Our ongoing molecular study of the endangered Sonora Tiger Salamander, Ambystoma tigrinum stebbinsi (Lowe), has uncovered evidence of introgression by the Barred Tiger Salamander, Ambystoma tigrinum mavortium. We conducted both mitochondrial DNA sequencing and analyses of nine microsatellite loci to (1) evaluate the distinctiveness of A. t. stebbinsi from the two other tiger salamander subspecies in Arizona; and (2) test for introgression in A. t. stebbinsi. Two mitochondrial haplotypes were found. One was undescribed for tiger salamanders, and the other was identical to that found in nearby A. t. mavortium. Microsatellite analyses, including assignment tests, diagnostic alleles, and high genetic distances, supported distinctness of A. t. stebbinsi. Thirty-nine animals that were putatively A. t. stebbinsi had mtDNA haplotypes identical to those in A. t. mavortium. These 39 individuals were distributed among six ponds, where a total of 73 individuals were sampled for microsatellites and considered “unknowns” because of the shared haplotype with A. t. mavortium. Assignment tests and diagnostic alleles of microsatellite data indicated that five of these 73 individuals may be hybrids of A. t. mavortium and A. t. stebbinsi. Some salamanders within the geographic range of A. t. stebbinsi were morphologically similar to A. t. mavortium or intermediate between the two subspecies. Our results suggest that introgression from introduced A. t. mavortium may be altering the gene pool of A. t. stebbinsi, thereby raising concerns about continued management of this endangered species.Abstract
Distribution of collecting sites. Collecting sites for the three subspecies. Inset are cattle tanks where salamanders were found during field surveys during 1994–2002 in SRV. Filled circles are Ambystoma tigrinum nebulosum localities, triangles are Ambystoma tigrinum mavortium localities, and open circles are Ambystoma tigrinum stebbinsi localities. Latitude-longitude coordinates for sites are under Specimens Examined
Neighbor-joining tree of salamander control region-insert sequences. Ambystoma texanum was the outgroup. Bootstrap replicates (n = 10,000) were performed and bootstrap support (percentage value) is indicated at internal nodes
(A) Principal components analysis of log-likelihood scores of Ambystoma tigrinum mavortium (triangles), Ambystoma tigrinum stebbinsi (open circles), and Ambystoma tigrinum nebulosum (closed circles) samples as assigned to subspecies after four iterations of Doh analysis software. PC1 explained 61% of the variation and PC2 explained 29% of the variation. Maximum-likelihood analyses were based on genotypes of nine microsatellite loci. (B) As above, and including “unknown” (putative hybrids—indicated by asterisks)
Frequency distributions of hybrid index scores for individuals belonging to Ambystoma tigrinum mavortium, Ambystoma tigrinum stebbinsi, and putative hybrids. Hybrid index scores were calculated following Campton and Utter (1985)
Contributor Notes
Washington State University, School of Biological Sciences, Pullman, Washington 99164-4236. Present Addresses: (SGM) Department of Biology, Albright College, Reading, Pennsylvania 19612; (MWR) Department of Biology, Queen's University, Kingston, Ontario K7L 3N6 Canada; (REZ) Department of Biology, Centre College, Danville, Kentucky 40422; (JW) Department of Biology, University of Miami, Coral Gables, Florida 33124; and (JPC) School of Life Sciences, Arizona State University, Tempe, Arizona 85287-4501. E-mail: (AS) astorfer@wsu.edu Send reprint requests to AS.