Lack of Cytonuclear Genetic Introgression Despite Long-Term Hybridization and Backcrossing between Two Poeciliid Fishes (Gambusia Heterochir and G. affinis)

We used the mitochondrial cytochrome-b gene and four microsatellite-DNA loci to assess levels of genetic introgression between two hybridizing poeciliid fishes in Texas, a locally endemic, endangered species, Gambusia heterochir, and a wide-ranging congener, G. affinis. Past morphological studies indicated a long history of hybridization and backcrossing. We detected ongoing, low-level hybridization, but no evidence of a long-term, evolutionary history of genetic introgression. Only one of 118 G. heterochir (0.9%) might have had an ancestor in G. affinis, but this was weakly supported. We detected one potential F₁ hybrid, but it was more likely a recent product of backcrossing between a hybrid and G. affinis. Two other fish were more distant products of such backcrossing. At present, there appears to be little cause for concern regarding the genetic integrity of G. heterochir. The widespread species, G. affinis, showed significant genetic structure over short geographic distances, including divergence between populations on two sides of a small dam with through-flowing water.

Figure 1

Collection localities. The San Saba River is a tributary of the Colorado River, which empties into the Gulf of Mexico. Dots in upper panel indicate Clear Creek (middle) and the Ft. McKavett (left) and Menard (right) collection sites. Site labels in lower panel follow Hubbs (1959); asterisks indicate sites at spring outflows.

Figure 2

Frequency distributions of microsatellite alleles for four loci in Gambusia from Clear Creek. Hatched = G. affinis below Dam 1 (n = 71), black = G. affinis above Dam 1 (n = 30), white = G. heterochir (n = 118). Each tick-mark and size designation represents a detected allele (some allele sizes are not shown because they were not detected), with the black bar centered over the tick-mark and the hatched and white bars on, respectively, the left and right.

Figure 3

Plot of scores for PC-I and PC-II from the principle components analysis of the multilocus, microsatellite genotype for individual G. heterochir, G. affinis, and four fish (labeled) with genotypes indicating potentially mixed genomes. Labels include a code for locality (capital letter) and specimen number. Arrows indicate coordinates for L17, which was embedded in the cluster for G. heterochir. Sample sizes are as in Fig. 2.

Figure 4

Mitochondrial DNA haplotype and a plot of the morphological index against PC-I microsatellite scores. Labels include a code for locality (capital letter) and specimen number. Sample sizes for the two groups of mtDNA are as follows: males (24 G. heterochir; 4 G. affinis), females (78 G. heterochir; 92 G. affinis).

Contributor Notes

Department Of Zoology And Oklahoma Cooperative Fish And Wildlife Research Unit, Oklahoma State University, Stillwater, Oklahoma 74078. E-mail: (AAE) echelle@okstate.edu. Send reprint requests to AAE.

Received: 04 Jun 2005

Accepted: 15 Mar 2006