Relationships between spatio-temporal environmental and genetic variation reveal an important influence of exogenous selection in a pupfish hybrid zone.

The importance of exogenous selection in a natural hybrid zone between the pupfishes Cyprinodon atrorus and Cyprinodon bifasciatus was tested via spatio-temporal analyses of environmental and genetic change over winter, spring and summer for three consecutive years. A critical influence of exogenous selection on hybrid zone regulation was demonstrated by a significant relationship between environmental (salinity and temperature) and genetic (three diagnostic nuDNA loci) variation over space and time (seasons) in the Rio Churince system, Cuatro Ciénegas, Mexico. At sites environmentally more similar to parental habitats, the genetic composition of hybrids was stable and similar to the resident parental species, whereas complex admixtures of parental and hybrid genotypic classes characterized intermediate environments, as did the greatest change in allelic and genotypic frequencies across seasons. Within hybrids across the entire Rio Churince system, seasonal changes in allelic and genotypic frequencies were consistent with results from previous reciprocal transplant experiments, which showed C. bifasciatus to suffer high mortality (75%) when exposed to the habitat of C. atrorus in winter (extreme temperature lows and variability) and summer (abrupt salinity change and extreme temperature highs and variability). Although unconfirmed, the distributional limits of C. atrorus and C. atrorus-like hybrids appear to be governed by similar constraints (predation or competition). The argument favouring evolutionary significance of hybridization in animals is bolstered by the results of this study, which links the importance of exogenous selection in a contemporary hybrid zone between C. atrorus and C. bifasciatus to previous demonstration of the long-term evolutionary significance of environmental variation and introgression on the phenotypic diversification Cuatro Ciénegas Cyprinodon.

Establishing a captive broodstock for the endangered bonytail chub (Gila elegans).

It is crucial for endangered species to retain as much genetic variation as possible to enhance recovery. Bonytail chub (Gila elegans) is one the most imperiled freshwater fish species, persisting as a declining population of large and old individuals primarily in Lake Mohave on the lower Colorado River. Establishment of a new captive broodstock from the 1981 F1 progeny of at most 10 wild fish plus any newly captured wild fish is evaluated and reviewed. The effective number of founders contributing to the 1981 F1 progeny appears quite small, varying from approximately 3.5, based on F1 allozyme data and supported by mtDNA data, to approximately 8.5, based on the original production records. Using a sample of these progeny to initiate a new broodstock further reduces the effective number of founders. With even the most optimistic evaluation of the amount of genetic variation in F1 progeny, it is obvious that including wild fish in the broodstock is essential to increase the amount of genetic variation. The approach given here could be applied to retain genetic variation in other endangered species in a captive broodstock until they have stable natural populations of adequate size.

DID VICARIANCE MOLD PHENOTYPES OF WESTERN NORTH AMERICAN FISHES? EVIDENCE FROM GILA RIVER CYPRINIDS.

Pairwise, two- and three-way Mantel tests were used to evaluate a null hypothesis of no significant covariation when morphological features of three cyprinid fish taxa of the genus Gila were compared. Tests involved ecological conditions and past and present hydrography in the Gila River Basin of western North America. A vicariance hypothesis was the only model statistically proficient in explaining diversity of fish phenotypes. Of paleohydrographic reconstructions compared, those of the mid-Miocene and Pliocene epochs were significantly associated with present-day distributions of phenotypes. Of these, the Pliocene was paramount.

Origin of Gila seminuda (Teleostei: Cyprinidae) through introgressive hybridization: implications for evolution and conservation.

Morphological and genetic characters from cyprinid fishes of the genus Gila were examined to assess a hypothesized hybrid origin of Gila seminuda from the Virgin River, Arizona-Nevada-Utah. The presumed parents, Gila robusta robusta and Gila elegans, are clearly differentiated from one another based on morphology, allozymes, and mtDNA haplotypes. G. seminuda is morphologically intermediate and polymorphic at allozyme loci diagnostic for the parental species. Restriction endonuclease analysis of mtDNA showed G. seminuda nearly identical to G. elegans. These results support an origin of the bisexual taxon G. seminuda through introgressive hybridization. The Gila population in the Moapa River, Nevada, also appears to be of hybrid origin and is considered a distinctive population of G. seminuda. Inter-specific hybridization is potentially an important mode of evolution among western North American fishes, and valid species of hybrid origin may exist in other groups as well. Consideration of this mode of evolution argues for the need to conserve entire species complexes.