| Fitness, heterozygosity, and potential for either inbreeding or outbreeding depression affect the survivability of a species most when it is already endangered. As a small live-bearing endangered fish with a short generation time, readily identifiable sexes, and original populations in environmentally differing habitats, the Gila topminnow (Pociliopsis occidentalis occidentalis ) offers an unparalleled opportunity to study problems of conservation genetics in an endangered vertebrate species.; Potential fitness differences among four populations (Bylas Spring, Cienega Creek, Sharp Spring, and Monkey Spring) were examined using juvenile survival, 12-week size, bilateral asymmetry, and brood size as fitness surrogates. Potential for inbreeding depression, heterosis, and outbreeding depression was assessed by comparison of fitness, utilizing the traits listed above, among cross-types within populations.; There were no significant fitness differences among populations for any of the examined traits. However, two significant results among crosstypes within populations were noted. Brood sizes for inbred Sharp Spring fish declined as levels of inbreeding rose and, in Monkey Spring, the first generation of sib-mating resulted in an extreme female-biased sex ratio.; Timing of male sexual development was also compared among populations. Monkey Spring males initiated gonopodial development significantly later than in males from Sharp Spring, Bylas Spring or Cienega Creek which did not differ significantly from each other.; Two main conclusions have been drawn from these results. First, populations with relatively higher levels of genetic variation such as Sharp Spring and Monkey Spring, may be more susceptible to inbreeding depression than populations like Cienega Creek or Bylas Spring which have relatively less genetic variation. It is possible that purging of deleterious alleles has occurred in Bylas Spring with no further loss of fitness upon inbreeding and no fitness gains upon outcrossing.; Second, the unique response of Monkey Spring to inbreeding and the significant difference in male development raises interesting questions regarding the genetics of sex determination (and development) in this population and suggests that we may wish to consider treating Monkey Spring as an evolutionarily significant unit. This view is supported by recent studies of genetic variation at an MHC locus and microsatellite loci. |
