Meristic Variation in Homozygous and Heterozygous Fish

The relationship between the level of genetic heterozygosity and variability for meristic characters was examined in a live-bearing poeciliid fish, Poeciliopsis lucida. Progeny from two highly homozygous inbred strains were compared to more heterozygous fish (reciprocal outcrosses between inbred strains and wild fish) for their capacity to canalize meristic characters despite experiencing different sustained temperatures during embryonic development. Meristic characters were thought to be determined during a relatively short period in embryonic development. Three of the characters included in this study were highly significantly correlated with standard length. This correlation was reduced to nonsignificance if smaller fish were deleted from the data set. Apparently, for these characters, elements continued to be added, or at least become visible, for some time after birth. Poeciliopsis does not seem to be especially sensitive to temperature variation. Although the between-temperature standard deviations were larger than those within-temperature in most cases, only I/,5 were significantly larger. The overall order of temperature-induced variability was consistent with the hypothesis of greater heterozygote stability, however most pairwise comparisons between experimental groups were not significant. The progeny of reciprocal outcrosses between the inbred strains had quite different levels of within- and betweentemperature variability despite the fact that they should be nearly genetically identical. Apparently factors other than overall level of heterozygosity were also affecting variability in these fish. This study therefore provides only weak support for the hypothesis that heterozygotes are better able to canalize their phenotype than are homozygotes. The highly homozygous inbred strains of Poeciliopsis used in this study seemed to be nearly as good at canalizing their phenotype as more heterozygous fish. They may have developed this capacity for two reasons. Selection on inbred strains in the laboratory may have promoted well-integrated and buffered homozygous genotypes. In addition, natural populations of Poeciliopsis inhabit highly unstable desert streams in which inbreeding seems to be frequent. Thus they exist under the precise conditions which could be expected to produce genomes with the capacity to canalize despite homozygosity if selection were to favor it.