Temporal changes in allele frequency, genetic variation and inbreeding depression in small populations of the guppy, Poecilia reticulata

We established three closed lines of N=10 for the guppy Poecilia reticulata, to evaluate the relationships among temporal changes in allele frequency, genetic variation and inbreeding depression for a fitness-related trait in small populations. Genetic variation at the allozyme loci, expressed by the proportion of polymorphic loci, number of alleles per locus and heterozygosity, decreased somewhat in two closed lines but it increased in one closed line over six generations. Effective population size (Ne) at each generation was estimated from the standardized variance in the allele frequencies. The average Ne was 24.4, 10.3 and 10.0 in the three closed lines. The inbreeding coefficient calculated from the Ne increased to 0.186, 0.321 and 0.414, respectively. As an index of the amount of inbreeding depression, changes in salinity tolerance were examined, because this trait is strongly sensitive to inbreeding depression and decreases linearly with an increase in inbreeding coefficient. The mean value of the salinity tolerance significantly decreased to 82.5%, 71.7% and 67.6% in the three closed lines during the six generations, suggesting inbreeding depression for salinity tolerance. Although a significant correlation was not observed between the amount of inbreeding depression and the genetic variation, the amount of inbreeding depression correlated with the inbreeding coefficient calculated from Ne. The regression line indicated an 8.4% decrease in the mean per 10% increase in the inbreeding coefficient and was similar to that obtained directly from full-sib matings. These results indicate that the temporal changes in the allele frequencies can provide an estimation of the amount of inbreeding depression during successive generations in small populations.