Pedigree analysis on small laboratory populations of the butterfly Bicyclus anynana: The effects of selection on inbreeding and fitness

Abstract

The effect of small population size and gene flow on the rate ofinbreeding and loss in fitness in Bicyclus anynana populationswas quantified by means of a pedigree analysis. Laboratorymetapopulations each consisted of four subpopulations with breeding sizeof N = 6 or N = 12 and migration rate of m = 0 or m= 0.33. Pedigrees were established by individually marking about35,000 butterflies. The increase in inbreeding coefficients(F-coefficients) over time was compared to that of simulated populationswith similar N and m. In the seventh generation, the level of inbreedingin larger subpopulations did not deviate significantly from the expectedvalues, but smaller subpopulations were less inbred than expected.Individuals in the small populations still showed considerableinbreeding depression, indicating that only a small proportion of therecessive deleterious alleles had been purged by selection. Two opposingprocesses potentially affected the rate of inbreeding and fitness: (1)Inbreeding depression increased the variance in family size and reducedthe effective population size. This will accelerate the rate ofinbreeding and is expected to selectively purge deleterious recessivealleles. (2) Variance in reproductive success of families was reducedbecause individuals which had a large number of siblings in thepopulation were more likely to mate with a full-sib than individualswith a smaller number of siblings. Subsequent inbreeding depressionreduced the number of viable offspring produced by these full-sibmatings. As a consequence, natural selection purged only some of thedeleterious alleles from the butterfly populations during sevengenerations with inbreeding. These findings emphasise the potentialproblems of using only small numbers of breeding individuals (N≤10) incaptive populations for conservation purposes.