COMPARISON OF INBREEDING AND OUTBREEDING IN HERMAPHRODITIC ARIANTA-ARBUSTORUM (L) (LAND SNAIL)

Inbreeding depression was studied in the outcrossing hermaphroditic land snail Arianta arbustorum, by comparing the reproductive performance of full-sib pairs (inbreeding) and pairs of unrelated individuals (outbreeding). Fecundity (number of eggs laid) was not found to differ significantly between inbreeding and outbreeding pairs under laboratory conditions, but fertility (the proportion of eggs that hatched) and the number of hatchlings produced by inbreeding pairs were significantly lower than those of outbreeding pairs. Estimates of reduction in the fertility of eggs (0.37) and in the number of hatchlings produced (0.56) suggested significant inbreeding depression in full-sib crosses. Inbred offspring reared in the garden had a higher mortality than outbred offspring reared in the same environment, with an inbreeding depression of 0.30 in the first year after hatching; but no significant difference was found when offspring from both groups were kept in the laboratory. The results support the hypothesis that cross-fertilization in simultaneous hermaphrodites is maintained by strong inbreeding depression.