Ecological and genetic evidence for clonal reproduction in Gyrodactylus gasterostei Glaser, 1974.

Reproduction of isolated Gyrodactylus gasterostei from the three-spined stickleback, Gasterosteus aculeatus, has been investigated. Three lineages of isolated parasites, without opportunity for cross- or self-fertilisation, continued to reproduce for up to 19 generations (224 births) until experiments were discontinued. Most parasites gave birth to two daughters only, although four individuals in one lineage gave birth three times. After the final birth the empty uterus expanded, while a large oocyte remained in the seminal receptacle and appeared capable of development. Reproduction by isolated flukes was further analysed using quantitative genetic techniques, applied to the hamuli and marginal hooks. The variance of hamulus and marginal hook dimensions in lineages of isolated flukes (which, if sexual must be highly inbred) was similar to that observed in natural populations. Crude heritability appeared high, but a significant relationship between hook size and environmental temperature was noted, and when this was controlled for, the heritability of all characters became negligible. Genetic drift was not observed over 17 generations and variance within isolated lines accounted for almost all of the total variance. Genes controlling hamulus or marginal hook dimensions did not progress to fixation, although the number of generations (more than 10) was sufficient to ensure homozygosity if self-fertilisation were occurring. These observations indicate that reproduction by isolated G. gasterostei continues indefinitely, but does not involve sex. The limited variance of natural populations suggests that these are also clonal with observed variation being largely environmental. Sexual reproduction may therefore be rare in G. gasterostei.