Résumé
Effective population size (Ne) is a measure of genetic drift and is thus a
central parameter in evolution, conservation genetics and invasion
biology. Interestingly, in native marine species, Ne is typically several
orders of magnitude lower than the census size. This pattern has often
been explained by high fecundity, variation in reproductive success and
pronounced early mortality, resulting in genetic drift across generations.
Data documenting genetic drift and/or Ne in marine invasive species are,
however, still scarce. We examined the importance of genetic drift in the
invasive species Crepidula fornicata by genotyping 681 juveniles sampled
during each annual recruitment peak over nine consecutive years in the Bay
of Morlaix (Brittany, France). Observed variations in genetic diversity
were partially explained by variation in recruitment intensity. In
addition, substantial temporal genetic differentiation was documented
(that is, genetic drift), and was attributed to nonrandom variance in the
reproductive success of different breeding groups across years in the
study species. Using a set of single-sample and temporal estimators for
Ne, we estimated Ne to be three or four orders of magnitude smaller than
the census size (Nc). On one hand, this reduction in Ne relative to Nc
appeared congruent with, although slight higher than, values commonly
observed in native marine species. Particular life-history traits of this
invasive species may play an important role in buffering genetic drift. On
the other hand, Ne still remained far below Nc, hence, possibly reducing
the efficiency of selection effects.