• density;
  • freshwater snails;
  • inbreeding depression;
  • Physa acuta;
  • self-fertilization

The genetic structure, selfing rate and inbreeding depression of the hermaphroditic freshwater snail Physa acuta were jointly analysed in a population near Montpellier, France. Allozymic markers revealed moderate gene diversity (0.138), and no heterozygote deficiency. The mean outcrossing rate, estimated by using progeny arrays, was 0.9, with substantial variation among families. This also suggests that the number of fathers among outcrossed offspring of a given mother is low. Inbreeding depression was estimated over more than one generation using 83 first-laboratory-generation (G1) families. The main parameters measured were parental (G1) fecundity, offspring (G2) survival and fecundity. Size and growth were also monitored. Parental fecundity was analysed under several conditions (isolation, pair and quadruplet outcrossing). The self-fertilization depression, including parental fecundity, offspring survival and fecundity, was about 0.9 at the population level. The genetic data obtained in the same population indicate a value of about 0.3 using Ritland’s (1990) technique, suggesting that the depression over the whole life-cyle might be even higher than 0.9. Grouping affected neither fecundity nor self-fertilization depression. Substantial variation in depression for survival was detected among individuals, from no survival in some selfed families to better survival than that of outbred families in others. The overall result (outbred population structure, high outcrossing rate and high self-fertilization depression) is consistent with what is expected in large outcrossing populations in which inbreeding depression is maintained by mutation-selection balance.