The influence of self-fertilization and grouping on fitness attributes in the freshwater snail Physa acuta: population and individual inbreeding depression

The genetic structure, selfing rate and inbreeding depression of the hermap hroditic freshwater snail Physa acuta were jointly analysed in a population near Montpellier, France. Allozymic markers revealed moderate gene diversi ty (0.138), and no heterozygote deficiency. The mean outcrossing rate, esti mated by using progeny arrays, was 0.9, with substantial variation among fa milies. This also suggests that the number of fathers among outcrossed offs pring of a given mother is low. Inbreeding depression was estimated over mo re than one generation using 83 first-laboratory-generation (G(1)) families . The main parameters measured were parental (G(1)) fecundity, offspring (G (2)) survival and fecundity. Size and growth were also monitored. Parental fecundity was analysed under several conditions (isolation, pair and quadru plet outcrossing). The self-fertilization depression, including parental fe cundity, 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 i n others. The overall result (outbred population structure, high outcrossin g rate and high self-fertilization depression) is consistent with what is e xpected in large outcrossing populations in which inbreeding depression is maintained by mutation-selection balance.