Predominance of outcrossing in Lymnaea stagnalis despite low apparent fitness costs of self‐fertilization

Abstract

We have quantified the natural mating system in eight populations of the simultaneously hermaphroditic aquatic snail Lymnaea stagnalis, and studied the ecological and genetic forces that may be directing mating system evolution in this species. We investigated whether the natural mating system can be explained by the availability of mates, by the differential survival of self‐ and cross‐fertilized snails in nature, and by the effects of mating system on parental fecundity and early survival. The natural mating system of L. stagnalis was found to be predominantly cross‐fertilizing. Density of snails in the populations had no relationship with the mating system, suggesting that outcrossing rates are not limited by mate availability at the population densities observed. Contrary to expectations for outcrossing species, we detected no evidence for inbreeding depression in survival in nature with inferential population genetic methods. Further, experimental manipulations of mating system in the laboratory revealed that self‐fertilization had no effect on parental fecundity, and only minor effects on offspring survival. Predominance of cross‐fertilization despite low apparent fitness costs of self‐fertilization is at odds with the paradigm that high self‐fertilization depression is necessary for maintenance of cross‐fertilization in self‐compatible hermaphrodites.

Natural range expansion and human-assisted introduction leave different genetic signatures in a hermaphroditic freshwater snail

“…Considering that the microsatellite data of molluscs are often encumbered by null alleles, our data suggests that inbreeding does not influence the resulting population structure. This suggestion is consistent with those earlier studies that suggest that the hermaphroditic L. stagnalis is able to selffertilize, but has a strong preference for outcrossing (Cain 1956;Puurtinen et al 2007). …”

“…It is not unusual that such populations exhibit extinctioncolonization dynamics where empty ponds are being recolonized by migrants from the regional population pool. Although we did not find support for significant on-going selffertilization in our population samples, self-fertilization in these hermaphroditic snails is possible and occurs in isolation (Puurtinen et al 2007). Both, self-fertilization, and the option of sperm storage allow establishment through a single individual after an extinction event and would enhance the genetic stochasticity associated with colonization.…”

“…Pulmonate species on the other hand are hermaphrodites, which generally show a tendency towards outcrossing or selfing in their native populations. L. stagnalis has been classified as outcrossing (Cain 1956;Coutellec-Vreto et al 1994;Puurtinen et al 2007) and seems to be an outlier when looking at its invasion success. Most of the studied invasive pulmonate snails have been shown to be preferentially selfing in their native range (Lymnaea truncatula, Biomphalaria pfeifferi).…”

Test No. 243: Lymnaea stagnalis Reproduction Test

“…During mating, an individual can only take the male or the female part. L. stagnalis, as all Basommatophora, can selffertilize and appears to show no or weak inbreeding depression (6,7). Yet, crossed fertilization is the most common reproductive pathway (8).…”

“…Cadmium chloride is a common reference substance which has been successfully used in the international validation studies for this Test Guideline, with an EC 50,reproduction range of 40 -230 µg Cd/L -1 (6,7).…”