Genetic Variability and Drift Load in Populations of an Aquatic Snail

Abstract

Abstract. Population genetic theory predicts that in small populations, random genetic drift will fix and accumulate slightly deleterious mutations, resulting in reduced reproductive output. This genetic load due to random drift (i.e., drift load) can increase the extinction risk of small populations. We studied the relationship between genetic variability (indicator of past population size) and reproductive output in eight isolated, natural populations of the hermaphroditic snail Lymnaea stagnalis. In a common laboratory environment, snails from populations with the lowest genetic variability mature slower and have lower fecundity than snails from genetically more variable populations. This result suggests that past small population size has resulted in increased drift load, as predicted. The relationship between genetic variability and reproductive output is independent of the amount of nonrandom mating within populations. However, reproductive output and the current density of snails in the populations were not correlated. Instead, data from the natural populations suggest that trematode parasites may determine, at least in part, population densities of the snails.

Inbreeding levels and prey abundance interact to determine fecundity in natural populations of two species of wolf spider

“…In addition to the now moderately large set of data on plants, two recent studies have examined the correlation between population size and fitness in animals. Puurtinen et al (2004) showed that snails from populations with less genetic diversity matured more slowly and had lower fecundity than from those populations with more genetic diversity, when reared in the laboratory environment. However, Davis and Roberts (2005) failed to find a significant effect of population size when comparing clutch size and embryonic survival in large and small populations of the frog Heleioporus albopunctatus.…”

Relationship between prevalence of trematode parasite Diplostomum sp. and population density of its snail host Lymnaea stagnalis in lakes and ponds in Finland

“…High prevalence of trematode parasitism in snail populations makes a remarkable proportion of snails infertile due to the host-castrating effect of trematodes (Dillon 2000). This may reduce the host density, as suggested by Lafferty (1993) and Puurtinen et al (2004). The current results, on the other hand, indicate that although trematode parasites should have a negative effect on snail population densities, the relationship between host density and prevalence of infection may greatly differ for individual trematode species, such as Diplostomum sp.…”

“…prevalence. This is in contrast to the negative correlation between L. stagnalis density and the prevalence of trematodes found by Puurtinen et al (2004). These authors included all trematode taxa except Diplostomum, which was absent in mid-July.…”

“…A negative association between the prevalence of trematode parasitism and the density of gastropod hosts at population level has been found by Lafferty (1993) for Cerithidea californica and by Puurtinen et al (2004) for L. stagnalis. These authors suggest that trematode parasites may determine population densities of the snails due to the host-castrating effect of the parasites (Lafferty 1993;Puurtinen et al 2004). As we wanted to study the association between snail density and trematode prevalence specifically for Diplostomum sp.…”

“…For example, the structure of a definitive host population may affect the abundance and species richness of trematodes in the intermediate gastropod hosts (Smith 2001;Hechinger and Lafferty 2005). A negative association between the prevalence of trematode parasitism and the density of gastropod hosts at population level has been found by Lafferty (1993) for Cerithidea californica and by Puurtinen et al (2004) for L. stagnalis. These authors suggest that trematode parasites may determine population densities of the snails due to the host-castrating effect of the parasites (Lafferty 1993;Puurtinen et al 2004).…”