The effects of habitat spatial structure on the accumulations of deleterious mutations : using freshwater snails

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We examined the effects of habitat spatial structure on the accumulations of deleterious mutations using field freshwater snails (Physa acta). The snails were collected from two different habitat : (1) (Large stable population) Rice fields in which the water levels might be stable and the snails might interbreed within large populations ; (2) (Alternate mixing populations) Small isolated temporally water pools which sometimes become large ponds in a heavy rain. Snails might inbreed at high densities in small isolated water pools in alternate mixing population ; while snails might inbreed at low densities in the large stable population. Thus, it can be predicted that inbreeding depression of snails from the alternate mixing populations might be low at high densities. The results showed that the inbreeding depression estimated from the number of eggs and the fitness (number of eggs x survival rates) was lower in the snails form the alternate mixing populations than those from the large stable populations at high densities but there was no significant differences in inbreeding depression between snails from two populations at low densities. The results were consistent with the above predictions.Furthermore, the individual -based simulations were conducted to examine the effects of the genetic neighborhood size and the shape of habitat on the accumulations of deleterious mutation and the risk of extinction. The results indicated that the total population size and the genetic neighborhood size affect linearly the numbers of accumulated homozygotic deleterious loci independently. In addition, the shape of habitat significantly affect the accumulation of deleterious mutations and the extinction probability.