Examining a possible advantage of diploidy in Potamopyrgus antipodarum in a phosphorus-limited environment

Chamberlain, Tonya
The persistence of sexual reproduction is a mystery in evolutionary biology. The benefits of reproducing asexually almost always outweigh the cost of reproducing sexually. The freshwater snail Potamopyrgus antipodarum is obligately sexual or asexual with asexual snails being either triploid or tetraploid polyploids. Polyploid genomes possess and require more phosphorus (P) per unit mass than their sexual conspecifics due to extra chromosomal set(s). We tested the hypothesis that an environment limited in P will favor faster growth in sexuals, and therefore favor sexual reproduction, in P. antipodarum. According to the Growth Rate Hypothesis, an animal in its juvenile life stage requires more P than in any other life stage. Therefore, effects of P limitation on growth should be most evident for juveniles. We raised juvenile diploid, triploid, and tetraploid P. antipodarum on diets with low and high P content and compared their growth rates. Tetraploid lineages grew more slowly than triploid lineages under low-P diets. Unexpectedly, asexuals grew larger than sexuals in both treatments. Because our sexual lineages were likely inbred, which may have contributed to their growth rates, we are repeating the experiment this year with wild caught sexual and asexual--triploid and tetraploid--snails.
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