The influence of genetic and extrachromosomal factors on population sex ratio in the marine isopod, Paracerceis sculpta

In the marine isopod, Paracerceis sculpta, three male morphs are distinct at a single genetic locus, Ams (- Alternative mating strategy). In the laboratory, interactions among Ams alleles, alleles at another autosomal locus (Tfr = Transformer), primary sex factors (ZW = females; ZZ = males), and a possible extrachromosomal factor (ECF), appear to influence the frequencies of the male morphs and cause deviations in family sex ratio. To determine whether ECF is a feminizing Wolbachia endosymbiont, we probed isopod tissues using Wolbachia -specific primers and PCR. To identify ECF’s effect on population sex ratio, we generated expected values using a stepwise model that included the effects of each known sex ratio-influencing factor. We then compared observed and expected sex ratios over a two-year sampling period. We found no evidence of DNA sequences characteristic of Wolbachia endosymbionts in the tissues of P. sculpta. However, simulations showed significant deviations of observed from expected population sex ratios when the presumed effects of ECF were excluded. When the effects of ECF were included in simulations, significant deviations in the expected population sex ratio occurred when P ECF exceeded 0.50. Observed and expected population sex ratios fit best at low to intermediate P ECF Under these conditions, observed fluctuations in population sex ratio were explained by observed population frequencies of primary sex factors and Ams alleles, with oscillating frequencies of Tfr 2 , an allele known to cause sex conversion of genetic males. At high frequencies of Tfr 2 , the effects of ECF could be excluded from the model without affecting the sex ratio. Our results show: 1) Wolbachia endosymbionts are not responsible for sex ratio devations in P. sculpta; 2) ECF has a recognizable effect on population sex ratio, but unlike most extrachromosomal sex factors, ECF appears to persist at intermediate population frequency. Moreover, when fluctuations in Tfr 2 are allowed, the effects of ECF are no longer necessary to explain the population sex ratio. Thus, 3) Frequency-dependent selection on primary sex factor- Ams - Tfr interactions alone may be responsible for female-biased as well as variable sex ratios in the northern Gulf of California P. sculpta population.