Sex allocation and population structure in apicomplexan (protozoa) parasites

Establishing the selfing rate of parasites is important for studies in clin ical and epidemiological medicine as well as evolutionary biology. Sex allo cation theory offers a relatively cheap and easy way to estimate selfing ra tes in natural parasite populations. Local mate competition (LMC) theory pr edicts that the optimal sex ratio (r*; defined as proportion males) is rela ted to the selfing rate (s) by the equation r*=(1-s)/2. In this paper, we g eneralize the application of sex allocation theory across parasitic protozo a in the phylum Apicomplexa. This cosmopolitan phylum consists entirely of parasites, and includes a number of species of medical and veterinary impor tance. We suggest that LMC theory should apply to eimeriorin intestinal par asites. As predicted, data from 13 eimeriorin species showed a female-biase d sex ratio, with the sex ratios suggesting high levels of selfing (0.8-1.0 ). Importantly our estimate of the selfing rate in one of these species, To xoplasma gondii, is in agreement with previous genetic analyses. In contras t, we predict that LMC theory will not apply to the groups in which syzygy occurs (adeleorins, gregarines and piroplasms). Syzygy occurs when a single male gametocyte and a single female gametocyte pair together physically or in close proximity, just prior to fertilization. As predicted, data from f our adeleorin species showed sex ratios not significantly different from 0. 5.