POLYMORPHISM IN BISEXUAL REPRODUCTIVE PATTERNS OF CYCLICAL PARTHENOGENS - A SIMULATION APPROACH USING A ROTIFER GROWTH-MODEL

A dynamic model based on six differential equations, developed in a pr evious paper by Serra and Carmona (1993) was modified and used to addr ess the theoretical possibility of polymorphism for the strategies of mixis - i.e., strategies for bisexual phase induction - in cyclical pa rthenogens. The model focuses on rotifers and describes the growth rat es for amictic (asexual) females, fertilizable mictic (sexual) females , mictic females that produce males, mictic females that produce resti ng eggs (i.e., sexual eggs), males and resting eggs, and takes into ac count the main constraints on mixis. Moreover, birth rates were assume d as time-dependent, which involved a limited period for population gr owth. By using computer simulation we explored the evolution of the ge notypes determining mixis ratio - defined as the proportion of the egg s produced from amictic females that develop into mictic females - and the timing of mixis induction. The assumptions on the genetic system did not qualitatively affect simulation conclusions. Our main findings are: (1) When mixis is induced, mixis ratios equal to one were select ed, regardless the phenotype for the timing of mixis induction. (2) A fairly high degree of polymorphism was maintained for the genes determ ining the timing of mixis induction. (3) As a result from (1) and (2), at the evolutionary equilibrium, the population as a whole showed a s teep increase of mixis ratio, from 0 to 1, during the sexual phase of the growth cycle. (4) If the moment of mixis induction is assumed to b e based on an one-locus, two-allele genetic system, results show that genes determining an early mixis were more frequent than expected from optimization criteria. Therefore, at equilibrium, the number of resti ng eggs produced by the population was not maximized. We suggest that this finding is a result of the selection involved in a mating race am ong the sexual individuals in the population.