ABSTRACT
The brittlestar (Echinoderms) Amphipholis squamata is a suitable model for studying microevolution. The species presents a huge inter-and intra-populational genetic variability since previous works have demonstrated that populations are constituted by different genetically determined colour morphs. An isolated population from Sicily has been seasonally characterized for more than 3 years. The occurrence of the colour morphs was constant through the years till June 1999. This equilibrium was a consequence of a trade-off between differential life-history traits (fecundity) and selective pressure (predation) for two colour morphs. An important infestation by the endo-parasite Rhopalura ophiocomae occurred in June 1999. Our survey confirms that the parasite selects a host according to its brooding state. Our observations also revealed that the parasite had quantitative and qualitative consequences on the population, inducing huge modifications in the genetic structure. A synthetic model will be presented, which describes of the effect of the different selective pressures (predation, parasitism and competition) on the genetic diversity of the population. Contribution to Cibim.
