ASYNCHRONY OF INFECTION TIMING, HABITAT PREFERENCE, AND SYMPATRIC SPECIATION OF SCHISTOSOME PARASITES.

Abstract

Sympatric speciation, which has been the subject of many theoretical studies based on principally genetic models, requires at least two fundamental conditions; (1) the establishment, through disruptive selection, of a stable polymorphism in a heterogeneous environment; and (2) the of reproductive isolation between the morphs (Maynard Smith 1966; Rice 1984, 1987). Studies of natural biological systems, however, remained restricted to a few taxonomically different groups, predominantly insects. Divergent host races in monophagous insects (Bush 1969; Tauber and Tauber 1989), where habitat diversification and asynchrony in seasonal timing of reproduction was implicated in the restriction of gene flow, have provided the best evidence for nonallopatric speciation. Considering that sympatric speciation was closely linked with a process of habitat (-host) shifts, Price (1980, p. 41) and others emphasized the potential applicability of this process to populations of parasites and stated that if host race formation can lead to speciation of phytophagous insects, it can also be important among animal parasites. It is now well accepted that coevolution of lineages of hosts and parasites (phylogenetic or parallel speciation), where host speciation has been accompanied by divergence of their parasites, is not the only pattern for the of hostparasite associations. Parasites, particularly those whose life cycle involves active dispersion (Combes 1991a), have the opportunity of lateral transfer to unrelated host species, a major feature of parasite evolution (Futuyma and Kim 1987, p. 441). Natural metazoan parasite populations, independently and sympatrically maintained by two or more different hosts, diverging to host-adapted populations have been little studied. Such a situation exists for the trematode Schistosoma mansoni on the Caribbean island of Guadeloupe (1528 kM2), where two species of vertebrate hosts (human and rat) are involved in the transmission dynamics of the parasite.