PARASITE SPECIATION WITHIN OR BETWEEN HOST SPECIES - PHYLOGENETIC EVIDENCE FROM SITE-SPECIFIC POLYSTOME MONOGENEANS

Partial nuclear 28S ribosomal RNA and mitochondrial cytochrome c oxida se subunit I (COI) gene sequences (953 and 385 nucleotides, respective ly) of one fish monogenean (outgroup) and six polystome monogeneans (f our Polystomoides spp. from the oral cavities and urinary bladders of freshwater turtles in Australia and Malaya, two Neopolystoma spp. from the urinary bladder and conjunctival sac of a freshwater turtle in Au stralia) were used to examine the question of whether congeneric speci es infecting different sites in the same host species have speciated i n that host by adapting to different sites, or whether species infecti ng a particular site in one host have given rise to species infecting the same site in different hosts. Results show unequivocally that cong eneric species infecting the same site, even of host species belonging to different suborders and occurring on different continents, are mor e closely related than congeneric species infecting different sites of the same host species. This is interpreted as meaning that speciation has not occurred in one host. Morphological evolution of polystomes h as been very slow: few differences between species and even genera hav e evolved over a period of at least 150 Myr, and this is matched by lo w substitution rates of nucleotides, and the ambiguous position of spe cies of different genera, depending on whether COI or 28S rDNA sequenc es are used. (C) 1997 Australian Society for Parasitology. Published b y Elsevier Science Ltd.