HOST-INDUCED MORPHOLOGY IN MITES - IMPLICATIONS FOR HOST-PARASITE COEVOLUTION

Coevolution of parasites and their hosts is presumed if the phylogeny of a group of parasites matches that of their hosts, producing a high value of a consistency index. Such comparisons are done using relatively host specific parasite species that are usually separated only by morphological characters. Parasitic, unionicolan mites in the subgenus Unionicolides are described, from sampling data, as highly host specific and are thought to have co-speciated with their freshwater mussel hosts. However, experimental observations of Unionicola Unionicolides poundsi and U. U. lasellai were inconsistent with these mites' supposed high host specificity. A transplant experiment, in which nymphs metamorphosed in the wrong host, demonstrated that the morphological characters used to differentiate these mite species are plastic and hostdetermined. Hence, when taxonomists use only morphological characters and host-induced morphological variation is present, morphological variants can be elevated to the status of species. Consequently, parasites can appear to be highly host specific. If morphs are then grouped, as species, into the same taxon, phylogenetic comparisons can produce high consistency indices independently of any coevolutionary processes. The conditions under which this bias can occur are commonly true of phylogenetic comparisons. Hence, host-induced morphology is a viable, alternative explanation for high consistency indices. © 1990 Society of Systematic Zoology.