EVOLUTIONARY PATTERNS IN LIFE-HISTORIES OF OXYURIDA

The Oxyurida comprises some 850 known species that occur in the intest ine of arthropods and vertebrates (one species in annelids). Important arthropod hosts include Diplopoda, Blattodea, Gryllotalpoidea, Passal idae, Scarabaeida and Hydrophilidae. The major vertebrate hosts are li zards, tortoises, primates, rodents and lagomorphs. An underlying char acteristic of the group is haplodiploid reproduction and like many hap lodiploid groups, pinworms tend to have life histories that involve hi gh levels of inbreeding. Unlike Strongylida, Ascaridida and Spirurida, which have diversified in tissue site and life cycle as well as hosts , pinworms show little variation in these features and have radiated o nly across host groups. Two explanations are advanced for this. Haplod iploidy and its concomitant inbreeding may act to canalise evolutionar y change, although diverse groups such as the Hymenoptera belie this. Alternatively, Strongylida, Ascaridida and Spirurida are presumed to h ave arisen from skin-penetrating ancestors that were forced to undergo a tissue migration before reaching their primitive tissue site, the g ut. This migration demanded they adapt to a variety of tissue sites an d thus acted as a preadaptation to further diversification. The Oxyuri da, in contrast, probably arose using oral contaminative transmission. The lack of exposure to other tissue sites may therefore have relegat ed pinworms to their position in the posterior gut.