Modes of transmission and evolution of life histories in zooparasitic nematodes

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Abstract
A hypothetical phylogeny of life-history patterns in zooparasitic nematodes is presented. It is suggested that the course of evolution of life histories has been strongly influenced by conditions under which parasitism arose; two factors appear particularly important, namely the primitive mode of transmission and the feeding habits of the ancestral populations. Two transmission modes were available at the outset: active penetration (usually cutaneous) and passive oral contamination. Only groups that arose using the former mode developed tissue parasitism and heteroxeneity. With percutaneous transmission, nematodes were forced to respond and adapt to the tissue environment from the beginnings of parasitism, and the penetrating habit probably facilitated experimentation with intermediate hosts. The Enoplida and Tylenchia arose from stylet-bearing predaceous or plant-parasitic fluid feeders adapted to feed on tissues. Cutaneous penetration was probably the primitive transmission mode and these groups were tissue dwelling at the outset. Zooparasitic Rhabditia and Diplogasteria are derived from free-living particulate feeders and the earliest parasitic forms probably fed on the intestinal microflora of the host; tissue parasitism arose secondarily and only in lineages that primitively used penetrative transmission.

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