THE EVOLUTION OF TISSUE MIGRATION BY PARASITIC NEMATODE LARVAE

Migration by nematode larvae through the tissues of their mammalian ho sts can cause considerable pathology, and yet the evolutionary factors responsible for this migratory behaviour are poorly understood. The b ehaviour is particularly paradoxical in genera such as Ascaris and Str ongylus in which larvae undergo extensive migrations which begin and e nd in the same location. The orthodox explanation for this apparently pointless behaviour is that a tissue phase is a developmental requirem ent following the evolutionary loss of skin penetration or intermediat e hosts. Yet tissue migration is not always necessary for development, and navigation and survival in an array of different habitats must re quire costly biochemical and morphological adaptations. Migrating larv ae also risk becoming lost or killed by the host. Natural selection sh ould therefore remove such behaviour unless there are compensating ben efits. Here we propose that migration is a selectively advantageous li fe-history strategy. We show that taxa exploiting tissue habitats duri ng development are, on average, bigger than their closest relatives th at develop wholly in the gastrointestinal tract. Time to reproduction is the same, indicating that worms with a tissue phase during developm ent grow faster. This previously unsuspected association between juven ile habitat and size is independent of any effects of adult habitat, l ife-cycle, or host size, generation time or diet. Because fecundity is intimately linked with size in nematodes, this provides an explanatio n for the maintenance of tissue migration by natural selection, analog ous to the pre-spawning migrations of salmon.