accidentally ingested the skin penetrators, who became oral infectors but
retained their migration through the host, and thus the odd migration was
explained (Durette-Desset, 1985; Adamson, 1989). This hypothesis was
tested in related species representing oral infectors, skin penetrators
and oral infectors that migrate in the host, using sequences of part of
the mitochondrial DNA (mtDNA) gene encoding cytochrome c oxidase
subunit I. These phylogenetic analyses supported the hypothesis that the
most primitive nematode parasites were skin penetrators (Sukhdeoet al.,
1997). The data suggest that the odd migration ofS. vulgarisis a legacy of a
past transmission strategy, and reminds us that evolution does not always
produce the optimal solutions.
These examples represent the effects of phylogeny at a gross level
in the habitat-selection behaviour of nematodes, but there are many
presumed effects at finer scales that have not been tested. For example,
a common assumption is that the restriction of the skin penetrators to
the gut may have been the result of their bacteria-feeding preference
(Durette-Desset, 1985; Adamson, 1989). However, there is now some
evidence suggesting that the earliest skin penetrators lived as adults in the
lungs and not the intestines (S.C. Sukhdeo, unpublished). Clearly, much236 M.V.K. Sukhdeoet al.
Oral ingestion (tissue migration)Oral ingestion (no migration)Skin penetration
Free-living
Fig. 11.2. The evolution of transmission strategies in intestinal nematodes
(Strongylida). Free-living soil nematodes first invaded amphibian hosts via skin
penetration. Subsequently, with the evolution of large herbivores, these skin
penetrators were ingested and the oral routes evolved. (After Sukhdeoet al., 1997.)