The evolution of tissue migration in parasitic nematodes (Nematoda: Strongylida) inferred from a protein-coding mitochondrial gene

Polymerase chain reaction was used to amplify and sequence a 710-bp region of the mtDNA protein-encoding gene, cytochrome c oxidase subunit I, in nine nematodes belonging to the order Strongylida, one nematode from the order Rhabditida and two nematodes from the order Ascaridida. This study tested the hypothesis that in the Strongylida, tissue migration in orally infecting nematodes reflects a legacy of skin penetrating strategies of host infection that were retained after an oral route of infection became established (percutaneous transmission model). Phylogenetic analyses (NJ, MP, ML) consistently produced a single tree topology with two clades within the Strongylida, one containing nematodes that incorporated tissue migration as part of their life cycles, and the other containing nematodes with no tissue migration. Our analysis concluded that the ancestral Strongylida possessed skin penetrating and tissue migrating characters that were modified as members of the group progressed along one of two evolutionary pathways. In one pathway, skin penetration was lost but tissue migration was retained as the nematodes evolved an oral infection route. In the other, both skin penetration and tissue migration characters were lost as the nematodes evolved an oral infection route. These data support the hypothesis of percutaneous transmission as being ancestral in the Strongylida.