Phylogenetic systematic analysis of the Trichostrongylidae (Nematoda), with an initial assessment of coevolution and biogeography.

Phylogenetic analysis of the subfamilies of the Trichostrongylidae based on 22 morphological transformation series produced a single cladogram with a consistency index (CI) = 74.2%. Monophyly for the family was supported by the structure of the female tail and copulatory bursa. Two major clades are recognized: the Cooperiinae clade with the basal Cooperiinae and Libyostrongylinae+Trichostrongylinae, and the Graphidiinae clade with the basal Graphidiinae and Ostertagiinae+Haemonchinae. Dendrograms presented by Durette-Desset (1985) (CI = 56.1%) and Lichtenfels (1987), based on the key to the Trichostrongylidae by Gibbons and Khalil (1982) (CI = 59.0%), were found to be relatively inefficient in describing character evolution and in supporting putative relationships among the subfamilies. Based on the current analysis, the intestine appears to have constituted the ancestral habitat for the trichostrongylids with the stomach/abomasum having been independently colonized in each clade. Assessment of host associations suggests extensive colonization but also a high degree of coevolution with Bovidae and Cervidae for Ostertagiinae+Haemonchinae. Biogeography for this assemblage is complex, but this analysis is compatible with a Palearctic or Eurasian origin for Cooperiinae, Haemonchinae, and Ostertagiinae.