Avian louse phylogeny (Phthiraptera: Ischnocera): a cladistic study based on morphology

The louse suborder Ischnocera (Phthiraptera) contains 3060 currently described species from over 150 genera. These lice are permanent obligatory ectoparasites of a diverse selection of birds and mammals with a worldwide distribution. Historically, they have played a major role in the development of our ideas on coevolution, and species hosted by mammals have been used extensively as model organisms for the study of cospeciation. In contrast, avian taxa comprising 90% of ischnoceran species have been neglected due to a lack of data on their wider systematics. A comparative study based on the adult and nymphal instar morphology of avian lice yielded 138 characters from 56 species (51 genera), all of which are illustrated or discussed here for the first time. A further five outgroup taxa were examined from the mammalian ischnoceran family Trichodectidae. Phylogenetic analyses of these data produced three most parsimonious cladograms, the strict consensus of which is highly resolved and broadly consistent with previous classifications. Morphological character variation is extensive, and nymphal character traits are useful in identifying instances of convergent evolution in adult morphology. The role of ontogeny in the development of the major character complexes of the head and abdomen is discussed, and its implications for further work on the phylogeny of avian Ischnocera is considered. Comparison with host taxonomy reveals a series of complex host-parasite associations that do not support a hypothesis of strict one to one cospeciation. However, extrapolation of these associations is compromised by the low sample size. The role of niche specialization to explain the presence of multiple unrelated lineages on the same host taxon is considered.