TABLE CONTENTS

Phylogenetic Analysis of Parasitism of Invertebrates by Rhabditid
Nematodes
Spiridonov, S.E.
Center of Parasitology, A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences,
Leninskii pr., 33, Moscow, 119071, Russia.
Aside from nematodes that use invertebrates as intermediate or paratenic hosts for vertebrate
parasitism, there are several types of parasitic associations characterized by whether the
nematodes inhabit the host’s intestinal lumen or parasitize the host’s coelom as adults or
larvae. About a dozen higher taxa (superfamilies, infraorders or suborders) are represented
among the nematodes parasitizing invertebrates as a single (definitive) host or killing an
invertebrate host to consume its body contents once they are processed by symbiotic bacteria.
Within the order Rhabditida (sensu De Ley and Blaxter, 2002), clades consisting entirely of
nematode associates of invertebrates are relatively common suggesting that radiations of
invertebrate-associated nematodes may have coevolved with certain terrestrial invertebrate
lineages more readily than less derived clades of freeliving nematodes. Molecular
phylogenetic analysis (SSU and D2D3 LSU rDNA) of nematode diversity in earthworms,
terrestrial molluscs, insects and diplopod millipedes demonstrates that the colonization by
nematodes of these invertebrates was not a singular evolutionary event, but happened at least
two or three times. Taxa comprising the nematode associates of invertebrates cluster in both
basal and terminal nodes of rhabditid phylogeny. In some cases, large radiations of
invertebrate-associated nematodes are sister groups for a set of genera in the Rhabditomorpha
or Cephalobomorpha. For example, angiostomatids of terrestrial molluscs cluster within
Rhabditidae whereas drilonematids are inferred to occur within the Cephalobidae.
Conversely, Thelastomatoidea (Oxyuromorpha) and Ransomnematoidea (Rhigonematomorpha)
show a basal position in Rhabditida phylogeny. Unlike drilonematids, two speciesrich
families of the nematodes that are parasitic in earthworms (Ungellidae and
Homungellidae) are basal to the Cephalobomorpha. Entomopathogenic nematodes form
clades within three levels of rhabditid phylogeny – on the generic level (Phasmarhabditis),
on the family level (Heterorhabditidae) and closer to the basal nodes (Steinernematidae). This
latter family is inferred with a basal node for the remaining Strongyloidoidea
(Strongyloides+Alloionema+Rhabditophanes).
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