The evolution of parasitism in Nematoda

Abstract

Nematodes are abundant and diverse, and include many parasitic species. Molecular phylogenetic analyses have shown that parasitism of plants and animals has arisen at least 15 times independently. Extant nematode species also display lifestyles that are proposed to be on the evolutionary trajectory to parasitism. Recent advances have permitted the determination of the genomes and transcriptomes of many nematode species. These new data can be used to further resolve the phylogeny of Nematoda, and identify possible genetic patterns associated with parasitism. Plant-parasitic nematode genomes show evidence of horizontal gene transfer from other members of the rhizosphere, and these genes play important roles in the parasite-host interface. Similar horizontal transfer is not evident in animal parasitic groups. Many nematodes have bacterial symbionts that can be essential for survival. Horizontal transfer from symbionts to the nematode is also common, but its biological importance is unclear. Over 100 nematode species are currently targeted for sequencing, and these data will yield important insights into the biology and evolutionary history of parasitism. It is important that these new technologies are also applied to free-living taxa, so that the pre-parasitic ground state can be inferred, and the novelties associated with parasitism isolated.

Keywords: horizontal gene transfer.

Fig. 1.

The phylogenetic structure of the Nematoda and the origins of parasitism (A) A cartoon of the phylogenetic structure of the Nematoda, based on nuclear small subunit ribosomal RNA analyses and interpretation of taxon relationships derived from morphology (De Ley and Blaxter, ; Blaxter and Denver, 2012). Taxon systematic names are given for the major nodes in the phylogeny. Clades I, II, C, III, IV and V were first defined in Blaxter et al. (1998). Helder and colleagues revised the numbering of clades (Holterman et al. ; van Megen et al. 2009), and their schema is given in smaller Arabic numerals beneath the relevant branches. For each ordinal/subordinal group named, the ecosystem and trophic habits are indicated by small icons. For the major clades, the numbers of published genomes, genomes in progress and the proportion of named species (Hallan, 2007) are given. (B) The utility of large scale nematode genome data for phylogenetic analyses. A phylogeny of Nematoda derived from 181 protein coding genes from 23 nematode species, and four ecdysozoan taxa as outgroup. The alignment was subjected to analysis with PhyloBayes (Lartillot et al. 2009), and all nodes had posterior probability of 1·00. The major clades in Rhabditida are resolved, and Enoplia is recovered at the base of Nematoda. The figure is adapted from Blaxter et al. (2014).