Non-homogenized ITS regions in the parasitic nematode Cooperia oncophora.

Abstract

Here, the validity of the assumption of concerted evolution of ribosomal regions in larval and adult Cooperia oncophora was assessed. In each of 4 individuals of this parasitic nematode, at least 78% of the sequences comprised different ITS variants. This implies that concerted evolution is not acting, which is corroborated by the scarcity of signatures of gene conversion and recombination. Mis-incorporation of nucleotides and illegitimate PCR-induced recombination turned out to be unlikely, and positions with substantial frequencies of alternative nucleotides corresponded to ambiguous positions in published ITS2 sequences of this and other Cooperia species based on direct sequencing. The ITS regions of each individual C. oncophora displayed a significant excess of unique mutations in agreement with expansion of the ribosomal gene family. Interesting corollaries of the inferred size changes of this gene family are genomic rearrangements that occur during larval development such as multiple rounds of endoduplication (in Rhabditidae), chromatin diminution (in Ascaris), and non-compensatory mutations on the secondary structure of the ITS2. It is yet unknown which process is important in trichostrongylids. Finally, although it can not be rigorously assessed in Cooperia, the ITS polymorphisms can readily be envisioned to affect phylogenetic reconstructions of closely related nematodes.