Widespread prevalence but contrasting patterns of intragenomic rRNA polymorphisms in nematodes: Implications for phylogeny, species delimitation and life history inference.

Ribosomal RNA genes have long been a favoured locus in phylogenetic and metabarcoding studies. Within a genome, rRNA loci are organized as tandem repeated arrays and the copies are homogenized through the process of concerted evolution. However, some level of rRNA variation (intragenomic polymorphism) is known to persist and be maintained in the genomes of many species. In nematode worms, the extent of rRNA polymorphism (RP) across species and the evolutionary and life history factors that contribute to the maintenance of intragenomic RP is largely unknown. Here, we present an extensive analysis across 30 terrestrial nematode species representing a range of free-living and parasitic taxa isolated worldwide. Our results indicate that RP is common and widespread, ribosome function appears to be maintained despite mutational changes, and intragenomic variants are stable in the genome and neutrally evolving. However, levels of variation were varied widely across rRNA locus and species, with some taxa observed to lack RP entirely. Higher levels of RP were significantly correlated with shorter generation time and high reproductive rates, and population-level factors may play a role in the geographic and phylogenetic structuring of rRNA variants observed in genera such as Rotylenchulus and Pratylenchus. Although RP did not dramatically impact the clustering and recovery of taxa in mock metabarcoding analyses, the present study has significant implications for global biodiversity estimates of nematode species derived from environmental rRNA amplicon studies, as well as our understanding of the evolutionary and ecological factors shaping genetic diversity across the nematode Tree of Life.