Reticulate evolution and the origins of ribosomal internal transcribed spacer diversity in apomictic Meloidogyne

Among root knot nematodes of the genus Meloidogyne, the polyploid obligate mitotic parthenogens M. arenaria, M. javanica, and M. incognita are widespr ead and common agricultural pests. Although these named forms are distingui shable by closely related mitochondrial DNA (mtDNA) haplotypes, detailed se quence analyses of internal transcribed spacers (ITSs) of nuclear ribosomal genes reveal extremely high diversity, even within individual nematodes. T his ITS diversity is broadly structured into two very different groups that are 12%-18% divergent: one with low diversity (<1.0%) and one with high di versity (6%-7%). In both of these groups, identical sequences can he found within individual nematodes of different mtDNA haplotypes (i.e.. among spec ies). Analysis of genetic variance indicates that more than 90% of ITS dive rsity can be found within an individual nematode, with small but statistica lly significant (5%-10%; P < 0.05) variance distributed among mtDNA lineage s. The evolutionarily distinct parthenogen M. hapla shows a similar pattern of ITS diversity with two divergent groups of ITSs within each individual. In contrast, two diploid amphimictic species have only one lineage of ITSs with low diversity (<0.2%). The presence of divergent lineages of rDNA in the apomictic taxa is unlikely to be due to differences among pseudogenes. Instead, we suggest that the diversity of ITSs in M. arenaria, M. javanica, and M. incognita is due to hybrid origins from closely related females (as inferred from mtDNA) and combinations of more diverse paternal lineages.