Low, but Strongly Structured Mitochondrial DNA Diversity in Root Knot Nematodes (Meloidogyne)

Root-knot nematodes (genus Meloidogyne) have been the subject of recent and numerous studies of genetic variation because of the need to develop molecular diagnostics for the four globally distributed, parthenogenetic species that are significant agricultural pests. Our analysis of Meloidogyne mtDNA improves on previous studies: (i) by examining restriction site polymorphism among a large number of isolates also characterized for standard morphological, host range and allozyme phenotypes; (ii) by using higher resolution electrophoretic techniques; and (iii) by mapping variable restriction sites with reference to the complete nucleotide sequence. This revealed fivefold less sequence divergence (<0.6%) between variants than estimated in previous restriction fragment length polymorphism (RFLP) studies, but perfect correspondence between mtDNA haplotype and allozyme (esterase) phenotypes. The mtDNA variation, although limited, is strongly structured with as much divergence between two lineages of Meloidogyne arenaria as between either of these and Meloidogyne javanica. The low diversity of mtDNAs suggests that these parthenogenetic lineages arose from distinct but closely related sexual females, a pattern seen in other parthenogenetic complexes. In contrast to the concordance between mtDNA and allozyme markers, there were several discrepancies between the traditional methods of identification. We suggest that further studies of these nematodes should focus on well defined genetic groups, whether or not these coincide with existing taxonomic units.