LOW, BUT STRONGLY STRUCTURED MITOCHONDRIAL-DNA DIVERSITY IN ROOT-KNOTNEMATODES (MELOIDOGYNE)

Root-knot nematodes (genus Meloidogyne) have been the subject of recen t and numerous studies of genetic variation because of the need to dev elop molecular diagnostics for the four globally distributed, partheno genetic species that are significant agricultural pests. Our analysis of Meloidogyne mtDNA improves on previous studies: (i) by examining re striction site polymorphism among a large number of isolates also char acterized for standard morphological, host range and allozyme phenotyp es; (ii) by using higher resolution electrophoretic techniques; and (i ii) by mapping variable restriction sites with reference to the comple te nucleotide sequence. This revealed fivefold less sequence divergenc e (<0.6%) between variants than estimated in previous restriction frag ment length polymorphism (RFLP) studies, but perfect correspondence be tween mtDNA haplotype and allozyme (esterase) phenotypes pes. The mtDN A variation, although limited, is strongly structured with as much div ergence between two lineages of Meloidogyne arenaria as between either of these and Meloidogyne javanica. The low diversity of mtDNAs sugges ts that these parthenogenetic lineages arose from distinct but closely related sexual females, a pattern seen in other parthenogenetic compl exes. In contrast to the concordance between mtDNA and allozyme marker s, there were several discrepancies between the traditional methods of identification. We suggest that further studies of these nematodes sh ould focus on well defined genetic groups, whether or not these coinci de with existing taxonomic units.