EVOLUTION OF THE AT-RICH MITOCHONDRIAL-DNA OF THE ROOT-KNOT NEMATODE,MELOIDOGYNE-HAPLA

Mitochondrial DNA of the root knot nematode Meloidogyne hapla was inve stigated for intraspecific diversity and divergence from other parthen ogenetic root knot nematodes. A 1,900-bp fragment containing COII, tRN A(His), 16S rRNA, ND3 and Cyt b genes has been cloned and sequenced fr om one individual and an 1,188-bp region within this region was sequen ced from four other Australian isolates. M. hapla mtDNA is more than 8 0% AT-rich, like other Meloidogyne spp. Nucleotide diversity within M. hapla is some 10-fold higher than across three other parthenogenetic species of root-knot nematode (M. arenaria, M. javanica, and M. incogn ita), implying an earlier origin for M. hapla. Nucleotide divergence b etween M. hapla and its congener M. javanica is as great as that betwe en Ascaris suum and Caenorhabditis elegans, members of different nemat ode subclasses, while amino acid sequence difference between Meloidogy ne is more than twice as great. This is interpreted as an AT-bias-indu ced acceleration of the amino acid substitution rate, over and above s aturation of nucleotide divergence in the strongly AT-biased DNA, on t hree lines of evidence: (1) in conserved blocks in 16S rDNA congeneric Meloidogyne have no more differences than between A. suum and C. eleg ans; (2) the Meloidogyne lineage has more amino acid changes relative to the Ascaris/Caenorhabditis lineage with respect to four of five out groups, the exceptional outgroup being the only species (Apis) as AT-r ich as Meloidogyne; and (3) between the two Meloidogyne there are more first and second but fewer third codon position changes than between the other nematode species. M. hapla is also found to contain a 102-bp tandem repeat of at least 40 copies; a size, arrangement, and positio n the same as in M. javanica, but sequence comparisons did not demonst rate homology between the two repeats.