MITOCHONDRIAL-DNA SEQUENCE DIVERGENCE AMONG MELOIDOGYNE-INCOGNITA, ROMANOMERMIS-CULICIVORAX, ASCARIS-SUUM, AND CAENORHABDITIS-ELEGANS

Mitochondrial DNA sequences were obtained from the NADH dehydrogenase subunit 3 (ND3), large rRNA, and cytochrome b genes from Meloidogyne i ncognita and Romanomermis culicivorax. Both species show considerable genetic distance within these same genes when compared with Caenorhabd itis elegans or Ascaris suum, two species previously analyzed. Caenorh abditis, Ascaris, and Meloidogyne were selected as representatives of three subclasses in the nematode class Secernentea: Rhabditia, Spiruri a, and Diplogasteria, respectively. Romanomermis served as a represent ative out-group of the class Adenophorea. The divergence between the p hytoparasitic lineage (represented by Meloidogyne) and the three other species is so great that virtually every variable position in these g enes appears to have accumulated multiple mutations, obscuring the phy logenetic information obtainable from these comparisons. The 39 and 42 % amino acid similarity between the M. incognita and C. elegans ND3 an d cytochrome b coding sequences, respectively, are approximately the s ame as those of C. elegans-mouse comparisons for the same genes (26 an d 44%). This discovery calls into question the feasibility of employin g cloned C. elegans probes as reagents to isolate phytoparasitic nemat ode genes. The genetic distance between the phytoparasitic nematode li neage and C. elegans markedly contrasts with the 79% amino acid simila rity between C. elegans and A. suum for the same sequences. The molecu lar data suggest that Caenorhabditis and Ascaris belong to the same su bclass.