Relationships among some ascaridoid nematodes based on ribosomal DNA sequence data

The nuclear ribosomal DNA (rDNA) region spanning the first (ITS-1) and seco nd (ITS-2) internal transcribed spacers was sequenced for 15 taxa of ascari doid nematodes. The length of the ITS-1 and ITS-2 sequences in the 15 taxa ranged from 392-500 bp and 240-348 bp, respectively. While nucleotide varia tion of 0-2.9% in the ITS-1 and/or ITS-2 sequences was detected within taxa where multiple samples were sequenced, significantly higher level of nucle otide difference (9.4-66.6%) was detected between the taxa, except for Asca ris suum and A. lumbricoides whose taxonomic status remains uncertain. Thes e interspecific differences were linked with the considerable size differen ces (0-108 bp) in the rDNA spacers. Phenograms based on the genetic differe nces among the 15 taxa showed some concordance with previous classification schemes derived from morphological data.