Submitted to: Journal of Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 17, 1997
Publication Date: N/A
Interpretive Summary: A DNA fragment correlated with citrus parasitism in the burrowing nematode, Radopholus citrophilus, was used as a genetic marker to demonstrate that R. citrophilus and its sibling species, R. similis were not reproductively isolated. This means that they are not sibling species, and are actually races of the same genus. This was further confirmed through assessment of the ability of the offspring of controlled matings to parasitize citrus. We demonstrated that citrus parasitism in burrowing nematodes appears to be inherited as a dominant trait. We also conducted several in-depth studies to identify the genetic sequence of the DNA fragment associated with citrus. The DNA fragment does not appear to be directly involved in citrus parasitism but will contribute to the identification of the genes that do confer citrus parasitism. These finding will improve the likelihood of developing diagnostics to identify citrus-parasitic burrowing nematodes, have clarified the taxonomic relationship of these quarantine pests, and will most importantly provide useful information toward development of effective control strategies which are currently unavailable.
A 2.4 kb DNA fragment (DK#1) from the burrowing nematode, Radopholus citrophilus was correlated with citrus parasitism. DK#1 contains numerous stop codons, and has no open reading frames of significant length, nor were any significant homologies detected. DK#1 is present as two or more alleles or may possibly be located at two independent sites in the genome of R. citrophilus. DK#1 is absent in the genome of R. similis DK6. The genetic sequence of DK#1 is highly conserved within R. citrophilus. DK#1 is not present in the plant parasitic nematode species, Belonolaimus longicaudatus, Hoplolaimus spp., Meloidogyne arenaria, M. javanica, Pratylenchus coffeae or Tylenchulus semipenetrans. We used DK#1 as a genetic marker to demonstrate interspecific hybridization in a cross of an R. similis female (DK#1 minus, not citrus parasitic) with an R. citrophilus male (DK#1 positive, citrus parasitic). The F1 progeny were hemizygous for the presence of the marker. These F1 inherited the ability to parasitize and reproduce in roots of rough lemon seedlings, demonstrating that citrus parasitism is a dominant trait conferred from the male parent in this cross. These findings indicate that the citrus parasitic burrowing nematodes are not a bonafide sibling species since they are not reproductively isolated and therefore should be considered as a race of R. similis.