Sequence Tag Site and Host Range Assays Demonstrate that Radapholus similis and R. citraphilus are not Reproductively Isolated

D. T. Kaplan, M. C. Vanderspool, C. H. Opperman


Males of citrus-parasitic Radopholus citrophilus (FL1) were mated with non-citrus-parasitic R. similis (FL5) females. Progeny inherited a 2.4-kb sequence tag site (DK#1) and the ability to reproduce in citrus from the paternal parent (FLl); both traits were absent in the maternal line (FL5). The hybrid progeny produced offspring in roots of citrus seedlings over an 8-month period and therefore were considered reproductively viable. Genomic DNA hybridization studies indicated that one or more copies of DK#1 were present in R. FL1. It is not likely that DK#1 represents a citrus parasitism gene because it was amplified from some burrowing nematode isolates that did not parasitize citrus and because DK#1 contains no open reading frames. Inability to reliably test individual nematodes for their ability to parasitize citrus was a constraint to obtaining F2 data required for definitive genetic characterization of citrus parasitism in burrowing nematodes, and alternate approaches will be required. Although the physical relationship of DK#1 and the citrus parasitism locus remains undefined, results of controlled mating studies using these parameters as genetic markers enabled us to identify hybrid F[sub1] progeny. Therefore, R. similis and R. citrophilus are not sibling species since gene flow between the two does not appear to be restricted via geographic isolation (sympatric in Florida) or by genetics. Key words: banana, citrus, Florida, genetics, hybrid, nematode, PCR, quarantine, Radopholus citrophilus, Radopholus similis, RAPD, STS, taxonomy.

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