Derivation of host and pathogen genotypes in the fusiform rust pathosystem on slash pine using a complimentary genetics model and diallel data
Seedlings from 20, full-sib families five-parent slash pine diallel were inoculated using two, single urediniospore-derived cultures of the fusiform rust fungus on two different dates during the 1994 growing season. Presence or absence of fusiform rust galls was recorded for each inoculated seedling at nine months post-inoculation and percent infection levels for each family:inoculum:date combination were calculated. The complimentary genetics model normally requires clonal material of either the host or pathogen in order to assign genotypes to identified complimentary gene pairs. Diallel data, however, allows these assignments to be made even when both the host and pathogen populations are segregating. For the July data, three pairs of complimentary genes are thought to explain the observed percent infection levels among the host families and fungus cultures. The putative genotypes of the fungus cultures WLP-2010 and CCA-2 are A1A1:A2a2:a3a3 and A1a1:A2a2:A3A3, respectively. The respective host genotypes for parents 8-7, 9-2, 18-26, 18-62, and 18-27 are R1R1:r3r3, R1r1:r3r3, r1r1:R2r2/r2r2:r3r3, r1r1:r2r2/R2r2:r3r3 and R1r1:R3r3. The homozygous dominant reaction gene of parent 8-7 at locus 1, complimentary by the families with 8-7 as one of the parents appear to be so "resistant" when challenged with WLP-10. Two additional complementary gene pairs can be hypothesized if the May data is included for families 9-2 x 18-25 and 18-27 x 18-26. The identification of these gene pairs using both the May and July inoculation, data suggest the existence of temperature-sensitive genes in this pathosystem. A Chi-square analysis of the July inoculation data using a complementary model with four gene pairs indicate a good fit between expected and observed percentage infection levels. The implications of these findings on rust screening and deployment are discussed.