Abstract
BackgroundPine wilt disease (PWD), which is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, is currently the greatest threat to pine forests in Europe and East Asian countries including Japan. Constructing a detailed linkage map of DNA markers and identifying PWD resistance genes/loci lead to improved resistance in Pinus thunbergii, as well as other Pinus species that are also susceptible to PWD.ResultsA total F1 mapping population of 188 individuals derived from a cross between the PWD-resistant P. thunbergii varieties ‘Tanabe 54’ (resistant rank 2 to PWD) and ‘Tosashimizu 63’ (resistant rank 4 to PWD) was inoculated with PWN, and was evaluated for disease symptoms. To perform linkage analysis for PWN resistance, a set of three maps was constructed; two parental maps generated using the integrated two-way pseudo-testcross method, and a consensus map with population-type cross-pollination. The linkage map of ‘Tanabe 54’ consisted of 167 loci, and covered 14 linkage groups (LGs), with a total genetic distance of 1214.6 cM. The linkage map of ‘Tosashimizu 63’ consisted of 252 loci, and covered 14 LGs, with a total genetic distance of 1422.1 cM. The integrated consensus map comprised 12 LGs with the basic chromosome number of P. thunbergii, and a total genetic distance of 1403.6 cM. Results from quantitative trait loci (QTL) analysis using phenotype data and linkage maps indicated that PWN resistance is controlled by a single dominant allele, which was derived from the ‘Tanabe 54’ female parent. This major QTL was located on linkage group 3 and was designated PWD1 for PINE WILT DISEASE 1.ConclusionsThe PWD1 locus is a major resistance QTL located on the Pinus consensus LG03 that acts in a dominant manner to confer pine wood nematode resistance. Information from the present study will be useful for P. thunbergii breeding programs to improve resistance to PWD, and also to help identify susceptibility genes in Pinus species.
Highlights
Pine wilt disease (PWD), which is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, is currently the greatest threat to pine forests in Europe and East Asian countries including Japan
The phenotypic segregation ratios were not statistically different from 1:1 (x2 = 0.00, p = 1.00), and were different from 3:1 (x2 = 62.67, p = 0.00). These results indicate that PWD resistance in the F1 population is controlled by one major gene
Results from the nematode inoculation test performed on F1 individuals indicated that PWN resistance is controlled by a single dominant allele, which is Construction of genetic linkage map Genetic linkage maps are important in genetic research and breeding for mapping desirable traits and identifying quantitative trait loci (QTL) and numerous genetic markers required for constructing linkage maps
Summary
Pine wilt disease (PWD), which is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, is currently the greatest threat to pine forests in Europe and East Asian countries including Japan. The selected clones were reared to develop PWD-resistant Pinus varieties. These varieties were evaluated based on the survival rate of open-pollinated progeny following inoculation with PWN; average rates of survival of openly pollinated progeny from resistant varieties were 51% for P. thunbergii and 65% for P. densiflora, which is respectively 35 and 18% higher than for unselected populations [10]. It was shown that the number of genetic factors for P. thunbergii resistance was 1.96 based on diallel analysis using a full diallel mating design that used three out of 16 varieties including ‘Tanabe 54’ [12]. The findings have shown that about two genetic factors are involved and that resistance is additive; the locus for PWD resistance has not been identified using a molecular genetic approach
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