Location of independent root-knot nematode resistance genes in plum and peach.

Prunus species express different ranges and levels of resistance to the root-knot nematodes (RKN) Meloidogyne spp. In Myrobalan plum ( Prunus cerasifera), the dominant Ma gene confers a high-level and wide-spectrum resistance to the predominant RKN, Meloidogyne arenaria, Meloidogyne incognita, Meloidogyne javanica and the isolate Meloidogyne sp. Florida which overcomes the resistance of the Amygdalus sources. In Japanese plum ( Prunus salicina), a similar wide-spectrum dominant resistance gene, termed R(jap), has been hypothesized from an intraspecific segregating cross. In peach, two crosses segregating for resistance to both M. incognita and M. arenaria were used to identify single genes that each control both RKN species in the Shalil ( R(Mia557)) and Nemared ( R(MiaNem)) sources. Localisation of these genes was made possible using the RFLP and SSR- saturated reference Prunus map TxE, combined with a BSA approach applied to some of the genes. The Ma1 allele carried by the Myrobalan plum accession P.2175 was localised on the linkage group 7 at an approximate distance of 2 cM from the SSR marker pchgms6. In the Japanese plum accession J.222, the gene R(jap) was mapped at the same position in co-segregation with the SSR markers pchgms6 and CPPCT022. The peach genes R(Mia557) and R(MiaNem), carried by two a priori unrelated resistance sources, were co-localized in a subtelomeric position on linkage group 2. This location was different from the more centromeric position previously proposed by Lu et al. (1999) for the resistance gene Mij to M. incognita and M. javanica in Nemared, near the SSR pchgms1 and the STS EAA/MCAT10. By contrast, R(Mia557) and R(MiaNem) were flanked by STS markers obtained by Yamamoto and Hayashi (2002) for the resistance gene Mia to M. incognita in the Japanese peach source Juseitou. Concordant results for the three independent sources, Shalil, Nemared and Juseitou, suggest that these peach RKN sources share at least one major gene resistance to M. incognita located in this subtelomeric position. We showed that plum and peach genes are independent and, thus, can be pyramided into interspecific hybrid rootstocks based on the plum and peach species.