Genetic linkage maps constructed by using an interspecific cross between Japanese and European pears

Genetic linkage maps of the European pear ( Pyrus communis L.) cultivar 'Bartlett' and the Japanese pear ( Pyrus pyrifolia Nakai) cultivar 'Housui' were constructed based on AFLPs, SSRs from pear, apple and Prunus, isozymes and phenotypic traits by using their F(1) progenies. The map of the female parent Bartlett consisted of 226 loci including 175 AFLPs, 49 SSRs, one isozyme and one S locus on 18 linkage groups over a total length of 949 cM, while that for 'Housui' contained 154 loci including 106 AFLPs, 42 SSRs, two phenotypic traits and the other four markers on 17 linkage groups encompassing a genetic distance of 926 cM. These maps were partially aligned using 20 codominant markers which showed segregating alleles in both parents. Compared with the reports of apple genetic maps, these pear maps were not saturated but were near saturation. Distorted segregation was observed in two and one regions of the genome of Bartlett and Housui, respectively. The position of 14 SSRs originating from apple could be successfully determined in pear maps, which enabled us to compare the two maps. Some SSRs developed from Prunus (peach, cherry) were also mapped. The relationships between pear and the other species belonging to the Rosaceae were discussed based on the position of SSRs.