Genetic Linkage Map of the Japanese Pear ‘Housui’ Identifying Three Homozygous Genomic Regions

A genetic linkage map of the Japanese pear (Pyrus pyrifolia Nakai) ‘Housui’ was constructed based on simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers. A mapping population derived from a cross between ‘Bartlett’ and ‘Housui’ was used to create the linkage map of ‘Housui’. Segregation data on 63 progeny were scored using a pseudo-testcross strategy. The linkage map of ‘Housui’ consisted of 17 linkage groups (LGs) spanned 1,174 cM with an average distance of 3.5 cM between markers. A total of 335 loci (105 SSRs, 224 AFLPs, and 6 others) were used to construct the genetic linkage map. We successfully aligned all LGs to European pear and apple reference maps with common SSR markers. Compared to the reference map of ‘Bartlett’, constructed using same mapping population, LG 4 and LG 5 of ‘Housui’ were small and covered 9.7 cM and 23.2 cM, respectively. Furthermore, LG 12 of ‘Housui’ was established with only one SSR marker. These results indicated that three particular genomic regions (LG 4, LG 5, and LG 12) of ‘Housui’ were homozygous because a pseudo-testcross strategy was applied to construct the linkage maps. Genotyping of parental and ancestral cultivars of ‘Housui’ demonstrated that homozygous regions were increased on particular LGs of Japanese pear, maybe due to biased crossing and selection of seedlings during the Japanese pear breeding program.