Inheritance of Sex Expression In Melon (Cucumis melo L.) and Molecular Mapping of Andromonoecy Using Microsatellite and AFLP Markers

At present melon (Cucumis melo L.) used in commercial production in China is predominantly andromonoecious. Development of monoecious melons is an important goal of many breeding programs to reduce hand pollination and improve fruiting in F₁ hybrids. Although it is well known that sex determination in melon is governed by two genes (the andromonoecious gene a and gynoecious gene g) and their interplay, environmental conditions also play important roles in melon sex expression. In the present study, we investigated sex expression in melon using plants of six generation (P₁, P₂, F₁, F₂, BC₁P₁ and BC₁P₂) derived from each of three mating schemes: 3-2-2 (monoecious, AAGG) × Top Mark (andromonoecious, aaGG), WI 998 (gynoecious, AAgg) × 3-2-2, and WI 998 × Top Mark. Segregation of sex types in all populations fitted the two-gene model except in those populations (F₂ and BC₁) involved in WI 998, in which in addition to parental sex types, significant proposrtions of hermaphroditic, gynomonoecious and trimonoecious flowers were also observed among the progeny suggesting effects of environmental or additional genetic factors on melon sex expression. To identify molecular markers for sex expression related traits in melon, we developed a linkage map using 152 F₆ recombinant inbred lines (RILs) derived from 3-2-2 × Top Mark. Among the 152 RILs, 72 were monoecious and 80 andromonoecious fitting the expected 1:1 segregation based on the two-gene model for sex expression in melon. Of a total of 428 SSRs and 256 AFLPs screened, 70 SSR and 100 AFLP markers were eventually mapped. The resulting genetic map consisted of 17 linkage groups with a total genetic distance of 1,223 cM and an average marker interval of 7.2 cM. Ten markers were identified which were linkage with the andromonoecy locus (a gene). The closest flanking markers were the SSR marker MU13328-3 and the AFLP marker e33m43-1 which were 4.8 cM and 6.0 cM away from the a locus, respectively.