Genetic Analysis of Melon Sex Expression

Melon (Cucumis melo L. 2n=2x=24) is an outcrossing horticultural crop belonging to subsp.melo in the Cucurbitaceae family. In melon sex expression is typically andromonoecious, but can also be monoecious, andromonoecious, hermaphrodite, gynomonoecious, trimonoecious, gynoecious and androecious. Since monoecious is the goal of our melon breeding program, we investigated the inheritance of monoecious sex expression utilizing molecular markers  linked to the monoecious trait. Parental lines were: 3-2-2 from China, thin-skinned melon, monoecious; Top Mark, from USA, thick-skinned and netted melon, andromonoecious; and WI998, from USA, netted melon, gynoecious. Sex expression of the F₁, F_2, F₆, BC₁P₁ and BC₁P₂ generations from the crosses 3-2-2 × Top Mark, WI998 × Top Mark, WI998 × 3-2-2, respectively, were evaluated. Using 350 individuals from the F₁ generation from the 3-2-2 × Top Mark cross, we acquired 152 S₂F₆ plants  to construct genetic maps. Sex expression was controlled by two main genes. Main-gene heritability was 83.3% and minor-gene heritability was 8.125%. Segregations of every generation were investigated and confirmed the genotype of 3-2-2（monoecious）as AAGG, the genotype of WI998 （gynoecious） was AAgg, and the genotype of Top Mark （andomonoecious）was aaGG. Using RILs of the 3-2-2 × Top Mark cross to construct a genetic map, a total of 70 SSR markers, 100 AFLP markers and one agronomic trait were grouped into 17 large linkage groups which spanned 1222.9cM with an average marker interval of 7.19 cM. The longest linkage group was LG1（spanning 197.4 cM）and the shortest group was LG3（spanning 11.0 cM）. Ten markers in the same linkage group with the A gene and the group spanning 55.7 cM, SSR marker MU13328-3 and AFLP marker e33m43-1 were linkage to A gene with 4.8 cM and 6.0 cM, respectively.