by Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609

Qualitative Traits

The inheritance of watermelon traits has been studied extensively, and single genes have been identified that are of value to plant breeding programs. Examples include A for monoecious vs. andromonoecious sex expression, Ar-1 and Ar-2 for resistance to anthracnose races 1 and 2, C for canary yellow flesh color, dw-1 and dw-2 for dwarf vines, E for non-explosive rind, F for non-furrowed fruit surface, Fo-1 for fusarium wilt resistance, gs for striped green rind pattern, Go for non-golden rind at maturity, M for non-mottled fruit skin, O for elongate rather than oval fruit shape, Pm for resistance to powdery mildew, s and l for short seeds, yo for orange flesh, and Y for red flesh (Table 3.2).

Non-lobed leaves is a mutant expressed beginning in the seedling stage that is controlled by a single recessive gene. The single-gene trait can be useful for indication of hybrid plants. Hybrid seeds can be produced on one inbred line used as the female parent and having non-lobed leaves. If it is pollinated using bee pollination in an isolation block, and the male parent has normal, lobed leaves, then it will be possible to distinguish hybrid from non-hybrid at the seedling stage in the commercial seed lot. The hybrid seeds can then be planted in excess in grower fields and the non-lobed seedlings (produced by self- or sib-pollination) can be removed to leave just hybrid plants. Alternatively, non-hybrid seedlings can be removed from the flats during transplant production.

Inbreeding Depression and Heterosis

Watermelon is monoecious, and is naturally cross-pollinated like maize. However, there is not as much inbreeding depression or heterosis as one might expect. This is similar to other cucurbits such as cucumber and melon. It has been suggested that the lack of inbreeding depression is due to the small population size used by farmers during the domestication of the species. Watermelon plants are large, so only a few plants probably were grown in each area. Therefore, even with monoecious sex expression and insect-pollinated flowers, there would have been considerable inbreeding among the few plants representing the population. Since there is little inbreeding depression in watermelon, inbred lines are developed using self-pollination with little loss of vigor from the parental population.

In studies of heterosis in watermelon, some estimates have shown a 10% advantage of the hybrid over the high parent, but only for some parental combinations. The small amount of heterosis observed in watermelon hybrids makes hybrids unnecessary for high yielding commercial varieties since inbreds should perform as well. However, hybrid varieties are useful for combining traits inherited in a dominant fashion from the two parents. Examples of such traits include red or canary yellow flesh, resistance to fusarium wilt and anthracnose, and lack of susceptibility to powdery mildew. Hybrids are necessary for protecting proprietary breeding lines from unauthorized use. However, one of the most important uses of hybrids is the production of seedless varieties. The primary method for production of seedless watermelons involves the cross of a tetraploid female parent with a diploid male parent to produce a triploid, which will be sterile, and therefore, seedless. Currently, triploid hybrids are the most practical method for production of seedless watermelons.