Watermelon
Crop Information
Genetics
- 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.
