Cucurbit Genetics Cooperative Report 10:66-68 (article 35) 1987
Outcrossing in Watermelons
B.B. Rhodes
Edisto Research and Education Center, Clemson University Blackville, SC
29817
W.C. Adamson
USDA Southern Regional Plant Introduction Station Experiment, GA 30212
W.C. Bridges
Department of Experimental Statistics Clemson University, Clemson, SC 29634-0367
The minimum distance to prevent pollen transfer between watermelon plants
is reported to be 0.25 mile (1). This distance is difficult to achieve when
a large number of isolations are required on a small research station. This
study attempted to estimate amount of pollen transfer by indigenous bees
at distances less than 0.25 mile.
Studies of natural pollen transfer among watermelon plants were made
in 1983, 1984 and 1985 at Blackville, SC and Experiment, GA. A dominant
gene, Spotted (Sp) (2) was used to measure how much pollen
was transferred and how far. From open pollinated fruit selected from specific
field sites, seed were removed for scoring the dominant seedling marker.
Usually more than 100 seedlings from each melon were scored at the third
or fourth true leaf stage.
We attempted to design studies to address the following questions: (1)
How far is pollen transferred? (2) What is the effect of interplanting muskmelons
between the pollen source and the female parent? The following studies were
conducted:
1983. At Blackville, 16-hill blocks of 13 watermelon varieties
were alternated with 32-hill blocks of muskmelons in four replicates. This
planting was actually a watermelon and muskmelon trial that lost a number
of seedlings in several blocks after a late freeze. Missing hills were replaced
by Sp seedlings. The location of each Sp plant was mapped.
A watermelon block was isolated on two sides by muskmelons and on two sides
by an unplanted strip 25 feet wide.
1984. At Experiment, eight plant introductions were replicated
four times in blocks of eight watermelon plants buffered on each side by
a 'Butternut' squash plant. Hills of watermelons were equidistant from a
row of Sp plants alternating with 'Butternut' squash plants. After
fruit set, the nearest distance from the melon to a pollen source was measured.
1985. Two studies were done - one at Blackville, SC and one at
Experiment, GA.
At Blackville, four hills of the Sp line were planted inside a
large block of 'Crimson Sweet', planted two feet-apart on the row, with
rows ten feet apart. The nearest distance from Sp hills to 'Crimson
Sweet' plants was 15 feet and the greatest distance was 125 feet. Seed were
saved from fruit from each quadrant: 21, 32, 19 and 25 for quadrants NW,
NE, SW and SE, respectively.
At Experiment, the study employed two blocks with 5 Sp plants
in the center and watermelon PI's planted at 12 and 48 feet from Sp in one block and at 12, 36 and 48 feet in the other block. Spaces between
blocks were 15 feet apart within the row and 12 feet apart between rows.
The same four PI's were planted at 12, 36 and 48 feet. There was only
one set of plants at 12 feet, four sets of the PI's at 36 feet and 8 sets
of the PI's at 48 feet.
To determine the effect of distance on per cent outcrossing, distances
were first bracketed into 12 groups and means from each of the four data
sets were considered replicates (Table 1). All four studies indicate that
outcrossing is rare beyond 90 feet. The presence of a nearby bee hive could
increase outcrossing.
To determine the effect of the muskmelon "barrier" in the 1985
Blackville field, outcrossing was compared within the block where the pollinator
was located, in another block of watermelons separated only by a 25 foot
unplanted area and in a watermelon block isolated from the pollinator by
a block of muskmelons (Table 2). It is interesting that outcrossing occurred
in only one case where watermelons were isolated by muskmelons. However,
outcrossing percentages of 0.72, 0.72 and 1.01 were found in blocks of watermelons
separated by two blocks of muskmelons. Thus, these small barriers are not
sufficient to prevent outcrossing.
Where only natural bees are present, outcrossing beyond 80 feet appears
to be rare. A final observation made with an entirely different genetic
system supports this idea. In 1986, a block of several hundred male sterile
watermelon plants were sited 450 feet from several hundred male fertile
watermelon plants. Adjacent rows of male fertile plants were removed after
the initial fruit set. Every fruit was then removed from male sterile plants.
The male fertile plants produced pollen for three more weeks but none of
the thousands of female blossoms on the male sterile plants were pollinated.
One- fourth mile (1320 feet) is a generous isolation and, in most cases,
half this distance is sufficient.
Table 1. Effect of distance from pollen source on per cent outcrossing
in watermelon.
z Per cent of seedlings showing dominant Sp trait. (B) is Blackville, South Carolina location and (E) is Experiment, Georgia location.
Table 2. Per cent outcrossing within blocks of watermelons and among
blocks isolated by unplanted blocks or muskmelon blocks.
z Four X 4-hill block of watermelons spaced 5 feet apart.
y Twenty-five feet wide.
x Eight X 4-hill block of muskmelons spaced 2.5 feet apart on the row with
5 feet between rows.
Literature Cited
- Lorenz, O.A. and D.N. Maynard. 1980. Knott's Handbook for Vegetable
Growers. 2nd Ed. p. 329.
- Rhodes, B.B. 1986. Genes affecting foliage color in watermelon. J.
of Heredity 77:134-135.
