Bisex Sterility Governed by a Single Recessive Gene in Cucurbita pepo L.

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Cucurbit Genetics Cooperative Report 20:46-47 (article 20) 1997

R. Bruce Carle
University of Florida, Central Florida Research & Education Center, Leesburg, FL 34748-8232

Bisex genetic sterility has been reported for both Cucurbita pepo L. (1) and C. maxima Dutch. (2). In C. pepo male flowers had rudimentary anthers containing no pollen and female flowers appeared normal but aborted a week after anthesis regardless of sib or cross pollination. In C. maxima, male flowers had stiff short petals and rudimentary anthers containing no pollen; the female flowers appeared normal but set fruit parthenocarpically by either controlled or natural pollination. In both C. pepo and C. maxima segregation ratios supported single recessive gene models for bisex sterility, designated s in C. maxima. Unfortunately genetic stock for either of these sterilities is no longer available.

During the Fall 1996 pollination season, male sterile plants were observed in a F8 C. pepo lines, YSN531PMR. The androecium of the male flowers failed to develop, appearing as a small green nub at anthesis. Five out of 15 plants were male sterile, suggesting a single gene heterozygote had been selfed in the previous F7 generation. Self pollinations were obtained on 9 of the fertile plazNts, and sibling pollinations (sterile x fertile) were made on all 5 male sterile plants. The fertile plants produced fruit and seed as expected. The sterile plants, however, set fruit parthenocarpically with no seed found in sibling or naturally pollinated fruit.

Thirty seed each of the original F8 line and the 9 F9 fertile selfs were sown in the Spring of 1997. The resulting plants were classified at peak bloom based on male flower development. Sterile plants were recovered in 6 of the 9 F9 lines, indicating that 2/3 of the 1996 normal F8 plants were heterozygous as would be expected if the F7 progenitor was a single gene heterozygote (25 plants, P.99 for correct classification of single recessive gene). Using 3:1 expected ratio, chi-square analyses were performed on the pooled (1996-97) F8 segregation data and the 6 segregating F9 lines. All populations fit a single gene recessive model (Table 1).

Phenotypically, sterility in YSN531-PMR resembles the bisex sterilities cited by previous investigators; the androecium is rudimentary, female flowers appear normal, but no viable seed are obtained by controlled or natural pollination. In the absence of available stocks to test allelism, assignment of gene symbol s-2 is appropriate. The association of parthenocarpy and sterility in YSN531-PMR has yet to be determined. Parthenocarpy has been observed in other fertile UF/CFREC breeding lines; it is likely happenstance in YSN531-PMR rather than a pleiotropic effect of sterility. Stock of YSN531-PMR segregation for sterility will be increased and made available to interested investigators as well as to the gene curators.

Table 1. Sterile segregation in an F8 Cucurbita pepo line, YSN531-PMR, and in the F9 lines derived from fertile F8 plants.

Line

Phenotype

Frequency

1:3
Sterile
Fertile
Sterile
X2 z
P
F8y 9 27 0.25 0.00 >.99
F9#1 4 20 0.17 0.89 .50-.25
F9#2 10 19 0.34 1.39 .25-.10
F9 #3 4 21 0.16 1.08 .50-.25
F9#4 8 19 0.30 0.31 .75-.50
F9 #5 5 22 0.19 0.60 .50-.25
F9#6 9 20 0.31 0.56 .50-.25
F9 Combined 40 121 0.25 0.00 >.99
Homogeneity 4.83 .50-.25

z X2df: individual; 1:F9 combined, 1: F9 homogentity, 5.
y F8 data combined from 1996 (5 sterile, 10 fertile) and 1997 (4 sterile 17 fertile) plantings.

Literature Cited

  1. Curtis, L.C. and J. Scarchuk. 1948. A bisex variant in Cucurbita pepo. J. Hered. 39:32.
  2. Hutchins, A.E. 1944. A male and female sterile variant in squash, Cucurbita maxima Duch. Proc. Amer. Soc. Hort. Sci. 44:494-496.