Cucurbit Genetics Cooperative Report 1:10 (article 7) 1978
R. W. Robinson
New York Agricultural Experiment Station, Geneva, NY 14456
It has been proposed that a single recessive gene (m) conditions andromonoecious sex expression, and a different but very closely linked locus governs round versus elongated fruit shape in the cucumber. I have never found a single crossover type in F2 populations of several thousand plants, raising the question if there really are two independent, but completely linked loci for sex expression and fruit shape, or if the two traits are pleiotropic effects of the same gene.
Crossovers were observed between m and the linked gene (l) for fruit locule number, and it was evident that gene interaction influences fruit shape; m+ and +l plants always had fruit length intermediate to the elongated fruit of ++ plants and the round fruit of m l plants. Thus, l and other background genes modify fruit shape, but andromonoecious plants always had shorter fruit than their monoecious counterparts. Fruit shape of hermaphrodite plants was similar to that of andromonoecious plants, and gynoecious and monoecious plants were similar in fruit shape.
The monoecious cultivar ‘Wisconsin SMR 18’ occasionally produces perfect flowers. Typically one flower on a plant will be perfect, while all other flowers on that plant are pistillate or staminate. The incidence of perfect flowers is low, less than one for every thousand pistillate flowers. Significantly, fruit developing from a perfect flower invariably were round, whereas fruit developing from pistillate flowers on the same plant would be elongated. The perfect flowers had normal male and female fertility, and produced only monoecious progeny with elongated fruit when self pollinated. The simultaneous change in both sex expression and fruit shape, therefore, was due to a nongenetic accident in development.
Ethephon application to the andromonoecious cultivar ‘Lemon’ induced it to develop pistillate flowers. Where as the perfect flower of untreated plants developed into round fruit, the pistillate flowers of treated plants developed into elongated fruit. Since nongenetic events such as growth regulator application and developmental change simultaneously effects sex expression and fruit shape, and round shape is always associated with perfect flowers, both sex expression and fruit shape are likely affected by the same endogenous hormone. The m gene probably affects the same hormone, and therefore has pleiotropic effects on sex expression and fruit shape.