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Sex expression in muskmelon (Cucumis melo) is controlled by two independent genes: a and g (Robinson et al, 1976; Mohr 1986). Andromonoecious is represented by a; g denotes gynomonoecy. Homozygous dominant genotypes, GG, reduce the frequency of perfect flowers. The a locus is similar to the m locus in cucumbers; it interacts with the g locus. The homozygous recessive genotype, aa, favors male expression: Genotype A -GaaG- A -gg aagg Plant Phenotype Monoecious Andromonoecious Gynomonoecious Hermaphroditic Evidence suggests that several other loci may interact with g and a to modify sex expression, but the details have not been determined.

Since Darwin's discovery of homostylous plants, species with monomorphic flowers have been found that have related, distylous taxa. Heterostyly is not correlated with habitat, since heterostylous plants occur from xeric to aquatic conditions. However, it is frequently correlated with perenniality; the only annual heterostyled species occur in the genera Amsinckia and Fagopyrum (Ganders 1979). In Lythrum, where all species possess some form of heterostyly, they are also all perennial. In Lithospermum and Cryptantha, where both annual and perennial species occur, only the perennials are heterostylous.

The female parent is required to be Srr. These principles have been used to develop other male-sterile lines (Fig. 9) whereas the male parents should have the genetic constitution ofFRR, FRr, or Frr. In horticultural crops such as onion, where the harvested portion is vegetative, segregation for male fertility among the progeny is not important (Fig. 9). Examples of schemes for developing single- and double-cross, and threeway hybrid seeds using cytoplasmic/nuclear male sterility can be seen in Fig.