" . . . . . . Nature has something more in view than that its own proper
males should fecundate each blossom. " Andrew Knight Philosophical
Transactions, 1799 Sterility implicating the male sex solely presents a
paradoxical situation in which universality and uniqueness are
harmoniously blended. It maintains a built-in outbreeding system but is
not an isolating mechanism, as male steriles, the "self-emasculated"
plants, outcross with their male fertile sibs normally. Both genes
(nuclear and cytoplasmic) and environment, individually as well as
conjointly, induce male sterility, the former being genetic and the
latter nongenetic. Genetic male sterility is controlled either
exclusively by nuclear genes (ms) or by the complementary action of
nuclear (lr) and cytoplasmic (c) genes. The former is termed genic and
the latter gene-cytoplasmic male sterility. Whereas genic male sterility
exhibits Mendelian inheritance, gene-cytoplasmic male sterility is
non-Mendelian, with specific transmissibility of the maternal cytoplasm
type. Genetic male sterility is documented in 617 species and species
crosses com- prising 320 species, 162 genera and 43 families. Of these,
genic male sterility occurs in 216 species and 17 species crosses and
gene-cytoplasmic male sterility in 16 species and 271 species crosses.
The Predominance of species exhibiting genic male sterility and of
species crosses exhibiting gene-cytoplasmic male sterility is due to the
fact that for the male sterility expression in the former, mutation of
nuclear genes is required, but in the latter, mutations of both nuclear
and cytoplasmic genes are necessary.