Monoecious and dioecious species produce unisexual flowers in which either male or female reproductive organs
develop. These species have evolved a mechanism to selectively abort or arrest
one of the sexes, a phenomenon named sex determination. On pages 481-493 of this
issue, Kater et al. describe a set of floral homeotic mutants of the monoecious
species cucumber, which provides novel information about the spatial control of
sex determination. The homeotic mutants were generated by altering the expression
of MADS box transcription factors, which resulted in mutants resembling the ABC
classes of flower mutants. In male homeotic mutant flowers, carpels developed in
the first (instead of sepals) and third whorl (instead of stamens) but they never
developed in the restricted fourth whorl. Similarly, male tissue was observed in
the second whorl of female flowers but never in the restricted third whorl.
Interestingly, the outgrowth of nonreproductive organs is not prevented in these
restricted floral whorls, as was observed in a class C mutant (female flower
shown on the cover). The petals in the center of this female flower developed
from the normally restricted third whorl, suggesting that the sex determination
machinery is able to sense the reproductive identity of the developing organs in
whorls 3 and 4. The authors suggest that the class C homeotic gene function is
required for the position-dependent arrest of reproductive organs.
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