Maize mutants and variants altering developmental time and their heterochronic interactions.

It is useful to envision two fundamentally different ways by which the timing of plant development is regulated: developmental stage-transition mechanisms and time-to-flowering mechanisms. The existence of both mechanisms is indicated by the behavior of various mutants. Shoot stage transitions are defined by dominant mutants representing at least four different genes; each mutant retards transitions from juvenile shoot stages to more adult shoot stages. In addition, dominant leaf stage-transition mutants in at least seven different genes have similar phenotypes, but the leaf rather than the shoot is the focus (and at least two of these genes encode homeodomain proteins.) One mutant, Hairy sheath frayed 1-O (Hsf1-O) simultaneously affects shoot and leaf; this mutant's behavior initiated our interest in plant heterochronism. The second type of timekeeping involves time-to-flowering. As with most plant but not animal species, cultivars of the maize species vary greatly for the time-to-flowering quantitative trait: between 6 and 14 weeks is common. It is via the 'slipping time frames' interaction that takes place between stage-transition mutants and time-to-flowering genetic backgrounds that unexpected and radical phenotypes occur. We see a reservoir of previously unsuspected morphological possibilities among the few heterochronic genotypes we have constructed, possibilities that may mimic the sort of variation needed to fuel macroevolution without having to posit (as done by Goldschmidt) any special macromutational mechanisms.