THE CONTROL OF PINNA MORPHOLOGY IN WILDTYPE AND MUTANT LEAVES OF THE GARDEN PEA (PISUM-SATIVUM L)

The compound leaf of the wildtype pea exhibits basal stipules, proxima l leaflets, and distal tendrils, which are profoundly modified by seve ral recessive genes thought to act as homeotic mutations. The acacia ( tl) gene appears to replace simple tendrils with leaflets in the dista l region, while the afila (af) gene appears to replace the leaflets wi th branched tendrils in the proximal region. Morphological analyses of all homozygous and heterozygous combinations of this genetic system i ndicate that these genes affect many other features of leaf morphology , including leaf length, petiole length, pinna number, and branching o rder. Moreover, these genes are apparently expressed within the contex t of preexisting proximal and distal regions in the developing leaf. P inna identity on the leaves from different genotypes is therefore regu lated by the interaction among relative pinna position, morphological region, and genetic background. Current models of pea leaf development fail to account for several characteristics of pea leaves, such as (1 ) the leaf phenotype of the double recessive mutant called pleiofila, which consists of novel compound pinnae with very small terminal leafl ets, (2) mixed pairs of proximal and distal pinnae, and (3) intermedia te narrow leaflets/broad tendrils in certain heterozygotes. Starting w ith the assumption that the pleiofila leaf represents the ground state of the pea leaf, the available evidence indicates that the dominant A f and TL genes are acting as heterochronic mutations that determine th e timing of lamina formation and axis termination, with the result tha t these genes cause the observed morphological changes in pinna identi ty.