CHARACTERIZATION OF DEVELOPMENT IN MAIZE THROUGH THE USE OF MUTANTS. I. THE POLYTYPIC (Pt) AND RAMOSA‐1 (ra 1 ) MUTANTS

The origin of morphological differences attributable to mutant genes c2.n be identified at certain switch points in the developmental pathway. The effect of the Polytypic (Pt) gene is a stimulus to meristems in the developing inflorescence to continuous initiation of differentiating structures. The most severe expression is a suppression of meristematic activity. The action of the gene is a superimposition of its effect on the normal developmental pathway. The ramosa-1 (rat) gene interrupts the normal sequence of events, at the switch point, which would normally result in spikelet formation, and there is produced instead a lateral branch. The response of numerous switch points in the developmental pathway of the maize inflorescence supports the conclusion that meristems are a plastic system genetically programmed at successive intervals. THE GENESIS of form is one of the foremost problems in biology, but its solution is evasive. It is necessary to study development at all levels to elucidate the factors underlying morphogenesis. Investigation at the organismal level is a challenging and necessary aspect of the study. An organism matures through a series of sequential steps ordered by the genetic complement. An interruption of these events resulting in a change in the mature form permits investigation of the effect of specific factors. The succession of stages in development can be diverted by hormones, surgery, control of nutrition, and by light and temperature. Each of these has provided information useful in understanding morphogenesis. The approach that is the basis for our study is to investigate divergent development resulting from the substitution of a mutant gene for its normal counterpart. This enables one to study plants which vary from the control in a minimal way. The effect of the gene should be traceable to a single biochemical lesion, and thus the investigation is not confused by treatments with chemicals and other devices. Studies of mutant lines can indicate the points in development at which normal alleles at certain loci supply additional information leading to normal development. Our studies with mutants of maize affecting ear and tassel development lead to the conclusion that these "switch points" are numerous. There is no point at which a meristem can be considered to be irrevocably determined for a particular course of development. The meristem is, rather, a plastic system which must be programmed at successive intervals. 1 Received for publication April 9, 1963. This work was supported in part by the National Science Foundation (Grant G-16036). The authors gratefully acknowledge the assistance of Evart Cable in the photography and microtechnique. Journal Paper No. 2085 of the Purdue University Agricultural Experiment Station. Zea mays L. provides material for this kind of investigation. Tassel and ear mutants are abundant, and many have been carefully studied genetically. The sequence of events which gives rise to the normal ear of corn is relatively uniform and predictable. The development of the normal ear and tassel has been described by numerous investigators (Weatherwax, 1917, 1935; Collins, 1919; Arber, 1934; Bonnett, 1940; Anderson, 1944; Mangelsdorf, 1945; Cutler and Cutler, 1948; Reeves, 1946, 1950; Kiesselbach, 1949; Nickerson, 1954; Heslop-Harrison, 1961). One can trace the developmental pathways in the maize inflorescence by noting changes in meristematic activity beginning with the vegetative meristem and ending with the initiation of the ovule. Potential switch points at which diversion from the normal pathway may occur are illustrated in Fig. 1. The vegetative meristem gives rise to a leaf primordium by forming a subterminal ridge of meristematic tissue perpendicular to the longitudinal axis (Fig. 1A). The switch to an ear meristem involves a change in size and shape of the axis meristem and the initiation of longitudinal rows of branch primordia in an acropetal sequence (Fig. IB, 7). The branch meristem produces 2 spikelet primordia (Fig. IC, 7), each giving rise to 2 glumes ID, 7). Upper and lower floret meristems are then derived from the spikelet meristem (Fig. 1E). The initiation of the palea, lemma, lodicules and stamens (Fig. IF, G) by the floret meristems varies in extent and sequence. The floret meristem may then give rise to the pistil primordium (Fig. 1H), and, finally, to the ovule primordium (Fig. 1I). The ultimate nature of the structure produced by this series of events is dependent on the suppression, stimulation, repetition and order of occurrence of each step. Heslop-Harrison (1961) showed that photoperiod, temperature, cr auxin will affect a switch in the developmental pathway