THE EXPRESSION OF ANDROMONOECY IN SOLANUM-HIRTUM (SOLANACEAE) - PHENOTYPIC PLASTICITY AND ONTOGENIC CONTINGENCY

Sex expression (the proportions of staminate and hermaphrodite flowers produced) in andromonoecious Solanum hirtum is phenotypically plastic , and there is genetic variation for sex expression plasticity. Change s in sex expression phenotype are inherently the result of altered dev elopment. However, the underlying developmental components of sex expr ession plasticity and of differences in plasticity among genotypes are unknown. This study takes an explicitly genetic and developmental app roach to the study of phenotypic plasticity and examines changes in se x expression of ten clonally replicated genotypes at three levels of o rganization: among inflorescences, within inflorescences, and at the l evel of developing floral meristems. Changes in sex expression of indi viduals and differences among individuals are the result of a predicta ble interplay of resource, architectural, and floral level response wi thin the hierarchical construction of the shoot system. Phenotypic pla sticity of whole plant sex expression is ultimately due to sexual labi lity of individual developing flowers: floral sex is not determined un til a primordium size of 9-10 mm. Until that time, sex expression rema ins labile and developing floral primordia can respond to changes in p lant resource status. This flower level developmental lability, howeve r, is expressed within the constraints set by the architecture and ont ogenetic history of the organism. Only those floral primordia produced in distal portions of each inflorescence are labile, capable of devel oping into either a staminate or hermaphrodite flower, whereas those p rimordia in basal positions invariably develop as hermaphrodite flower s. The genotypes differ with respect to the architectural components o f phenotypic plasticity and it is this architectural variation that re sults in differences in plasticity among genotypes. The phenomenon, in which the developmental fate of a primordium depends upon where and w hen it is produced within the architecture of an organism and what eve nts have preceded it during ontogeny, can be termed ''ontogenetic cont ingency.''