Developmental and morphological analyses of homeotic cytoplasmic male sterile and fertile carrot flowers

Abstract

Floral development and morphology were observed for two homeotic cytoplasmic male sterile carrot lines and their isonuclear fertile maintainers. For one sterile line, W33A stamens are replaced by petal-like organs; for the other, W259A, both stamens and petals are replaced by green bract-like structures. Both isonuclear maintainers, W33B and W259B respectively, have stamens and white petals. The different sterile phenotypes result from the interactions of distinct nuclear genotypes with one sterility-inducing cytoplasm. Early stages of floral development were similar among all four lines; the third whorl primordia were radial while those in the second whorl were dorsiventral. However, the third whorl primordia were splayed outward in the sterile lines and inward in the fertile lines. Subsequently, radial anthers on filaments differentiated in fertile lines and dorsiventral hastate and cordate shaped structures appeared in W33A and W259A, respectively. In the mature flower, the third whorl organs were cordate in W33A and ovate in W259A. Based on epidermal cell morphology, the second whorl organs of the two sterile lines had characteristics of both petals and bracts, but of opposite degrees; cells of W33A and W259A were most similar to those of petals and bracts, respectively. The third whorl organs of the sterile lines had characteristics of their respective second whorl organs; however, structures of W33A also had filament-like cells and those of W259A were more bract-like than their respective second whorl organs. The cytoplasm affected when homeosis was manifested during development. Nuclear factors interacting with cytoplasm were most important for determining differentiation. The significance of cytoplasm to current models of nuclear-gene-controlled homeosis is discussed.