A Comparative Developmental Study of the Selfing and Outcrossing Flowers ofClarkia tembloriensis(Onagraceae)

Abstract

Clarkia tembloriensis (Onagraceae) contains both highly outcrossing and highly self‐pollinating populations. Except for the ovary, flowers on plants from self‐pollinating populations typically have shorter floral organs and have anthers that produce less pollen. A comparative study of flower development, including organogenesis, histogenesis, organ growth, and allometry, was undertaken to determine the stage in development at which differences in floral form arise. Growth rates of flower buds from 3.0 mm in length to maturity are the same in the two flower types. Earlier stages were not accessible for direct measures of growth. Growth rates of ovaries from 1.0 mm in length to anthesis are the same for both flower types. Since ovaries achieve the same final size in both flower types at anthesis, the ovary was used as an index organ by which to measure the growth of other organs in the flower from a bud size of 3.0 mm. The size of sepals, petals, long anthers, and styles in selfing flowers are smaller than in the outcrossing flowers of the same ovary length as measured using dissected buds, scanning electron micrographs, and histological sections. Flower primordia are initially the same size in the two floral types, so divergence between them can be documented in a developmental study. The earliest detected differences in organ size and form between flower types occurs after all the organs are initiated in a bud ca. 1 mm long with an ovary length of 0.3 mm. Since the allometric relationship of the organs to the ovary is the same in both flower types in buds >3 mm, the mature differences must arise during this developmental window after organ initiation and just before organ differentiation. Significant differences in pollen production in the two flower types can be attributed to formation of fewer sporogenous cells in the self‐pollinating flower.