Homeosis, heterochrony, and the evolution of floral form

Homeosis and heterochrony have both been cited as important mechanisms for morphological change during evolution. Homeosis has been reported as the one-for-one replacement of organs, leading to morphological novelty by moving organs to positions where they normally do not occur. Heterochrony is a change in ontogenetic timing during evolution, causing new forms to arise in a descendant. This dissertation reports results of two comparative developmental studies conducted to evaluate the suitability of homeosis and heterochrony in explaining patterns of variation in floral form.; Pistillata is a homeotic floral mutant of Arabidopsis thaliana reported to have sepals replacing petals due to a single gene mutation. Developmental study using light and electron microscopy showed that sepal-like organs in the position of petals are not one-for-one organ replacements. Petals are initiated and then switched into the ontogenetic pathway characteristic of sepals, leading to hybrid sepal/petal organ forms. Temporal patterns of organ determination in plants imply that a change in timing, or heterochrony, may underlie this homeotic phenotype.; Populations of Arenaria uniflora represent a potential case of heterochronic evolution in floral morphology associated with the evolution of self-pollination. Selfing races consist of smaller plants bearing smaller flowers. These flowers typically self-pollinate due to a loss of protandry characterizing outcrossing populations. A growth study was combined with a study of flower development to determine if changes in floral form associated with a shift to autogamy could be attributed to heterochrony.; Growth data on inflorescences from four races showed that plants from two selfing populations grew slower than those from two outcrossing sites. Selfing races also had longer flower plastochrons. The growth study allowed rates of floral development to be inferred by generalizing the method of the plastochron index for use on heteroblastic shoots.; Flower development was studied in one outcrossing/selfing racial pair. Most floral organs were smaller in the selfer. Inter-racial differences in sepal and stamen filament lengths were due to differences in cell length; differences in petal size were due to differences in cell number.; Flower primordia at sepal initiation had the same form and size. Petal primordia were smaller at initiation in the selfer. Differences in anther length were not detectable until meiosis, but selfing anthers were morphologically precocious at earlier stages. Stamen meiosis occurred approximately twelve hours earlier in the selfer. Gynoecial ontogeny was similar in both races, except for early stigma receptivity in the selfer, which caused selfing to occur. The slower rate of growth in the selfer and equal times of flower development in both races make the selfing flower neotenous.