Both cross-pollinating and self-pollinating sub-species of Clarkia xantiana (Gray) grow in the southern Sierra Nevada mountains of California. Selfing plants (ssp. parviflora) have evolved from crossers (ssp. xantiana) in hotter, more arid regions of the species’ distribution. Crossing and selfing plants are most easily distinguished by differences in flower morphology, time to flower opening, and plant size. Selfers tend to be smaller plants that flower earlier and produce smaller flowers with little or no protandry. The developmental basis of early flowering in selfers was investigated in a growth study in which leaves, internodes and flowers were measured daily. Hypothetically, early flowering can be accomplished in different ways, i) flowers can form at more basal nodes or, ii) flowers form at equivalent nodal positions but after a shorter duration of vegetative growth or with relatively rapid development of floral organs. Selfers of C. xantiana adopt a combination of these strategies. In particular, they flowered 2.6 nodes earlier than crossers and had shorter duration of leaf and internode growth which resulted in smaller plants that produced flowering nodes earlier. In an associated anatomical investigation of flower development, rates of ovary, petal, stamen, and pistil growth were determined. Allometric differences were found when ovary growth was compared in the two sub-species. Ovary elongation and ovule development proceed faster relative to organs of the corolla in selfing flowers. Taken together, the earlier flowering node and the relatively rapid maturation of ovaries in the selfing plants indicate that the evolution of self-pollination in C. xantiana has been progenetic, i.e. the rate of sexual development has been accelerated.

Key words: Clarkia xantiana, heterochrony, Onagraceae, self-pollination