Developmental stability and floral ontogeny in self- and cross-pollinating populations of Clarkia tembloriensis (Onagraceae).

摘要

Four natural populations of Clarkia tembloriensis, whose levels of heterozygosity and rates of outcrossing were previously found to be correlated, are used to examine the relationship of developmental stability to breeding system and to elucidate developmental mechanisms for the evolution of self-pollinating floral forms. From the northern end of the species range, a predominantly selfing population (t = 0.26) is compared with a more outcrossed population (t = 0.84), which is genetically similar. From the southern end of the range, a highly selfing population (t = 0.03) is compared with a neighboring more outcrossed population (t = 0.58).; In Chapter I, developmental stability in leaves is estimated using two measures of within plant variation in leaf length as well as calculations of fluctuating asymmetry (FA) for several leaf traits. Plants from more homozygous populations tend to have greater within-plant variance in leaf length over developmentally comparable nodes than plants from more heterozygous populations. In addition, the decline in leaf length with plant age is less stable in plants from selfing populations than in plants from outcrossing populations.; FA is significantly greater in the southern selfing population than in the neighboring outcrossed population, but the same is not true for the northern populations which show lower FA overall. FA can vary significantly between nodes of the plant. The data suggest that large differences in breeding system are associated with differences in stability, with more inbred populations being the least stable.; In Chapter II, FA is examined in flowers collected from the same plants of the two northern populations used in Chapter I. Floral stability values are about half those of leaves, but no differences were found between selfing and outcrossing populations.; Flowers on plants from self-pollinating populations are typically smaller in the lengths of all organs except the ovary and produce less pollen. In Chapter III, a comparative study of flower development, including organogenesis, histology, organ growth rates, and allometric comparisons, is undertaken to determine the stage in development at which these differences arise. Comparative growth rates of entire flower buds are the same in the two flower types, but in selfing flowers, sepals grow slightly slower and ovaries grow slightly faster than in outcrossing flowers. Since the allometric growth rate of the organs relative to the ovary (the index organ) is the same in both flower types in later stages of development, the mature differences in size must arise early during organ differentiation. Flower primordia appear to be the same size at inception in the two flower types. Heterochrony is invoked as a mechanism for the evolution of the selfing flower type from the ancestral outcrossing form.