ULTRAVIOLET FLORAL PATTERNING, REPRODUCTIVE ISOLATION AND CHARACTER DISPLACEMENT IN THE GENUS RUDBECKIA (COMPOSITAE)

KENNETH DUNCAN MCCREA, Purdue University

Abstract

The purpose of this investigation was to determine whether differences in ultraviolet floral patterning among species of Rudbeckia act as a mechanism to strengthen reproductive isolation in sympatry. To indirectly demonstrate the importance of two aspects of ultraviolet floral pattern geometry, ray length and the percent of the ray length that reflects ultraviolet light (%UVRL), data were collected to show (1) that conditions exist which would promote sympatric divergence, i.e., displacement, in these characters, and (2) that such displacement has occurred. Selection for displacement requires a cost for interspecific pollination. Analysis of breeding systems indicated that although two of six species are apomictic, all either require or benefit from conspecific pollination. Hand crosses indicated that each species experiences a significant loss of ovules (averaging 14 to 74 percent) due to hybridization and/or parthenocarpy caused by interspecific pollinations. This indicates that generally there may be strong selection for displacement and especially for R. hirta which suffers the largest potential loss with interspecific pollination. The rate of change in a character in response to selection is proportional to the heritability of the character. The narrow sense heritabilities of ray length and %UVRL in R. hirta were estimated using the parent-offspring regression method. The heritability estimates were 0.20 for ray length and 0.55 for %UVRL, which could allow for a rapid response to strong selection. For character displacement to occur in sympatry, there must be sufficient heritable variation on which selection can act. Analysis of a total of 238 populations of six Rudbeckia species, whose geographic ranges overlap in the eastern United States, has shown there is considerable variation in ray length and %UVRL both within and among populations. Significant differences in means were found between some of the species, but there was considerable overlap in the variation among all of the species. In direct comparisons of allopatric and sympatric populations of each species, the only significant differences observed was for mean %UVRL in R. hirta populations. However, multiple linear regression analysis revealed significant latitudinal clines for %UVRL in R. hirta and ray length in R. fulgida tetraploid. After correction for clinal variation, significant differences between allopatric and sympatric population means for these features were detected in both cases. In most sympatric associations studied, the species differed more than would average populations of the same species. To test the null hypothesis that no displacement had occurred, all possible interspecific pairwise comparisons were made between allopatric population means and then compared with the differences in sympatry. The differences in %UVRL between R. hirta and R. fulgida tetraploid and between R. hirta and R. triloba diploid were found to be significantly larger than those predicted by random founder events, indicating that character displacement has occurred. Theoretical analysis of the process of character displacement indicated that selection will act to increase pre-existing differences between species but will not cause divergence in the absence of such differences. The evidence for ultraviolet pattern displacement in Rudbeckia conforms to this hypothesis since in all cases divergence occurred in the direction of pre-existing species differences. For ultraviolet floral pattern displacement to have occurred in Rudbeckia, pollinators must discriminate between species based on differences in ray length and %UVRL. Although such discrimination was not demonstrated directly, it can be inferred from the existence of displacement. This study establishes, for the first time, that differences in the geometry of ultraviolet floral patterns act as a reproductive isolating mechanism between species.

Degree

Ph.D.

Subject Area

Ecology

COinS