Term of Award

Fall 1999

Degree Name

Master of Science

Document Type and Release Option

Thesis (restricted to Georgia Southern)

Department

Department of Biology

Committee Chair

Lorne M. Wolfe

Committee Member 1

Donald J. Drapalik

Committee Member 2

Lissa M. Leege

Committee Member 3

Denson K. McLain

Abstract

A major focus of evolutionary biology is understanding the processes that maintain genetic variation in natural populations. Genetic variation is the foundation of evolutionary change, and investigations of polymorphic systems can provide excellent tools for understanding the evolutionary forces acting to maintain genetic variation. Linaria canadensis (L) Dumont is a weedy, winter annual plant (Scrophulariaceae) that grows in disturbed habitats (i.e., road-side ditches and agricultural fields) and exhibits a floral color polymorphism. The flowers of L. canadensis are most commonly found in pale blue and dark purple (hereafter referred to as the light and dark morphs respectively). A rare albino morph has also been reported, and in 1998 I discovered a pink morph in South Carolina. The overall goal of my research was to investigate the ecological and evolutionary dynamics of floral polymorphisms in L. canadensis in Florida, Georgia, and South Carolina. The objectives were: 1) to determine if there was variation in the frequency of the common light and dark morphs over temporal and spatial scales and 2) quantify factors which may influence the reproductive success and ultimately the frequency of the floral morphs.

Morph frequency was determined by comparing morph frequency from 161 populations sampled in Florida, Georgia, and South Carolina during the 1998 and 1999 flowering season. Although morph frequency did not vary significantly temporally, there was a marginally significant difference in spatial variation. Populations in South Carolina contained an almost equal light to dark morph ratio while there were few individuals of the pink morph. In Georgia, the light morph was most abundant and the pink morph increased in frequency. In Florida, the dark morph plants were most abundant, the light morph plants dropped to below a 10% frequency, and there was a dramatic increase in the frequency of the pink morph plants (25% of overall morph frequency).

Populations of this species vary dramatically in size: from three individuals to approximately 14 million individuals. Twenty nine of 96 small-sized populations censored were monomorphic (< 400 individuals), three of 29 medium (400-3,000 individuals), and three of 36 large-sized (> 3,000 individuals) monomorphic populations. The fact that small populations were more likely to be monomorphic may be the result of founder effect.

Significant variation in floral traits and rewards between the morphs were also found. Light flowers were larger, had longer nectar spurs, and produced a higher volume and concentration of nectar (% sucrose equivalents) than individuals of the dark morph. Individuals of the dark morph produced significantly more flowers and fruit than individuals of the light morph. However, there was not a significant difference in percent fruit set: approximately 90% of the flowers of plants of both morphs successfully produced fruit.

Pollinators were extremely rare in both years of this study. Only 10 individuals from two different orders were seen: Hymenoptera (Apidae, two species) and Lepidoptera (Hesperiidae and Picridae, one species each). A pollinator exclusion experiment revealed there was not a significant difference in percent fruit set between bagged and unbagged plants: over 90% of the flowers of both morphs set fruit regardless of whether they were bagged or unbagged. Therefore, the lack of variation in fruit set is an indication of high levels of autogamy in L. canadensis. Thus, there do not seem to be reproductive consequences to these variable floral traits in contemporary populations of L. canadensis. So, variation in morph frequency is probably better explained through stochastic process, such as genetic drift, founder effect, and bottle neck.

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