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Susan Mazer >
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The Evolutionary Significance of Variation in Traits Subject to Ontogenetic Change and Maternal Environmental Effects
Faculty Research: 3) The Evolutionary Significance of Variation in Traits Subject to Ontogenetic Change and Maternal Environmental EffectsMy interest in ontogenetic sources of variation derives from the observation that traits expressed by modular organisms can present special difficulties when estimating the genetic component of their variation. Simply put, if the phenotype of sequentially-produced organs (e.g., leaves, flowers, seeds, fruits) changes as an individual ages, then ontogenetic and genetic sources of phenotypic variation can be badly confounded. For many years, I have been concerned with detecting the magnitude of these influences in wild plant species (Mazer, Snow, and Stanton, 1986; Mazer, 1987a,b; Mazer et al., 1989; Byrne and Mazer, 1990; Mazer and Schick, 1991a,b; Mazer, 1992; Mazer and Wolfe, 1992; Mazer and Delesalle, 1996a,b,c; Mazer and Wolfe, 1998). It is now well-known that both ontogenetic and maternal environmental effects contribute strongly to variation in fitness-related traits. The strength and ubiquity of these sources of variation are important to evolutionary biologists because they can mask the expression of genetic variation in such traits. If so, ontogenetic and maternal effects can constrain the rate of evolutionary change and contribute to the maintenance of genetic variation in traits under strong selection. Until recently, ontogenetic and maternal environmental variation were considered to be "nuisances" to the study of natural selection because they often obscure additive genetic variation in fitness-related traits, thereby retarding or constraining the rate or direction of evolution. Over the last few years, however, ontogenetic and maternal environmental sources of variation have been recognized as traits that are themselves potentially the targets of natural selection. For example, within natural populations, some genotypes may express ontogenetic variation in sequentially produced organs while others do not. Or, for some genotypes, the maternal environment may have a strong affect on progeny phenotype, while for others the maternal environment may have little affect. Four recent publications on unrelated taxa provide evidence that justifies this perspective (Mazer and Delesalle, 1996a,b,c; Mazer and Wolfe, 1998). I am now extending this research in two projects to ask whether natural selection itself may operate to influence the evolution and magnitude of ontogenetic and maternal environmental effects on fitness-related traits. Project I: Ontogenetic Variation in Floral Traits in Clarkia unguiculata and C. exilis First, does ontogenetic variation in floral traits mask underlying genetic variation, thereby diminishing the expression of heritable variation and constraining the efficacy of natural selection on these traits? We have found that, while there is significant ontogenetic variation in all of the traits we've examined (floral display area and the numbers of petals, ovules, and anthers per flower), we are nevertheless able to detect significant genetic variation in all of these traits. The low degree of canalization of floral traits does not preclude these traits from appearing to be heritable when averaged over each genetic lineage. Second, is there evidence that the expression of ontogenetic change itself can evolve? Here, we view ontogenetic change as analogous to phenotypic plasticity. Unlike phenotypic plasticity, however, in which the phenotypic expression of a trait depends upon the environment in which the plant is grown (i.e., an "environment-specific phenotype"), ontogenetic sources of variation involve "age-specific phenotype". To date, I have detected evidence for variation among genotypes in the degree of ontogenetic variation in a variety of floral traits. This work indicates that natural selection (or random genetic drift) may operate to mold the expression of ontogenetic variation, as assumed by one component of our NSF-funded work. This work indicates that natural selection (or random genetic drift) may operate to mold the expression of ontogenetic variation. Project II: Ontogenetic Variation in Gender-Related Traits in Clarkia unguiculata and Clarkia exilis (Onagraceae): does ontogenetic change mirror size-related change among genotypes? Recent theoretical models of sex allocation have predicted that individual plants or flowers that acquire high levels of limiting resources should exhibit higher proportional investment in traits related to female function (e.g., egg or seed production) than those of low resource-status. I have recently completed two experiments in the outcrossing C. unguiculata and the autogamous C. exilis to determine whether size-dependent changes in sex allocation among flowers within a plant and among individuals conform to the predictions of these models. Previous studies have been limited in two ways. First, they have examined, in a given population, either variation in gender expression among flowers within plants or phenotypic correlations (those observed in a heterogeneous environment) among plants between size and gender expression, but not both. Second, phenotypic correlations measured in heterogeneous environments are problematic because they confound potential environmental and genetic causes of covariation between size and gender. If we want to know whether a particular size-dependent change in gender has evolved, we need to seek direct evidence for a genetic basis to it. Consequently, I designed an experiment to detect, in a single population, both temporal changes in sex allocation among flowers within individuals (as they grow) and correlations among maternal families between size and sex allocation (Mazer and Dawson, in press). We found that, as plants grow, flowers produced later on tend to be more male-biased than flowers produced relatively early in an individual's reproductive phase. As maternal resources decline with plant age, flowers invest less in female function while maintaining relatively constant levels of pollen production. By contrast, plants representing large and vigorous maternal families produce flowers that do not differ in gender expression from those produced by small and less vigorous maternal families. Ontogenetic changes in sex allocation are much greater than size-related changes in sex allocation in C. unguiculata. Current Research Projects
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| Department of Ecology, Evolution and Marine Biology • University of California, Santa Barbara |
