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Current Research: 1) Testing the assumptions of sex allocation theory: quantitative genetic variation and covariation among floral traits in the Sand-Spurrey, Spergularia marina (Caryophyllaceae)
I have been examining empirically two assumptions shared by all theoretical models of the evolution of sex allocation in hermaphroditic organisms. First is the relatively trivial assumption that gender expression (the relative allocation of biomass or some limiting resource to traits that reflect female vs. male function ) is heritable. That is, theoreticians assume that an individual's gender is reliably transmitted through the nuclear genome from parents to their offspring. Second, these models assume that there are genetic "trade-offs" - detectable as negative genetic covariances - between the absolute allocation of reproductive resources to male and female functions. In other words, controlling for differences among genotypes in resource acquisition, genotypes that allocate a high level of reproductive resources to female function (e.g., ovule number, seed number, or seed yield) must allocate a relatively low level of resources to male function (e.g., to pollen production). This assumption is based on the belief that male and female function depend on a shared and limited resource. To date, very few studies have properly estimated genetically based covariances between components of male and female investment. Consequently, the view that resource-sharing enforces a trade-off between male and female function has not been widely tested.
Using a combination of approaches - sampling wild populations, conducting artificial selection experiments, cultivating distinct genetic lineages in a controlled environment, and manipulating abiotic environmental conditions - I have been investigating environmental, ontogenetic, and genetic sources of variation in gender expression and other floral traits. Spergularia marina has been particularly suitable for this work because it exhibits remarkably high levels of phenotypic variation in gender allocation. (Flowers produce 0 - 10 stamens per flower, 40 - 180 ovules per flower and ~30 - 100 seeds per fruit). In collaboration with Dr. Veronique Delesalle (Associate Professor, Gettysburg College), I have discovered that wild populations differ in the degree of maternally-transmitted variation in primary and secondary sexual traits, and in their genetic architecture (the magnitude of genetic covariance between traits) (Delesalle and Mazer, 1996; Mazer and Delesalle, 1996). These results suggest that populations are evolving along different evolutionary trajectories and may be subject to different evolutionary "constraints". In addition, our recently published work (Mazer, Delesalle, and Neal, 1999) is the first study to detect a negative genetic correlation (or "trade-off") between pollen and ovule production per flower. This paper presents the results of a three-generation artificial selection experiment and is the first study to date that has detected a negative genetic correlation between male and female investment at the individual flower level, thereby supporting this major assumption of many theoretical models.
In collaboration with my graduate student, Ed Lowry, I have just completed an experiment to determine whether the genetic correlation between pollen and ovule production is sensitive to the environmental conditions in which plants are raised. One prediction is that, as resources become more limiting, negative genetic correlations (or "trade-offs") among developmentally related traits will become more strongly expressed. These data will also determine whether pairs of traits such as total lifetime flower production and flower size, the rate of flower production and flower size, and petal number and petal size, express such trade-offs more strongly under adverse (resource-limited) conditions. Current Research Projects
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| Department of Ecology, Evolution and Marine Biology • University of California, Santa Barbara |
