Faculty Research: 2) The evolution of gender-related traits in species with different mating systems: a quantitative genetic comparison of selfing and outcrossing species of Clarkia (farewell-to-spring: Onagraceae)

The realization that hermaphroditic organisms do not necessarily contribute equally to future generations through male and female function has dramatically changed the study of plant evolutionary ecology. This insight has motivated much work in sex allocation theory, which in turn has provided important conceptual models for understanding breeding system evolution.

One of the best-supported predictions of sex allocation theory in plants is that allocation to male function (e.g., pollen production per flower) should be lower in selfing than in outcrossing taxa. My research on Clarkia begins with this observation, of which the likely explanation is that in species in which flowers are pollinated by their own pollen, there is strong selection to produce no more pollen per flower than is necessary to ensure full seed set. Extending this argument, Dr. Veronique Delesalle and I have developed a set of novel predictions that are logical long-term consequences of mating-system-specific selection (Mazer and Delesalle, 1998).

We are currently testing these predictions using the following three pairs of Clarkia (Onagraceae) sister taxa with different mating systems (selfing vs. outcrossing):

Clarkia unguiculata (outcrossing) vs. Clarkia exilis (selfing)
Clarkia concinna concinna (outcrossing) vs. Clarkia concinna automixa (selfing)
Clarkia xantiana xantiana (outcrossing) vs. Clarkia xantiana parviflora (selfing)

C. unguiculata

C. concinna

C. xantiana

Over the next four years, we will conduct a series of field and greenhouse experiments in these species to explore the evolution and developmental stability of the primary sex ratio (pollen:ovule production per flower); ovule, pollen and seed production; seed mass; and flower size. In addition, we will seek evidence for potential genetic trade-offs among reproductive components in these taxa.

First, we predict that, as a result of selection favoring efficient patterns of resource allocation in autogamous (self-fertilizing) taxa, they should exhibit little variation in gender expression (e.g., the pollen:ovule ratio) relative to outcrossers. Specifically, we expect selfing taxa to exhibit lower levels of genetic variation in the pollen:ovule ratio of individual flowers than outcrossers.

Second, selection against the production of flowers with inefficient P:O ratios should result in greater canalization of the P:O ratio in selfers than in outcrossers. In other words, phenotypic variation in gender expression among flowers within individuals should be lower in plants with self-fertilizing than with outcrossing flowers.

Third, due to selection against autogamous genotypes producing excess or insufficient pollen relative to the number of ovules in a flower, highly selfing taxa should experience stronger stabilizing selection on the pollen:ovule (P:O) ratio than outcrossers, and we aim to detect this in the field.

Fourth, in spite of strong selection on the P:O ratio, genetic variation in both pollen and ovule production per flower might easily persist within both selfing and outcrossing populations. In selfing taxa, genotypes producing relatively few flowers (each with high levels of pollen and ovule production) and genotypes producing comparatively high numbers of flowers (each with relatively few gametes) could be expected to coexist if selection on lifetime flower production is weak. This may be the case for selfers, in which the secondary costs of producing flowers may be quite low. In outcrossers, genetic variation in ovule and pollen production per flower might easily persist if female- and male-biased genotypes coexist, each contributing differentially to future generations through male vs. female function. Female-biased genotypes are those producing relatively large numbers of ovules (but relatively few pollen grains); male-biased genotypes are those with relatively few ovules but high pollen production.

Given that there is genetic variation in ovule and pollen production per flower, a final prediction is that, in perfect-flowered angiosperms, the genetic correlation between investment in male and female gametes per flower should differ between selfers and outcrossers. As mentioned above, central to sex allocation models is the assumption that negative genetic correlations should characterize the relationship between male and female investment. In contrast, we predict that, in taxa with self-fertilizing flowers (autogamous selfers), a positive genetic correlation between pollen and ovule production per flower should evolve if reproductive efficiency is to be maintained. That is, autogamously selfing genotypes that produce relatively large numbers of ovules per flower should be under selection to produce relatively large numbers of pollen grains per flower (or otherwise risk insufficient pollination) and vice versa. In other words, selection in autogamous selfers will select against gender "specialists", resulting in the evolution of a positive genetic correlation between ovule and pollen production per flower. By contrast, in outcrossing taxa, the maintenance of gender specialists may contribute to the expression of negative genetic correlations between pollen and ovule production per flower (as predicted by theory).

This project will require labor-intensive artificial selection experiments and the use of allozyme or molecular markers to establish selfing rates in natural populations. In addition, we aim to develop microsatellite markers for paternity analyses that will allow us to assess the effects of variation in gender allocation on both paternal and maternal fitness in natural populations. In a related project, Dr. Scott Hodges (UCSB), Dr. Robert Nakamura (Cal State Los Angeles), and I are collaborating to detect Quantitative Trait Loci influencing ovule and pollen production in Clarkia unguiculata. The identification of QTLs is an alternative way to detect possible genetic correlations between pollen and ovule production due to pleiotropy or genetic linkage.

Current Research Projects

1. Testing the assumptions of sex allocation theory: quantitative genetic variation and covariation among floral traits in the Sand-Spurrey, Spergularia marina (Caryophyllaceae)
2. The evolution of gender-related traits in species with different mating systems: a quantitative genetic comparison of selfing and outcrossing species of Clarkia (farewell-to-spring: Onagraceae)
3. The Evolutionary Significance of Variation in Traits Subject to Ontogenetic Change and Maternal Environmental Effects
4. Genetic and environmental influences on life history, floral traits, and sex allocation in Raphanus sativus (Brassicaceae; wild radish): the stability of genetic parameters across environments.
5. The Detection of the Long-term Outcome of Natural Selection and the Ecological Sorting of Species Among Habitats: Comparative Studies of Plant Reproductive Characters
6. Ecological adaptation, gene flow, and the potential for hybrid breakdown in restoration projects.

Susan Mazer | Research | Publications | Curriculum Vitae

<< Return to Faculty Page