Daniel J. Schoen

THE BREEDING SYSTEM OF GILIA ACHILLEIFOLIA: VARIATION IN FLORAL CHARACTERISTICS AND OUTCROSSING RATE

The outcrossing rate is a key component of the breeding system. While estimates of outcrossing rate exist for some plants, less information is available on variation in outcrossing rate within species and on its causes. Since many features of flowers affect the level of outcrossing, plant breeding systems are often inferred from observations on floral morphology, physiology, and phenology. Typically, such observations include whether plants are unisexual or cosexual, have perfect or unisexual flowers, are self-compatible or selfincompatible (Grant, 1958; Baker, 1959), whether flowers are protandrous, protogynous, or homogamous (Faegri and van der Pijl, 1971), degree of stigma exsertion (Grant, 1954; Breese, 1959), flower size (Rollins, 1963), pollen-ovule ratio (Lloyd, 1965; Cruden, 1977), and amount of fruit or seed set in the absence of pollinators (Lloyd, 1965; Harding et al., 1974). Few studies of plant breeding systems have addressed the problem of how well the presence of these floral features can be used to predict outcrossing rates of plant populations, or whether variation among populations in the expression of these features is heritable. This latter question is especially relevant to studies concerned with the evolution of plant breeding systems (e.g., Lloyd, 1965; Moore and Lewis, 1965; Baker, 1966; Ornduff, 1972; Arroyo, 1973; Rick et al., 1978; Solbrig and Rollins, 1977). This paper presents the results of an investigation of breeding system variation in the California annual plant, Gilia

A global perspective of the richness and evenness of traditional crop-variety diversity maintained by farming communities

Varietal data from 27 crop species from five continents were drawn together to determine overall trends in crop varietal diversity on farm. Measurements of richness, evenness, and divergence showed that considerable crop genetic diversity continues to be maintained on farm, in the form of traditional crop varieties. Major staples had higher richness and evenness than nonstaples. Variety richness for clonal species was much higher than that of other breeding systems. A close linear relationship between traditional variety richness and evenness (both transformed), empirically derived from data spanning a wide range of crops and countries, was found both at household and community levels. Fitting a neutral “function” to traditional variety diversity relationships, comparable to a species abundance distribution of “neutral ecology,” provided a benchmark to assess the standing diversity on farm. In some cases, high dominance occurred, with much of the variety richness held at low frequencies. This suggested that diversity may be maintained as an insurance to meet future environmental changes or social and economic needs. In other cases, a more even frequency distribution of varieties was found, possibly implying that farmers are selecting varieties to service a diversity of current needs and purposes. Divergence estimates, measured as the proportion of community evenness displayed among farmers, underscore the importance of a large number of small farms adopting distinctly diverse varietal strategies as a major force that maintains crop genetic diversity on farm.

Variation in male reproductive investment and male reproductive success in white spruce

An experimental population composed of ramets of white spruce clones was studied to determine the influence of different levels of clonal male reproductive investment on clonal male reproductive success, where the male reproductive success of different clones in the population was defined as the proportion of the seed crop sired by each clone. We present a multilocus estimation procedure which provides unbiased estimates of the proportion of seed sired by each parental genotypic class from pollen gamete frequency data, whenever gametic segregation frequencies are known.

An atlas of over 90,000 conserved noncoding sequences provides insight into crucifer regulatory regions

Despite the central importance of noncoding DNA to gene regulation and evolution, understanding of the extent of selection on plant noncoding DNA remains limited compared to that of other organisms. Here we report sequencing of genomes from three Brassicaceae species (Leavenworthia alabamica, Sisymbrium irio and Aethionema arabicum) and their joint analysis with six previously sequenced crucifer genomes. Conservation across orthologous bases suggests that at least 17% of the Arabidopsis thaliana genome is under selection, with nearly one-quarter of the sequence under selection lying outside of coding regions. Much of this sequence can be localized to approximately 90,000 conserved noncoding sequences (CNSs) that show evidence of transcriptional and post-transcriptional regulation. Population genomics analyses of two crucifer species, A. thaliana and Capsella grandiflora, confirm that most of the identified CNSs are evolving under medium to strong purifying selection. Overall, these CNSs highlight both similarities and several key differences between the regulatory DNA of plants and other species.

The evolution of self-incompatibility when mates are limiting

Self-incompatibility (SI) is a genetic barrier to inbreeding that is broadly distributed in angiosperms. In finite populations of SI plants, the loss of S-allele diversity can limit plant reproduction by reducing the availability of compatible mates. Many studies have shown that small or fragmented plant populations suffer from mate limitation. The advent of molecular typing of S-alleles in many species has paved the way to address quantitatively the importance of mate limitation, and to provide greater insight into why and how SI systems breakdown frequently in nature. In this review, we highlight the ecological factors that contribute to mate limitation in SI taxa, discuss their consequences for the evolution and functioning of SI, and propose new empirical research directions.

Mutation Rates and Dominance Levels of Genes Affecting Total Fitness in Two Angiosperm Species

Theories about the evolution of sex and the effects of inbreeding depend on knowledge of the mutation rate and dominance level of deleterious alleles affecting total fitness. In two species of largely self-fertilizing annual plants, minimal estimates of such mutation rates were found to be 0.24 to 0.87 per sporophyte genome per generation, but confidence intervals exceeded 1.0 in each of the four populations. Dominance levels were near zero in one species and intermediate (0.28 to 0.35) in the other. These results suggest that the detrimental effects of inbreeding are a result of new partially recessive mutations rather than overdominance.

RELATIVE FITNESSES OF SELFED AND OUTCROSSED PROGENY IN GILIA ACHILLEIFOLIA (POLEMONIACEAE)

The evolution of self-pollination is a recurrent theme in the history of the angiosperms, and has long been of interest to evolutionists (Darwin, 1876; Darlington, 1939; Mather, 1943; Baker, 1955; Stebbins, 1957; Allard et al., 1968; Jain, 1976; Maynard Smith, 1977; Lloyd, 1979). Yet despite the widespread attention that the topic has received, the factors important to the evolution of self-pollination (autogamy) are poorly understood. As noted by Jain (1976), there are many hypothesized advantages associated with autogamy, but few data bearing on these hypotheses. One of the commonly cited advantages of selfing, and the most general, has been referred to as the automatic selection advantage (Jain, 1976). This advantage arises from the fact that while an outcrossing individual contributes on the average, one haploid genome through its ovules and one through its pollen, a selfing variant contributes up to three haploid genomes on the average, two through its selfed offspring, and one by outcrossing as a pollen parent. Unless the inbred progeny have reduced fitness, mutations promoting selfing are expected to increase in frequency by selection (Fisher, 1941; Maynard Smith, 1977; Lloyd, 1979, 1980). Inbreeding depression in the progeny of selfed parents may impart a reduction in viability, fecundity or both, sufficient to impede the selection of genes for self-pollination. According to Lloyd (1979), in situations where a selfing mutant arises in an outcrossing population, the critical magnitude of inbreeding depression above which mutant genes promoting selfing are

THE EVOLUTION OF FLORAL LONGEVITY: RESOURCE ALLOCATION TO MAINTENANCE VERSUS CONSTRUCTION OF REPEATED PARTS IN MODULAR ORGANISMS

The component parts of modular organisms often show interspecific variation in their longevity. In plants, the flower is an example of such a structure. Models are developed in this paper to predict optimal floral longevity (the optimal length of time that flowers should remain open and functional) under a variety of conditions. A tradeoff involving allocation of resources to floral construction versus floral maintenance is assumed. The main model variables are the rate at which pollen and seed fitness accrue over time (fitness‐accrual rates) and the daily cost of maintaining an existing flower relative to the cost of constructing a new one (floral maintenance cost). Long‐lived flowers are selected when fitness‐accrual rates and floral maintenance costs are low, whereas short‐lived flowers are selected when fitness‐accrual rates and floral maintenance costs are high. Dichogamy favors longer‐lived flowers relative to homogamy, whereas nonindependence among flowers in their attractiveness to pollinators (attraction to flower clusters) selects for shorter‐lived flowers. Reduction in floral maintenance costs later on in the flowers life favors longer‐lived flowers. Observations on the dissemination and receipt of pollen in individual flowers over time, together with measurements of corolla respiration and nectar sugar production rate are required to test the model quantitatively. The parameters important to the evolution of optimal floral longevity (i.e., maintenance and construction costs, and fitness‐accrual rates) may be general features of evolution of optimal longevities of other repeated structures.

Whole- and part-flower self-pollination in glycine clandestina and G. argyrea and the evolution of autogamy

The overall rate of self‐fertilization can be viewed as the sum of two distinct processes: 1) self‐pollination of all ovules in a flower (whole‐flower self‐pollination); and 2) self‐pollination of some of the ovules in a flower, occurring together with outcrossing of the remaining ovules (part‐flower self‐pollination). In some situations these processes may be equated with different modes of self‐pollination. A model of the mating system in which the progeny of separate fruits serve as the unit of observation is presented. The model partitions the overall rate of self‐pollination into components attributable to whole‐ and part‐flower selfing. When the mating system is estimated using information on marker genotypes from chasmogamous fruits in two species of Glycine together with the whole‐ and part‐flower selfing model, the results indicate that the chasmogamous flowers in a subalpine population of G. clandestina underwent a significant level of whole‐flower selfing, whereas in another, lower elevation population of G. clandestina and in a subtropical population of G. argyrea, they did not. This difference is thought to be related to the contrast in the variability of environmental conditions for insect‐mediated pollination between the habitats sampled. In particular, the large component of whole‐flower selfing observed in the subalpine population of G. clandestina may be due to self‐pollination that is induced during periods unfavorable to insect‐mediated pollination. It can be demonstrated that such induced selfing will be selected whenever environmental conditions are such that pollinator activity limits seed set, and moreover that induced selfing can result in the selection of overall levels of self‐pollination that are intermediate between 0 and 1. Monte Carlo simulation is employed to show that ignoring the correlation of self‐fertilization events that result from whole‐ and part‐flower selfing may lead to biased estimates of mating system parameters.

THE EVOLUTION OF SELF-FERTILIZATION IN PERENNIALS

Many plants are perennials, but studies of self‐fertilization do not usually include features of perennial life histories. We therefore develop models that include selfing, a simple form of perenniality, adult inbreeding depression, and an adult survivorship cost to seed production. Our analysis shows that inbreeding depression in adults diminishes the genetic transmission advantage associated with selfing, especially in long‐lived perennials that experience inbreeding depression over many seasons. Perennials also pay a cost when selfing increases total seed set at the expense of future survivorship and reproduction. Such life‐history considerations shed new light on the generalization that annuals self‐fertilize more than perennials. Past research suggested reproductive assurance as an explanation for this association, but common modes of selfing offer equal reproductive assurance to annuals and perennials. Instead, perennials may avoid selfing because of adult inbreeding depression and the cost to future survivorship and reproduction.