BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT GAILLARDIA PULCHELLA (ASTERACEAE)

Biparental inbreeding is thought to be a common feature of plant populations with restricted pollen dispersal. It is generally assumed that the inbreeding depression frequently observed to accompany self-fertilization can be extrapolated to the lesser degrees of consanguinity involved in biparental inbreeding, but this is virtually untested. To test this assumption, seeds collected from a single natural population of the self-incompatible annual Gaillardia pulchella were used to generate full-sib families derived by crossing either noninbred full-sibs (inbred families) or noninbred nonrelatives (outbred families). Members of each family were divided between high-stress and low-stress treatments that differed in soil volume and nutrient level. Inbred seedlings had a lower chance of survival, were more likely to be morphologically abnormal, and grew more slowly than outbred seedlings, indicating the presence of biparental inbreeding depression. Stress treatment had no significant effect on inbreeding depression, and no family stress-environment interactions were detected. Inbreeding did not increase the among-family variance in growth rate, suggesting that inbreeding depression of growth rate is caused by many genes with small individual effects. Relative to direct estimates of inbreeding depression, observed levels of near-neighbor outcrossing depression, presumed to be biparental inbreeding depression, are surprisingly high in many plant species.     

ASSOCIATIONS BETWEEN ALLOZYME FREQUENCIES AND SOIL CHARACTERISTICS IN GAILLARDIA PULCHELLA (COMPOSITAE)

Selection favoring different alleles in different environments frequently has been suggested as an explanation for allozyme variation within and among populations. This hypothesis predicts that allozyme frequencies will be correlated with environmental variables. Previous studies on allozyme frequency-environment covariation in plants often have relied on qualitative assessments of the environment and have emphasized highly autogamous species. We have examined allozyme frequency-soil associations in Gaillardia pulchella, an obligately outcrossed annual plant, by regressing the frequencies of 15 common allozymes representing six polymorphic enzyme loci on principal components from a set of 20 quantitative soil variables. Fifty-one populations, representing four taxonomic varieties, were included in the analysis. Among the 26 populations representing the var. pulchella, allozymes Adh-2f and Pgm-1c were significantly associated with a block of highly inter-correlated soil characteristics which serve to discriminate between soils derived from calcareous vs. non-calcareous rock types. This geographically complex pattern of allozyme frequency-soil covariation is not likely to be spurious and, thus, indicates the presence of adaptively differentiated soil races, or ecotypes. However, these results are not sufficient to conclude that the allozyme frequency divergence between ecotypes was mediated by selection, either directly or through genetic hitchhiking. The pattern of allozyme frequency-soil covariation within var. pulchella was not found among the other taxonomic varieties. Patterns of genotype-environment covariation often may be recognizable only within geographically or environmentally restricted groups of populations because of the confounding influences of other environmental variables.     

GENETIC VARIATION AND COVARIATION FOR CHARACTERISTICS ASSOCIATED WITH CADMIUM TOLERANCE IN NATURAL POPULATIONS OF THE SPRINGTAIL ORCHESELLA CINCTA (L.)

Heavy metals can be strong and stable directional selective agents for metal-exposed populations. Genetic variation for the metal-tolerance characteristic “cadmium excretion efficiency” was studied in populations of the collembolan Orchesella cincta from a reference- and a metal-contaminated forest soil. Previously it has been shown that “excretion efficiency” influences tolerance through midgut-mediated immobilization and excretion of toxic metal ions, and that an increased mean excretion efficiency is present in animals inhabiting metal-contaminated litter. In the present research, offspring-parent regressions showed that additive genetic variation for cadmium excretion efficiency was present in the population from the reference site. The heritability estimate was 0.33. In the natural population exposed to heavy metals from an industrial source, additive genetic variation was not significantly different from zero. Differences in the heritability between the reference and the exposed population were not significant. Genetic variation for cadmium excretion efficiency allows for a response to selection in the reference population. Such a response has probably occurred in the metal-exposed population. Half-sib analysis with animals from the reference population was used to estimate genetic variation and maternal effects for excretion efficiency, relative growth rate and molting frequency, and to determine genetic correlations between these characteristics. Additive genetic variation was demonstrated for all three characteristics, heritability estimates were 0.48, 0.75 and 0.46, respectively. Maternal effects were low for excretion efficiency and molting frequency, but may be present for relative growth rate. Phenotypic and genetic correlations among these characteristics were positive. The environmental correlation between relative growth rate and molting frequency was positive, others were negative. Direct selection for any of the characteristics, or genetic correlations between tolerance characteristics and growth characteristics, or both may have caused the responses previously observed in field populations.     

GENOTYPIC VARIATION AND CLONAL STRUCTURE IN CORAL POPULATIONS WITH DIFFERENT DISTURBANCE HISTORIES

Genotypic diversity in six populations of the endemic Hawaiian reef coral, Porites compressa, was directly related to habitat-disturbance history. The highest diversity (lowest amount of clonal proliferation) was found in populations that had been intensely or recently disturbed. In these populations, space was not limited and mean colony size was small (< 500 cm²), suggesting early stages of recolonization. In an undisturbed, protected habitat, lower genotypic diversity was a result of a significant degree of clonal replication of established genotypes. Unoccupied substratum was rare in this habitat, and average colony size was large (> 2500 cm²). Populations in intermediately disturbed habitats showed intermediate levels of diversity and clonal structure as a result of the combined contributions of sexual and asexual reproduction. Individual clones were distributed over small areas (< 4 m²) or distances (< 1 m) in young populations, and more broadly (> 256 m²) and over longer distances (> 90 m) in the older, undisturbed population. Interpretations of life-history parameters and estimates of total genetic variability in species that have the potential to reproduce asexually are dependent upon an assessment of the overall clonal structure of populations. The power of genotypic assays to reliably detect clonal versus unique colonies, as well as the spatial scales over which clonal populations are sampled, are critical to such assessments.     

VARIATION IN REACTION NORMS AMONG POPULATIONS OF THE GRASS BOUTELOUA RIGIDISETA

Recent research has emphasized the importance of investigating the reaction norms of quantitative traits to understand evolution in natural environments. In this study, genetic differences in reaction norms among eight populations of the grass Bouteloua rigidiseta were examined using clonal replicates of genotypes planted in a common garden with two levels of competition (single B. rigidiseta without competition and single B. rigidiseta surrounded by four Erioneuron pilosum). The populations were found to be genetically differentiated for a variety of traits. Differences in reaction norms of size-specific fecundity (spikelet clusters per tiller number) were detected among the populations: some showed little response to competition; in others size-specific fecundity was much greater in the absence of competition. This divergence in reaction norms among these populations may be the result of past selection (including the cost of plasticity), or genetic drift.     

REPRODUCTION AND VARIATION IN ACONITUM COLUMBIANUM (RANUNCULACEAE), WITH EMPHASIS ON CALIFORNIA POPULATIONS

Nectary depth in Aconitum columbianum Nutt. in T. & G. shows little variation within populations but much continuous variation among populations. Mean nectary depth in populations studied ranges from 3.4 mm (SD = ±0.32) to 9.4 mm (SD = ±0.75). Correlations of nectary depths with the foraging behaviors and tongue lengths of bees visiting A. columbianum flowers indicate that populations with shallow nectaries are adapted to pollination by both short- and long-tongued bees. Bumblebee species with short tongues are not usually pollinators of flowers in populations with deep nectaries. Nectary depth is geographically correlated in California, and populations over large areas have similar nectary depths. Nectary depth is also correlated with bulbifery. Bulbiferous plants have strictly shallow nectaries, and are confined to two regions near the western extreme of the range of A. columbianum. The range of bulbiferous Aconitum in California is contiguous with the range of non-bulbiferous populations with shallow nectaries.