LIFETIME ESTIMATES OF BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT RAPHANUS SATIVUS

Studies of inbreeding depression in plant populations have focused primarily on comparisons of selfing versus outcrossing in self-compatible species. Here we examine the effect of five naturally occurring levels of inbreeding (f ranging from 0 to 0.25 by pedigree) on components of lifetime fitness in a field population of the self-incompatible annual, Raphanus sativus. Pre- and postgermination survival and reproductive success were examined for offspring resulting from compatible cross-pollinations. Multiple linear regression of inbreeding level on rates of fruit and seed abortion as well as seed weight and total seed weight per fruit were not significant. Inbreeding level was not found to affect seed germination, offspring survival in the field, date of first flowering, or plant biomass (dry weight minus fruit). The effect of inbreeding on seedling viability in the greenhouse and viability to flowering was significant but small and inconsistently correlated with inbreeding level. Maternal fecundity, however, a measure of seed yield, was reduced almost 60% in offspring from full-sib crosses (f = 0.25) relative to offspring resulting from experimental outcross pollinations (f = 0). Water availability, a form of physiological stress, affected plant biomass but did not affect maternal fecundity, nor did it interact with inbreeding level to influence these characters. The delayed expression of strong inbreeding depression suggests that highly deleterious recessive alleles were not a primary cause of fitness loss with inbreeding. Highly deleterious recessives may have been purged by bottlenecks in population size associated with the introduction of Raphanus and its recent range expansions. In general, reductions in total relative fitness of greater than 50% associated with full-sib crosses should be sufficient to prohibit the evolution of self-compatibility via transmission advantage in Raphanus.     

THE ROLES OF PLANT HORMONES IN STYLE AND STIGMA GROWTH IN GAILLARDIA GRANDIFLORA (ASTERACEAE)

Style and stigma elongation and stigma unfolding, and the roles of plant hormones in these processes in Gaillardia grandiflora Van Houtte were investigated. Style and stigma elongation in vivo began just after anthesis, and style elongation was accompanied by epidermal cell elongation (greatest near the stigma) and a fresh weight increase, but not by cell division or a dry weight increase. The stigma unfolded after the style and stigma elongated. Style-stigma units excised from young disc flowers of this composite were measured as they responded to plant growth regulators applied singly, as well as in sequential and simultaneous combinations, in vitro. Style elongation was promoted by auxin, was inhibited by gibberellins and ethylene, and was unaffected by other growth regulators. Stigma elongation followed a similar pattern of response. Endogenous auxin levels and ethylene production showed parallel variation and endogenous gibberellin levels showed inverse variation with style and stigma elongation. Stigma unfolding was more sensitive to auxin applications and was promoted by applied ethylene. Ethylene production showed parallel variation and endogenous auxin levels showed inverse variation with stigma unfolding. AVG and Co²⁺ applications decreased auxin-induced style elongation and fusicoccin promoted all of the growth responses of style-stigma units in vitro. A gibberellin-auxin-ethylene-acid growth interaction mode of control is proposed for these three growth processes.     

THE ROLES OF PLANT HORMONES IN THE GROWTH OF THE COROLLA OF GAILLARDIA GRANDIFLORA (ASTERACEAE) RAY FLOWERS

Corolla elongation and the roles of plant hormones in this process in Gaillardia grandiflora Van Houtte ray flowers were examined. The sterile ray flowers elongated during a 2-day period, and corolla growth was accompanied by fresh and dry weight increases and epidermal cell elongation (greatest near the base of the corolla) but not by cell division. Corollas excised from young ray flowers were measured during treatment in vitro with solutions of plant growth regulators. They elongated in response to gibberellins and fusicoccin but did not respond to auxins, cytokinins, abscisic acid, ethylene, or inhibitors of ethylene biosynthesis. Sequential and simultaneous hormone applications indicated no additive or synergistic effects between hormones, but auxin did reduce gibberellin-promoted growth. Analyses of endogenous auxins showed no significant variation, and ethylene production decreased prior to elongation, while a 20-fold increase in endogenous gibberellin activity was observed just prior to rapid corolla elongation. It appears that corolla growth in Gaillardia is accomplished by an increase in gibberellin activity alone, that multiple hormone interactions are not important in the control of corolla growth, and that part of the mode of action of gibberellin is acid-induced growth.     

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 CONSEQUENCES OF RARITY IN ASTER FURCATUS (ASTERACEAE), A THREATENED,SELF-INCOMPATIBLE PLANT

Aster furcatus is a rare, self-incompatible plant with fewer than 50 known populations throughout its range. We verified self-incompatibility in A. furcatus by conducting experimental self- and cross-pollinations and by examining seed set in a small population comprised of a single clonal genet. We examined variation at 22 electrophoretic loci in 23 populations of A. furcatus from across its range in Wisconsin, Illinois, Indiana, and Missouri. Except for two rare alleles found in single individuals in three populations, all loci but one of those examined were fixed for single alleles. The only variable locus (triosephosphate isomerase, TPI-1) tended to exhibit genotype frequencies in Hardy-Weinberg equilibrium or with a slight excess of heterozygotes. Although overall gene diversity was extremely low, TPI genotype frequencies were indicative of an outcrossing plant. We examined the subpopulation genetic structure among clonal plants within one Wisconsin population in greater detail. F statistics indicated that much of the genetic variation at the polymorphic TPI locus was due to differentiation among populations. We discuss the implications of self-incompatibility and low levels of genetic variation for the evolution and conservation of Aster furcatus and other rare plants with similar breeding systems.     

INBREEDING AND THE COST OF MEIOSIS: THE EVOLUTION OF SELFING IN POPULATIONS PRACTICING BIPARENTAL INBREEDING

The effect of biparental inbreeding on the conditions governing the evolution of selfing is examined using recursions in mating-type frequencies. Sibmating in combination with random outcrossing influences two key determinants of the adaptive value of selfing: 1) the meiotic cost of biparental reproduction and 2) the level of inbreeding depression due to deleterious mutations. Biparental inbreeding serves to maintain biparental reproduction by increasing relatedness between parents and their biparentally derived offspring and introduces the possibility of an optimal mating system that incorporates both modes of reproduction. Biparental inbreeding serves to promote uniparental reproduction by reducing the relative inbreeding depression suffered by uniparental offspring. The net effect of these two antagonistic trends depends upon the extent to which mutational load accounts for differences in the numbers of the two types of offspring. A brief summary of the empirical literature suggests that: 1) biparental inbreeding may occur in populations exhibiting mixed mating systems; 2) while inbreeding depression represents an important factor, it does not account entirely for differences in offspring number between the two modes of reproduction.     

CLINAL VARIATION ASSOCIATED WITH EDAPHIC ECOTONES IN HYBRID POPULATIONS OF GAILLARDIA PULCHELLA

The variety pulchella of the outcrossing annual plant species Gaillardia pulchella consists of two edaphic races in central Texas which are divergent for one morphological and four electrophoretic characters. Reduced pollen stainability in F₁ hybrids suggests the races are also divergent in chromosome structure. The recent proliferation of this species on roadsides and in pastures has led to hybridization between these races. An analysis of character variation in three hybrid populations revealed significant clinal variation associated with edaphic ecotones, and the width of these clines was found to vary among characters in a consistent pattern. It is argued that this pattern is the result of different characters experiencing different effective selection regimes, with narrower clines reflecting greater differentials in effective selection. Several mechanisms are discussed by which selection may impede the transgression of alleles across the ecotones in these populations. The results of this study are compared to the results of parallel studies on the autogamous annual species Avena barbata in California, and it is suggested that the difference between these two species in the width of clines separating edaphic ecotypes may be accounted for by their different breeding systems.     

THE ROLES OF AUXIN,ETHYLENE, AND ACID GROWTH IN FILAMENT ELONGATION IN GAILLARDIA GRANDIFLORA (ASTERACEAE)

Filament elongation and the role of auxin in this process in Gaillardia grandiflora was investigated. Filament elongation in vivo occurred just prior to anthesis and was accompanied by cell elongation and fresh weight increase. Filaments isolated and exposed to auxin in vitro grew more rapidly than controls and their growth was comparable to that of filaments in vivo. Furthermore, the natural auxin content of disc flowers (determined by double-standard isotope dilution analyses) increased just prior to anthesis and filament elongation. These results imply that auxin controls filament elongation. Applied ethylene slightly promoted filament elongation in vitro, and ethylene production of the flowers (determined by gas chromatography) slightly increased prior to filament growth. Fusicoccin and acidic buffers also stimulated elongation of isolated filaments. Thus, the role of auxin in controlling filament elongation in Gaillardia may involve stimulation of ethylene biosynthesis and acid growth.     

INBREEDING DEPRESSION IN TWO MIMULUS TAXA MEASURED BY MULTIGENERATIONAL CHANGES IN THE INBREEDING COEFFICIENT

In mixed-mating plant populations, one can estimate the relative fitness of selfed progeny w by measuring the inbreeding coefficient F and selfing rate s of adults of one generation, together with F of adults in the following generation (after selection). In the first application of this multigenerational method, we estimated F and s for adults over three consecutive generations in adjacent populations of two annual Mimulus taxa: the outbreeding M. guttatus and the inbreeding M. platycalyx. This gave estimates of w for the last two generations. Although average multilocus selfing rates were high in both taxa (0.63 in M. guttatus; 0.84 in M. platycalyx), the relative fitness of selfed progeny averaged only 0.19 in M. guttatus and 0.32 in M. platycalyx. An alternative estimator for w that incorporates biparental inbreeding gave even lower estimates of w. These values are significantly below the 0.5 threshold thought to favor selfing, and show that partially selfing populations can harbor substantial genetic load. In accordance with the purging hypothesis, the more highly selfing M. platycalyx showed marginally lower inbreeding depression than M. guttatus in both years (P = 0.08). Inbreeding depression and selfing rates also varied among years in concert among taxa. Several sources of bias are discussed, but computer simulations indicate it is unlikely that w is biased downwards by linkage of marker loci to load loci.     

GEOGRAPHICAL VARIATION IN THE INBREEDING DEPRESSION OF SCOTS PINE

The magnitude of inbreeding depression caused by recessive mutations in a population is dependent on the mutation rate and on the intensity of selection against the mutations. We studied geographical differences in the level of early inbreeding depression of Scots pine in a common garden experiment. The mean abortion rate of experimentally self-pollinated seeds was significantly lower (75.4%) among trees that originated from northern populations (66–69°N) than among trees from more southern (60–62°N) populations (86.5%). Thus, the number of embryonic lethal equivalents was lower in the northern populations (4.5) than in the southern ones (6.9). The outcrossing rate at the mature seed stage was slightly lower in the northern populations (average 0.93) than in the southern one (0.99). The estimated selfing rate at the zygote stage varied from 0–0.28 in the populations. The reduction in the magnitude of inbreeding depression in the north may have been caused by increased levels of self-fertilization in the northern populations. The proportion of self-fertilized seedlings and adults was very small in all populations (F ≈ 0), indicating high inbreeding depression also in later life stages. The high level of inbreeding depression in the partially selfing Scots pine can be explained by mutation-selection balance only if the mutation rate is high.     

BIPARENTAL INBREEDING AND INTERREMNANT MATING IN A PERENNIAL PRAIRIE PLANT: FITNESS CONSEQUENCES FOR PROGENY IN THEIR FIRST EIGHT YEARS

Despite fundamental importance to population dynamics, mating system evolution, and conservation management, the fitness consequences of breeding patterns in natural settings are rarely directly and rigorously evaluated. We experimentally crossed Echinacea angustifolia, a widespread, perennial prairie plant undergoing radical changes in distribution and abundance due to habitat fragmentation. We quantified the effects of both biparental inbreeding and crossing between remnant populations on progeny survival and reproduction in the field over the first eight years. Lifetime fitness is notoriously difficult to assess particularly for iteroparous species because of the long sequence and episodic nature of selection events. Even with fitness data in hand, analysis is typically plagued by nonnormal distributions of overall fitness that violate the assumptions of the usual parametric statistical approaches. We applied aster modeling, which integrates the measurements of separate, sequential, nonnormally distributed annual fitness components, and estimated current biparental inbreeding depression at 68% in progeny of sibling‐mating. The effect of between‐remnant crossing on fitness was negligible. Given that relatedness among individuals in remnant populations is already high and dispersal very limited, inbreeding depression may profoundly affect future dynamics and persistence of these populations, as well as their genetic composition.     

INBREEDING DEPRESSION IN ANDRODIOECIOUS POPULATIONS OF DATISCA GLOMERATA (DATISCACEAE)

Several workers have suggested that the rarity of androdioecy (the presence of males and hermaphrodites in a breeding population) in nature is due to the large fitness gain required by male plants in order to be maintained by selection. As part of an ongoing investigation of this hypothesis, we tested the effects of selfing on fitness in functionally androdioecious populations of Datisca glomerata. We compared progeny from self-fertilizations, cross-fertilizations with pollen from male plants only, cross-fertilizations with pollen from hermaphrodite plants only, and open-pollinated flowers for several measures of progeny fitness including seed weight, germination rate, and seedling weight. Significant inbreeding depression was observed for androdioecious populations of D. glomerata for both seed and seedling weights. However, no significant differences were observed across treatments for seed germination percentages. The observation of significant levels of inbreeding depression in this study, combined with prior evidence of threefold greater pollen production by males, may at least partially account for the large fitness increase required by males to be maintained by selection.     

DROUGHT STRESS AND INBREEDING DEPRESSION IN LYCHNIS FLOS-CUCULI (CARYOPHYLLACEAE)

Interactions between drought stress and inbreeding depression were studied in Lychnis flos-cuculi. Four inbreeding levels (F = 0, 0.25, 0.50 and 0.75), and three watering treatments were used. Performance was scored for germination rate and proportion, survival, plant size, proportion of plants flowering, flowering date, stem height, number of flowers, flower size, anther weight, fruiting proportion and number of capsules. Multiplicative fitness values were estimated from these traits. Inbreeding affected most of the traits studied, and a severe inbreeding depression was found for the combined fitness estimates. The higher inbreeding depression found here relative to the same family groups in a former experiment may reflect greater dominance and suppression in the present experiment at higher density.     

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING PHLOX

Inbreeding depression was measured in terms of the relative survivorship of self versus cross seed in 14 Phlox drummondii cultivars and 10 populations of P. cuspidata. The cultivars are predominantly outcrossing; P. cuspidata is predominantly self-fertilizing. The relative survivorship of self versus cross seed averages 0.84 in the cultivars and 0.99 in P. cuspidata. In wild P. drummondii, the average is 0.83. The mean number of lethal equivalents per zygote averages 0.69 in the cultivars and 0.05 in P. cuspidata, versus 0.79 in the wild P. drummondii. There is a significant negative correlation between the amount of automatic self-seed produced by a cultivar and the number of lethal equivalents it carries (r = ?0.60). Phlox cuspidata has a lower level of cross-seed abortion than P. drummondii, which probably is not due to a difference in the incidence of lethal genes in their populations.     

SEVERE INBREEDING DEPRESSION AND RAPID FITNESS REBOUND IN THE BUTTERFLY BICYCLUS ANYNANA (SATYRIDAE)

We established inbred laboratory lines of the satyrid Bicyclus anynana with one, three and 10 pairs of butterflies, which were subsequently allowed to increase freely to a maximum size of 300 butterflies. Minimally inbred control lines were established with 300 randomly selected virgin butterflies of equal sex ratio. We measured fecundity, egg weight, egg hatching, adult emergence, adult size, and the proportion of crippled adults in generations F₂, F₃, F₅, and F₇ (the latter two for the one pair bottleneck lines only). The most striking result was an unexpectedly large decrease in egg hatching with increase in inbreeding (25% per 10% increase in inbreeding). Such a level of inbreeding depression has not been reported previously for any insect. The distribution of egg hatching rate for individual clutches within inbred lines was markedly skewed, with a large fraction of clutches producing no eggs at all. This is interpreted as a relatively lower ratio of detrimental to lethal (or sterile) mutation loads than is found in Drosophila, the only insects for which mutation loads have been well characterized. Possible explanations for this severe inbreeding depression include a relatively high rate of mutation to recessive alleles with substantial damaging effects and infrequent episodes of inbreeding in nature. In the experiments, average egg hatching rate recovered rapidly between F₂ and F₇ in three of the six one-pair lines. We discuss the implications of these results for survival of populations through extreme bottlenecks in nature and in captivity.     

POLLEN TUBE GROWTH AND INBREEDING DEPRESSION IN AMSINCKIA GRANDIFLORA (BORAGINACEAE)

To discriminate between the roles of differential pollen tube growth and inbreeding depression in causing reduced seed production following self-pollination of distylous Amsinckia grandiflora (Boraginaceae), callose plug and pollen tube production were compared following cross- and self-pollinations of A. grandiflora. Pin (long-styled morph) × thrum (short-styled morph) crosses yielded more pollen tubes than pin × pin crosses or pin self-pollinations; no difference could be detected for the pin × pin or pin self-pollination categories. Using thrums as the female parent, no significant differences were detected in any pollination category. These results indicate that reduced seed production following self-fertilization of A. grandiflora is likely to be the result of inbreeding depression rather than inhibition of self pollen tubes.     

THE GENETIC INTERPRETATION OF INBREEDING DEPRESSION AND OUTBREEDING DEPRESSION

Inbreeding with close relatives and outbreeding with members of distant populations can both result in deleterious shifts in the means of fitness-related characters, most likely for very different reasons. Such processes often occur simultaneously and have important implications for the evolution of mating systems, dispersal strategies, and speciation. They are also relevant to the design of breeding strategies for captive populations of endangered species. A general expression is presented for the expected phenotype of an individual under the joint influence of inbreeding and crossbreeding. This expression is a simple function of the inbreeding coefficient, of source and hybridity indices of crossbreeding, and of specific forms of gene action. Application of the model may be of use in identifying the mechanistic bases for a number of evolutionary phenomena such as the shift from outbreeding enhancement to outbreeding depression that occurs with population divergence.