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.     

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE): POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken the harmful effects of inbreeding by exposing genetic load to selection. This study examines the maintenance of inbreeding depression in partially self-fertilizing populations of the long-lived, herbaceous wetland plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through self-fertilization. Population-genetic estimates of inbreeding depression (δ = 1 – relative mean fitness of selfed progeny) based on changes in the inbreeding coefficient for the same ten populations were uniformly high, ranging from 0.49 to 1.79 and averaging 1.11 ± 0.29 SE. Although confidence intervals of individual population estimates were large, estimates were significantly greater than 0 in six populations and greater than 0.5 in four. Inbreeding depression was also estimated by comparing growth, survival, and flowering of experimentally selfed and outcrossed offspring from two of these populations in a 1-yr glasshouse experiment involving three density regimes; after which offspring were transplanted into garden arrays and two field sites and monitored for two consecutive growing seasons. Overall for survival averaged 0.27 ± 0.01 in the glasshouse, 0.33 ± 0.04 in the garden, and 0.46 ± 0.04 in the field. The glasshouse experiment also revealed strong inbreeding depression for growth variables, especially above-soil dry weight ( = 0.42 ± 0.03). The fitness consequences of inbreeding depression for these growth variables approximately doubles if survival to maturity is determined by severe truncation selection. Despite substantial selfing, inbreeding depression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.     

DIFFERENTIAL SUCCESS OF POLLEN DONORS IN A SELF-COMPATIBLE LILY

If pollen donors are equally effective at siring seeds, the presence of equal proportions of pollen from two pollen donors on a stigma will lead to equal proportions of seeds sired by each pollen donor. Variation in germination rates, pollen-tube growth, and embryo viability may cause one donor to sire more seed than another. We looked for differential donor success in the field by simultaneously applying equal amounts of pollen from two pollen donors. We simultaneously applied equal amounts of self and outcross pollen to receptive stigmas and simultaneously applied pollen from two donors at different physical distances from the recipient. Following simultaneous application of self and outcross pollen, significantly more of the seeds were sired by outcross pollen donors. Seed set following simultaneous application of two outcross donors was also nonrandom. Pollen donors from 100 m were more likely to sire seeds when competing with pollen from plants nearby (1 m). To determine whether pollen-tube growth rates were responsible for these patterns of paternity, we varied the timing of deposition of outcross pollen allowing self pollen tubes a head start on the stigma. Outcross pollen was applied 3 or 24 h after self pollen. In spite of this time delay, the majority of the seeds were again sired by outcross pollen. There was no significant difference in the amount of seeds sired by self pollen between the two delay treatments. This result suggests that mechanisms operating after ovule fertilization may contribute to the discordance between the proportions of the pollen present and the proportions of seeds sired.     

FIVE GENERATIONS OF ENFORCED SELFING AND OUTCROSSING IN MIMULUS GUTTATUS: INBREEDING DEPRESSION VARIATION AT THE POPULATION AND FAMILY LEVEL

The focus of this study was to examine the consequences of five sequential generations of enforced selfing and outcrossing in two annual populations of the mixed-mating Mimulus guttatus. Our primary goal was to determine whether purging of deleterious recessive alleles occurs uniformly between populations and among families, and thus gain insights into the mode of gene action (dominance, overdominance, and/or epistasis) governing the expression of inbreeding depression at both the population and family levels across the life cycle.     

BADGE SIZE,PHENOTYPIC QUALITY,AND REPRODUCTIVE SUCCESS IN THE HOUSE SPARROW: A STUDY ON HONEST ADVERTISEMENT

Honest signaling theory suggests that advertising traits must be costly to their bearer; thus, only individuals of high phenotypic quality can exhibit maximal expression of these traits. Males of the sexually dichromatic house sparrow, Passer domesticus, have a black throat patch that functions as a badge of status. I investigated whether badge size honestly shows phenotypic quality. Badge size increases with age and decreases with advancing fledging date in yearling males; thus, badge size was larger in older individuals even though age differences were small. Badge size also increased with physical condition independent of age. These results indicate that badge size functions as an honest signal, possibly because there are costs involved in its production. I also found that males with enlarged badges acquired more nest sites than either control males or males with reduced badges. However, males with enlarged badges possessing a nest site raised fewer fledglings per year than did males with reduced badges, suggesting that cheating has no selective benefit. Further studies that accurately measure the energy expenditure allocated to badge production and that quantify additional fitness components are needed to clarify how reliable badges are maintained.     

COMPLEX INTERACTIONS BETWEEN PATERNAL AND MATERNAL EFFECTS: PARENTAL EXPERIENCE AND AGE AT REPRODUCTION AFFECT FECUNDITY AND OFFSPRING PERFORMANCE IN A BUTTERFLY

Parental effects can greatly affect offspring performance and are thus expected to impact population dynamics and evolutionary trajectories. Most studies have focused on maternal effects, whereas fathers are also likely to influence offspring phenotype, for instance when males transfer nutrients to females during mating. Moreover, although the separate effects of maternal age and the environment have been documented as a source of parental effects in many species, their combined effects have not been investigated. In the present study, we analyzed the combined effects of maternal and paternal age at reproduction and a mobility treatment in stressful conditions on offspring performance in the butterfly Pieris brassicae. Both paternal and maternal effects affected progeny traits but always via interactions between age and mobility treatment. Moreover, parental effects shifted from male effects expressed at the larval stage to maternal effects at the adult stage. Indeed, egg survival until adult emergence significantly decreased with father age at mating only for fathers having experienced the mobility treatment, whereas offspring adult life span decreased with increasing mother age at laying only for females that did not experience the mobility treatment. Overall, our results demonstrate that both parents’ phenotypes influence offspring performance through nongenetic effects, their relative contribution varying over the course of progeny's life.     

ALTERNATIVE PATHS TO SUCCESS IN A PARASITE COMMUNITY: WITHIN‐HOST COMPETITION CAN FAVOR HIGHER VIRULENCE OR DIRECT INTERFERENCE

Selection imposed by coinfection may vary with the mechanism of within‐host competition between parasites. Exploitative competition is predicted to favor more virulent parasites, whereas interference competition may result in lower virulence. Here, we examine whether exploitative or interference competition determines the outcome of competition between two nematode species (Steinernema spp.), which in combination with their bacterial symbionts (Xenorhabdus spp.), infect and kill insect hosts. Multiple isolates of each nematode species, carrying their naturally associated bacteria, were characterized by (1) the rate at which they killed insect hosts, and by (2) the ability of their bacteria to interfere with each other's growth via bacteriocidal toxins called “bacteriocins.” We found that both exploitative and interference abilities were important in predicting which species had a selective advantage in pairwise competition experiments. When nematodes carried bacteria that did not interact via bacteriocins, the faster killing isolate had a competitive advantage. Alternatively, nematodes could gain a competitive advantage when they carried bacteria able to inhibit the bacteria of their competitor. Thus, the combination of nematode/bacterial traits that led to competitive success depended on which isolates were paired, suggesting that variation in competitive interactions may be important for maintaining species diversity in this community.     

MALE ALTERNATIVE REPRODUCTIVE STRATEGIES IN A MARINE ISOPOD CRUSTACEAN (PARACERCEIS SCULPTA): THE USE OF GENETIC MARKERS TO MEASURE DIFFERENCES IN FERTILIZATION SUCCESS AMONG α-, β-, AND γ-MALES

Three discrete male morphs coexist in Paracerceis sculpta, a marine isopod crustacean inhabiting the northern Gulf of California. Ornamented α-males establish themselves in the spongocoels of intertidal sponges, where females congregate to breed. Smaller β-males, resembling sexually mature females, enter spongocoels by deception, while tiny γ-males invade spongocoels by stealth. Isopods breed year-round, and the operational sex ratio fluctuates widely over short durations. When females are abundant, receptive females accumulate in spongocoels, and these spongocoels are preferentially invaded by β- and γ-males. To test the hypothesis that the density of receptive females affects relative fertilization success among male morphs, individual β- and γ-males, heterozygous for a dominant cuticular pigmentation allele, were placed in artificial spongocoels with an unmarked α-male and densities of one, two, and three unmarked, receptive females. The fertilization success of each male was determined by counting the number of marked and unmarked progeny each female produced. Alpha-males guard females effectively and sire nearly all young when one female is in a spongocoel. The success of β- and γ-males increases, however, and may even exceed the success of α-males when two or three females are present. The regular occurrence of more than one receptive female in the harems of α-males may contribute to the persistence of β- and γ-males in this species.