The inbreeding depression cost of selfing: Importance of flower size and population size in Collinsia parviflora (Veronicaceae)

Inbreeding depression should evolve with selfing rate when frequent inbreeding results in exposure of and selection against deleterious alleles. The selfing rate may be modified by plant traits such as flower size, or by population characteristics such as census size that can affect the probability of biparental inbreeding. Here we quantify inbreeding depression (δ) among different population sizes of Collinsia parviflora, a wildflower with interpopulation variation in flower size, by comparing fitness components and multiplicative fitness of experimentally produced selfed and outcrossed offspring. Selfed offspring had reduced multiplicative fitness compared to outcrossed offspring, but inbreeding depression was low in all combinations of population size and flower size (δ ≤ 0.05) except in large populations of large-flowered plants (δ = 0.45). The decrement to multiplicative fitness with inbreeding was not affected by population size nested within flower size, but differed between small- and large-flowered plants: small-flowered populations had lower overall inbreeding depression (δ = 0.04) compared to large-flowered populations (δ = 0.25). The difference in load with flower size suggests that either selection has removed deleterious recessive alleles or these alleles have become fixed in small-flowered, potentially more selfing populations, but that purging has not occurred to the same extent in presumably outcrossing large-flowered populations.     

The mechanism of delayed selfing in Collinsia verna (Scrophulariaceae)

Collinsia verna, blue-eyed Mary, has floral attributes of an outcrossing species, yet most flowers readily self-pollinate under greenhouse conditions. Here we describe the mechanism of self-pollination in C. verna via changes in relative positions of the stigma and anthers and late timing of receptivity, resulting in delayed selfing. Each flower contains four anthers that dehisce sequentially over ∼1 wk. Pollen that is not collected by pollinators accumulates in the keel petal and retains high viability (>80% pollen germination) up to the time of corolla abscission. The stigmatic surface does not become receptive until after the third anther dehisces. This overlap in the sexual phases is concurrent with a change in herkogamy during floral development. In most flowers (70%), the stigma has moved to the front of the keel and is positioned near the anthers when the third anther dehisces. Under field conditions, fruiting success of plants within pollinator exclosures was ∼75% of the fruiting success in open-pollinated plants (33% fruiting success via autogamy vs. 44% fruiting success, respectively). Collinsia verna plants in pollinator exclosures exhibit variation in autogamy rates within natural populations (range 0–80%). In addition, only half of naturally pollinated, receptive flowers examined had pollen tubes growing in their styles. In contrast, shortly after corolla abscission, nearly all flowers examined (96%) had pollen tubes in their styles. Thus we find that in C. verna, autogamy occurs late in floral development, which has the potential to provide substantial reproductive assurance, and that individuals vary in their ability to set fruit through this mechanism. We suggest that delayed selfing mechanisms may be overlooked in other species and that variable pollinator availability may play a significant role in the maintenance of mixed mating in species with delayed selfing, such as C. verna.     

Variation in sex allocation and male-female trade-offs in six populations of Collinsia parviflora (Scrophulariaceae s.l.)

Assumed trade-offs between male and female functions in hermaphroditic flowers have been difficult to demonstrate. Collinsia parviflora (Scrophulariaceae) is a winter annual that exhibits significant among-population variation in corolla size in British Columbia, Canada. We asked whether reduction in secondary male allocation (i.e., the attractive corolla), a preliminary indicator of mating system, was matched by a reduction in primary male allocation (i.e., pollen production), and whether there were allocation trade-offs between male and female function both within and among six study populations. Larger-flowered populations allocated more to male function (androecium and corolla biomass), and because populations did not vary in female biomass allocation (gynoecium and calyx), population differences were not due to simple allometric scaling. Populations also differed in per-flower gamete production (pollen and ovules). We found male-female trade-offs within populations between androecium and gynoecium mass and between corolla and calyx mass. Among populations, there was a marginal trade-off between pollen and ovule production and a significant within-male trade-off between pollen grain size and number. Trade-offs between the sexes were primarily apparent when we controlled for flower size in the analysis. Variation among populations in sex allocation may reflect different optima related to the mating system.     

Population differentiation in Collinsia verna Nuttal (Scrophulariaceae): a multifaceted approach

Three species of Collinsia (commonly called Blue-eyed Mary, Innocence, Chinese Houses) including 12 populations of C. verna were examined for genetic variability and differentiation using three investigative techniques: enzyme electrophoresis, chiasma frequencies, and morphological measurements. Analysis of isozyme data showed higher heterozygosity was maintained in the C. verna populations (previously reported to be self pollinating) than in C. heterophylla and C. tinctoria, which are outcrossers. C. heterophylla and C. tinctoria were separated by a significant genetic distance from each other and especially from C. verna. Within C. verna isozyme, morphological, and chiasma frequency analyses separated the populations into subsets showing some geographical trends, possibly influenced by water dispersal of seeds. High genetic variability was maintained and differentiation has occured in the C. verna populations. The explanation may lie in a combination of factors associated with the heterozygosity paradox including microgeographical influence and inappropriate mating system classification.     

Reproductive strategies in two arctic Pedicularis species (Scrophulariaceae)

A study of a number of reproductive traits in two sympatric species of Pedicularis in northern Swedish Lapland, the subarctic-alpine P lapponica and the artic P hursuta , revealed that the life-history strategies of the two species differ profoundly High fruit set and low seed abortion rate, as m P hursuta , is common in arctic plants in late-thawing habitats and represents a case of extreme adversity selection rather than an indication of a ruderal life-history strategy Pedicularis lapponica , on the other hand, is a typical K-strategist (or stress-tolerator) requiring a longer period of growth for optimal reproduction Occuring at both low and high altitudes in the area, P lapponica tends to increase in self-compatibility with altitude, which is interpreted as an adaptation to lower pollinator visitation frequency in arctic environments The variation in length of the protruding part of the style in P lapponica is shown to be correlated with exposure to light Predispersal seed predation is severe m P lapponica at low altitudes, where the capsules are attacked by fly and moth larvae At high altitudes, a minor proportion of the capsules of P lapponica experience predation and only from flies, while P hursuta is completely unpredated     

The reproductive assurance benefit of selfing: importance of flower size and population size

Autonomous selfing can provide reproductive assurance (RA) for flowering plants that are unattractive to pollinators or in environments that are pollen limited. Pollen limitation may result from the breakdown of once-continuous habitat into smaller, more isolated patches (habitat fragmentation) if fragmentation negatively impacts pollinator populations. Here we quantify the levels of pollen limitation and RA among large and small populations of Collinsia parviflora, a wildflower with inter-population variation in flower size. We found that none of the populations were pollen limited, as pollen-supplemented and intact flowers did not differ in seed production. There was a significant effect of flower size on RA; intact flowers (can self) produced significantly more seeds than emasculated flowers (require pollen delivery) in small-flowered plants but not large-flowered plants. Population size nested within flower size did not significantly affect RA, but there was a large difference between our two replicate populations for large-flowered, small populations and small-flowered, large populations that appears related to a more variable pollination environment under these conditions. In fact, levels of RA were strongly negatively correlated with rates of pollinator visitation, whereby infrequent visitation by pollinators yielded high levels of RA via autonomous selfing, but there was no benefit of autonomous selfing when visitation rates were high. These results suggest that autonomous selfing may be adaptive in fragmented habitats or other ecological circumstances that affect pollinator visitation rates.     

Reproductive strategy as population density varies in Daphnia magna (Cladocera)

• 1 In this study an experiment was carried out to test whether Daphnia magna (Cladocera) changed its reproductive strategy in response to varying population density but constant food supply.
• 2 As population density increased there was a reduction in the age at which females reproduced. Early reproduction was offset by smaller body length and clutch size (number of eggs per female) relative to later reproducing females grown at lower population densities.
• 3 This pattern was interpreted as a strategy carried out by females to reduce the time at which they release their offspring. One potential advantage of early reproduction is to reduce the risk of severe food limitation for neonates born during a period of rapid population increase.

     

Light and scanning electron microscopic studies of flower development inLindenbergia macrostachya Benth. (Scrophulariaceae)

All the floral primordia are homologous to leaves in their development inLindenbergia macrostachya. The sepals follow an anterior to posterior sequence of initiation. The petals and stamens are initiated almost simultaneously but sequentially in order of petals followed by stamens. There is no sign of development of fifth posterior stamen. p ]The calyx tube is formed by interprimordial growth followed by zonal growth. The combined interprimordial growth between the petal primordia and growth on the abaxial side of stamen primordia results in the formation of upper corolla tube whereas lower corolla tube is formed only by zonal growth. The zonal growth extends below the bases of stamen primordia also due to which they become epipetalous. The placentae arise from the carpellary margins, move inwards and get fused in the lower half and remain free in the upper part of the ovary. Thus the ovary appears biloeular with axile plaeentation in the lower haler and unilocular with parietal placentation in the upper half.     

Identification of microsatellite loci in Collinsia verna (Veronicaceae)

We developed eight polymorphic microsatellite loci for Collinsia verna (Veronicaceae). In a sample of 18–35 individuals from a single population, we found two to 15 alleles per locus (mean 8.3). We also tested these loci for cross‐amplification in all 22 species in the tribe Collinseae. Overall, more than half the species in the tribe amplified one microsatellite while three species most closely related to C. verna (Collinsia violacea, Collinsia parviflora and Collinsia grandiflora) amplified multiple microsatellite loci. These microsatellite loci will be used in future studies of mating system in this tribe and other quantitative genetic and population genetic studies.     

Reproductive assurance through self-fertilization does not vary with population size in the alien invasive plant Datura stramonium

Autonomous self-fertilization is suggested to be associated with invasiveness in plants because it offers reproductive assurance when there is a shortage of suitable mates or pollinators. Given that shortages of mates and pollinators are a common cause of Allee effects in small plant populations, we predict that the benefits of self-fertilization in terms of reproductive assurance should be greatest in small populations. We tested this idea for the invasive herb Datura stramonium , a self-fertilizing species which is also cross-pollinated to some extent by insects (mainly hawkmoths and honeybees). During two consecutive years, we studied 20 and 55 populations, respectively, of different sizes. Untreated flowers of D. stramonium showed high levels of fruit and seed set in all populations studied. Although, fruit and seed set were generally reduced by about 90% in flowers in which self-fertilization was prevented through emasculation, this effect did not vary according to population size. By using a natural color (anthocyanin) dimorphism in 12 populations, we showed that the average outcrossing rate was low (1.3%) and that there was no relationship between outcrossing rate and population size. Pollen removal from flowers also did not vary according to population size, suggesting that the pollinator visitation rate is not lower in small populations. However, decreasing deviations of observed from expected fruit set with population size imply that small populations may have an increased chance of extinction due to demographic stochasticity. Overall, our results suggest that reproductive assurance through self-fertilization in invasive plants may be important for all stages of population establishment, and not just in the founder population.     

How pollinator-mediated mating varies with population size in plants

In most higher plants sexual interactions are mediated by animal pollinators that affect the number and differential reproductive success of mates. The number and sex of breeding individuals in populations are central factors in evolutionary theory, but the quantitative effect of plant population size on pollinator-mediated mating is understudied. We investigated variation in pollen removal (male function) and fruit set (female function) among flowering populations of different size of two bumblebee-and one butterfly-pollinated, rewardless, pollen-limited, hermaphroditic orchid species in Sweden. As the amount of pollen removed from plants by insects (either absolute or proportional) increased, so did the number of pollinations, whereas the proportions of plants with different pollinator-designated functional sex (male, female, hermaphrodite) depended primarily on the ratio between the amount of fruit set and pollen removed within populations. A larger population size was found to have several effects: (1) the total numbers of pollinia removed and fruits set increased; (2) the proportion of pollen removed from plants decreased; (3) the proportion of flowers pollinated decreased in the butterfly-but was not affected in the bumblebee-pollinated species; (4) the ratio between fruits set and pollinia removed increased linearly in the bumblebee-pollinated species but reached a maximum at c. 80 individuals in the butterfly-pollinated species; (5) the numbers of pollinator-designated pure male and hermaphrodite individuals increased; and (6) the variance in pollinium removal, but not fruit set, increased among individuals. These findings empirically verify the basic importance of population size for the mating structure of outcrossing plants, and indicate that selection for female sexual traits is reinforced when population size is smaller while selection for male sexual traits is reinforced when population size is larger.     

Reproductive assurance and the evolutionary ecology of self-pollination in Clarkia xantiana (Onagraceae)

The reproductive assurance hypothesis posits that selection favors self-pollination in flowering plants where mates and/or pollinators are scarce. A corollary is that self-pollinating populations are expected to be superior colonizers of mate- and pollinator-scarce environments. The California annual Clarkia xantiana includes outcrossing populations (ssp. xantiana) and autogamously self-pollinating populations (ssp. parviflora). Outcrossing is ancestral, and the subspecies have parapatric distributions with a narrow contact zone. We tested aspects of the reproductive assurance hypothesis by examining geographic and subspecies variation in the densities of mates and pollinators (native bees) and the density dependence of pollinator visitation and pollen receipt. Plant and flower densities, pollinator density, and pollinator visitation rates were lowest in the region of exclusively self-pollinating populations. Pollinator assemblages there lacked Clarkia-associated pollinator taxa that were common elsewhere. Self-pollinating populations in the contact zone generally had densities and visitation rates intermediate between allopatric self-pollinating populations and outcrossing populations. Visitation rate and pollen receipt increased significantly with plant density. These findings suggest that selection for reproductive assurance influenced the origin of self-pollination and/or that reproductive assurance influenced the geographic distribution of self-pollination. Geographic variation in pollinator assemblages may have generated variation in the value of reproductive assurance.     

Chromosome numbers in AustralianEuphrasia (Scrophulariaceae)

Chromosome numbers for six Australian taxa ofEuphrasia have been determined. Improved staining techniques have shown that numbers for four of the taxa published previously by the first author were incorrect. The investigated taxa show high ploidy levels with an apparent base number of x = 11, the same as for the genus outside Australia.Dedicated to Prof. DrElisabeth Tschermak-Woess on the occasion of her 70th birthday.     

Chromosome numbers in chelone (Scrophulariaceae)

Meiotic and mitotic counts for all major taxa withinChelone show a euploid series: 2n = 28, 56, 84. A base number ofx = 14 is suggested. The distribution of numbers supports some previous taxonomic decisions. It raises questions about others, particularly those involving theC. glabra-C. obliqua complex. A root-tip technique involving digestion by “Pectinase” is described.     

Genome size of chrysophytes varies with cell size and nutritional mode

The cellular content of nuclear DNA varies up to 200,000-fold between eukaryotes. These differences can arise via different mechanisms. In particular, cell size and nutritional mode may influence evolution of the nuclear DNA content. Chrysophytes comprise organisms with different cell organizations and nutritional modes. Heterotrophic clades evolved independently several times from phototrophic or mixotrophic ancestors. Thus, chrysophytes are an ideal model taxon for investigating the effect of the nutritional mode on cellular DNA content. We investigated the genome size of heterotrophic, mixotrophic, and phototrophic chrysophytes. We demonstrate that cell sizes and genome sizes differ significantly between taxa with different nutritional modes. Phototrophic strains tend to have larger cell volumes and larger genomes than heterotrophic strains do. The investigated mixotrophic strains had intermediate cell volumes and small to intermediate genome sizes. Heterotrophic chrysophytes had the smallest genomes and cell volumes compared to other chrysophytes. In general, genome size increased with cell volume, but cell volume only partially explained the variation in genome size. In particular, genome sizes of mixotrophic strains were smaller than expected based on cell sizes.     

The interaction between pollinator size and the bristle staminode of Penstemon digitalis (Scrophulariaceae)

Penstemon digitalis, a prairie species whose flowers possess a large bristle staminode, is visited by eight bee species of varying size. Two sets of field experiments involving staminode removal were performed to test pollination efficiency in relation to bee size. Our data indicate that bristle staminode presence and function are influenced by size-dependent selection on bee body size and associated pollen transporting attributes. The first experiment compared staminode presence and removal in open-pollinated flowers. Staminode removal significantly reduced total pollen deposition but had no effect on total pollen removal. The second experiment utilized single bee visits to assess the interaction between pollinator size and staminode presence on the rate of pollen deposition and removal. This experiment indicated that staminode removal resulted in fewer pollen grains deposited on stigmas and less pollen removed from anthers for both large and small bees (the contrary was true for medium bees). Although the number of pollen grains deposited was greatly reduced for large bees, staminode removal reduced deposition efficiency most notably for small bees by 41.3%, reducing female reproductive success. Staminode removal increased pollen removal efficiency most notably for medium bees by 22.7%, reducing male reproductive success. Mechanisms of staminode function are discussed.     

Limosella subulata Ives (Scrophulariaceae) in Venezuela

DIAZ-MIRANDA, D., 1992. Limosella subulata Ives (Scrophulariaceae) in Venezuela.
Field and herbarium studies revealed that the genus Limosella is represented in Venezuela by a single species, Limosella subulata Ives. A distinct character of the Venezuelan material studied is the presence of 4-merous flowers, which has not been reported previously for this species.     

Pollen competition reduces inbreeding depression in Collinsia heterophylla (Plantaginaceae)

We tested two predictions of the hypothesis that competition between self-pollen may mitigate negative genetic effects of inbreeding in plants: (1) intense competition among self-pollen increases offspring fitness; and (2) pollen competition reduces the measured strength of inbreeding depression. We used Collinsia heterophylla (Plantaginaceae), an annual with a mixed mating system, to perform controlled crosses in which we varied both the size of the pollen load and the source of pollen (self vs. outcross). Fitness of selfed offspring was higher in the high pollen-load treatment. Our second prediction was also upheld: inbreeding depression was, on average, lower when large pollen loads were applied (11%) relative to the low pollen-load treatment (28%). The reduction was significant for two fitness components relatively late in the life-cycle: number of surviving seedlings and pollen-tube growth rate in vitro. These findings suggest that intermittent inbreeding, which leads to self-fertilization in plants with genetic loads, may select for traits that enhance pollen competition.