Paulette Bierzychudek

Pollinator Limitation of Plant Reproductive Effort

In many recent studies there have been attempts to describe and explain patterns of resource allocation in plants (e.g., Abrahamson and Gadgil 1973; Newell and Tramer 1978). Nearly all such studies are based on the unstated assumption that the reproductive output of a plant is resource limited. However, if hand-pollinated plants produce more seeds than naturally pollinated controls, then reproduction is being limited, not by energy levels, but by pollinator activity. As part of a study on the population biology of Arisaema triphyllurm, a herbaceous forest perennial (Bierzychudek 1981), I hand pollinated some plants in a natural population. The difference between hand-pollinated and natural seed production was remarkably large. Hand-pollinated plants produced over an order of magnitude more seeds (the mean for medium-sized plants was 43.2 seeds/plant, N = 12) than did controls of similar size (mean = 1.0, N = 20). While there was no relationship between the size of a plant and the number of seeds it produced when naturally pollinated (r = .11, N = 35), this same relationship was significant for hand-pollinated plants (r = .81, N = 33, P < .01), indicating that only the hand-pollinated individuals were resource limited. There is no reason to assume that this level of seed production is unusually low for Arisaema; I observed only slightly higher levels at several other sites and in other years (with a maximum of 10.3 seeds/plant). Arisaemas pollinators (mycetophilid and sciarid flies) seem abundant, if not particularly efficient. Others have observed similar low rates of seed production by Arisaema (Meehan 1886; Sakamoto 1961; Treiber 1980). Is this an isolated case, or do pollinators limit the reproductive output of many plants? The yield of many cultivated crop plants has been found to be pollinator limited (McGregor 1976), but this might be expected when large plantings are made of a single crop species. Information on native species under natural conditions is more difficult o find. Schemske et al. (1978) note that only 33% of naturally pollinated flowers of Eiythronium albidum set seed, compared with 78% of flowers outcrossed by hand. Willson et al. (1979) show that 82.3% of handoutcrossed Phlox divaricata produced some seed, compared to 58% of naturally pollinated plants at the same site. In Costa Rica 29.7% of hand-outcrossed flowers of Combretumfrtuticosurn set fruit, compared to 7% of naturally pollinated flowers (Schemske, in press). Brassavola nodosa, a Central American orchid, also seems to be pollinator limited: 67% of hand-outcrossed flowers set fruit, versus 12% of naturally pollinated flowers (Schemske 1980). Weller (1980) found the fecundity of hand-pollinated Lithospermum caroliniense averaged 17% compared to 9% for naturally pollinated individuals, a significant difference. Finally, Encyclia cordigera, another orchid, may also be pollinator limited: Among hand-pollinated inflorescences, the proportion producing no fruit ranged from 5% to 22%, depending on the identity of the pollen donors. Seventy-eight percent of naturally pollinated inflorescences set no fruit (Janzen et al. 1980). The results of some hand pollination experiments are quite different. Stephenson (1979) found that hand-pollinated Catalpa speciosa produced significantly

Patterns in plant parthenogenesis

Plant taxa that reproduce asexually display some distinct geographical and ecological patterns. A literature review reveals that such taxa 1) tend to have larger ranges, 2) tend to range into higher latitudes, and 3) tend to range to higher elevations than do their sexual relatives. Asexual taxa have a greater tendency than sexual taxa do to colonize once-glaciated areas. These trends have previously been identified as characteristic of parthenogenetic animals as well. While many authors have interpreted these trends as providing support for the ‘biotic uncertainty’ hypothesis for the maintenance of sex, these trends are consistent with several other interpretations as well. Furthermore, all of these interpretations have ignored the positive correlation that exists between ploidy level and breeding system: asexual plant and animal taxa are generally polyploid, while their sexual relatives are generally diploid. Evidence is presented for plants, and by extension for animals as well, that high ploidy levels alone could (independent of breeding system) endow individuals with the ability to tolerate these ‘extreme’ environments. For this reason, it appears premature to interpret observed distribution patterns as evidence to support hypotheses about what forces maintain sexual reproduction. Only experimental tests, using sexuals and asexuals of comparable ploidy levels, can permit us to discriminate among the alternatives.

PERSPECTIVE: EVOLUTION OF FLOWER COLOR IN THE DESERT ANNUAL LINANTHUS PARRYAE: WRIGHT REVISITED

Abstract.— Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a longstanding debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long‐term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue‐flowered plants produced more seeds than white‐flowered plants in years of relatively low seed production, whereas white‐flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the 11 years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue‐flowered plants typically had a fitness advantage in years of low spring precipitation, whereas white‐flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower‐color polymorphism at Pearblossom site 1, whereas gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water‐use efficiency. Although the mechanism of selection remains elusive, our results refute Wrights conclusion that the flower color polymorphism in L. parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.

How do plant ecologists use matrix population models

Matrix projection models are among the most widely used tools in plant ecology. However, the way in which plant ecologists use and interpret these models differs from the way in which they are presented in the broader academic literature. In contrast to calls from earlier reviews, most studies of plant populations are based on < 5 matrices and present simple metrics such as deterministic population growth rates. However, plant ecologists also cautioned against literal interpretation of model predictions. Although academic studies have emphasized testing quantitative model predictions, such forecasts are not the way in which plant ecologists find matrix models to be most useful. Improving forecasting ability would necessitate increased model complexity and longer studies. Therefore, in addition to longer term studies with better links to environmental drivers, priorities for research include critically evaluating relative/comparative uses of matrix models and asking how we can use many short-term studies to understand long-term population dynamics.

SPATIAL DIFFERENTIATION FOR FLOWER COLOR IN THE DESERT ANNUAL LINANTHUS PARRYAE: WAS WRIGHT RIGHT?

Abstract Understanding the evolutionary mechanisms that contribute to the local genetic differentiation of populations is a major goal of evolutionary biology, and debate continues regarding the relative importance of natural selection and random genetic drift to population differentiation. The desert plant Linanthus parryae has played a prominent role in these debates, with nearly six decades of empirical and theoretical work into the causes of spatial differentiation for flower color. Plants produce either blue or white flowers, and local populations often differ greatly in the frequencies of the two color morphs. Sewall Wright first applied his model of “isolation by distance” to investigate spatial patterns of flower color in Linanthus. He concluded that the distribution of flower color morphs was due to random genetic drift, and that Linanthus provided an example of his shifting balance theory of evolution. Our results from comprehensive field studies do not support this view. We studied an area in which flower color changed abruptly from all-blue to all-white across a shallow ravine. Allozyme markers sampled across these regions showed no evidence of spatial differentiation, reciprocal transplant experiments revealed natural selection favoring the resident morph, and soils and the dominant members of the plant community differed between regions. These results support the hypothesis that local differences in flower color are due to natural selection, not due to genetic drift.

LOOKING BACKWARDS: ASSESSING THE PROJECTIONS OF A TRANSITION MATRIX MODEL

Analyses of population projection models are increasingly being used by conservation biologists and land managers to assess the health of sensitive species and to evaluate the likely effects of management strategies, harvesting, grazing, or other manipulations. Here I describe some of the limitations of this approach and illustrate how these limitations may affect its usefulness. I do this by comparing the results of such an analysis, performed in 1979 on two populations of a perennial plant, Arisaema triphyllum, with new information about the size and structure of these same populations gathered in 1994, 15 years later. While one population changed as the model projected it would, the other behaved quite differently from the projection. Instead of increasing in size, this population decreased between 1979 and 1994. Possible shortcomings in the data and in the model include: too few plants to provide accurate transition probabilities; too few years to capture accurately the complete range of year-to-year ...

Determinants of gender in Jack-in-the-pulpit: the influence of plant size and reproductive history

SummaryJack-in-the-pulpit, Arisaema triphyllum, is a perennial forest herb with the ability to change sex. At two sites in upstate New York, plant sex was correlated with plant size: males were smaller than females, nonflowering plants were smaller than males. Changes in plant size were accompanied by changes in sex. Sex change occurred quite frequently; at one site, 8% of nonflowering plants, 64% of males, and 63% of females changed their sex from one season to the next. The probability that a plant will change size and sex between years was altered by artificial defoliation and by the production of seeds, but was not affected by supplementing plants with nutrient fertilizer. Discriminant analysis indicated that several historical factors significantly affected plant sex: a model including the variables of current plant size, previous years plant size, and previous years sex was significantly better at predicting the current sex of individuals than was a model containing only current plant size. However, even the consideration of these three variables left up to a third of the plants misclassified with respect to gender. This analysis explains in part why plants at the two sites changed sex at different sizes, but it is likely that other factors-e.g. genetic differences-are involved.

STABLE TWO-ALLELE POLYMORPHISMS MAINTAINED BY FLUCTUATING FITNESSES AND SEED BANKS: PROTECTING THE BLUES IN LINANTHUS PARRYAE

Abstract.— Motivated by data demonstrating fluctuating relative and absolute fitnesses for white‐ versus blue‐flowered morphs of the desert annual Linanthus parryae, we present conditions under which temporally fluctuating selection and fluctuating contributions to a persistent seed bank will maintain a stable single‐locus polymorphism. In L. parryae, blue flower color is determined by a single dominant allele. To disentangle the underlying diversity‐maintaining mechanism from the mathematical complications associated with departures from Hardy‐Weinberg genotype frequencies and dominance, we successively analyze a haploid model, a diploid model with three distinguishable genotypes, and a diploid model with complete dominance. For each model, we present conditions for the maintenance of a stable polymorphism, then use a diffusion approximation to describe the long‐term fluctuations associated with these polymorphisms. Our protected polymorphism analyses show that a genotype whose arithmetic and geometric mean relative fitnesses are both less than one can persist if its relative fitness exceeds one in years that produce the most offspring. This condition is met by data from a population of L. parryae whose white morph has higher fitness (seed set) only in years of relatively heavy rain fall. The data suggest that the observed polymorphism may be explained by fluctuating selection. However, the yearly variation in flower color frequencies cannot be fully explained by our simple models, which ignore age structure and possible selection in the seed bank. We address two additional questions–one mathematical, the other biological–concerning the applicability of diffusion approximations to intense selection and the applicability of long‐term predictions to datasets spanning decades for populations with long‐lived seed banks.

Environmental sensitivity of sexual and apomictic Antennaria: do apomicts have general-purpose genotypes?

The fact that apomictic taxa typically occupy a wider range of environments than their sexual relatives has generated the hypothesis that apomicts are more likely to possess “general‐purpose genotypes,” i.e., genotypes whose performance is relatively insensitive to changes in environmental conditions. This hypothesis was tested by cloning sexual and apomictic females of Antennaria parvifolia (Asteraceae) and growing each genotype in six growth‐chamber environments varying in temperature and moisture levels. A joint regression analysis revealed that the survival of apomictic genotypes was significantly less sensitive to environmental conditions than that of sexual genotypes but demonstrated no differences with regard to flowering or biomass. However, the coefficient of variation in biomass across the six environments was significantly lower for apomicts than for sexuals, and the geometric mean of survival over the six environments was significantly higher for apomicts. Apomicts significantly exceeded sexuals in mean survival, mean flower‐head production, and mean biomass. These results support the hypothesis that apomictic genotypes are more “general‐purpose” than sexuals, and increase the difficulty of explaining the persistence of sexual reproduction in A. parvifolia.

Assessing «optimal» life histories in a fluctuating environment: the evolution of sex-changing by jack-in-the-pulpit

Arisaema triphyllum plants change sex: small individuals are males while large plants are females. Contrary to the expectations generated by the size-advantage model, though, reproductive success is independent of size for existing males and females, because it is pollinator-limited rather than resource-limited. Estimates of the likely reproductive effort that would be borne by nonexistent small females suggest, however, that there is a size threshold below which female reproduction is prohibitively costly. This threshold creates selection for sex-changing, but because of temporal fluctuations in seed production and sex ratio, the magnitude and even the direction of this selection are not constant. Such fluctuations make it unlikely that jack-in-the-pulpits ever attain the simple sort of optimum sex-changing strategy described by theory predicated on assumptions of population stability.