INTERSPECIFIC INTERACTIONS AND SCARCITY OF A TROPICAL LIMPET

The øcmaeid limpet Lottia stipulata occurs in the midintertidal zone of rocky Pacific shores from Colombia to ElSalvador. It is regularly present but rare (<1 m^–2)at wave-exposed to semi-protected sites in Panama. The verticaldistribution of Lottia overlaps that of a pulmonate limpet,Siphonaria gigas, the most abundant molluscan grazer in themid zone (28. m^–2). Manipulations of the density of Siphonariasuggest the pulmonate negatively affects the recruitment, growth,and abundance of Lottia. Observations of populations of limpetsin an area undergoing succession support these results –Lottia recruits quickly and reaches high levels of abundance,but cannot maintain high densities as other benthic consumerslike S. gigas settle and increase in numbers over time. Paradoxically,examination of radulae suggests that if ability to use algalcrusts (the major available food) were of primary importance,Lottia would be the superior competitor. However, competitionneed not be invoked, and results can be explained by a one-wayrelationship between these two limpets. Lottia apparently hasno effect on Siphonaria, but the larger pulmonate probably actsas an agent of disturbance by interfering with foraging by Lottiaand by bulldozing or consuming newly-settled individuals. Predationon Lottia by a predaceous gastropod, Purpura columellaris, andby the American oystercatchcr, Haematopus palliatus, is insignificantat most sites, but may be important where these consumers areabundant. The abundance of Lottia appears to be controlled primarilyby disturbance by Siphonaria and by physical factors, not bypredation or competition. ¹Present address: Department of Zoology, University of Massachusetts,Amherst, Massachusetts 01003 ²Present address: Department of Zoology, University of RhodeIsland, Kingston, Rhode Island 02881 ³Order of authorship determined by coin toss. (Received 31 July 1984;     

SELECTION ON COPULATION DURATION IN DROSOPHILA MELANOGASTER: PREDICTABILITY OF DIRECT RESPONSE VERSUS UNPREDICTABILITY OF CORRELATED RESPONSE

Estimates of heritabilities and genetic correlations for seven reproductive attributes had previously been obtained from parent-offspring regression (Gromko, 1987, 1989). Copulation duration was shown to have a heritability of 0.23 and to be genetically correlated with courtship vigor (r_A = ?0.41) and with fertility (r_A = 0.27). These observations form the basis for the prediction of direct and correlated responses to selection for increased and decreased copulation duration, which are reported here. The direct response corresponded closely to prediction, but the correlated responses did not provide consistent qualitative fit. A hypothesis is proposed to explain this difference in predictability of direct and correlated response to selection. The major postulate is that the different polygenes involved in the direct response to selection for copulation duration have different pleiotropic effects.     

家畜多个显性性状半同胞家系选择的育种进展分析

本文提出了固定家畜多个不相连锁显性性状的半同胞家系选择体制,并从群体基因型频率、配子频率及表型频率等方面分析了该体制的育种进展．研究证明,半同胞家系选择效果取决于每头公畜交配母畜数K、母畜产仔数N及群体育种水平;K、N越大,群体育种水平越低,选择效果越好．当K=1时,半同胞家系选择就退化成全同胞家系选择;当K=1且N=1时,退化成“理想型横交固定”;当K较大且群体育种水平较低时半同胞家系选择效果与“多元测交”相当．最后讨论了半同胞家系选择在家畜育种实践中的应用问题.     

POPULATION GENETIC STRUCTURE OF CECROPIA OBTUSIFOLIA,A TROPICAL PIONEER TREE SPECIES

Theoretical analyses of the genetic organization of pioneer species have postulated two very different scenarios. Some models have predicted that such species would show strong population substructuring, whereas other models have suggested that extinction and recolonization can augment gene flow and reduce interpopulation differentiation. We tested these alternative scenarios by analyzing the genetic structure of eight loci from populations of the pioneer dioecious tree, Cecropia obtusifolia, in the tropical rain forest region of Los Tuxtlas, México. The populations studied exhibit low overall F_ST values, no clear pattern of isolation by distance, and high estimates of gene flow. These results suggest either that the species is not at a genetic equilibrium under present levels of gene flow with populations derived from each other in the recent past, or that pollen and seed dispersal in this species occur over long distances (up to more than 100 km). Mating among relatives appears higher than expected by chance based on significantly positive fixation indices (F) and F_IS values at some loci. However, no direct evidence for biparental inbreeding was found. The multilocus and single-locus outcrossing rates for C. obtusifolia were estimated at t_m = 0.974 (SE = 0.024) and t_s = 0.980 (SE = 0.035), respectively. These are not significantly different from 1, and the difference, t_m — t_s = — 0.006 (SE = 0.018), is not significantly different from 0. These estimates, however, could be biased because in all enzymes, except PGM-1, we found statistically significant departures from the mixed-mating model used to estimate them. Two rare alleles were found only in seeds collected from the soil, and the greatest number of different alleles were found also in soil seeds. It is hypothesized that the seed bank may play an important role in the genetic buffering of C. obtusifolia. Significantly positive or negative fixation indices in adults at some loci and significantly different heterozygosities among different life stages (from seeds to adults) suggest the action of selection at some loci.     

EFFECTS OF SINGLE AND MULTIPLE EXPOSURES OF FERULIC ACID ON THE VEGETATIVE AND REPRODUCTIVE GROWTH OF PHASEOLUS VULGARIS BBL-290

Phaseolus vulgaris BBL-290 plants were grown in growth chambers in the Southeastern Plant Environment Laboratory and exposed to either single (at seedling, flower, or podfill) or multiple (biweekly or weekly) treatments of ferulic acid (FA). In the first experiment, plants were harvested one week after FA treatment (0, 1.0, 2.0 mM) and at final harvest (56 days old). FA delayed leaf expansion during the seedling and flowering stages. The total plant leaf area and the plant dry weight of plants treated with 1.0 and 2.0 mM FA as seedlings were reduced one week after treatment by 38–48%. The total plant leaf area and the plant dry weight of plants treated at flowering with 2.0 mM FA were reduced by 25% one week after treatment. Treatment with 2.0 mM FA at podfill caused the senescence and abscission of older leaves and reduced total plant leaf area, plant dry weight and mean pod dry weight by 54, 40, and 48%, respectively, one week after treatment. The plants treated at the seedling and flowering stages recovered by final harvest. In a subsequent experiment, FA (0, 0.50, 1.0, 1.5 mM) reduced total plant leaf area at the seedling and flowering stages but not at podfill. The youngest expanding leaves were most sensitive to FA at flowering. The leaf area of these leaves was reduced by 35 and 25%, one and two weeks after treatment, respectively. Their absolute growth rates were reduced from 31 to 56% one week after treatment at flowering. Their relative growth rates were reduced by 50% one week after treatment. Growth rates then recovered within two weeks after treatment. In the final experiment, biweekly exposures of FA (0.25, 0.50, 0.75, 1.0) reduced total plant leaf area but did not affect any other growth parameters. Weekly exposures of FA (0.25, 0.50, 0.75, 1.0) reduced total plant leaf area up to 34%, absolute growth rate up to 58%, leaf number up to 31% and pod number up to 58%. As the frequency of exposure to FA increased, the concentration necessary to affect bean plant growth and development decreased.     

PHYTOPLANKTON COMMUNITY STRUCTURE: TEMPORAL VARIABILITY IN A TROPICAL UPWELLING ECOSYSTEM

The Cariaco Basin (Southeastern Caribbean Sea) is the largest anoxic basin of oceanic character. Its surface waters are affected by coastal upwelling during boreal winter and spring. Historical information on the phytoplankton communities in the basin is scarce. In November 1995, an oceanographic time-series station was established at 10.5°N - 64.66°W. In this study changes in the structure of the phytoplankton community in the upper 100-m layer were studied. Monthly samples to determine phytoplankton abundance and pigment composition (HPLC) were collected from November 1995 to January 1999; water temperature was used as proxy for upwelling. Surface waters reached temperatures ≤24° C during the upwelling season, and surpassed 26° C during the rest of the year. A total of 300 species were found. Generally, the highest number of cells (>500 cells ml⁻¹) were measured from January to April every year. Diatoms were the dominant group in terms of abundance, species composition (168 species) and pigments (Chl c and fucoxanthin) during this period. The abundance and marker pigment concentration (peridinin, but- and hex-fucoxanthin) for other groups support this observation. During the rest of the year diatom and total abundance decreased markedly (<100 cells ml⁻¹). Small organisms (<5 μm) became dominant. The maximum concentration values of zeaxanthin and But- and Hex-Fuco in this period indicated the presence of cyanobacteria and prymnesiophytes. The sparse and relatively low concentration of Chl b , lutein and prasinoxanthin indicated that chlorophytes and its allies are a minor floral component.     

MEASURING SELECTION ON A POPULATION OF DAMSELFLIES WITH A MANIPULATED PHENOTYPE

The estimation of the relationship between phenotype and fitness in natural populations is constrained by the distribution of phenotypes available for selection to act on. Because selection is blind to the underlying genotype, a more variable phenotypic distribution created by using environmental effects can be used to enhance the power of a selection study. I measured selection on a population of adult damselflies (Enallagma boreale) whose phenotype had been modified by raising the larvae under various levels of food availability and density. Selection on body size (combination of skeletal and mass at emergence) and date of emergence was estimated in two consecutive episodes. The first episode was survival from emergence to sexual maturity and the second was reproductive success after attaining sexual maturity. Female survival to sexual maturity was lower, and therefore opportunity for selection greater, than males in both years. Opportunity for selection due to reproductive success was greater for males. The total opportunity for selection was greater for males one year and for females the other. Survival to sexual maturity was related to mass gain between emergence and sexual maturity. Females gained more mass and survived less well than males in both years but there was no linear relationship between size at emergence and survival for females in either year. However, females in the tails of the phenotype distribution were less likely to survive than those near the mean. In contrast, small males consistently gained more mass than large males and survived less well in one year. There was significant selection on timing of emergence in both years, but the direction of selection changed due to differences in weather; early emerging females were more successful one year and late emerging males and females the other. The number of clutches laid by females was independent of body size. Because the resources used to produce eggs are acquired after emergence and this was independent of size at emergence, female fitness did not increase with size. Small males may have had lower survival to sexual maturity but they had higher mating success than large males. Resources acquired prior to sexual maturity are essential for reproductive success and may in some species alter their success in inter- and intrasexual competition. Therefore, ignoring the mortality associated with resource acquisition will give an incomplete and potentially misleading picture of selection on the phenotype.     

A PATH-ANALYTIC MODEL FOR THE MEASUREMENT OF SELECTION ON MORPHOLOGY

This paper describes a path model for the analysis of phenotypic selection upon continuous morphological characters. The path-analysis model assumes that selection occurs on unmeasured general size and shape allometry factors that summarize linear relations among sets of ontogenetically, phylogenetically, or functionally related traits. An unmeasured factor for general size is considered the only aspect of morphometric covariance matrices for which there is an a priori biological explanation. Consequently, selection coefficients are derived for each measured character by holding constant only a general size factor, rather than by using multiple regression to adjust for the full covariance matrix. Fitness is treated as an unmeasured factor with loadings, representing directional selection coefficients, computed as the covariances of the size-adjusted characters with the measured fitness indicator. The magnitudes and signs of the selection coefficients, combined with biological insight, may suggest hypotheses of selection on one or more shape allometry factors. Hypotheses of selection on general size and shape allometry factors are evaluated through cycles of measurement, analysis, and experimentation, designed to refine the path diagram depicting the covariances among the measured characters, the measured indicator of fitness, and unmeasured factors for morphology and fitness. The path-analysis and multiple-regression models were applied to data from remeasurement of Lande and Arnold's (1983) pentatomid bugs and to Bumpus's (1899) data on house sparrows. The path analysis suggested the hypothesis that variation in bug survivorship was an expression of directional selection on wing loading. Bumpus's data are consistent with a hypothesis of stabilizing selection on general size in females and directional selection for small wing size relative to body size in males.     

ESTIMATING THE FORM OF NATURAL SELECTION ON A QUANTITATIVE TRAIT

The fitness function f relates fitness of individuals to the quantitative trait under natural selection. The function is useful in predicting fitness differences among individuals and in revealing whether an optimum is present within the range of phenotypes in the population. It may also be thought of as describing the ecological environment in terms of the trait. Quadratic regression will approximate the fitness function from data (e.g., Lande and Arnold, 1983), but the method does not reliably indicate features of f such as the presence of modes (stabilizing selection) or dips (disruptive selection). I employ an alternative procedure requiring no a priori model for the function. The method is useful in two ways: it provides a nonparametric estimate of f, of interest by itself, and it can be used to suggest an appropriate parametric model. I also discuss measures of selection intensity based on the fitness function. Analysis of six data sets yields a variety of forms of f and provides new insights for some familiar cases. Low amounts of variation and a low density of data points near the tails of many phenotype distributions emerge as limitations to gaining information on fitness functions. An experimental approach in which the distribution of a quantitative trait is broadened through manipulation would minimize these problems.     

THE EFFECT OF SOFT SELECTION ON THE VARIABILITY OF A QUANTITATIVE TRAIT

The equilibrium phenotypic variance of a normally distributed quantitative character P under soft selection is studied. This character is assumed to undergo Gaussian stabilizing selection W(p, x) = exp[–(p – x)²/2w²]. The environmentally determined optimum (x) is a normal variable with variance s². A stable equilibrium with is found, so that increases both with increasing environmental heterogeneity and with increasing local intensity of stabilizing selection. It is shown that both genetic and environmental components of the variance are selected until this equilibrium is reached. Habitat selection, supposed to be normal (with variance H²) around the optimum, also increases the value. Nevertheless, relatively intense local stabilizing selection (w < s) and accurate habitat choice (H < s) are required for the initial spread and the evolutionary stability of this habitat selection.