Showing papers in "American Journal of Botany in 1995"

Dioecy and its correlates in the flowering plants

Abstract: Considerable ffort has been spent documenting correlations between dioecy and various ecological and morphological traits for the purpose of testing hypotheses about conditions that favor dioecy. The data analyzed in these studies, with few exceptions, come from local floras, within which it was possible to contrast he subsets of dioecious and nondioecious taxa with regard to the traits in question. However, if there is a strong phylogenetic component o the presence or absence of dioecy, regional sampling may result in spurious associations. Here, we report results of a categorical multivariate analysis of the strengths ofvarious associations of dioecy with other traits over all flowering plants. Families were scored for presence of absence of monoecy or dioecy, systematic position, numbers of species and genera, growth forms, modes of pollination and dispersal, geographic distribution, and trophic status. Seven percent of angiosperm genera (959 of 13,500) contain at least some dioecious species, and ;6% of angiosperm species (14,620 of 240,000) are dioecious. The most consistent associations in the data set relate the presence of dioecy to monoecy, wind or water pollination, and climbing rowth. At both the family and the genus level, insect pollination is underrepresented among dioecious plants. At the family level, a positive correlation between dioecy and woody growth results primarily from the association between dioecy and climbing growth (whether woody or herbaceous) because neither the tree nor the shrub growth forms alone are consistently correlated with a family's tendency to include dioecious members. Dioecy appears to have evolved most frequently via monoecy, perhaps through divergent adjustments of floral sex ratios between individual plants. Monoecy itself is related to abiotic pollination and climbing rowth as revealed by multivariate analysis. Dioecy and monoecy are concentrated in the less advanced superorders of Thorne (1992) and subclasses of Cronquist (1988). The frequency of dioecy found in a local flora therefore flects the level of dioecy in its particular pool of families as much as, or more than, local selective factors. The positive associations of dioecy with abiotic pollination and monoecy are related to floral developmental nd morphological attributes, as is the negative association with bird and bat pollination; the positive association of dioecy with climbing growth is tentatively explained in terms of differential se ection for optimal resource allocation to sexual function. If rapid upward growth is at a premium in climbers and if fruit set at least temporarily inhibits growth or requires the production of thicker, more slowly growing stems to support heavy fruits, itmight be advantageous to postpone femaleness. Ifthe effect is strong, this may favor male plants.

Seed-Seedling Conflicts, Habitat Choice and Patterns of Plant Recruitment

Abstract: The ecological forces determining where within a landscape plants recruit and grow has been termed proximal habitat choice. Habitat choice is imposed first by a heterogeneous pattern of seed dispersal across the patches that make up the landscape and second by environmental variation that favors plant survival in some patches more than in others. Seedseedling conflicts can occur during both of these phases of habitat choice if conditions or traits that are favorable for seeds are unfavorable for seedlings. During the dispersal phase, smaller seeds may have a greater probability of dispersal than larger seeds, and thus a greater probability of escape from predation, but they contain fewer reserves for support of the establishing seedling. After dispersal, environmental characteristics of a given patch type that lead to disproportionately high seed survival may lead to disproportionately low seedling survival. Considering three hypothetical landscapes, each composed of five patch types, I demonstrate that seed-seedling conflicts can have a major impact on both the overall quantity of recruitment at the landscape level and on the distribution of recruitment among patches. Available empirical evidence suggests these conflicts may be widespread in natural systems. Given their potential importance and extent, seed-seedling conflicts may play a previously unrecognized role in habitat choice.

Interannual variability in primary production in tallgrass prairie: climate, soil moisture, topographic position, and fire as determinants of aboveground biomass

Abstract: From 1975 to 1993, aboveground net primary production (NPP) at the Konza Prairie Research Natural Area in NE Kansas varied from 179 g/m2 to 756 g/m2. Across a variety of sites, NPP was significantly related to precipitation (r2 = 0.37), but much variability was unexplained. Thus, we evaluated the relationship between NPP with meteorological variables and soil moisture measurements in tallgrass prairie sites that varied in fire frequency and topographic position. Annually burned lowland sites had significantly higher NPP than either annually burned upland or unburned sites. NPP in burned sites was more strongly related to meteorological variables and soil moisture when compared to unburned sites. The lack of significant correlation between soil moisture with NPP on unburned sites suggests that factors other than water availability limit production in these sites. When NPP data were analyzed separately by life forms, interannual variability in forb NPP was not correlated with any meteorological variables, but was negatively correlated with grass NPP (r = -0.49). The inability of a single factor, such as precipitation to explain a large portion of the interannual variability in NPP is consistent with the concept that patterns of NPP in tallgrass prairie are a product of spatial and temporal variability in light, water, and nutrients, driven by a combination of topography, fire history, and climate.

The role of hybridization in evolution: old wine in new skins

Abstract: Perhaps no one has had a greater influence on the development ofhybridization theory than Dr. Ledyard Stebbins. However, his contributions have not been restricted to his writings or lectures alone. Many of you have marveled at the great authority with which Dr. Stebbins identified the hybrid status, parentage, and ploidal level of plants in the field or on herbarium sheets. He didn't seem to need all the modem techniques that the current generation of botanists is both blessed and burdened with. For example, I remember a short field trip with Dr. Stebbins in the fall of 1984 to look at some sunflower hybrids near Davis. The plants were dead, only a few dried leaves, stalks, and floral heads remained, yet he confidently identified parental, hybrid, and backcrossed generations. Ironically, after 2 yr of careful molecular studies, I essentially confirmed his initial observations (Rieseberg, Soltis, and Palmer, 1988). Thus, I came to the conclusion that he was able to see things in plants that escaped less careful or less knowledgeable observers such as myself. The interest of the botanical community in plant hybridization appears to be somewhat cyclical. From the late 1940s through the middle 1960s, much of the morphological and cytological variation in plant populations was attributed to introgressive hybridization (e.g., Anderson, 1949; Stebbins, 1959). Over the next two decades, these interpretations were viewed with greater skepticism (Barber and Jackson, 1957; Gottlieb, 1972; Heiser, 1973; Doebley, 1984), and I think appropriately so, leading to perhaps less interest in hybridization and its evolutionary role. In the early 1990s, however, there appears to have been a renewed interest in hybrids, stimulating several recent reviews (e.g., Abbot, 1992; Arnold, 1992, 1994; Rieseberg and Brunsfeld, 1992; Rieseberg and Wendel, 1993; Strauss, 1994). This resurgence in interest has led to a reevaluation of some of the central tenets or dogmas ofhybridization theory (e.g., Rieseberg and Ellstrand, 1993; Arnold and Hodges, in press), as well as to the origin of some new ideas and approaches to the study of hybrids (e.g., Whitham, 1989; Rieseberg et al., 1993). The purpose of this paper is to discuss some general misconceptions that many botanists and zoologists appear to hold about hybrids, as well as to present several of the

Exceptional seed longevity and robust growth: ancient Sacred Lotus from China

Abstract: A 1,288 + 271-yr-old (1,350 ? 220 yr BP, radiocarbon age) seed of Sacred Lotus (Nelumbo nucifera Gaertn.) from an ancient lake bed at Pulantien, Liaoning Province, China, has been germinated and subsequently radiocarbon dated. This is the oldest demonstrably viable and directly dated seed ever reported, the preserved relict of one of the early crops of lotus cultivated by Buddhists at Pulantien after introduction of the religion into the region prior to 372 A.D. A small portion of the dry pericarp of a second lotus fruit from the same locale has been dated as being 332 + 135-yr-old (270 ? 60 yr BP, radiocarbon age) by accelerator mass spectroscopy at the Lawrence Livermore National Laboratory. This polycentenarian seed not only germinated but is still growing (since March, 1994). Of six old lotus fruits tested, two-thirds germinated, almost all in fewer than 4 d, as rapidly as fruits harvested from the progeny of Pulantien Sacred Lotus plants (under cultivation by the National Park Service in Washington, DC), and more rapidly than fresh fruits of Yellow Lotus [N. lutea (Willd.) Pers.]. Growth of the old lotus is robust: rhizome formation and leaf emergence at rhizome nodes are more rapid than those of the Pulantien progeny, although the leaf width is smaller. Activity of the protein-repair enzyme L-isoaspartyl methyltransferase in the old lotus seed is persistent during germination and is as robust as that in the progeny, and the degree of aspartyl racemization in proteins of the two groups of plants is minimal and essentially identical. The six dated ancient Sacred Lotus fruits range in age from 95 to 1,288 yr (with a mean age of 595 ? 380 yr), evidently reflecting their production, deposition, and preservation at varying times during the intervening millennium.

Interspecific variation in growth, biomass partitioning, and defensive characteristics of neotropical mangrove seedlings: response to light and nutrient availability

Abstract: Seedlings of three mangrove species -Rhizophora mangle L., Avicennia germinans (L.) Steam., and Laguncularia racemosa (L.) Gaertn. f. -were subjected to different light and nutrient regimes in two separate growth chamber experiments. At higher nutrient or light availability, relative growth rate, leaf production, and branch growth differed significantly among species in the following order: Rhizophora < Avicennia < Laguncularia. At lower nutrient or light levels, however, species' differences were greatly minimized. Lower nutrient or light levels caused greater investment in root biomass by all species, whereas higher nutrient availability resulted in greater investment in leaf area and maximized species' differences in total leaf area, number of leaves, and leaf area ratio. Mangrove leaves also differed among species in quantity and composition of secondary compounds that may protect seedlings against herbivores or stress factors such as excessive solar radiation. Relative amounts of condensed tannins, gallotannins, and nitrogen were significantly affected by light and nutrient regimes, but patterns of response differed among species. The results indicate that these sympatric species differ substantially in their potential for growth, acquisition of resources, stress tolerance, and susceptibility to herbivores during the seedling stage, but that these characteristics are significantly modified by availability of resources.

Phylogenetic relationships in tribe Cardueae (Asteraceae) based on ITS sequences

Abstract: The Cardueae pose vexing systematic problems at several taxonomic levels. Tribal delimitation is controversial, with views ranging from a broadly defined Cardueae comprising subtribes Echinopsidinae, Carlininae, Carduinae, and Centaureinae to a more restrictive Cardueae (comprising only Carduinae and Centaureinae) with tribal rank also given to Echinopsideae (or Echinopeae) and Carlineae. Furthermore, the circumscription of groups within a broad Cardueae is also controversial, with divergent views emphasizing differences in floral or achene morphology; the distinction between Carduinae and Centaureinae is particularly weak. The generic limits of Centaurea, comprising 500-600 species and great diversity in both chromosome number and pollen morphology, are also unclear. Patterns of morphological, chromosomal, and pollen diversity in Centaurea do not completely support either the current taxonomy or any of the proposed modifications. Clearly, the Cardueae in general and Centaurea in particular require a phylogenetic approach to resolve problems of taxonomic circumscription and rank and to understand patterns of pollen and chromosomal evolution. We therefore analyzed DNA sequences of the internal transcribed spacers (ITS) of the nuclear ribosomal RNA genes (rDNA) of 35 taxa to reconstruct the phylogeny of Cardueae and assess 1) the positions of Echinopsidinae and Carlininae, 2) the circumscription of the Carduinae-Centaureinae complex, and 3) the limits of Centaurea and related genera. Phylogenetic analysis of ITS sequence variation strongly supports the monophyly of Cardueae relative to outgroup taxa from Mutisieae and Cichorieae. Both Carlininae and Echinopsidinae occupy basal branches in Cardueae. Carduinae are paraphyletic, forming a basal grade in the tribe. The Centaureinae, however, form a well-supported clade. As typically circumscribed, Centaurea is paraphyletic, comprising species of several other genera in addition to those currently recognized as Centaurea. Proposed evolutionary trends in pollen morphology and, to a lesser extent, base chromosome numbers are largely supported by the ITS phylogeny for Cardueae.

Estimating volume flow rates through xylem conduits

Abstract: We discuss the errors in common approximations of the volume flow rate for laminar flow through conduits with noncircular transverse sections Before calculating flow rates, ideal geometric shapes are chosen to represent the noncircular transverse sections The Hagen-Poiseuille equation used with hydraulic diameter underestimates the volume flow rate for laminar flow through conduits even with such ideal shapes Correction factors that have been proposed for the Hagen-Poiseuille equation also lead to underestimates of the volume flow rate for those shapes The exact solutions are sometimes difficult to attain, but rates calculated using the exact solutions for the ideal shapes may be as much as five times higher than the approximated rates for common transversely elongated shapes Either the exact solutions or the approximations may be used to calculate volume flow rates through the xylem of plants Both of these methods actually approximate flow through the original conduits because the shapes used are approximations of the conduits' transverse sections We recommend using the exact solutions whenever possible; they should be closer to the real solution than other approximations We give tables of correction factors for use in the cases where calculating volume flow rate from the approximate solution, the Hagen-Poiseuille equation, is more feasible Obtaining theoretical volume flow rates that are larger than previously thought highlights the need to clarify the causes of differences between the theoretical rates and the smaller measured volume flow rates in plant xylem As part of an ongoing project to understand the dynamics of water flow through the xylem we discuss exact equations to calculate water flow through noncircular ideal capillaries and investigate the errors in frequently used approximations In determining theoretical volume flow rates through the xylem of plants, the Hagen-Poiseuille equation for laminar flow through conduits is often used The Hagen-Poiseuille equation provides the exact solution for laminar flow in individual ideal capillaries with circular transverse sections, but the application of this equation to conduits with noncircular transverse sections using the hydraulic diameter results in errors in the calculated volume flow rates Exact solutions for the determination of volume flow rate through capillaries with some common noncircular transverse sections have recently been published in English (White, 1991) In general, errors in calculated flow rates may result from anatomical deviations of the xylem conduits from ideal capillaries, or from methods used to calculate the volume flow rate This paper investigates the latter more completely than previous botanical literature (eg, Leyton, 1975; Nonweiler, 1975; Petty, 1978; Pickard, 1981; Zimmermann, 1983; Calkin, Gibson, and Nobel, 1986; Lewis, 1992), discusses some exact solutions for volume flow rates through some common noncircular conduits, and recommends methods for the calculation of theoretical volume flow rates based on desired accuracy, speed, and ease of calculation

Ten-year seed bank and vegetation dynamics of a tidal freshwater marsh

Abstract: A decade-long examination was made of recruitment and establishment in a tidal freshwater high marsh along the Delaware River. Over the 10 yr of the study, seed bank, field seedlings, and vegetation showed variable patterns and significant year-to-year fluctuations. Patterns of each species were unique, perhaps the result of specific germination and/or establishment requirements and seedling morphology. For a given species there was little correlation among seed bank, seedling, and vegetation patterns, and germination success did not guarantee establishment. Species diversity showed significant year-to-year fluctuation, but there was no trend, and perennials did not change in importance during the 10-yr period. Because four annual species (Bidens laevis, Cuscuta gronovii, Impatiens capensis, and Polygonum arifolium) composed over 90% of the seed bank and field seedlings, and 58-89% (x = 70 ? 4) of the cover, community dynamics were dependent on seedling recruitment. For a given species life history stages (seed bank, field seedlings, and vegetation) were not predictable over the temporal scale of a decade. It is predicted, however, that if hydrology remains unchanged, the same suite of species will persist. The importance of the parasite Cuscuta gronovii is noteworthy.

Disturbance, life history strategies, and seed fates in alpine herbfield communities.

Abstract: Species responses to disturbance are governed primarily by their life history and physiological traits and by the characteristics of the disturbance. Species reproductive traits are especially important in determining the potential of species to establish and to persist following disturbance. Herein, I review available literature on relationships among disturbance, species life histories, and seed fates in tundra environments. Research conducted on these relationships in alpine herbfield vegetation on the Beartooth Plateau, Montana, over the past 9 yr by my colleagues and myself is synthesized. In tundra environments, species reproductive capacities are often similar to those in more temperate environments, but short, cool growing seasons constrain seed production and reduce seedling growth and survival. Highly variable growing season conditions result in large differences in seed production and seedling establishment among years. On disturbed sites, disturbance characteristics determine the seed and seedling environment and influence rates of establishment. In these windy environments, relationships among soil surface characteristics and seed morphological attributes determine both the horizontal and vertical movement of seeds on exposed soils. Once seeds are incorporated into the soil, soil physical and chemical properties determine temperature and nutrient regimes and have the greatest effects on seed germination and seedling growth and survival. Examining the seed fates of herbfield species with varying life histories illustrates that the identities of species that establish following disturbance are largely predictable from their reproductive traits. Disturbance characteristics determine the success of different reproductive strategies and significantly influence community structure.

Characterization of invasive Conyza species (Asteraceae) in Europe: quantitative trait and isozyme analysis

Abstract: The genus Conyza (Asteraceae) represents one of the foremost examples of intercontinental plant invasions from the New World to the Old World that has resulted in a number of taxonomic problems owing to the dearth of knowledge concerning the biosystematics of the genus. In this study, quantitative trait and enzyme electrophoresis analysis was used to assess the pattern of variation within and among five short-lived taxa of Conyza (C. blakei, C. bonariensis, C. canadensis, C.floribunda, C. sumatrensis), introduced into Europe, and to test the hypothesis of a recent hybrid origin of C. floribunda. The five taxa exhibit marked differences in morphological and life-history characteristics that are concordant with divergence in genes encoding isozymes. In addition, a recent hybrid origin of C. floribunda is clearly not supported by either quantitative or electrophoretic data. These findings suggest that the five taxa differentiated genetically in their native range prior to their introduction in Europe. As formerly suggested by Arthur Cronquist (Bulletin of the Torrey Botanical Club 70: 629-632, 1943), C. canadensis appears to be more closely related to the genus Erigeron than the other taxa. The electrophoretic results also confirm that this particular species is diploid, while the presence of multiple bands and fixed heterozygosity demonstrates that the other Conyza taxa are all genetic allopolyploids. Both quantitative and electrophoretic data suggest the presence of two groups among the polyploid taxa: one comprising C. bonariensis and C. sumatrensis, the other C. blakei and C.floribunda. The latter grouping is supported by the observation that C. blakei and C. floribunda, both native to Argentina, exhibit a striking ability to shift from semelparous to iteroparous reproduction, while the other taxa are strictly semelparous.

Seed germination timing patterns in intermountain penstemon (scrophulariaceae)

Abstract: Regulation of seed germination timing is an important component of population persistence, especially for short-lived plants in seasonal environments with unpredictable year-to-year weather variation. Seed germination patterns were examined in the laboratory for seeds from 135 populations belonging to 38 species and 13 sections of the genus Penstemon (Scrophulariaceae), from habitats ranging from warm desert to alpine tundra. Seeds of most species were dormant at dispersal and required a period of moist chilling to become germinable. Response to chilling was related to probable chilling duration at the collection site. Populations from habitats with severe winters produced seeds with long chilling requirements, while those from habitats with mild winters produced seeds with short chilling requirements. Populations from midelevation habitats produced seeds with intermediate chilling requirements but with a sizeable fraction whose dormancy was not broken by chilling of any length. Another pattern observed mostly in warm desert populations was little primary dormancy at autumn temperatures combined with induction of a fraction into secondary dormancy by short chilling. Species with a wide habitat range included populations with contrasting germination patterns. Parallel habitat-correlated patterns were observed in different sections. Most species showed germination patterns that combined predictive mechanisms with potential for carryover of a persistent seed bank. Results of common garden experiments suggested that germination differences had a strong genetic basis both among populations and among plants in a population. Adaptive radiation in the genus Penstemon has thus included the evolution of habitat-specific germination timing strategies in multiple lineages within the genus.

Tests of a mechanistic model of one hormone regulating both sexes in Cucumis sativus (Cucurbitaceae)

Abstract: A mechanistic model of sex determination in flowering plants, which assumes that one hormone has male and female cell receptors to inhibit one sex and induce the other independently, was tested in cucumbers (Cucumis sativus) by applications of hormones and their inhibitors. Applications of gibberellic acid (GA) and an inhibitor of its synthesis (paclobutrazol) showed that GA had dual effects on sex expression of promoting maleness and inhibiting femaleness. Conversely, applications of Ethrel (an ethylene release agent) and AgNO3 (an ethylene action inhibitor) indicated that ethylene induced femaleness and inhibited maleness. Results of various combined applications of the two hormones and their inhibitors suggested that ethylene had overriding effects on GA and acted more directly on sex expression in cucumber. These experiments indicated that there is only one hormone, not two, regulating sex expression in cucumber, and that the sex hormone is likely to be ethylene. Results thus supported the assumption of the model that one hormone can regulate both sexes by inducing one and inhibiting the other independently. Model predictions were confirmed that from a monoecious line, female plants could be induced by increasing the ethylene level, and male plants induced by decreasing the ethylene level. On the other hand, as the model predicts, applications of ethylene or its inhibitor induced gynomonoecious, female, and trimonoecious plants from a hermaphroditic line. In conclusion, all assumptions and predictions of the one-hormone model were confirmed in the experiments.

The response of five tropical dicotyledon species to solar ultraviolet‐B radiation

Abstract: Tropical regions currently receive the highest levels of global solar ultraviolet-B radiation (UV-B, 280-320 nm) even without ozone depletion. The influence of natural, present-day UV-B irradiance in the tropics was examined for five tropical species including three native rain forest tree species (Cecropia obtusifolia, Tetragastrispanamensis, Calophyllum longifolium) and two economically important species (Swietenia macrophylla, Manihot esculenta). Solar UV-B radiation conditions in a small clearing on Barro Colorado Island, Panama (90 N), were obtained using either a UV-B-excluding plastic film or a film that transmits most of the solar UV-B. Significant differences between UV-B-excluded and near-ambient UV-B plants were often exhibited as increased foliar UV-B absorbing compounds and, in several cases, as reduced plant height with exposure to solar UV-B. Increases in leaf mass per area and reductions in leaf blade length under solar UV-B occurred less frequently. Biomass and photosystem II function using chlorophyll a fluorescence were generally unaffected. The results of this study provide evidence that tropical vegetation, including native rain forest species, responds to the present level of natural solar UV-B radiation. This suggests that even minor ozone depletion in the tropics may have biological implications.

Differential staining with orcein, giemsa, cma, and dapi for comparative chromosome study of 12 species of australian drosera (droseraceae)'

Abstract: Differential stainings with orcein, Giemsa, CMA, and DAPI were compared in 12 species of Western Australian Drosera. Chromosome numbers of D. roseana, D. barbigera, D. leioblasta, D. oreopodion, D. mannii, D. walyunga, D. sewelliae, D. helodes, and D. echinoblasta are reported here for the first time. A marked difference regarding chromosome number was observed in Drosera dichrosepala (2n = 12) from that of the previous report (2n = 18). The karyotypes of the species showed commonly that degree of asymmetry in chromosome length was directly proportional to the mean chromosomal length whereas the number of chromosomes was inversely proportional. Bimodal karyotypes were observed in D. oreopodion, D. walyunga, D. barbigera, and D. echinoblasta, which perhaps resulted from interspecific hybridization of the former two and fragmentation in the latter two. Sat-chromosomes found in D. falconeri, D. sewelliae, D. helodes, and D. echinoblasta responded differently in differential staining. The C and fluorescent bands at the mostly terminal region revealed that maximum C-positive heterochromatin-rich segments, GC-rich segments, and AT-rich segments were accumulated at the ends of Drosera chromosomes. Some chromosomes could be identified by their specific staining property. On the basis of chromosome number and C- and fluorescent-banding pattern, we suggest that D. helodes and D. sewelliae are closely related. The genus Drosera L., with 90 species, belongs to the family Droseraceae, which has a world-wide distribution (Diels, 1906). About two-thirds of Drosera species grow in Australia (Kondo, Segawa, and Nehira, 1976). The Australian Drosera taxa are mostly endemic to local areas (Diels, 1906; Conn, 1980; Marchant and George, 1982; Kondo and Lavarack, 1984). Most of the earlier cytologists concentrated on a conventional orcein karyomorphological study of this genus (Kondo, 1966, 1969, 1970, 1973, 1976, 1984; Kondo, Segawa, and Nehira, 1976). Thus, an attempt was undertaken to clarify relationships among the Australian

Parsimony analysis of cpDNA restriction site variation in subfamily Nepetoideae (Labiatae)

Abstract: Parsimony analysis of cpDNA restriction site variation supports monophyly of subfamily Nepetoideae. However, a close relationship among Nepetoideae and other gynobasic-styled Labiatae is not supported, indicating that a gynobasic style has evolved independently in at least two clades of Labiatae. The inferred relationships are congruent with the classification of Cantino, Harley, and Wagstaff (1992, Advances in labiate sciences, 27-37, Royal Botanic Gardens, Kew) but conflict to varying degrees with traditional classifications. Monophyly of four tribes of Nepetoideae also is supported.

Population genetics of Cordia alliodora (Boraginaceae), a neotropical tree. 2. Mating system

Abstract: A multilocus mixed mating model was used to evaluate the mating system of a natural population of Cordia alliodora (Boraginaceae), a neotropical tree. The population was highly outcrossed (tin = 0.966 ? 0.027), in agreement with results from controlled crosses. Departures from the mixed mating model were evident, suggesting some nonrandom, correlated mating. Pollen pool heterogeneity and variation in estimates of individual outcrossing rates indicated that the population may be genetically substructured. Individual outcrossing rates obtained for the samples taken from within different parts of the same tree indicated reduced levels of outcrossing due to limited sampling of the pollen pool. The incompatibility mechanism in C. alliodora, combined with variation in flowering and stand density, appears to lead to both temporal and spatial substructuring of the population.

Testing hypotheses of functional relationships: a comparative survey of correlation patterns among floral traits in five insect-pollinated plants

Abstract: To test hypotheses on the evolution of functional relationships, phenotypic correlations among floral traits were estimated in five species of insect-pollinated plants. The species studied were wild radish (Raphanus raphanistrum), canola (Brassica napus), phlox (Phlox divaricata), dame's rocket (Hesperis matronalis), and black mustard (Brassica nigra). Six floral traits were measured on each species. In three of the five species the correlations between the filaments and corolla tube were significantly greater than the other floral correlations. This result is consistent with the hypothesis that selection for proper anther placement to enhance pollination has increased the filament-corolla tube correlations in these three species. In contrast, none of the species showed evidence of selection for an optimal stigma placement; the correlations between the pistil and corolla tube lengths were not greater than the other floral correlations in any of the five species. Organisms are not collections of independent traits. Groups of traits must work together as organs to perform essential functions, and these organs must in turn work together in organ systems to produce an organism that is adapted to its environment. A major unresolved question in evolutionary biology is how genetic, developmental, and phenotypic relationships among different traits evolve to produce functionally integrated organs or or­ ganisms. One approach to answering this question is to examine the evolution of phenotypic and genetic corre­ lations among traits (Olsen and Miller, 1958; Cheverud, 1982, 1984). Selection on the correlation between traits can occur when certain combinations of trait values con­ fer higher fitness than other combinations. Given ade­ quate genetic variance, this selection can in turn alter the correlations over evolutionary time. Very little empirical evidence concerning the evolution of correlations exists, but a few studies demonstrate selection for increased cor­ relation between traits that are functionally related (Ar­ nold and Bennett, 1988; Moore, 1990; Brodie, 1992). On the other hand, some traits or groups of traits may func­ tion more effectively if they are decoupled from each other, so that they vary independently (Berg, 1960). One way to study the evolution of correlations is to look for evidence of past selection in present-day corre­ lation patterns (Berg, 1960; Kingsolver and Wiernasz, 1987, 1991; Armbruster, 1991; Conner and Via, 1993). Based on patterns of correlations among morphological traits in wild radish, Conner and Via (1993) hypothesized that pollinators might have caused selection for increased

Population genetics of Cordia alliodora (Boraginaceae), a neotropical tree. 3. Gene flow, neighborhood, and population substructure

Abstract: The utility of a) rare allozyme markers and b) paternity exclusion in estimating gene dispersal in tropical forest trees was demonstrated for a population of Cordia alliodora (R. & P.) Oken in Costa Rica. Based on the number of alleles in common, near neighbors were more highly related genetically than were more distant trees. The spatial clustering of relatives was within 50 m, coinciding to a large extent with the seed dispersal curve. Most pollen originated from within 75 m of the mother tree, although a low but substantial proportion of pollen movement was from as far as 280 m. Overall gene flow was extensive, with neighborhood areas as large as 7 ha. Neighborhood sizes at any moment in time and space were, however, relatively small, such that overall variation is maintained by continuous gene flow between neighborhoods. The results indicate that deforestation and fragmentation could curtail such gene flow leading to an erosion of genetic variation within fragments.

Ecological constraints on fruit initiation by Calyptrogyne ghiesbreghtiana (Arecaceae): FLORAL HERBIVORY, POLLEN AVAILABILITY, AND VISITATION BY POLLINATING BATS

Abstract: Failure of a flower to initiate fruit can be attributed to an insufficiency of resources, genetic incompatibilities, or ecological constraints on pollination. Flower visitors can increase fruit initiation by pollinating flowers, or decrease fruit initiation by damaging them. If, however, resources limit fruit initiation, variation in visitation may be relatively unimportant. Visitors to Calyptrogyne ghiesbreghtiana, a rain forest understory palm, leave a record of their activity by feeding on floral tissue. I recorded variation in inflorescence visitation, floral display, and fruit initiation during 7 mo over two field seasons. On average, bats visited 50-60% of flowers; katydids and scarab beetles visited and damaged the remaining 40-50%. Four species of bat (Phyllostomidae) captured near inflorescences were found to be carrying C. ghiesbreghtiana pollen. Approximately 18% of flowers visited by bats and not damaged by insects initiated fruit, whereas insect-damaged flowers set a negligible number. Some variation in fruit initiation was explained by nightly variation in the proportion of inflorescences at each site that were potential pollen donors (male-phase inflorescences). Visitors responded to variation in floral display; katydids damaged more flowers on taller inflorescences, and in one season bats visited a greater proportion of flowers on inflorescences with many flowers. However, floral display only explained a small amount of variation in visitation. A similar amount of variation in flower visitation was explained by the degree to which inflorescences were obstructed by surrounding vegetation. I used path analysis to summarize the overall effect of floral display on visitation by both pollinators and florivores and the effect of visitation and site sex ratio on fruit initiation. Because almost all flowers were visited by bats or damaged by katydids, the amount of bat visitation was strongly negatively correlated with katydid damage. In spite of the low fruit set from bat-visited flowers, variation in bat visitation explained 27-37% of the variation in fruit initiation.

Leaf optical properties along a vertical gradient in a tropical rain forest canopy in Costa Rica.

Abstract: Leaf optical properties (400-1,100 nm) were compared for four species of rain forest trees with crowns in understory, mid-canopy, and canopy positions to test whether optical properties change with light environment. The species tested represent a spectrum of regeneration patterns ranging from shade tolerant to light demanding. Overall, leaf optical properties of the four species were similar. Differences in absorptance were small, but statistically significant among the species and positions along the canopy gradient. Species absorptance differences corresponded somewhat to shade tolerance; two of the shade species showed higher absorptance in lower light environments, while the sun species showed the reverse pattern. Specific leaf mass (leaf weight per unit area) and chlorophyll content per unit leaf weight also changed along the canopy gradient. Specific leaf mass was positively correlated and chlorophyll per unit leaf weight was negatively correlated with increasing light environment. Consequently, the efficiency of absorption, as represented by the absorptance per unit leaf weight, increased as light level decreased, largely due to changes in specific leaf mass. In contrast, efficiency of absorption per unit leaf chlorophyll was relatively constant with light environment for the two species measured for chlorophyll.