MATING SYSTEM OF A TROPICAL RAIN FOREST TREE SPECIES

Canopy tree species are the dominant elements of the species-rich, fragile and endangered tropical rain forest ecosystems, yet little is known about the genetics of these species. We provide an estimate of the outcrossing rate in a population of Pithecellobium pedicellare, a large canopy tree in the tropical rain forests of Costa Rica. The outcrossing rate was high (t = 951 ± 0.021) and the pollen pool contributing to the progeny arrays used to measure outcrossing rate showed departure from homogeneity. The high outcrossing rate indicates that individuals scattered over a large area in this low density population could be bound with each other via outcrossing. However, despite a high outcrossing rate, the potential for inbreeding in this population is not negligible. We found a relatively large number of albino and chlorotic seedlings among the progeny arrays of several seed parents. Heterogeneity of pollen pool further indicates that the population, though widely outcrossed, may be structured. A more detailed study of mating systems in tropical rain forest trees may provide additional insights into the mating patterns of these trees. Such studies will be useful not only in understanding the dynamics of micro-evolutionary processes, but also in the conservation and management of tropical forest trees.     

MODELING OF MERISTEMATIC GROWTH OF ROOT APICES IN A NATURAL COORDINATE SYSTEM

A curvilinear, orthogonal coordinate system, which resembles the pattern of periclines and anticlines in the cellular network of root apices, is presented. The system makes possible an analysis of the dynamics of apical growth: the relationship between growth rates and tensile stresses in cell walls. In this paper the coordinate system is used in modeling the growth and cell partitioning in the apical domes. The symplastic growth is described by means of the growth tensor which is assumed to have diagonal form in the system, so its coordinate lines represent the principal directions of growth rate. The coordinate system and the growth tensor in diagonal form assure temporal stability of form and cellular structure of the modeled apex including transition from the dome to the cylindrical part of the apex. The spatial and temporal aspects of the dome part of two types of root apices—one with maximum of volumetric relative growth rate at the center region of the apex, and another with minimum of the rate (quiescent center)—are described. The maximum of the rate at the center results in the cellular pattern with an apical cell and merophytes, the minimum results in ribs of cells (inside the root proper) converging toward the quiescent center.     

MATING SYSTEM IN NATURAL POPULATIONS OF JEFFREY PINE

The mating system and allozyme variation at 20 loci in three Klamath Mountains and two Sierra Nevada populations of Jeffrey pine (Pinus jeffreyi Grev. & Balf.) were investigated. On average, multilocus estimates of the proportion of viable progeny due to outcrossing (t_m) were high in all populations (mean t_m = 0.935, range 0.881 to 0.971). Despite differences in stand structure, t_m did not differ (P > 0.05) between the Klamath (mean t_m = 0.933) and Sierra Nevada (mean t_m = 0.937) populations. At all but one locus in one population and at two in another, genotype frequencies fit (P > 0.05) Hardy-Weinberg expectations. Mean estimates of observed heterozygosity in Klamath (0.182) and Sierra Nevada (0.327) populations were comparable to values reported for other conifers.     

树鼩(Tuapaia belangeri chinensis)的动脉

本文在对中国树鼩的动脉系作系统解剖的基础上,记述了树鼩动脉分支的主要特征。同时,还与其他有关种类作了比较研究。结果表明,中国树鼩动脉系的主要特征与树鼩科中的其他种类如笔尾树鼩、普通树鼩和地树鼩等的动脉系极为相似,而与灵长类中的猿猴类有较大差异。     

BREEDING SYSTEM EVOLUTION IN LEAVENWORTHIA: BREEDING SYSTEM VARIATION AND REPRODUCTIVE SUCCESS IN NATURAL POPULATIONS OF LEAVENWORTHIA CRASSA (CRUCIFERAE)

Populations of Leavenworthia crassa (Cruciferae) studied for 3 years exhibited among- and within-population genetic variation for a suite of floral and reproductive traits (flower width, petal length, anther position, ability to set seeds in the absence of pollinators, time to first flowering) associated with breeding system. We used electrophoretic markers to show that a population with small, monomorphically colored flowers with introrse anthers had a significantly lower outcrossing rate (t = 0.03) than did a population with larger, polymorphically colored flowers with extrorse anthers (t = 0.33). In the more-outcrossing population the correlation between higher maternal plant outcrossing rate and the suite of six traits approached significance (P < 0.067), with greater petal size, greater flower width, and reduced ability to set seeds in the absence of pollinators contributing significantly. Plants in selfing populations had a generally higher reproductive success, with a higher number of flowers per plant, a smaller proportion of unfertilized ovules, a smaller proportion of fertilized ovules aborted, a higher rate of fruit set, and overall a larger number of seeds matured than did plants from the more outcrossing populations. Pollen limitation did not appear to account for lower reproductive success in outcrossing populations. Resource limitation did not differ substantially between populations. However, within-ovary patterns of fertilization, abortion, and seed weight were significantly less random in outcrossing populations than in selfing populations, suggesting that differential gamete and embryo success may be responsible for lower reproductive success in outcrossing populations.     

TOWARD A NATURAL SYSTEM OF SPECIES IN CHLOROGONIUM (VOLVOCALES, CHLOROPHYTA): A COMBINED ANALYSIS OF MORPHOLOGICAL AND rbcL GENE SEQUENCE DATA

The taxonomy of species of Chlorogonium (Volvocales, Chlorophyta) was studied based on comparative light and electron microscopy and DNA sequence data of 23 strains from five major algal culture collections. All of the 23 strains showed pyrenoids under photoautotrophic conditions, but 17 of the 23 exhibited marked reduction in size of pyrenoids, or pyrenoids were absent under photoheterotrophic conditions. The strains could clearly be delineated into six species, C. euchlorum, C. elongatum, C. fusiforme, C. capillatum, C. neglectum, and C. kasakii on the basis of differences in cell shape, number of contractile vacuoles, number and stability of pyrenoids, and ultrastructure of pyrenoids and stigmata. This distinction of species based on morphology was also supported by analyses of rbc L gene sequences. The later strongly showed that each species, C. euchlorum (seven strains), C. elongatum (three strains), and C. capillatum (10 strains), forms a robust clade. Although some morphological differences were noted within different strains of C. euchlorum and C. capillatum, these features were regarded as strain-specific because they were not reflected in the rbc L gene phylogenies. In addition, the rbc L gene trees strongly suggested that C. neglectum and C. kasakii are closely related to each other, consistent with the similarity of the ultrastructure of pyrenoids and stigmata between the two species. However, C. kasakii can be distinguished clearly from C. neglectum by its multiple pyrenoids in the chloroplast and acute anterior and posterior ends in the vegetative cell.     

MATING SYSTEM AND GENETIC STRUCTURE OF THE DISTYLOUS TROPICAL TREE PSYCHOTRIA FAXLUCENS (RUBIACEAE)

The population genetics and mating system of the understory tropical rain forest tree Psychotria faxlucens were studied at two plots in Los Tuxtlas, Veracruz, México, on the Gulf of México coast. This species is distylous and is pollinated by moths, mainly small hawkmoths. The seeds are dispersed by gravity and by frugivorous birds. Controlled pollinations indicate that the trees are self-incompatible and that intramorph pollinations have lower compatibilities than intermorph pollinations. The pollen: ovule ratio is high, suggesting obligate xenogamy. Using electrophoretic allozyme methods we found that eight out of 20 loci were polymorphic (P = 0.400), the observed heterozygosity (H) was 0.198, and the mean expected heterozygosity (H) was 0.495, both relatively high values compared with that reported for tropical trees. The genetic differentiation between the two plots is low, as shown by the heterogeneity in allele frequencies and the F_st (mean F_st seedlings = 0.031, mean F_st adults = 0.026), although for some loci the plot differentiation is statistically significant. The studied populations are near Hardy-Weinberg proportions, both for seedlings (mean F = 0.128) and adults (mean F = 0.075). From the fixation rate, an indirect estimate of the outcrossing rate at equilibrium gave a mean of t = 0.898 for plot 1 and 0.685 for plot 2. Direct single loci and multiloci outcrossing rate estimates were generally not statistically different from 1.0.     

PEARL BODIES OF A NEOTROPICAL TREE,OCHROMA PYRAMIDALE: ECOLOGICAL IMPLICATIONS

Multicellular, spherical or club-shaped pearl bodies characterized by large intracellular lipid vesicles are produced on leaves and stems of juvenile Ochroma pyramidale, a tree of second growth vegetation in lowland wet forest of the neotropics. Several lines of evidence suggest that pearl bodies are linked to maintenance of foraging ants on the plant: (1) their production is closely associated with foliar nectar secretion; (2) they are abundant on saplings grown in the glasshouse (averaging 402 bodies leaf^–1) but were not observed in the field where ants are predictably associated with Ochroma samplings; (3) pearl bodies are energy- and lipid-rich averaging 27.80 kJ/g dry wt^–1 and 74.4% lipid; (4) they are constricted at the base and easily detach from the leaf; (5) four ant species collect pearl bodies from artificial depots and return them to their nests. Chelaner sp. detaches pearl bodies from the leaf and returns them to the nest. Production of pearl bodies represents about 25% of the energy allocated to foliar nectar by saplings. Characteristics of the pearl bodies of Ochroma are consistent with those of a widespread group of trichomes and leaf emergences suggesting a common ecological role as ant food for these structures.     

REPRODUCTIVE ECOLOGY OF CARYA (JUGLANDACEAE): PHENOLOGY,POLLINATION, AND BREEDING SYSTEM OF TWO SYMPATRIC TREE SPECIES

Observations and experiments on both shagbark (Carya ovata; 2n = 32) and mockernut (C. tomentosa; 2n = 64) hickories in a New Jersey forest were performed over a 2-year period to determine the mechanisms operating both to maintain reproductive fidelity and to regulate fruit initiation. Phenological observations indicated significant differences between the species for almost all vegetative and reproductive characters examined; however, a large amount of overlap was also observed. Pollination experiments indicated that pollen limitation is not a significant factor in reducing fruit set in these anemophilous species. Pollen-ovule ratios ranged from 1 to 25 × 10⁶. Pollen was 80–90% viable at time of shedding. The large surface area of vegetative structures (e.g., leaves) at the time of pollination may retard the flow of pollen into and out of the canopy and thus increase self-pollination. A full diallel cross, replicated 5 times, was made among 8 trees (4 trees of each species) to characterize combining ability and compatibility relationships. All trees were found to be both geitonogamous and xenogamous to varying degrees. A high frequency of putative interploidal cross-compatibility was also observed. Fruits from these interspecific crosses were found to be germinable. Observations on individual trees suggested that dichogamy may help to reduce selfing and promote outcrossing in this breeding system. While most trees exhibited a temporal separation of genders, the trait was found to be quite plastic (i.e., trees switched between protogyny and protandry between years). Pistillate flowers were receptive for all trees within a species during the same general time period; however, pollen shedding varied considerably among trees and between years. Both species appear to be utilizing a broad array of mechanisms in order to maintain reproductive fidelity and adequate fruit set.