ECOLOGICAL CORRELATES OF REPRODUCTIVE TRAITS OF MEXICAN RAIN FOREST TREES

Sexual systems of 139 tree species from a tropical rain forest at Los Tuxtlas, Mexico were investigated to: 1) estimate the relative proportions of hermaphroditic, monoecious, and dioecious species; 2) describe flowers, fruits, and seeds in terms of size and weight; 3) describe flowering and fruiting phenology; and 4) correlate sexuality to pollination and dispersal syndromes, and the successional status occupied in the forest. Hermaphroditism occurred in 63% of the species, monoecism in 9%, and dioecy in 27%. Nondioecious species had larger flowers, but dioecious species had more seeds per fruit. The dioecious condition was associated with small flowers pollinated with unspecialized insects and fleshy fruits dispersed by animals at both species and generic levels. Reproductive traits were more correlated among nondioecious species than dioecious species. Pioneer species had more seeds per fruit, and longer flowering and fruiting periods, but persistent species produced heavier seeds and fruits. Flower and fruit morphological traits, sexual systems, and tree guilds are related in a comprehensive way, and a flow model based on data from this study is presented.     

THE EVOLUTIONARY ECOLOGY OF AN ANTIPREDATOR REACTION NORM: DAPHNIA PULEX AND CHAOBORUS AMERICANUS

Ponds containing the parthenogenetic zooplankter Daphnia pulex with and without chaoborid predators were sampled over the course of a season. A significant (P < 0.05) Spearman rank correlation was found between predator density and the expression of an antipredator defense (neckteeth) by the Daphnia. The reaction norms (percent induction of a single genotype versus predator density) of clones isolated from predator-free and predator-rich habitats were determined in a laboratory setting. There was a statistically significant different response among the six clones tested (P < 0.05). Clones isolated from chaoborid ponds showed significantly greater sensitivity to the presence of predator than clones from predator-free ponds (P < 0.05). In the laboratory, food levels under which prey were cultured affected induction of the antipredator response. Highest induction was found at the lowest food level used.     

THE DISTRIBUTION OF LIFE-HISTORY VARIATION IN THE DAPHNIA PULEX COMPLEX

In the midwestern United States the Daphnia pulex complex consists of a mosaic of sexual and asexual populations, providing a useful model system for studying the evolutionary forces underlying the maintenance of sex. One asexual and two sexual populations were surveyed for genetic variation for isozymes, mitochondrial DNA, and life-history characters. While the sexual populations exhibited substantial levels of genetic variance for fitness characters, no variation was detected in the asexual population at any level. However, a parallel survey among asexual clones derived from other ponds revealed large amounts of quantitative variation among clones, even among those with the same molecular profile. As a group, the asexuals are more variable for life histories than are the sexual populations. The molecular data indicate a relatively recent origin for the extant asexual D. pulex. The polyphyletic origin of these clones, combined with their microevolutionary potential, provides an explanation for their broad geographic distribution. The distribution of sex in the complex cannot be explained with the standard models that assume an invariant asexual population in reproductive isolation from the parental species. Although the frequency of asexuality may be driven by the spread of a sex-limited meiosis suppressor through sexual populations, the complete displacement of sexuality may be prevented by ecological distinctions between the two classes of individuals. On average, the asexuals are larger but produce smaller clutches than the sexuals.     

Ligand recognition in μ opioid receptor: experimentally based modeling of μ opioid receptor binding sites and their testing by ligand docking

For three-dimensional understanding of the mechanisms that control potency and selectivity of the ligand binding at the atomic level, we have analysed opioid receptor-ligand interaction based on the receptor's 3D model. As a first step, we have constructed molecular models for the multiple opioid receptor subtypes using bacteriorhodopsin as a template. The S-activated dihydromorphine derivatives should serve as powerful tools in mapping the three-dimensional structure of the μ opioid receptor, including the nature of the agonist-mediated conformational change that permits G protein-coupling to ‘second messenger’ effector molecules, and in identifying specific ligand-binding contacts with the μ opioid receptor. The analyses of the interactions of some opioid ligands with the predicted ligand binding sites are consistent with the results of the affinity labeling experiments.     

The importance of carotenoids in signaling during aggressive interactions between male firemouth cichlids (Cichlasoma meeki)

Male firemouth cichlids, Cichlasoma meeki, have red pigmentationcovering large areas of their ventral surface, which is displayedduring aggressive interactions. We manipulated the levels ofred pigmentation by assigning the fish to one of two diets,which were as similar as possible except that one was high incarotenoids while the other was low in carotenoids. During diadictrials under white light, fish kept on the high carotenoid dietwon a higher proportion of contests than fish kept on the lowcarotenoid diet Under green light, where differences in rednesscannot be discriminated, there was no effect of diet on theoutcome of contests. These experiments demonstrate that it isthe effect of the diet on red pigmentation that is importantrather than some confounding variable such as differential growthrates. The weight of the two fish was also important; therewas a tendency for the heavier fish to win more contests. Themass effect was subordinate to color under white light but wasthe dominant factor under green light The nature of the contestsunder the different light conditions also varied; the displayin which the red pigmentation is most obvious was not used undergreen light, but was common under white light This suggeststhat the display strategies are flexible and can be alteredaccording to which displays are most effective in a given environmentPrevious studies of other species of fish and birds have shownthat the degree of redness influences mate choice and is affectedby parasite infestations. We propose that carotenoid pigmentationis likely to reflect a general quality, influenced by severalfactors, rather than a context-specific quality such as fightingability.     

Inhibitory effect of oxygen accumulation on the growth of Spirulina platensis

Summary The growth of Spirulina platensis was studied in a light-limited culture under various dissolved oxygen (DO) concentrations. At high DO concentration, e.g. at 1.25 mM DO, the growth rate was decreased up to 36 % compared with that of 0.063 mM DO. The retarded growth rate at high DO concentrations seemed to be coupled with the degeneration of photosynthetic activity in terms of O₂ evolution. Under higher DO concentrations, superoxide dismutase and ascorbate peroxidase activities tended to increase, while the contents of photosynthetic pigment, such phycocyanin, carotenoid and chlorophyll-a decreased distinctly.     

Water Soluble Pigments Containing Xylose and Glucose in Gametangia of the Green Alga, Bryopsis maxima

Several water-soluble pigments were purified from gametangiaof Bryopsis maxima by liquid chromatography and characterizedby pyridylamination and high-performance anion-exchange chromatography.The structure of the main red pigment is proposed based on thedata of infrared spectrum, Mass spectrum, ¹H and ¹³C NMR spectraand pyridylamino analysis. As a consequence, this pigment containeda tetrapyrrole with phytol and a sugar chain comprised of xyloseand glucose. The sequence of the sugars in the chain was determinedbased on its Mass spectrum. The pigment was similar to chlorophyll-originpigments observed in other plants. No aldehyde group, however,was present at C5 in the open tetrapyrrole chain. (Received August 3, 1994; Accepted November 10, 1994)     

Differential Effects of Hypoxia on Ligand Binding Properties of Nicotinic and Muscarinic Acetylcholine Receptors on Cultured Bovine Adrenal Chromaffin Cells

Abstract: Hypoxia is known to disturb neuronal signal transmission at the synapse. Presynaptically, hypoxia is reported to suppress the release of neurotransmitters, but its postsynaptic effects, especially on the function of neurotransmitter receptors, have not yet been elucidated. To clarify the postsynaptic effects, we used cultured bovine adrenal chromaffin cells as a model of postsynaptic neurons and examined specific binding of l -[³H]nicotine (an agonist for nicotinic acetylcholine receptors: nAChRs) and ²²Na⁺ flux under control and hypoxic conditions. Experiments were performed in media preequilibrated with a gas mixture of either 21% O₂/79% N₂ (control) or 100% N₂ (hypoxia). Scatchard analysis of the specific binding to the cells revealed that the K_D under hypoxic conditions was twice as large as that under control conditions, whereas the B _max was unchanged. When the specific [³H]nicotine binding was kinetically analyzed, the association constant ( k ₁) but not the dissociation constant ( k ₋₁) was decreased to 40% of the control value by hypoxia. When the binding assay was performed using the membrane fraction, these changes were not observed. Nicotine-evoked ²²Na⁺ flux into the cells was suppressed by hypoxia. In contrast, specific [³H]quinuclidinyl benzilate binding to the intact cells was unaffected by hypoxia. These results demonstrate that hypoxia specifically suppresses the function of nAChRs (and hence, neuronal signal transmission through nAChRs), primarily by acting intracellularly.     

Block by 5-hydroxytryptamine of neuronal acetylcholine receptor channels expressed inXenopus oocytes

Summary 1. Effects of 5-hydroxytryptamine (5-HT) on neuronal nicotinic acetylcholine (ACh) receptor channels were investigated by expressing cloned channel subunits inXenopus oocytes.2. When channels were expressed with a combination of ₃ and ₄ subunits, 5-HT (10 to 300 µM) reversibly inhibited an inward current activated by 100 µM ACh in a concentration-dependent manner. The inhibition was also observed when ₃ subunit was combined with ₂ subunit instead of ₄ subunit, or ₄ subunit was combined with ₂ or _4–1 subunit instead of ₃ subunit to express channels.3. Compounds known to antagonize at 5-HT receptors (LY53857, metoclopramide and propranolol) exhibited an agonistic effect: they inhibited the ACh-activated current.4. The results suggest that 5-HT inhibits recombinant neuronal nicotinic receptor channels through a binding-site distinct from conventional 5-HT receptors. The binding-site may not be attributed to a unique type of channel subunits.