Seed rain and advanced regeneration in a tropical rain forest

By comparing seed rain, seedling and sapling community structures we assessed the possible role played by vertebrate seed dispersal as a structuring factor in advanced regeneration of closed-canopied sites in the tropical rain forest of Los Tuxtlas, Mexico. Seed weight, initial morphology of seedlings and species abundance were also analyzed to determine if these traits influenced the probability of establishment in the shade.About half of the seed species falling in five closed forest sites (25×25 m) during one year came from fruiting trees growing within the sites (local seeds) and half from fruiting trees found outside the sites (immigrant seeds). Seeds of liana and upper-canopy species were over-represented among immigrant seeds compared with seeds of understory tree species. This probably reflects the activity of frugivorous arboreal mammals, bats, and birds. Species with immigrant seeds had both a lower abundance and a narrower spatial distribution than locally produced seeds. Therefore, immigrant seeds showed higher diversity values than locally produced seeds.Average seed size and the proportion of epigeous seedlings were similar in local and immigrant species. Under closed-canopied sites, factors affecting community organization seem to operate selectively, favoring the establishment of large-seeded, local abundant species in the advanced regeneration. However, the fact that some saplings of immigrant species were found in the plots suggest that a slow species infiltration may be occurring leading to a slow shift in the advanced regeneration species composition. We propose that the influence of seed dispersal on advanced regeneration structure depends on the disturbance history of the patches where seeds land.     

天然次生林中水曲柳种子的扩散格局

 种子扩散是种群动态中的一个关键环节,研究其扩散格局对了解种子库及更新格局有重要意义。于1999～2000年在天然次生林中选择样地并设置样带,共设取样点1376个,观察和估计了水曲柳(Fraxinus mandshurica)的种子雨格局。结果发现,母树附近,尤其是冠幅范围内,各方向的种子雨没有显著差别。但种子雨密度随距离有明显变化,冠幅范围内平均可达462～903个·m-2。距冠缘10～20 m急剧减少,为50～190个·m-2。72.5%～96.4%的种子散落在距母株树干10 m 的范围内。饱满种子顺风     

蚂蚁对澜沧舞花姜种子散布及种苗空间分布格局的影响

 在野外系统观测了澜沧舞花姜（Globba lancangensis）18个果实共216粒种子的散布过程。共有10种蚂蚁参与了澜沧舞花姜的种子散布,距离为0.01～3.35 m,平均距离(0.47±0.03) m（平均值±SE,n=216）。其中最重要的３种蚂蚁是横纹齿猛蚁（Odontoponera transversa）、大头蚁(Pheidole sp.)和黄足厚结猛蚁(Pachycondyla luteipes),其出现频率分别为61%、50%和28%,散布的平均距离分别为(0.60±0.09) m、(0.20±0.01) m和(0.32±0.05) m。从总体上看,蚂蚁促进了种子的分散,降低了种子的聚集程度。横纹齿猛蚁对于种子上的油质体最为敏感,对人工去除了油质体的种子不搬运,对种子散布距离较远,暗示了其与澜沧舞花姜之间可能存在更紧密的互惠关系。野外样方调查结果表明,在3种舞花姜属植物中,以种子繁殖为主的澜沧舞花姜种苗之间的平均最近距离为(36.8±1.45) cm（平均值±SE,n=74）,显著大于以珠芽繁殖为主的毛舞花姜（Globba barthiri）的(29.8±2.70) m（n=34）（t73,33=2.11,p=0.037）和异果舞花姜（Globba racemosa）的(28.7±3.16) cm（n=32）（t73,31=2.33,p=0.022）;澜沧舞花姜的种苗聚集程度（Z=-1.70±0.19）显著小于毛舞花姜（Z=-2.58±0.37,t73,33=2.36,p=0.020）和异果舞花姜（Z=-3.28±0.53）（t73,31=3.54,p=0.001）。这说明相对于毛舞花姜和异果舞花姜,蚂蚁对种子的散布作用显著增加了澜沧舞花姜种苗间的平均最近距离,降低了居群的聚集度。     

Complete separation along matrilines in a social spider metapopulation inferred from hypervariable mitochondrial DNA region

The distribution and quantity of genetic diversity may be profoundly influenced by the emergence and dynamics of social groups. Permanent social living in spiders has resulted in the subdivision of their populations in more or less isolated colony lineages that grow, proliferate and become extinct without mixing with one another. A newly discovered hypervariable mitochondrial DNA region allowed us to examine the fine scale metapopulation structure in the social Anelosimus eximius. We sampled 39 colonies in Ecuador and French Guiana and identified 25 haplotypes. The majority of colonies contained one haplotype. Additional haplotypes occurred in approximately 15% of the colonies, and were always closely related to the common colony haplotype. Our findings confirm that colonies consist of single matrilines, with within‐colony variation explained by mutations within the matriline. We thus found no evidence of mixing of matrilines. Likewise, colonies in a cluster often shared a haplotype, implying common colony ancestry. In few cases, however, haplotypes were shared between more distant colonies, providing evidence for occasional longer distance dispersal and/or widespread colony lineages. The geographical localities of colonies were incongruent with phylogenetic trees and haplotype networks, showing that some areas contained two or more matrilines. Hence, females do not migrate into foreign colonies, but faithfully remain within their own colony lineage, even when they disperse into new areas. These results indicate that the fine scale metapopulation structure of pure matrilines is maintained over the long term and that colony turnover is not extensive or radical enough to homogenize entire geographical areas. Genetic diversity is thus preserved to some extent at the metapopulation level.     

Removal of seeds from frugivore defecations by ants in a Costa Rican rain forest

At our Costa Rican field site, seeds defecated by frugivorous birds usually do not remain where they have been deposited. Many species of ants are attracted to frugivore defecations and remove seeds and/or pulp. Pheidole species selectively remove seeds, fungus-growing species (tribe Attini) remove both pulp and seeds. Seeds of many Melastomataceae have an appendage, which we hypothesized is an elaiosome. Indeed, preference trials demonstrated that two species of Pheidole selected seeds with the appendage over seeds of the same species in which the appendage had been removed. However, we found that these ants did not take the appendage when it was offered by itself. We conclude that the appendage is not an elaiosome. In further trials, different ant species preferentially selected different seed species. These ants consumed some seeds and deposited others unharmed in refuse piles. We conclude that because the composition of leaf-litter ant communities is highly variable between neighboring square meter plots, and the probability of seed predation depends upon the species of ant, the over-all effect of ants on seed shadows and seed banks is spatially unpredictable. Addendum: The names of the two Pheidole emphasized in this study. P. nebulosa and P. nigricula, are unpublished names from a generic revision being prepared by E. O. Wilson and W. L. Brown. Their use here is not intended to constitute taxonomic publication but is solely for more precise indentification in future ecological research of similar nature     

Stable linkage disequilibrium owing to sexual antagonism

Linkage disequilibrium (LD) is an association between genetic loci that is typically transient. Here, we identify a previously overlooked cause of stable LD that may be pervasive: sexual antagonism. This form of selection produces unequal allele frequencies in males and females each generation, which upon admixture at fertilization give rise to an excess of haplotypes that couple male-beneficial with male-beneficial and female-beneficial with female-beneficial alleles. Under sexual antagonism, LD is obtained for all recombination frequencies in the absence of epistasis. The extent of LD is highest at low recombination and for stronger selection. We provide a partition of the total LD into distinct components and compare our result for sexual antagonism with Li and Nei''s model of LD owing to population subdivision. Given the frequent observation of sexually antagonistic selection in natural populations and the number of traits that are often involved, these results suggest a major contribution of sexual antagonism to genomic structure.     

Effetto della Presenza di Zuccheri Sull'attivita Esochinasica in Cotiledoni Germinanti di Ricino

Abstract

The effects of sugars on the development of hexokinase and fructokinase activities in isolated cotyledons from germinating castor bean seeds. — The possibility of an inductive effect of hexose concentration on the rate of synthesis of enzymes involved in the phosphorylation of sugars has been investigated. Cotyledons were removed from castor bean seeds germinated 48 h at 27 °C in the dark, and incubated 12 h in water or in 0,05–0,1 M glucose or fructose. The activities of hexokinase and of fructokinase (determined spectrophotometrically in the soluble fraction from cell free extracts) was found to increase, upon incubation, at a rate more than 100% higher for the cotyledons incubated in the presence of sugars than for those in water. The results suggest some specificity of the effect of fructose on fructokinase and of glucose on hexokinase. « Insoluble » hexokinase was not affected by the sugars. Protein synthesis inhibitors such as actinomycin D and puromycin inhibited any increase of kinase activities in the isolated cotyledons.