Evolutionary divergence in the pan-Atlantic mangrove Avicennia germinans

Cuticular hydrocarbon composition was determined for 18 populations of Avicennia germinans (Avicenniaceae) collected from West Africa, Florida, the Pacific coast of Mexico and French Guyana. Variation in covariance structure among the hydrocarbons was evaluated from correlations among the traits for each of the populations and from a hierarchical common principal components analysis. Principal components ordinations of populations based on the 171 z * transformed correlation coefficients, onto which a minimal spanning tree was superimposed, suggested a network in which French Guyanan populations formed a nexus from which African populations differentiated biochemically from Floridan and Mexican groups. Common principal components analysis provided further support for the differentiation in covariance structure among these biochemical traits of African populations. Pacific coast Mexican populations appeared to have diverged less in covariation among traits from Atlantic American sources than did the northern (Floridan) and southern (French Guyanan) Atlantic American populations from one another. The complete isolation of Pacific coast sources after emergence of the Central American isthmus during the Pliocene provides a reference for comparison of levels of differentiation among regions that suggests an underlying pattern of divergence in this species. Partial Mantel matrix tests revealed significant geographic effects, with no significant climatic effects. Our results provide support for recognition of a major differentiation between Old and New World sources within the taxon and indicate the need for a full revision of this species.     

Can ecosystem functional recovery be traced to decomposition and nitrogen dynamics in estuaries of the Lower Laguna Madre,Texas?

The biggest incentive to attempt the restoration and protection of estuaries is their widely acknowledged ecological and economic importance. Assessing estuary health and recovery can most accurately come from examining ecosystem processes. The purpose of this study was to explore the potential of mass loss and nitrogen (N) dynamics during leaf litter decomposition, to detect signs of functional recovery in two estuarine systems in south Texas. Submerged litterbags with black mangrove (Avicennia germinans) leaves were retrieved at various dates over 320 days. Decomposition was about 50% slower in one of the recovering systems compared to a reference site. Nitrogen immobilization and release from decaying leaf litter also discriminated among sites. Nitrogen immobilization potentials ranged from 4.15 to 6.89 mgN/g leaf litter, with the reference site exhibiting the highest value and thus the highest potential to conserve N during litter decomposition. The reference site also had a N immobilization time twice as long as the recovering sites, and a slower net release after the immobilization, appearing again as the most conservative in this part of the N cycling, possibly pointing to a less disturbed, or more stable ecosystem. Overall, the N dynamics during decomposition of mangrove leaf litter were similar in both recovering sites, whereas the reference site had a more conservative nutrient dynamics with more N being retained for longer in decomposing litter, coupled with a slower net release. Metrics derived from N dynamics may provide a finer resolution assessment of functional recovery, than only decomposition metrics.     

A combination of molecular markers identifies evolutionarily significant units in Cedrela odorata L. (Meliaceae) in Costa Rica

The necessity for conservation of the geneticcomponent of biodiversity is now widelyrecognised. A broad genetic base is required tomaintain evolutionary potential and thepopulation erosion occurring in much of theworld's forests threatens the genetic integrityof many tree species. Spanish Cedar (Cedrela odorata L.) has been under severepressure for generations and is now the focusof a study aimed at assessing the levels anddistribution of genetic diversity in remainingpopulations. Ten Costa Rican populations wereanalysed using chloroplast and AFLP markers.The overall level of diversity was as expectedfor an outcrossing, long-lived, woody species(H _T = 0.27). However, this concealeda deep divergence within the species, forchloroplast and AFLP (_CT = 0.83)markers. Populations were differentiated in twogroups that exhibited contrasting habitatpreferences and two ecotypes, wet and dry, wereidentified. Within the ecotypes, all but onepopulation were fixed for a single chloroplasthaplotype and within populations, total genomicdiversity levels were low (H _S= 0.03–0.13). Populations possessing the dryecotype maintained significantly more diversitythan those from wet regions. Within the wetecotype group, pairwise genetic distancebetween populations fitted an isolation bydistance model. The group was stronglysubdivided and showed isolation by distancearound the southern edge of the centralmountain ranges. The genetic divergence of thetwo ecotypes, observed at both organellar andnuclear loci, identifies evolutionarilysignificant units that, taken together withprevious studies of the species, provide arational basis on which to build a conservationpolicy for the species.     

锥栗种群在鼎湖山三个群落中的遗传分化研究

采用扩增片段长度多态性（AFLP,Amplified fragmen length polymorphism)方法对锥栗（Castanopsis chinensis)在3个不同群落亚种群的遗传分化进行了研究。研究地点位于广东省鼎湖山,3个群落分别是：针叶林群落,针阔叶混交林群落,常绿阔叶林群落。采集的叶要进行DNA提取,酶切、连接、PCR扩增后进行变异性聚丙烯酰胺的凝脉电泳,银染后把所得带进行1／0标记。结果表明,四组引物对分别扩增出27,20,33,39条带,其中分别有15,15,18,26条是多态性带,说明其多样性较高。用AMOVA（Analysis of molecular variance) 分析表明75．36％的遗传变异表现在种群内,24．64％的遗传变异表现在种群间,极显著（P＜0．001）。使用Nei遗传距离进行了算术平均数的非加权成组配对法(UPGMA）。其聚类结果表明常绿阔叶林群落亚种群和针阔叶混交林群落亚种群的种群聚合在一起。上述结果说明锥栗不同亚种群有不同的遗传多样性,种群间遗传分化较大。这是种群本身遗传特性的反映,并与群落演替密切相关,即不同群落微环境对种群遗传的影响。     

Genetic diversity of Ranunculus acris L. (Ranunculaceae) populations in relation to species diversity and habitat type in grassland communities

Correlates between genetic diversity at intra- and interpopulation levels and the species diversity in plant communities are rarely investigated. Such correlates may give insights into the effect of local selective forces across different communities on the genetic diversity of local plant populations. This study has employed amplified fragment length polymorphism to assess the genetic diversity within and between 10 populations of Ranunculus acris in relation to the species diversity (richness and evenness) of grassland communities of two different habitat types, 'seminatural' and 'agriculturally improved', located in central Germany. Within-population genetic diversity estimated by Nei's unbiased gene diversity (HE) was high (0.258-0.334), and was not correlated with species richness (Pearson's r = -0.17; P = 0.64) or species evenness (Pearson's r = 0.15; P = 0.68) of the plant communities. However, the genetic differentiation between R. acris populations was significantly correlated with the difference in species evenness (Mantel's r = 0.62, P = 0.02), but not with difference in species richness of plant communities (r = -0.17, P = 0.22). Moreover, we also found that populations of R. acris from the 'seminatural' habitat were genetically different (amova, P < 0.05) from those in 'agriculturally improved' habitats, suggesting that gene flow between these habitat types is limited. The results reported in this study may indicate that habitat characteristics influence the genetic diversity of plant species.     

Genetic structure at range edge: low diversity and high inbreeding in Southeast Asian mangrove (Avicennia marina) populations

Understanding the genetic composition and mating systems of edge populations provides important insights into the environmental and demographic factors shaping species' distribution ranges. We analysed samples of the mangrove Avicennia marina from Vietnam, northern Philippines and Australia, with microsatellite markers. We compared genetic diversity and structure in edge (Southeast Asia, and Southern Australia) and core (North and Eastern Australia) populations, and also compared our results with previously published data from core and southern edge populations. Comparisons highlighted significantly reduced gene diversity and higher genetic structure in both margins compared to core populations, which can be attributed to very low effective population size, pollinator scarcity and high environmental pressure at distribution margins. The estimated level of inbreeding was significantly higher in northeastern populations compared to core and southern populations. This suggests that despite the high genetic load usually associated with inbreeding, inbreeding or even selfing may be advantageous in margin habitats due to the possible advantages of reproductive assurance, or local adaptation. The very high level of genetic structure and inbreeding show that populations of A. marina are functioning as independent evolutionary units more than as components of a metapopulation system connected by gene flow. The combinations of those characteristics make these peripheral populations likely to develop local adaptations and therefore to be of particular interest for conservation strategies as well as for adaptation to possible future environmental changes.     

光皮桦6个南方天然群体的遗传多样性

为揭示中国特有珍贵用材树种光皮桦(Betula luminifera)天然群体的遗传多样性和遗传结构, 采用AFLP分子标记, 分析了采自浙江、福建、江西、广西和贵州5个省区6个天然群体的120份样品。9对引物获得了355个位点, 其中多态性位点323个。分析结果表明光皮桦天然群体具有较高的遗传多样性, 多态位点百分率(PPL)达90.99%, 各群体的PPL和Nei’s基因多样性(h_j)分别为93.20-98.60%和0.3143-0.3645; 总群体遗传多样性指数(H_t)为0.3616, 群体间遗传分化系数(F_st)为0.0650, 群体间总的基因流较高(N_m= 3.5962)。AMOVA分析表明群体间的遗传变异占总变异的11.49%, 浙江临安群体和贵州修文群体间的遗传距离最大(0.0665), 江西龙南群体和广西龙胜群体间的遗传距离最小(0.0173), 且遗传结构分析显示这两个群体的部分个体可能来自同一近祖。Mantel检测发现, 群体间的遗传距离与地理距离没有显著相关性(r = 0.423, P = 0.113), 而与两两群体所在地的均温差呈显著相关(r = 0.449, P = 0.017)。结合群体实地调查, 可以得出光皮桦天然群体的遗传多样性和遗传结构的形成不仅与其广域分布、自然杂交、种子特性以及生活史有关, 而且与群体被人为砍伐、生境片断化等因素有重要关系。基于上述结果我们提出了光皮桦天然种群的保护策略。     

Ecology of Mangrove Growth and Recovery in the Lesser Antilles: State of Knowledge and Basis for Restoration Projects

Whereas the increasing knowledge on tropical coastal wetlands highlights the ecological and economical importance of such ecosystems, anthropogenic activities within the coastal zone have caused substantial, irreversible losses of mangrove areas in the Lesser Antilles during the last decades. Such a paradox gives strength to compensatory policy efforts toward mangrove restoration. We review the available knowledge on the ecology of mangrove growth and recovery in the Lesser Antilles as a contribution to possible restoration projects in such islands. Distribution of species follows a general pattern of seaward/landward zonation according to their respective tolerance to flooding and to pore‐water salinity. An experimental study of seedling growth following simulated oil spill has documented the tolerance of Rhizophora mangle and Avicennia germinans seedlings to oil concentration in soils and the effects of natural biotic and abiotic factors on seedlings growth and survival. Monitoring mangrove recovery following hurricane Hugo has given information on growth patterns, from seedling to sapling stages, according to species and site conditions. Forest recovery was mostly due to pre‐established seedlings. For the large Rhizophora propagules, buoyancy appears to be a quite inefficient way of dispersal far inland from the sea shore or riversides. Causes of recovery failure are discussed. From these results we attempt to answer the questions when, where, how to plant mangroves, and what species to use.     

Genetic consequence of restricted habitat and population decline in endangered Isoetes sinensis (Isoetaceae)

BACKGROUND AND AIMS: Isoetes sinensis (Isoeteaceae) is a critically endangered aquatic quillwort in eastern China. Rapid decline of extant population size and local population extinction have occurred in recent years and have raised great concerns among conservationists. METHODS: Amplified fragment length polymorphisms (AFLPs) were used to investigate the genetic variation and population structure of seven extant populations of the species. KEY RESULTS: Eight primer combinations produced a total of 343 unambiguous bands of which 210 (61.2 %) were polymorphic. Isoetes sinensis exhibited a high level of intra-population genetic diversity (H(E) = 0.118; hs = 0.147; I = 0.192; P = 35.2 %). The genetic variation within each of the populations was not positively correlated with their size, suggesting recent population decline, which is well in accordance with field data of demographic surveys. Moreover, a high degree of genetic differentiation (F(ST) = 0.535; G(ST) = 0.608; theta(B) = 0.607) was detected among populations and no correlation was found between geographical and genetic distance, suggesting that populations were in disequilibrium of migration-drift. Genetic drift played a more important role than gene flow in the current population genetic structure of I. sinensis because migration of I. sinensis is predominantly water-mediated and habitat range was highly influenced by environment changes. CONCLUSIONS: Genetic information obtained in the present study provides useful baseline data for formulating conservation strategies. Conservation management, including both reinforcement for in situ populations and ex situ conservation programmes should be carefully designed to avoid the potential risk of outbreeding depression by admixture of individuals from different regions. However, translocation within the same regional population should be considered as a measure of genetic enhancement to rehabilitate local populations. An ex situ conservation strategy for conserving all extant populations to maximize genomic representation of the species is also recommended.     

AFLP markers reveal high polymorphic rates in ryegrasses (Lolium spp.)

An evaluation was performed of the potential use of AFLP markers to reveal polymorphisms among Lolium perenne plants with different degrees of kinship. Radioactive and fluorescent detection techniques were applied. The use of a fluorescent detection approach contributed greatly to the speed and ease of conducting and interpreting the AFLP patterns. The great discriminative power of AFLP markers and their capacity to represent genetic relationships among ryegrass plants was shown. Despite the high polymorphic value of the AFLP markers, standard statistical tests could not differentiate between two gene pools derived from different breeding programmes. It proved also impossible to correlate fodder and turf phenotypes with AFLP distance data. A very important point revealed by our data is the high degree of genetic diversity within commercial ryegrass varieties. Our findings are relevant to any outcrossing crop with a breeding strategy based on the production of synthetic populations.