Influence of environmental heterogeneity on genetic diversity and structure in an endemic southern Californian oak

Understanding how specific environmental factors shape gene flow while disentangling their importance relative to the effects of geographical isolation is a major question in evolutionary biology and a specific goal of landscape genetics. Here, we combine information from nuclear microsatellite markers and ecological niche modelling to study the association between climate and spatial genetic structure and variability in Engelmann oak (Quercus engelmannii), a wind-pollinated species with high potential for gene flow. We first test whether genetic diversity is associated with climatic niche suitability and stability since the Last Glacial Maximum (LGM). Second, we use causal modelling to analyse the potential influence of climatic factors (current and LGM niche suitability) and altitude in the observed patterns of genetic structure. We found that genetic diversity is negatively associated with local climatic stability since the LGM, which may be due to higher immigration rates in unstable patches during favourable climatic periods and/or temporally varying selection. Analyses of spatial genetic structure revealed the presence of three main genetic clusters, a pattern that is mainly driven by two highly differentiated populations located in the northern edge of the species distribution range. After controlling for geographic distance, causal modelling analyses showed that genetic relatedness decreases with the environmental divergence among sampling sites estimated as altitude and current and LGM niche suitability. Natural selection against nonlocal genotypes and/or asynchrony in reproductive phenology may explain this pattern. Overall, this study suggests that local environmental conditions can shape patterns of genetic structure and variability even in species with high potential for gene flow and relatively small distribution ranges.     

Incomplete lineage sorting or secondary admixture: disentangling historical divergence from recent gene flow in the Vinous‐throated parrotbill (Paradoxornis webbianus)

Although Pleistocene glaciations had a major impact on the population genetic patterns of many species in North America and Europe, it remains unclear how these climatic fluctuations contributed to species diversification in East Asia. One reason for this is the difficulty of distinguishing genetic admixture following secondary contact from incomplete lineage sorting, both of which can generate similar patterns of genetic variation. Using a combination of multilocus analyses and coalescent simulation, we explore how these two processes occurred in the Pleistocene evolutionary history of a widespread East Asian bird, the Vinous‐throated parrotbill, Paradoxornis webbianus. Maximum likelihood (ML) tree identified two major mitochondrial lineages, which are geographically separated in most parts of its range, but are sympatric at a few sampling sites. NJ tree and Structure analysis of microsatellite data set revealed an extensive level of admixture and little population structure, suggesting recent admixture between two formerly separated groups. Networks from nuclear DNA data sets, however, did not indicate any geographically isolated groups but rather a panmictic population, thus support incomplete lineage sorting. By using coalescent simulation approaches, we show that both processes did occur, although at different temporal scales. During the Pleistocene glaciations, probably around 0.1–0.5 Ma (the Marine Isotope Stage 6, MIS6), P. webbianus contracted into two separate refugia, and subsequently accumulated genetic divergence. During the interglacial MIS5, the species expanded into previously glaciated areas allowing the once separated groups to come into contact and become admixed. Taken together, our results indicate the current genetic variation within P. webbianus is a combination pattern of widespread distribution in pre‐Pleistocene, then contraction and fragmentation into separated refugia during glacial advance, followed by recently postglacial expansion and admixture.     

Latitude, elevational climatic zonation and speciation in New World vertebrates

Many biodiversity hotspots are located in montane regions, especially in the tropics. A possible explanation for this pattern is that the narrow thermal tolerances of tropical species and greater climatic stratification of tropical mountains create more opportunities for climate-associated parapatric or allopatric speciation in the tropics relative to the temperate zone. However, it is unclear whether a general relationship exists among latitude, climatic zonation and the ecology of speciation. Recent taxon-specific studies obtained different results regarding the role of climate in speciation in tropical versus temperate areas. Here, we quantify overlap in the climatic distributions of 93 pairs of sister species of mammals, birds, amphibians and reptiles restricted to either the New World tropics or to the Northern temperate zone. We show that elevational ranges of tropical- and temperate-zone species do not differ from one another, yet the temperature range experienced by species in the temperate zone is greater than for those in the tropics. Moreover, tropical sister species tend to exhibit greater similarity in their climatic distributions than temperate sister species. This pattern suggests that evolutionary conservatism in the thermal niches of tropical taxa, coupled with the greater thermal zonation of tropical mountains, may result in increased opportunities for allopatric isolation, speciation and the accumulation of species in tropical montane regions. Our study exemplifies the power of combining phylogenetic and spatial datasets of global climatic variation to explore evolutionary (rather than purely ecological) explanations for the high biodiversity of tropical montane regions.     

Strong influence of regional species pools on continent-wide structuring of local communities

There is a long tradition in ecology of evaluating the relative contribution of the regional species pool and local interactions on the structure of local communities. Similarly, a growing number of studies assess the phylogenetic structure of communities, relative to that in the regional species pool, to examine the interplay between broad-scale evolutionary and fine-scale ecological processes. Finally, a renewed interest in the influence of species source pools on communities has shown that the definition of the source pool influences interpretations of patterns of community structure. We use a continent-wide dataset of local ant communities and implement ecologically explicit source pool definitions to examine the relative importance of regional species pools and local interactions for shaping community structure. Then we assess which factors underlie systematic variation in the structure of communities along climatic gradients. We find that the average phylogenetic relatedness of species in ant communities decreases from tropical to temperate regions, but the strength of this relationship depends on the level of ecological realism in the definition of source pools. We conclude that the evolution of climatic niches influences the phylogenetic structure of regional source pools and that the influence of regional source pools on local community structure is strong.     

Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.     

基于生态位理论的典型草原铁杆蒿种群化感作用

运用改进的Levins生态位宽度指数和Pianka生态位重叠指数,研究了典型草原草地建群种与优势种之间的生态竞争关系;并利用建群种(铁杆蒿)茎叶浸提液对不同优势种的化感种子进行发芽试验,分析铁杆蒿的化感潜力及其在封育草地中的生态地位.结果表明:封育草地中,本氏针茅的生态位最宽(0.99),其次为百里香(0.94)、铁杆蒿(0.82)和大针茅(0.76),赖草最窄(0.73);铁杆蒿与本氏针茅、本氏针茅与百里香、百里香与大针茅、铁杆蒿与百里香之间的生态位重叠值分别为0.90、0.95、0.94和0.86.不同浓度的铁杆蒿浸提液对植物的化感作用强度不同,表现为“低促高抑”.铁杆蒿浸提液对本氏针茅幼苗根系生长的化感促进作用要强于百里香,而对百里香幼苗芽生长的抑制作用要强于本氏针茅.甲醇浸提液的化感作用要强于水浸提液.铁杆蒿、本氏针茅、百里香和大针茅之间高的生态位重叠,说明该草地群落将继续向本氏针茅群落演替,铁杆蒿群落仅是一个重要的过渡演替阶段.铁杆蒿的化感作用在其中担负着驱动力的角色.     

基于地形因子的天童地区常绿树种和落叶树种共存机制研究

位于亚热带的浙江天童和古田山常绿阔叶林大样地分布有较高比例的落叶树种,那么它们与常绿树种的共存机制是什么?常绿树种和落叶树种生态习性差异较大,二者对生境的选择应有所不同,我们推测生境分化可能是两类植物实现共存的主要机制。为检验该假设,我们以天童20ha动态样地调查数据为依托,选择个体数≥20的55个常绿树种和42个落叶树种作为分析对象,用典范对应分析(CCA)研究了地形因子对二者分布的影响差异,用torus转换检验来分析常绿树种和落叶树种与各类地形生境的关联。结果如下:(1)CCA分析表明地形因子对常绿树种分布的解释量为19.2%,对落叶树种分布的解释量为7.0%。(2)torus转换检验结果表明,与沟谷成正关联的常绿树种和落叶树种的比例分别为16.4%和28.6%,成负关联的比例分别为40%和7%;与山脊成正关联的常绿树种和落叶树种的比例分别为41.8%和4.8%,成负关联的比例分别为10.9%和47.6%;与受干扰生境成正关联的常绿树种和落叶树种的比例分别为16.4%和42.9%。上述结果说明地形对常绿树种分布的影响大于落叶树种;两个植物类群对生境的选择多呈现相反格局,尤其是在沟谷生境和山脊生境,这进一步表明生境分化是常绿树种和落叶树种共存的重要机制之一,生态位理论在一定程度上能较好地解释亚热带常绿阔叶林物种多样性的维持。     

The effect of small-scale resource origin on trophic position estimates in Perca fluviatilis

The influence of small-scale differences in resource origin on trophic position estimates was evaluated with the stable isotopes method. Perch Perca fluviatilis , a widespread freshwater predator, was used as a model organism. High individual variability was found in the contribution of benthic resources to the isotopic signatures of perch caught in the littoral zone, suggesting substantial within-habitat individual specialization. Small-scale resource origin and δ¹³C enrichment should be accounted for to avoid misleading estimates of both the absolute values and the ontogenetic trajectories of trophic position. A conceptual framework using end-member pathways is proposed to estimate trophic position with the stable isotope technique, particularly when marked ontogenetic niche shifts are expected.     

Development of the male germline stem cell niche in Drosophila

Stem cells are found in specialized microenvironments, or "niches", which regulate stem cell identity and behavior. The adult testis and ovary in Drosophila contain germline stem cells (GSCs) with well-defined niches, and are excellent models for studying niche development. Here, we investigate the formation of the testis GSC niche, or "hub", during the late stages of embryogenesis. By morphological and molecular criteria, we identify and follow the development of an embryonic hub that forms from a subset of anterior somatic gonadal precursors (SGPs) in the male gonad. Embryonic hub cells form a discrete cluster apart from other SGPs, express several molecular markers in common with the adult hub and organize anterior-most germ cells in a rosette pattern characteristic of GSCs in the adult. The sex determination genes transformer and doublesex ensure that hub formation occurs only in males. Interestingly, hub formation occurs in both XX and XY gonads mutant for doublesex, indicating that doublesex is required to repress hub formation in females. This work establishes the Drosophila male GSC niche as a model for understanding the mechanisms controlling niche formation and initial stem cell recruitment, as well as the development of sexual dimorphism in the gonad.