Comparative Population Dynamics of Two Closely Related Species Differing in Ploidy Level

Background

Many studies compare the population dynamics of single species within multiple habitat types, while much less is known about the differences in population dynamics in closely related species in the same habitat. Additionally, comparisons of the effect of habitat types and species are largely missing.

Methodology and Principal Findings

We estimated the importance of the habitat type and species for population dynamics of plants. Specifically, we compared the dynamics of two closely related species, the allotetraploid species Anthericum liliago and the diploid species Anthericum ramosum, occurring in the same habitat type. We also compared the dynamics of A. ramosum in two contrasting habitats. We examined three populations per species and habitat type. The results showed that single life history traits as well as the mean population dynamics of A. liliago and A. ramosum from the same habitat type were more similar than the population dynamics of A. ramosum from the two contrasting habitats.

Conclusions

Our findings suggest that when transferring knowledge regarding population dynamics between populations, we need to take habitat conditions into account, as these conditions appear to be more important than the species involved (ploidy level). However, the two species differ significantly in their overall population growth rates, indicating that the ploidy level has an effect on species performance. In contrast to what has been suggested by previous studies, we observed a higher population growth rate in the diploid species. This is in agreement with the wider range of habitats occupied by the diploid species.     

Estimates of Gene Flow among Populations, Geographic Races, and Species in the Ipomopsis Aggregata Complex

Interpopulational gene flow within a species can reduce population differentiation due to genetic drift, whereas genetic exchange among taxa can impede speciation. We used allozyme data to estimate gene flow within and among geographic races and species of perennial herbs in the Ipomopsis aggregata complex (Polemoniaceae). Estimates of interpopulational gene flow within taxa from two methods (F statistics and private alleles) were correlated with one another. Gene flow among populations within each geographic race (subspecies) of I. aggregata was relatively high (Nm greater than approximately 1.0). Gene flow was also high among populations of I. arizonica and among four northern populations of I. tenuituba. However, gene flow was low (Nm less than 1.0) for I. tenuituba when a population representing subsp. macrosiphon was included. This is consistent with previous findings that subsp. macrosiphon has had an independent origin and is reproductively, as well as geographically, isolated. A recently developed model, based on hierarchical F statistics, was employed to estimate genetic exchange among taxa. Gene flow estimates were generally high among races of I. aggregata (dNmrace greater than 1.0) but were low among subspecies of I. tenuituba (dNmrace less than 1.0). Consistent with morphological evidence, estimates of interspecific gene flow were moderate between I. aggregata and I. tenuituba, which hybridize in several areas. However, contrary to morphological evidence, we estimated relatively high levels of interspecific gene flow involving I. arizonica. Our results suggest that I. arizonica has hybridized with other species without the transfer of morphological traits.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gene Flow and Hybridization between Numerically Imbalanced Populations of Two Duck Species on the Subantarctic Island of South Georgia

Hybridization is common between species of animals, particularly in waterfowl (Anatidae). One factor shown to promote hybridization is restricted mate choice, which can occur when 2 species occur in sympatry but one is rare. According to the Hubbs principle, or "desperation hypothesis," the rarer species is more likely to mate with heterospecifics. We report the second of 2 independent examples of hybridization between 2 species of ducks inhabiting island ecosystems in the Subantarctic and South Atlantic Ocean. Yellow-billed pintails (Anas georgica) and speckled teal (Anas flavirostris) are abundant in continental South America, where they are sympatric and coexist in mixed flocks. But on South Georgia, an isolated island in the Subantarctic, the pintail population of approximately 6000 pairs outnumbers a small breeding population of speckled teal 300∶1. Using 6 genetic loci (mtDNA and 5 nuclear introns) and Bayesian assignment tests coupled with coalescent analyses, we identified hybrid-origin speckled teal alleles in 2 pintails on South Georgia. While it is unclear whether introgression has also occurred into the speckled teal population, our data suggest that this hybridization was not a recent event, but occurred some time ago. We also failed to identify unequivocal evidence of introgression in a much larger sample of pintails and speckled teal from Argentina using a 3-population "Isolation-with-Migration" coalescent analysis. Combined with parallel findings of hybridization between these same 2 duck species in the Falkland Islands, where population ratios are reversed and pintails are outnumbered by speckled teal 1:10, our results provide further support for the desperation hypothesis, which predicts that scarcity in one population and abundance of another will often lead to hybridization. While the South Georgia pintail population appears to be thriving, it''s possible that low density of conspecific mates and inverse density dependence (Allee effect) may be one factor limiting the reproductive output of the speckled teal population, and this situation may persist unless speckled teal increase in abundance on South Georgia.     

Measuring Gene Flow among Populations Having High Levels of Genetic Fragmentation

We present an analysis of the genetic structures of 22 species of salamanders, with regard to levels of gene flow among populations. We estimate the gene flow parameter, Nm (the product of the effective population number and rate of migration among populations) using two alternative methods described by Wright and Slatkin. For most species, these two methods give approximately congruent estimates of Nm; when estimates differ, the method of Wright produces values slightly larger than those derived by the method of Slatkin. We analyze these results in light of independently derived historical inferences of the fragmentation of populations. This analysis suggests that the Nm values calculated from protein polymorphisms may contain information more relevant to historical patterns of gene exchange than to the current population dynamics; moderately large values of Nm may be calculated for species containing populations known to be no longer exchanging genes. Application of a method for estimating the maximum possible rate of gene exchange among populations indicates that, for most species studied here, gene flow among populations probably is no greater than the mutation rate. We suggest that most plethodontid species cannot be viewed as units whose cohesion is maintained by continuing gene exchange. Furthermore, we suggest that phenotypic uniformity among populations is not easily explained by hypotheses of continual stabilizing selection and propose that future work concentrate upon clarification of the genetic and epigenetic factors conferring self-maintenance or autopoietic properties on living systems.     

两种濒危水韭植物迁地保护居群的基因流动态及回归重建保育遗传管理策略

采用微卫星分子标记对中华水韭（Isoetessinensis）安徽休宁、浙江建德和东方水韭（I.orientalis）浙江松阳三个孑遗居群的迁地保护居群开展了遗传多样性检测与遗传结构分析。7对多态性微卫星引物在36个迁地保护亚居群的720个样本中共检测到59个等位基因,每位点平均等位基因数（A）为8·43。迁地保护亚居群均维持很高的遗传多样性,多态信息含量（PIC）平均为0·707。迁地保护亚居群间遗传分化较低,遗传分化系数GST仅为0·070,居群间具有较大基因流（Nm=3·59）。单因素方差分析发现水韭孢子或孢子体在沿主要水流方向上的长距离传播能力要强于弱水流方向上的短距离传播能力,水流动态对水韭植物的基因流有重要影响。这与UPGMA聚类分析中迁地保护亚居群按邻近位置或水流相通程度优先聚类的结果相一致,水流所带动的强大基因流导致了不同孑遗居群来源的迁地保护亚居群间的遗传混杂。建议在开展水韭植物的迁地保护或回归自然重建时,对具有地方适应分化或者显著性进化的水韭植物居群应相互隔离而不宜配置在一起,以避免远交衰退的遗传风险。     

蕨类植物rbcL基因正选择和负选择位点的鉴定

基于分支模型、位点模型及分支-位点模型对蕨类的rbcL基因所受到的选择压力进行了分析.结果显示:分支模型下检测到大部分分支处于负选择,仅4个分支处于正选择压力下,并且仅2分支具有统计上的显著性;在位点模型下,通过比较模型M1a/M2a和M7/M8,在氨基酸水平上模型M2a和模型M8均鉴定出98L位点被正选择;在模型M8下,鉴定负向选择位点共228个,占总序列的83.82%,从而揭示出负选择对rbcL基因的进化起着非常重要的作用;在分支位点-模型c下鉴定出262A被正选择.98L、262A位点分别位于rbcL羧基末端α/β桶结构域的第3和第8个α螺旋上.蕨类通过该结构域的适应性进化,适应白垩纪被子植物兴起而引发的陆地生态系统改变,研究结果为以后实验分析提供了首选位点.     

西双版纳热带雨林中两种生态型蕨类植物的光合特性比较研究

通过比较分布于西双版纳热带雨林林下生境中的附生鸟巢蕨(Neottopteris nidus)和地生网脉铁角蕨(Asplenium finlaysonianum)的光合特征和光合诱导特性, 来研究不同生态型蕨类植物的光斑利用策略。研究结果表明, 2种蕨类植物的最大净光合速率、暗呼吸速率、表观量子效率、光饱和点和光补偿点没有显著差异, 但网脉铁角蕨的最大气孔导度远远高于鸟巢蕨, 表明后者具有更强的光合水分利用效率。在暗处理3小时接着光照(光强为20 mmol .m-2.s-1)30分钟后, 网脉铁角蕨的初始气孔导度显著高于鸟巢蕨。连续照射饱和强光后, 网脉铁角蕨达到最大净光合速率50%(T50%)和90%的时间(T90%)比鸟巢蕨短: 网脉铁角蕨和鸟巢蕨的T50%分别为0.57和5.31分钟, T90%分别为5.85和26.33分钟。诱导过程中, 气孔导度对强光的响应明显滞后于净光合速率。鸟巢蕨达到最大气孔导度的时间明显比网脉铁角蕨慢, 但在光合诱导消失过程中2种蕨类植物的 光合诱导维持能力却没有显著差异。上述结果表明, 与大多数地生林下植物(如网脉铁角蕨)相比, 附生鸟巢蕨的水分保护比碳获得更重要, 但却限制了附生蕨对光斑的利用。     

Parallel Patterns of Morphological and Behavioral Variation among Host-Associated Populations of Two Gall Wasp Species

A powerful approach to address the general factors contributing to ecological speciation is to compare distantly related taxa that inhabit the same selective environments. In this design, similarities among taxa can elucidate general mechanisms of the process whereas differences may uncover specific factors important to the process for individual taxa. Herein, we present evidence of parallel patterns of morphological and behavioral variation among host-associated populations of two species of cynipid gall wasps, Belonocnema treatae and Disholcaspis quercusvirens, that each exhibit a life cycle intimately tied to the same two host plant environments, Quercus geminata and Q. virginiana. Across both gall-former species we find consistent differences in body size and gall morphology associated with host plant use, as well as strong differences in host plant preference, a measure of habitat isolation among populations. These consistent differences among taxa highlight the important role of host plant use in promoting reproductive isolation and morphological variation among herbivorous insect populations–a prerequisite for ecological speciation.     

西双版纳热带雨林中两种生态型蕨类植物的光合特性比较研究

通过比较分布于西双版纳热带雨林林下生境中的附生鸟巢蕨（Neottopteris nidus）和地生网脉铁角蕨（Asplenium finlaysonianum）的光合特征和光合诱导特性,来研究不同生态型蕨类植物的光斑利用策略。研究结果表明,2种蕨类植物的最大净光合速率、暗呼吸速率、表观量子效率、光饱和点和光补偿点没有显著差异,但网脉铁角蕨的最大气孔导度远远高于鸟巢蕨,表明后者具有更强的光合水分利用效率。在暗处理3／J＇,时接着光照（光强为20I~mol-m-2,s。‘）30分钟后,网脉铁角蕨的初始气孔导度显著高于鸟巢蕨。连续照射饱和强光后,网脉铁角蕨达到最大净光合速率50％（T50％）和90％的时间（T90%）比鸟巢蕨短：网脉铁角蕨和鸟巢蕨的T50%分别为0．57和5．31分钟,T90％分别为5．85和26．33分钟。诱导过程中,气孔导度对强光的响应明显滞后于净光合速率。鸟巢蕨达到最大气孔导度的时间明显比网脉铁角蕨慢,但在光合诱导消失过程中2种蕨类植物的光合诱导维持能力却没有显著差异。上述结果表明,与大多数地生林下植物（如网脉铁角蕨）相比,附生鸟巢蕨的水分保护比碳获得更重要,但却限制了附生蕨对光斑的利用。     

Persistence of Common Alleles in Two Related Populations or Species

Mathematical studies are conducted on three problems that arise in molecular population genetics. (1) The time required for a particular allele to become extinct in a population under the effects of mutation, selection, and random genetic drift is studied. In the absence of selection, the mean extinction time of an allele with an initial frequency close to 1 is of the order of the reciprocal of the mutation rate when 4Nv << 1, where N is the effective population size and v is the mutation rate per generation. Advantageous mutations reduce the extinction time considerably, whereas deleterious mutations increase it tremendously even if the effect on fitness is very slight. (2) Mathematical formulae are derived for the distribution and the moments of extinction time of a particular allele from one or both of two related populations or species under the assumption of no selection. When 4Nv << 1, the mean extinction time is about half that for a single population, if the two populations are descended from a common original stock. (3) The expected number as well as the proportion of common neutral alleles shared by two related species at the tth generation after their separation are studied. It is shown that if 4Nv is small, the two species are expected to share a high proportion of common alleles even 4N generations after separation. In addition to the above mathematical studies, the implications of our results for the common alleles at protein loci in related Drosophila species and for the degeneration of unused characters in cave animals are discussed.     

High Levels of Genetic Divergence among Populations in a Weedy Fern, Pteris multifida Poir.

Abstract Intragametophytic selfing in the homosporous ferns has been viewed as an advantageous mechanism permitting colonization of distant, open habitats. However, there is little evidence for the generality of colonization through intragametophytic selfing. In this study, the genetic structure of Pteris multifida was determined using Wright's F-statistics in order to detect the occurrence of this type of colonization. A high level of genetic divergence among populations (F_ST= 0.543) was found in P. multifida. This genetic divergence seems to be caused by frequent occurrences of colonization through intragametophytic selfing.     

圈养大熊猫群体间的基因流状况分析

圈养群体的遗传管理一个非常重要的手段就是实现不同群体间的基因交流.为了全面评估大熊猫圈养群体间的基因流状况,本研究以卧龙中国大熊猫保护中心的31只圈养大熊猫(简称卧龙群体)和成都大熊猫繁育研究基地与楼观台陕西省珍稀野生动物抢救饲养研究中心的37只圈养大熊猫(简称成都群体,其中楼观台1只)为研究对象,以7个大熊猫微卫星位点为分子标记,发现卧龙群体和成都群体间的遗传分化水平很低(Fst=0.041,P=0.001);尽管整个圈养群体的近交程度较低(Fis=0.026),但是成都群体的近交系数(Fis=0.045)远高于卧龙群体的近交系数(Fis=0.002);谱系分析、贝叶斯分析和系统进化法分析均显示,这两个群体间存在着基因流,但是这种基因流是单向的.此结果提示各个大熊猫饲养单位之间必须实现更多的合作,将大熊猫群体作为一个管理单元进行管理,从而实现更多的基因流.     

Contrasting Genetic Structure among Populations of Two Amphidromous Fish Species (Sicydiinae) in the Central West Pacific

Both present-day and past processes can shape connectivity of populations. Pleistocene vicariant events and dispersal have shaped the present distribution and connectivity patterns of aquatic species in the Indo-Pacific region. In particular, the processes that have shaped distribution of amphidromous goby species still remain unknown. Previous studies show that phylogeographic breaks are observed between populations in the Indian and Pacific Oceans where the shallow Sunda shelf constituted a geographical barrier to dispersal, or that the large spans of open ocean that isolate the Hawaiian or Polynesian Islands are also barriers for amphidromous species even though they have great dispersal capacity. Here we assess past and present genetic structure of populations of two amphidromous fish (gobies of the Sicydiinae) that are widely distributed in the Central West Pacific and which have similar pelagic larval durations. We analysed sections of mitochondrial COI, Cytb and nuclear Rhodospine genes in individuals sampled from different locations across their entire known range. Similar to other Sicydiinae fish, intraspecific mtDNA genetic diversity was high for all species (haplotype diversity between 0.9–0.96). Spatial analyses of genetic variation in Sicyopus zosterophorum demonstrated strong isolation across the Torres Strait, which was a geologically intermittent land barrier linking Australia to Papua New Guinea. There was a clear genetic break between the northwestern and the southwestern clusters in Si. zosterophorum (φ_ST = 0.67502 for COI) and coalescent analyses revealed that the two populations split at 306 Kyr BP (95% HPD 79–625 Kyr BP), which is consistent with a Pleistocene separation caused by the Torres Strait barrier. However, this geographical barrier did not seem to affect Sm. fehlmanni. Historical and demographic hypotheses are raised to explain the different patterns of population structure and distribution between these species. Strategies aiming to conserve amphidromous fish should consider the presence of cryptic evolutionary lineages to prevent stock depletion.     

Temperature Sensitivity of a Hybrid between Two Species of Sea Urchin Differing in Thermotolerance

Eggs of Dendraster excentricus can be fertilized with sperm of Strongylocentrotus purpuratus at a high success rate according to Whiteley and Baltzer's method. The resulting hybrids are well suited for investigations of the factors determining embryonic thermosensitivity, because the embryos of both the above species differ significantly with regard to egg size, blastula shape, and particularly thermotolerance and the temperature dependence of their developmental rate. Morphologically, the hybrids show some paternally inherited traits, but their thermosensitivity is strictly determined by the egg cytoplasm.     

Influence of Gene Flow and Breeding Tactics on Gene Diversity within Populations

Expressions describing the accumulation of gene correlations within and among lineages and individuals of a population are derived. The model permits different migration rates by males and females and accounts for various breeding tactics within lineages. The resultant equations enable calculation of the probabilistic quantities for the fixation indices, rates of loss of genetic variation, accumulation of inbreeding, and coefficients of relationship for the population at any generation. All fixation indices were found to attain asymptotic values rapidly despite the consistent loss of genetic variation and accumulation of inbreeding within the population. The time required to attain asymptotic values, however, was prolonged when gene flow among lineages was relatively low (less than 20%). The degree of genetic differentiation among breeding groups, inbreeding coefficients, and gene correlations within lineages were found to be primarily functions of breeding tactics within groups rather than gene flow among groups. Thus, the asymptotic value of S. Wright's island model is not appropriate for describing genetic differences among groups within populations. An alternative solution is provided that under limited conditions will reduce to the original island model. The evolution of polygynous breeding tactics appears to be more favorable for promoting intragroup gene correlations than modification of migration rates. Inbreeding and variance effective sizes are derived for populations that are structured by different migration and breeding tactics. Processes that reduce the inbreeding effective population size result in a concomitant increase in variance effective population size.     

植物居群的基因流动态及其相关适应进化的研究进展

生物自然居群间的基因流不但可以阻止遗传分化以维持物种的完整性, 而且也能积极响应生物多样化的进程。理解与基因流相关的适应性进化及其内在机理将有助于我们更好地认识生物物种形成和多样化的原始动力以及真正原因。该文通过对植物种内和种间居群基因流动态进行讨论, 阐述了近年来有关植物基因流动态的一些重要理论观念和研究进展, 以期为相关领域动态及趋势研究提供参考。     

植物居群的基因流动态及其相关适应进化的研究进展

生物自然居群间的基因流不但可以阻止遗传分化以维持物种的完整性,而且也能积极响应生物多样化的进程。理解与基因流相关的适应性进化及其内在机理将有助于我们更好地认识生物物种形成和多样化的原始动力以及真正原因。该文通过对植物种内和种间居群基因流动态进行讨论,阐述了近年来有关植物基因流动态的一些重要理论观念和研究进展,以期为相关领域动态及趋势研究提供参考。     

Rare, Fragile Species, Small Populations, and the Dilemma of Collections

Considerable sums of money are aimed at biodiversity conservation programs at present, and although collection of specimens is necessary for the advancement of taxonomy and systematics, we detected a dilemma between conservation and collection of rare species with small populations. Collecting may act synergistically with other factors to increase the risk of, or even drive species to extinction. We present some examples we believe show the conflict between collecting rare species with a small population and the conservation programs for those species. We also stress the need to review the Minimum Viable Population Size concept as it can be used as a justification for collecting small, rare, and declining population species.