昆虫同域物种形成机制及检验

从生物学、生态和遗传的角度阐述昆虫同域物种形成过程中涉及到的可能性机制。昆虫同域种的分化与作用于同域初始种群的歧化选择密切相关,歧化选择间接导致种群生态特征和遗传特征的分化,促进同域近缘种群间的生殖隔离。同域物种形成的过程中涉及到性状替换、性选择、同型交配等机制。寄主专化型多见于昆虫同域种的分化过程中,一般以植食性昆虫为主。有关昆虫同域物种形成的检验机制有多种,归纳起来主要包括同型交配的检验、遗传漂流的量化、遗传分化程度和连锁不平衡(LD)的检测、杂交后代适合度的估算等。目前发现在许多昆虫种类中存在同域物种形成的可能性,但是有关其隔离机制并没有得到充分的解释。     

基因流存在条件下的物种形成研究述评:生殖隔离机制进化

物种形成过程是生物多样性形成的基础, 长期以来一直是进化生物学的中心议题之一。传统的异域物种形成理论认为, 地理隔离是物种分化的主要决定因子, 物种形成只有在种群之间存在地理隔离的情况下才能发生。近年来, 随着种群基因组学的发展和溯祖理论分析方法的完善, 种群间存在基因流情况下的物种形成成为进化生物学领域新的研究焦点。物种形成过程中是否有基因流的发生?基因流如何影响物种的形成与分化?基因流存在条件下物种形成的生殖隔离机制是什么?根据已发表的相关文献资料, 作者综述了当前物种形成研究中基因流的时间和空间分布模式、基因流对物种分化的影响以及生殖隔离机制形成等问题, 指出基因流存在条件下的物种形成可能是自然界普遍发生的一种模式。     

动物物种形成研究进展

物种形成研究是人们关于新物种如何产生和维持的思考和探索,是进化生物学最重要的组成部分之一。对物种形成方式、生殖隔离产生过程及其内在遗传机制的探究和揭示,是物种形成研究领域的重要命题和主要研究内容,也是认识和理解自然界中物种多样性现象的关键线索和重要依据。本文聚焦于动物类群,首先介绍了目前人们关于物种概念的不同定义,在此基础上,围绕动物物种形成方式、生殖隔离分子机制等所取得的研究进展和重要突破进行阐述,并分析了目前动物物种形成相关研究的局限;最后,探讨了今后动物物种形成研究中潜在的新机遇和新突破。     

动物物种形成的生殖隔离机制研究进展

物种形成过程及其机制不仅是理解自然界生物多样性现象的关键,也是长期困扰达尔文的\"谜中之谜\",是进化生物学研究领域的核心命题之一.物种形成是指新物种从祖先物种中分化出来的演化过程,也是种群间生殖隔离效应或屏障建立的演化过程,对这一过程及其内在遗传机制的探究和揭示是我们理解生物多样性现象的关键所在.本文聚焦于动物类群,系统阐述了动物物种生殖隔离的产生方式及其分子机制,并探讨了目前动物物种生殖隔离相关研究所面临的瓶颈、挑战和潜在机遇,为后续动物物种形成相关研究提供参考和启发.     

生态物种形成及其研究进展

物种形成是基本的进化过程, 也是生物多样性形成的基础。自然选择可以导致新物种的产生。生态物种形成是指以生态为基础的歧化选择使不同群体分化产生生殖隔离的物种形成过程。本文首先回顾了生态物种形成的研究历史, 并详细介绍了生态物种形成的3个要素, 即歧化选择的来源、生殖隔离的形式以及关联歧化选择与生殖隔离的遗传机制。歧化选择的来源主要包括不同的环境或生态位、不同形式的性选择, 以及群体间的相互作用。生殖隔离的形式多种多样, 我们总结了合子前和合子后隔离的遗传学机制以及在生态物种形成中起到的作用。控制适应性性状的基因与导致生殖隔离的基因可以通过基因多效性或连锁不平衡相互关联起来。借助于第二代测序技术, 研究者可以对生态物种形成的遗传学与基因组学基础进行研究。此外, 本文还总结了生态物种形成领域最新的研究进展, 包括平行进化的全基因组基础, 以及基因流影响群体分化的理论基础。通过归纳比较由下至上和由上至下这两种不同的研究思路, 作者认为这两种思路的结合可以为生态物种形成基因的筛选提供更有力也更精确的方法。同时, 作者还提出生态物种形成的研究应该基于更好的表型描述以及更完整的基因组信息, 研究的物种也应该具有更广泛的代表性。     

物种形成基因及其功能

什么是物种?新物种是如何形成的？这些问题是生命科学研究的重大问题.物种的形成是在生殖隔离的基础上某些新的生物学性状的形成和保留,是生物进化的最基本过程,其实质是基因结构突变的积累与功能的分化. 地理隔离使群体中的基因不能交流,基因突变也会影响个体间交配趣向,从而造成交配隔离或者交配后杂合体的基因组不亲和、杂交不育甚至杂交不活,使不同的群体逐渐分化为新物种. 随着分子生物学与基因组学的飞速发展,进化生物学家已经发现一些与物种形成有关的基因-物种形成基因（speciation genes）,鉴定并了解这些基因的功能,不仅能使我们在分子水平上理解新物种形成的实质和规律、而且对于我们突破种间屏障进行远缘杂交育种也有重要的理论指导意义.本文综述了目前对几个物种形成基因及其功能的研究进展,为该领域的进一步研究提供资料.     

同域分布的专食性近缘跳甲生殖隔离机制及物种形成研究取得进展

物种形成是种群间生殖隔离演化的产物,探究昆虫种间生殖隔离的机制有助于我们深入理解物种形成的模式。跳甲属（昆虫纲：鞘翅目：叶甲科）昆虫多为专食性,种间寄主隔离显著,是探讨这一问题提供了理想材料。     

二化螟寄主种群及种群间生殖隔离形成机制研究

水稻和茭白是二化螟的两种主要寄主植物。经过长期的适应进化,二化螟已分化出水稻种群和茭白种群。国内外的大量研究表明,二化螟水稻种群和茭白种群在形态特征、交配行为、寄主选择性、寄主适应性、田间种群动态、越冬生物学、生理生化及遗传多样性等方面均存在一定差异。二化螟水稻种群和茭白种群的发生期差异、交配节律差异和交配后配子不亲合等可能造成了寄主种群的部分生殖隔离。目前认为,二化螟两寄主种群间仍存在基因交流,处于物种形成的早期阶段,但尚需从寄主种群鉴定、表型差异区分和种群间生殖隔离机制等方面开展深入研究来进一步明确,为理解昆虫同域物种形成机制提供参考。     

3种同域分布的杓兰属植物的种子及杂交种子特征分析

杂交种子研究在一定程度上能说明是否存在杂种不活机制,在植物生殖隔离研究中具有重要意义。通过对同域分布的西藏杓兰(Cypripedium tibeticum)、黄花杓兰(C.flavum)和褐花杓兰(C.calcicola)的自交、异交、杂交种子的形态特征及活性进行分析,发现3种杓兰属植物两两之间均可产生杂交种子,且杂交种子活性较高,杂交种子与其他处理所得种子的外观、表面纹饰无显著性差异;种子宽度、种子长度、有胚率、着色率并没有比自交或异交种子显著低。这一结果表明这3种同域杓兰属植物种与种之间具有相当高的亲和性,它们之间不存在明显的杂种不活机制。黄花杓兰与西藏杓兰或褐花杓兰间的传粉者大小明显不同,黄花杓兰由丽蝇和熊蜂工蜂传粉,而西藏杓兰和褐花杓兰由体形较大的熊蜂蜂王传粉,传粉者隔离已使得它们之间的物种界限比较清晰,因此已经没有必要再产生杂种不活等其他隔离机制。而西藏杓兰与褐花杓兰的传粉者相同,又没有明显的杂种不活隔离机制,暗示它们之间有其他合子后隔离机制或应将其合并为一个种。     

Divergence with gene flow as facilitated by ecological differences: within-island variation in Darwin's finches

Divergence and speciation can sometimes proceed in the face of, and even be enhanced by, ongoing gene flow. We here study divergence with gene flow in Darwin''s finches, focusing on the role of ecological/adaptive differences in maintaining/promoting divergence and reproductive isolation. To this end, we survey allelic variation at 10 microsatellite loci for 989 medium ground finches (Geospiza fortis) on Santa Cruz Island, Galápagos. We find only small genetic differences among G. fortis from different sites. We instead find noteworthy genetic differences associated with beak. Moreover, G. fortis at the site with the greatest divergence in beak size also showed the greatest divergence at neutral markers; i.e. the lowest gene flow. Finally, morphological and genetic differentiation between the G. fortis beak-size morphs was intermediate to that between G. fortis and its smaller (Geospiza fuliginosa) and larger (Geospiza magnirostris) congeners. We conclude that ecological differences associated with beak size (i.e. foraging) influence patterns of gene flow within G. fortis on a single island, providing additional support for ecological speciation in the face of gene flow. Patterns of genetic similarity within and between species also suggest that interspecific hybridization might contribute to the formation of beak-size morphs within G. fortis.     

Sympatric speciation: when is it possible?

This paper is written to compare the results of theoretical investigations of sympatric speciation with the relevant experimental data. We understand sympatric speciation as a formation of species out of a population whose spatial structure is not important genetically. A necessary prerequisite for speciation is an action of disruptive selection on sufficiently polymorphic traits. The present analysis confirms the view that such a selection is ecologically realistic. The genetical part of speciation begins with a development of reproductive isolation between those individuals that are opposed in some characters. It is shown that selection for reproductive isolation may be quite strong. Extinction of intermediate individuals, which completes speciation, proceeds under a wide range of conditions, including those when the newly formed species differ in quantitative characters, though most of the genes arc likely to remain the same in both species. The whole process seems possible if differences in several (up to 10) loci are sufficient to adapt the forming species to different niches and to establish reproductive isolation. It is shown that populations with bimodal distributions of some genetically determined quantitative characters can have a considerable life-time. Such distributions may be formed either as a transition stage of sympatric speciation or represent a stationary state under conditions close to those necessary to complete speciation. They are very important for experimental investigations. Sympatric speciation always follows the same principal course; it does not contradict the idea of a genome coadaptedness. The occurrence of sympatric speciation is different for different taxa depending rather on how frequently populations are subjected to the appropriate kind of selection than on their ability to obey it.     

Competitive speciation

A new mode of speciation, competitive speciation, is suggested. It assumes that fitness is depressed by the density of a phenotype's competitors, and that the adaptive landscape of phenotypes is complex. From this it follows that some intermediate forms may be fit if and only if some extreme forms are rare or absent. Subsequent to the evolution and population growth of both extreme forms, the intermediate may disappear and homogamy evolve among each of the extremes because of disruptive selection If so, sympatric speciation has occurred and niche space has been rendered into discrete segments.
The limitations of the forces leading to competitive speciation are explored. Competitive speciation is discussed in relation to stasipatric speciation and host race formation. It may be responsible for both. Finally the rates of geographical speciation and polyploidy are compared to those of competitive speciation. The latter should be almost as fast as polyploidy and may be at the root of adaptive radiation. Unlike either polyploidy or geographical speciation, competitive speciation accelerates when species diversity declines.     

INCIPIENT REPRODUCTIVE ISOLATION BETWEEN TWO SYMPATRIC MORPHS OF THE INTERTIDAL SNAIL LITTORINA SAXATILIS

The study of speciation in recent populations is essentially a study of the evolution of reproductive isolation mechanisms between sub-groups of a species. Prezygotic isolation can be of central importance to models of speciation, either being a consequence of reinforcement of assortative mating in hybrid zones, or a pleiotropic effect of morphological or behavioral adaptation to different environments. To suggest speciation by reinforcement between incipient species one must at least know that gene flow occurs, or have recently occurred, and that assortative mating has been established in the hybrid zone. In Galician populations of the marine snail Littorina saxatilis, two main morphs appear on the same shores, one on the upper-shore barnacle belt and the other in the lower-shore mussel belt. The two morphs overlap in distribution in the midshore where hybrids are found together with pure forms. Allozyme variation indicates that the two parental morphs share a common gene pool, although within shores, gene flow between morphs is less than gene flow within morphs. In this study, we observed mating behavior in the field, and we found that mating was not random in midshore sites, with a deficiency of heterotypic pairs. Habitat selection, assortative mating, and possibly sexual selection among females contributed to the partial reproductive isolation between the pure morphs. Sizes of mates were often positively correlated, in particular, in the upper shore, indicating size-assortative mating too. However, this seemed to be a consequence of nonrandom microdistributions of snails of different sizes. Because we also argue that the hybrid zone is of primary rather than secondary origin, this seems to be an example of sympatric reproductive isolation, either established by means of reinforcement or as a by-product to divergent selection acting on other characters.     

Asymmetric introgression between the M and S forms of the malaria vector, Anopheles gambiae, maintains divergence despite extensive hybridization

The suggestion that genetic divergence can arise and/or be maintained in the face of gene flow has been contentious since first proposed. This controversy and a rarity of good examples have limited our understanding of this process. Partially reproductively isolated taxa have been highlighted as offering unique opportunities for identifying the mechanisms underlying divergence with gene flow. The African malaria vector, Anopheles gambiae s.s., is widely regarded as consisting of two sympatric forms, thought by many to represent incipient species, the M and S molecular forms. However, there has been much debate about the extent of reproductive isolation between M and S, with one view positing that divergence may have arisen and is being maintained in the presence of gene flow, and the other proposing a more advanced speciation process with little realized gene flow because of low hybrid fitness. These hypotheses have been difficult to address because hybrids are typically rare (<1%). Here, we assess samples from an area of high hybridization and demonstrate that hybrids are fit and responsible for extensive introgression. Nonetheless, we show that strong divergent selection at a subset of loci combined with highly asymmetric introgression has enabled M and S to remain genetically differentiated despite extensive gene flow. We propose that the extent of reproductive isolation between M and S varies across West Africa resulting in a 'geographic mosaic of reproductive isolation'; a finding which adds further complexity to our understanding of divergence in this taxon and which has considerable implications for transgenic control strategies.     

Sympatric speciation in a genus of marine reef fishes

Sympatric speciation has been contentious since its inception, yet is increasingly recognized as important based on accumulating theoretical and empirical support. Here, we present a compelling case of sympatric speciation in a taxon of marine reef fishes using a comparative and mechanistic approach. Hexagrammos otakii and H. agrammus occur in sympatry throughout their ranges. Molecular sequence data from six loci, with complete sampling of the genus, support monophyly of these sister species. Although hybridization occurrs frequently with an allopatric congener in an area of slight distributional overlap, we found no F₁ hybrids between the focal sympatric taxa throughout their coextensive ranges. We present genetic evidence for complete reproductive isolation based on SNP analysis of 382 individuals indicating fixed polymorphisms, with no shared haplotypes or genotypes, between sympatric species. To address questions of speciation, we take a mechanistic approach and directly compare aspects of reproductive isolation between allopatric and sympatric taxa both in nature and in the laboratory. We conclude that the buildup of reproductive isolation is strikingly different in sympatric vs. allopatric taxa, consistent with theoretical predictions. Lab reared hybrids from allopatric species crosses exhibit severe fitness effects in the F₁ or backcross generation. No intrinsic fitness effects are observed in F₁ hybrids from sympatric species pairs, however these treatments exhibited reduced fertilization success and complete pre‐mating isolation is implied in nature because F₁ hybrid adults do not occur. Our study addresses limitations of previous studies and supports new criteria for inferring sympatric speciation.