近交衰退:我们检测到了吗

近交衰退产生的原因是近交增加了有害等位基因纯合几率,导致个体适应能力下降。种群变小是导致近交衰退的主要原因,但在实际研究中发现近交衰退并非一定明显表现出来,这可能有以下几个主要的原因遗传负荷的淘汰、在较好的环境下近交衰退会表现不明显、并非能在所有性状中检测到近交衰退、近交衰退只出现在某些发育阶段和不同家系、种群、个体中的近交衰退程度不同。这提示我们近交衰退与生态及遗传密切相关。     

室内饲养的小菜蛾种群近交衰退分析

【目的】分析近交繁殖对小菜蛾Plutella xylostella种群的影响,为室内繁殖小菜蛾种群提供依据。【方法】在室内条件下,连续饲养6代小菜蛾种群,比较近交种群和杂交种群在交配行为、繁殖能力、蛹重、成虫寿命及形态等方面的差异。【结果】近交繁殖各代蛹重达极显著差异,尤其第3代后,蛹重显著下降;近交各代的雌雄成虫在交配次数和交配持续时间上显著减小,尤其是第4代后,下降幅度极其明显,F6代雌(雄)成虫的交配次数和交配持续时间分别是F0代的12.9%(13.8%)和8.3%(8.9%),表明自F4代后,近交小菜蛾种群已呈现出明显的衰退现象。同样,F4代后近交种群的产卵前期、产卵期、产卵量、卵孵化率、成虫寿命等均极显著下降,但世代历期显著延长。同时,近交对雌雄成虫体长有显著影响,F6代雌雄成虫体长是F0代的60%左右。【结论】自近交繁殖3代始,会导致小菜蛾种群交配行为、繁殖及形态的变化,尤其是近交繁殖4代以后变化更为明显,使种群出现明显的衰退现象。因此,在小菜蛾室内繁殖时,隔一段时间(至多3代)采集远源虫源,避免种群衰退。     

回交对小菜蛾近交种群适合度衰退的清除效应

研究近交种群的清除效应对深入探索昆虫近交衰退的遗传机理具有重要作用。本文在连续近交10代的基础上,采用年龄-龄期、两性生命表方法分析回交对小菜蛾近交衰退的清除效应,结果显示,回交1代即可显著提升近交小菜蛾卵的孵化率、成虫前期存活率与繁殖力,雌、雄成虫寿命也显著延长,回交小菜蛾的种群适合度达到杂交种群的水平。其中,回交小菜蛾的单雌产卵量甚至显著高于杂交种群。回交小菜蛾种群的内禀增长率、周限增长率、净增殖率也显著提高,与杂交种群相比无明显差异。同时,对种群数量的模拟发现,回交与杂交小菜蛾的种群数量变化趋势均较一致,60 d时回交与杂交各代种群数量均达到108以上。该结果为进一步研究揭示小菜蛾近交衰退的遗传机理奠定基础。     

厦门木麻黄种群交配系统及近交衰退

木麻黄耐沙生,盐碱,是沿海优良的防护林树种,从20世纪80年代开始,木麻黄林呈现出衰退现象。采用等位酶分析技术研究木麻黄种群的交配系统及近交衰退,木麻黄种群异交率为0．622,表明为混合酱类型,与其亲缘种比较来看,引种降低了木麻黄异交率,增加了近交,采用电是接估算的近交衰退程度很高。结果表明,引种过程中的建立者效应引起的近交及其后的近交衰退确定在木麻黄林衰退中起了重要作用,根据基因型有选择地引起木麻黄以减轻衰退。     

Differences in floral traits and flower visitation rates in mating systems in Prunella vulgaris (Lamiaceae)

夏枯草交配系统对花特征和访花频率差异的影响 植物花特征和传粉者的访问次数与交配系统类型密切相关。唇形科植物夏枯草(Prunella vulgaris)存 在两种植株类型,分别为柱头伸出花冠和柱头在花冠内部的植株,而且两种植株的比例在不同种群中存在差异。本研究选择柱头伸出花冠外植株占绝大多数、柱头伸出花冠外植株占多数和柱头在花冠内部植株占多数的3个种群,通过比较每个种群中两种植株类型的开花物候、花形态特征、昆虫访问频率、自交能力、传粉者对结实的贡献以及近交衰退的水平,以检验花特征和传粉者访问次数与交配系统类型的关系。研究结果表明,与柱头在花冠内部的植株相比,柱头伸出花冠外的植株具有更大和更多的花,产生更多的花粉和花蜜,具有更高的访花频率,并主要通过异交产生种子。在种群水平,柱头伸出花冠外的植株占多数种群的访花频率显著高于柱头在花冠内部植株占多数的访花频率。柱头在花冠内部的植株比柱头伸出花冠外的植株具有更强的自动自交能力,在传粉者缺乏时为其提供了繁殖保障,但繁殖保障和异交率在不同种群中差异不显著,表明较低的昆虫访问能够满足夏枯草的授粉需求以产生种子,这可能与夏枯草较少的胚珠数量(每朵花仅有4个胚珠)有关。柱头在花冠内部植株的近交衰退水平低于柱头伸出花冠外植株的近交衰退水平,但两种植株类型的近交衰退水平均低于0.5,说明近交衰退不足以阻止该物种中自交的进化。综上所述,柱头在花冠内部的植株能够通过自交为夏枯草提供繁殖保障,而柱头伸出花冠外的植株能够利用昆虫传粉确保异交,表明混合交配系统在该物种中是一个稳定的状态。     

濒危植物居群恢复的遗传学考量

本文针对濒危植物居群的遗传多样性、生殖适合度、基因流、近交和远交衰退等遗传学问题在居群恢复过程中的应用进行了探讨。濒危植物居群的回归重建,既面临遗传多样性的迅速丧失、近交衰退等遗传风险,还因回归引种地存在较多近缘种而带来远交衰退的风险,最终导致遗传适应性降低,生境适应性变窄,繁殖和竞争能力减弱。为提高濒危物种保护的质量和效率,在构建回归居群时,应分批次从同一来源居群的不同母株采集材料,确保种源的遗传纯正性和遗传组成的多样性,还应使回归居群尽可能远离近缘广布种。另外,还需要对回归种群进行持续的监测和管理才能保证回归引种的成功。     

黄山钓桥青冈种群的交配系统与近交衰退

采用垂直板型不连续聚丙烯酰胺凝胶电泳检测了黄山钓桥青冈种群的遗传变异、交配系统及近交衰退程度。黄山钓桥青冈群维持有中等程度的遗传变异,多态位点百分比和期望杂合度分别为５０％和０．１８１４。采用ＰＯＤ－２、ＥＳＴ－１和ＥＳＴ－２等３个多态位点,利用ＭＬＴ程序估计的花粉库和母株基因频率之间存在一些差异,可能是由于成熟个体花粉产量不同和种群内个体非随机分布造成的。单位点杂交率平均为０．３９２,略高于多位     

中国麋鹿遗传多样性现状与保护对策

通过对麋鹿野生种群的绝灭过程、圈养历史、种群增长及遗传多样性状况的分析研究,认为麋鹿脱离野生种群成为完全的圈养群体约有100多年的历史,捕猎和栖息地丧失是其绝灭的根本原因。麋鹿最初引入欧洲时曾经历了严重的近交衰退阶段,目前其耐受近交的能力显著增强。截至1994年我国麋鹿已达近500只,其遗传变异量约为其野生种群的70%。在我国重建麋鹿自然种群不仅完全可能,而且也只有如此才能使麋鹿在自然中进化并丰富其受损的遗传多样性。     

马边大风顶自然保护区大熊猫种群生存力模拟分析

采用漩涡模型Vortex mondel 713 , 模拟了马边大风顶自然保护区大熊猫种群在未来100年内的变动趋势。结果显示: 在无交配限制、无密度制约、无近亲交配衰退等条件下, 马边大风顶自然保护区大熊猫种群数量呈下降趋势; 在考虑近交衰退的影响后, 遗传多样性水平降低, 灭绝率提高; 竹子开花虽能加速大熊猫种群的绝灭, 但由于保护区分布有多个竹种,因此并不会对大熊猫种群产生灾难性影响; 但是人为捕杀可迅速减少大熊猫种群数量, 加速其灭绝过程。因此, 对该保护区大熊猫进行保护的最重要措施就是严格控制人为捕杀, 并保护栖息地及走廊带。     

被子植物早期近交衰退与晚期自交不亲和

被子植物自交后结籽率的降低通常由早期近交衰退(early-acting inbreeding depression)与晚期自交不亲和(self-incompatibility)导致.早期近交衰退是严格的合子后的作用机制,通常由多位点隐性有害基因的纯合导致,并在合子发育成成熟种子的过程中发生.发生在柱头表面或花柱中的自交不亲和是合子前的作用机制,而发生在子房内的晚期自交不亲和(late-acting ovarian self-incompatibility)可在合子前或合子后发生作用,与早期近交衰退很难区分.在合子前的子房内自交不亲和机制下,尽管花粉管能生长到子房甚至穿透胚珠,但通常不能形成合子.合子后的子房内自交不亲和能形成合子,但由于自交不亲和通常由单位点控制,合子败育集中发生在受精作用后的很短时间内.基于早期近交衰退和子房内自交不亲和的这种差异,己有研究提出了8种区分方法.通过解剖学方法观察授粉后生物学过程,比较自交和异交先后授粉和自交授粉处理的结籽率,以及通过一元线性回归模型考察自交和异交处理下成熟种子和败育种子数之和是否保持恒定,可以区分是合子前还是合子后过程导致的自交后种子产量降低.通过全同胞间杂交实验判断败育的遗传基础是单基因控制还是多位点有害隐性等位基因的纯和,可以区分合子后自交不亲和机制与早期近交衰退;或者通过这两种不同遗传基础导致的种子大小等表型性状的差异来区分.     

Inbreeding depression and mixed mating in Leptosiphon jepsonii: a comparison of three populations

BACKGROUND AND AIMS: Inbreeding depression is thought to play a central role in the evolution and maintenance of cross-fertilization. Theory indicates that inbreeding depression can be purged with self-fertilization, resulting in positive feedback for the selection of selfing. Variation among populations of Leptosiphon jepsonii in the timing and rate of self-fertilization provides an opportunity to study the evolution of inbreeding depression and mating systems. In addition, the hypothesis that differences in inbreeding depression for male and female fitness can stabilize mixed mating in L. jepsonii is tested. METHODS: In a growth room experiment, inbreeding depression was measured in three populations with mean outcrossing rates ranging from 0.06 to 0.69. The performance of selfed and outcrossed progeny is compared at five life history stages. To distinguish between self-incompatibility and early inbreeding depression, aborted seeds and unfertilized ovules were counted in selfed and outcrossed fruits. In one population, pollen and ovule production was quantified to estimate inbreeding depression for male and female fitness. KEY RESULTS: Both prezygotic barriers and inbreeding depression limited self seed set in the most outcrossing population. Cumulative inbreeding depression ranged from 0.297 to 0.501, with the lowest value found in the most selfing population. Significant inbreeding depression for early life stages was found only in the more outcrossing populations. Inbreeding depression was not significant for pollen or ovule production. CONCLUSIONS: The results provide modest support for the hypothesized relationship between inbreeding depression and mating systems. The absence of early inbreeding depression in the more selfing populations is consistent with theory on purging. Differences in male and female expression of inbreeding depression do not appear to stabilize mixed mating in L. jepsonii. The current estimates of inbreeding depression for L. jepsonii differ from those of previous studies, underscoring the effects of environmental variation on its expression.     

Evidence for an epigenetic role in inbreeding depression

Inbreeding depression (i.e. negative fitness effects of inbreeding) is central in evolutionary biology, affecting numerous aspects of population dynamics and demography, such as the evolution of mating systems, dispersal behaviour and the genetics of quantitative traits. Inbreeding depression is commonly observed in animals and plants. Here, we demonstrate that, in addition to genetic processes, epigenetic processes may play an important role in causing inbreeding effects. We compared epigenetic markers of outbred and inbred offspring of the perennial plant Scabiosa columbaria and found that inbreeding increases DNA methylation. Moreover, we found that inbreeding depression disappears when epigenetic variation is modified by treatment with a demethylation agent, linking inbreeding depression firmly to epigenetic variation. Our results suggest an as yet unknown mechanism for inbreeding effects and demonstrate the importance of evaluating the role of epigenetic processes in inbreeding depression.     

Effect of inbreeding depression on outcrossing rates among populations of a tropical pine

Inbreeding depression is common among plants and may distort mating system estimates. Mating system studies traditionally ignore this effect, nonetheless an assessment of inbreeding depression that may have occurred before progeny evaluation could be necessary. In the neotropical Pinus chiapensis inbreeding depression was evaluated using regression analysis relating progeny F-values with seed germinability, the mating system was analysed in three populations with contrasting size, using isozymes, obtained a corrected outcrossing rate. Selfing decreased seed viability by 19%, relative to an outcrossed plant. Multilocus outcrossing rates, t(m), varied widely among populations. In the two smallest populations t(m) congruent with 1. Therefore, inbreeding depression did not affect the estimates, but overestimated t(m) by 10% in the third population, which has a true mixed mating system (selfing was the major source of inbreeding), and an unusually low t(m) for pines (t(m) = 0.54, uncorrected, t(m) = 0.49, corrected). Inbreeding depression may be an uneven source of bias for outcrossing estimates even at the infraspecific level. Accuracy [corrected] but not precision [corrected] may be gained by including inbreeding depression in outcrossing estimates. Therefore, caution should be taken when comparing t(m) among species or even populations within the same species.     

Inbreeding Depression and Inbreeding Avoidance in a Natural Population of Guppies (Poecilia reticulata)

Maintenance of genetic variation in the face of strong natural selection is a long‐standing problem in evolutionary biology. One of the most extreme examples of within‐population variation is the polymorphic, genetically determined color pattern of male Trinidad guppies (Poecilia reticulata). Female mating preference for rare or novel patterns has been implicated as a factor in maintaining this variation. The origin of this preference is not understood, although inbreeding avoidance has been proposed as a mechanism. Inbreeding avoidance is advantageous when populations exhibit inbreeding depression and the opportunity for mating between relatives exists. To determine whether these conditions are met in a natural guppy population, we assessed mating and reproductive patterns using polymorphic molecular markers. Females produced more offspring with less‐related males than with more‐related ones. In addition, females were more likely to have mated with less‐related males, but this trend was only marginally significant. Male heterozygosity was positively correlated with mating success and with the number of offspring sired, consistent with strong inbreeding depression for adult male fitness. These results provide substantial insight into mating patterns of a wild guppy population: strong inbreeding depression occurs, and individuals tend to avoid mating with relatives.     

Reduced inbreeding depression in peripheral relative to central populations of a monocarpic herb

Many temperate taxa were confined to warmer latitudes during the last glacial maximum. As their ranges expanded when climates warmed, genetic drift and inbreeding in relatively small peripheral populations are expected to have reduced genetic diversity and the segregating genetic load. Therefore, inbreeding depression in peripheral populations might be lower than in centrally located sites. We evaluated the consequences of inbreeding for fitness traits in six central and six northern peripheral populations of the herb Campanulastrum americanum. Inbreeding reduced performance for all traits. Inbreeding depression in peripheral populations was lower than in central populations. This difference increased across the life cycle from similar levels for germination, to central populations having three times the inbreeding depression for adult traits. Geographical patterns of inbreeding depression suggest that mating system variation and potential future mating system evolution in many temperate taxa might reflect, at least in part, nonequilibrium conditions associated with historic range changes.     

No correlation between inbreeding depression and delayed selfing in the freshwater snail Physa acuta

Inbreeding depression, one of the main factors driving mating system evolution, can itself evolve as a function of the mating system (the genetic purging hypothesis). Classical models of coevolution between mating system and inbreeding depression predict negative associations between inbreeding depression and selfing rate, but more recent approaches suggest that negative correlations should usually be too weak or transient to be detected within populations. Empirical results remain unclear and restricted to plants. Here, we evaluate, for the first time, the within-population genetic correlation between inbreeding depression and a trait that controls the amount of self-fertilization (the waiting time) in a self-fertile hermaphroditic animal, the freshwater snail Physa acuta. Using a large quantitative-genetic design (36 grand-families and 348 families), we observe abundant within-population family-level genetic variation for both inbreeding depression (estimated for survival, fecundity, and size) and the degree of behavioral selfing avoidance. However, we detected no correlation between waiting time and inbreeding depression across families. In agreement with recent models, this result shows that mutational variance rather than differential purging accounts for most of the genetic variance in inbreeding depression within a population.     

STRONG INBREEDING DEPRESSION IN TWO SCANDINAVIAN POPULATIONS OF THE SELF‐INCOMPATIBLE PERENNIAL HERB ARABIDOPSIS LYRATA

Inbreeding depression is a key factor influencing mating system evolution in plants, but current understanding of its relationship with selfing rate is limited by a sampling bias with few estimates for self‐incompatible species. We quantified inbreeding depression (δ) over two growing seasons in two populations of the self‐incompatible perennial herb Arabidopsis lyrata ssp. petraea in Scandinavia. Inbreeding depression was strong and of similar magnitude in both populations. Inbreeding depression for overall fitness across two seasons (the product of number of seeds, offspring viability, and offspring biomass) was 81% and 78% in the two populations. Chlorophyll deficiency accounted for 81% of seedling mortality in the selfing treatment, and was not observed among offspring resulting from outcrossing. The strong reduction in both early viability and late quantitative traits suggests that inbreeding depression is due to deleterious alleles of both large and small effect, and that both populations experience strong selection against the loss of self‐incompatibility. A review of available estimates suggested that inbreeding depression tends to be stronger in self‐incompatible than in self‐compatible highly outcrossing species, implying that undersampling of self‐incompatible taxa may bias estimates of the relationship between mating system and inbreeding depression.     

Environment-dependent inbreeding depression: its ecological and evolutionary significance

Inbreeding depression is a major evolutionary and ecological force that influences population dynamics and the evolution of inbreeding-avoidance traits such as mating systems and dispersal. There is now compelling evidence that inbreeding depression is environment-dependent. Here, we discuss ecological and evolutionary consequences of environment-dependent inbreeding depression. The environmental dependence of inbreeding depression may be caused by environment-dependent phenotypic expression, environment-dependent dominance, and environment-dependent natural selection. The existence of environment-dependent inbreeding depression challenges classical models of inbreeding as caused by unconditionally deleterious alleles, and suggests that balancing selection may shape inbreeding depression in natural populations; loci associated with inbreeding depression in some environments may even contribute to adaptation to others. Environment-dependent inbreeding depression also has important, often neglected, ecological and evolutionary consequences: it can influence the demography of marginal or colonizing populations and alter adaptive optima of mating systems, dispersal, and their associated traits. Incorporating the environmental dependence of inbreeding depression into theoretical models and empirical studies is necessary for understanding the genetic and ecological basis of inbreeding depression and its consequences in natural populations.     

Inbreeding depression along a life-history continuum in the great tit

Inbreeding resulting from the mating of two related individuals can reduce the fitness of their progeny. However, quantifying inbreeding depression in wild populations is challenging, requiring large sample sizes, detailed knowledge of life histories and study over many generations. Here we report analyses of the effects of close inbreeding, based on observations of mating between relatives, in a large, free-living noninsular great tit (Parus major) population monitored over 41 years. Although mating between close relatives (f > or = 0.125) was rare (1.0-2.6% of matings, depending on data set restrictiveness), we found pronounced inbreeding depression, which translated into reduced hatching success, fledging success, recruitment to the breeding population and production of grand offspring. An inbred mating at f = 0.25 had a 39% reduction in fitness relative to that of an outbred nest, when calculated in terms of recruitment success, and a 55% reduction in the number of fledged grand offspring. Our data show that inbreeding depression acts independently at each life-history stage in this population, and hence suggest that estimates of the fitness costs of inbreeding must focus on the entire life cycle.     

Domestication reshaped the genetic basis of inbreeding depression in a maize landrace compared to its wild relative,teosinte

Inbreeding depression is the reduction in fitness and vigor resulting from mating of close relatives observed in many plant and animal species. The extent to which the genetic load of mutations contributing to inbreeding depression is due to large-effect mutations versus variants with very small individual effects is unknown and may be affected by population history. We compared the effects of outcrossing and self-fertilization on 18 traits in a landrace population of maize, which underwent a population bottleneck during domestication, and a neighboring population of its wild relative teosinte. Inbreeding depression was greater in maize than teosinte for 15 of 18 traits, congruent with the greater segregating genetic load in the maize population that we predicted from sequence data. Parental breeding values were highly consistent between outcross and selfed offspring, indicating that additive effects determine most of the genetic value even in the presence of strong inbreeding depression. We developed a novel linkage scan to identify quantitative trait loci (QTL) representing large-effect rare variants carried by only a single parent, which were more important in teosinte than maize. Teosinte also carried more putative juvenile-acting lethal variants identified by segregation distortion. These results suggest a mixture of mostly polygenic, small-effect partially recessive effects in linkage disequilibrium underlying inbreeding depression, with an additional contribution from rare larger-effect variants that was more important in teosinte but depleted in maize following the domestication bottleneck. Purging associated with the maize domestication bottleneck may have selected against some large effect variants, but polygenic load is harder to purge and overall segregating mutational burden increased in maize compared to teosinte.