遗传群体偏分离研究进展

偏分离是指观察到的基因型比例偏离预期的孟德尔分离频率方式,无法用传统的遗传理论和方法加以分析。偏分离被认为是一种重要的进化动力,并对遗传连锁图谱的构建造成影响。本文针对偏分离的现象、偏分离的影响因素和形成原因,以及对QTL定位的影响等方面进行综合分析,系统阐述了植物分离群体偏分离的研究进展,为后续研究提供有益的参考。     

植物杂交后代中基因偏分离的产生原因及其进化意义

在天然杂交-渐渗的过程中,来自双亲的等位基因在后代的不均等分布导致偏分离的产生。大量研究表明,偏分离在自然界中广泛存在。但是目前的研究主要集中在遗传连锁图谱构建过程中发现的偏分离现象及导致偏分离的原因,很少有研究关注偏分离带来的基因频率或基因型频率变化对进化产生的影响。本文介绍了导致偏分离的主要因素,如基因相互作用、双亲间遗传分化、细胞质遗传和环境影响等,分析了偏分离的进化意义,认为偏分离的产生对群体进化潜力、性别比例、遗传多样性及维持分化稳定性等方面具有重要的影响。在未来的研究中,应关注转基因进入野生近缘种群体后通过偏分离对群体长期进化潜力的影响,以及偏分离现象在群体世代间有何变化等内容。     

亚洲棉种内群体异常偏分离的分子标记检测

利用3个形态标记、20个SSR和11个SRAP多态性标记,研究它们在“如东鸡脚桠果”与“美国中棉971”杂交的F2群体中的分离情况。结果表明,77.42%的分子标记表现为偏分离,所有的偏分离标记都偏向母本“如东鸡角桠果”;如此高的偏分离比例以及偏分离标记都偏向一个亲本,这种现象在棉花中是比较罕见的。3种类型的SSR标记和非物种特异性的SRAP标记都表现偏分离,而形态标记则表现为正常分离,这表明该异常偏分离现象是由材料本身的遗传特性所决定的。通过分析偏分离的共显性标记的等位基因频率(p=q)以及各种基因型频率(p2:2pq:q2)的F2分布,发现大多数标记的等位基因频率差异显著而F2基因型频率分布正常,表明这些标记产生偏分离可能是配子体选择的结果。     

玉米F2群体分子标记偏分离的遗传分析

以优良玉米杂交组合 (综 3× 87 1)的F2 群体为材料 ,构建了包含 15 0个SSR标记和 2 4个RFLP标记的玉米分子标记连锁图。通过对 174个分子标记的分析 ,发现有 4 9个分子标记表现偏分离 (P <0 0 5 ) ,占总标记数的2 8 2 %。这些偏分离标记有 11个偏向父本综 3,占 2 2 5 % ;12个偏向母本 87 1,占 2 4 5 % ;2 5个偏向杂合体 ,占5 1 0 %。还有 1个标记同时偏向双亲。同时在 9条不同的染色体上发现 14个偏分离的热点区域 ,其中 4个与已经定位的配子体基因的位置相近 ,由此表明配子体基因是导致偏分离的部分原因。所发现的SDR6 1和SDR7 2似乎是两个新的偏分离热点区域。进一步讨论了引起偏分离的原因 ,以及偏分离标记对QTL定位的影响。对于单位点的QTL分析而言 ,偏分离标记一般不会影响QTL定位的位置和效应 ;对于两位点的上位性分析而言 ,则要求较少的偏分离标记和较大的群体     

杂种偏分离的遗传和分子机理研究进展

杂种偏分离是指杂交后代群体在某个位点的基因型分离比偏离了预期的孟德尔分离比例的一种现象,是来自不同杂交亲本基因之间的不兼容性所致。功能缺失型和功能获得型的基因间互作都可以导致杂种偏分离,其中前者的机理比较简单,即缺陷型的基因组合导致原有功能丧失而造成细胞死亡。功能获得型杂种偏分离系统是由多基因控制的遗传系统,包含两个基本成分：杀手(killer)因子和护卫(protector)因子,此外还有增强子(enhancer)、抑制基因(repressor)等修饰因子。功能获得型杂种偏分离有通用的遗传模型：具有传递优势的单倍型含有高活性的killer+和protector+;传递劣势的单倍型含有低活性的killer-和protector-;中性的单倍型(广亲和型)则含有killer-和protector+。该系统通过killer和protector间的紧密连锁、修饰因子的积累等途径得以在自然选择中保存下来。尽管不同功能获得型杂种偏分离系统的遗传机理有较高的相似性,但分子机制则大相径庭。文章综述了杂种偏分离的遗传和分子机理以及其与杂种不育的关系,以期为后续杂种偏分离研究提供参考。     

栽培大豆与半野生大豆杂种F2九体中RFLP标记的偏分离及其形成原因…

本文研究了大豆５６个ＤＮＡ限制性片长度多态性（ＲＦＬＰ）村记在一栽培大豆／半野生大豆杂种Ｆ２群体中的分离,结果表明,２５％的ＲＦＬＰ标记表现了偏分离,偏离的方向主要趋于栽培大豆亲本,其形成原因主要是存在着配子体选择。这对研究大豆的遗传及育种选择等有着重要的指导意义。     

栽培大豆与半野生大豆杂种F_2群体中RFLP标记的偏分离及其形成原因的分析

本文研究了大豆５６个ＤＮＡ限制性片段长度多态性（ＲＦＬＰ）标记在一栽培大豆／半野生大豆杂种Ｆ２群体中的分离。结果表明,２５％的ＲＦＬＰ标记表现了偏分离,偏离的方向主要趋于栽培大豆亲本,其形成原因主要是存在着配子体选择。这对研究大豆的遗传及育种选择等有着重要的指导意义。     

B交配型因子对香菇双核体核型分离比的影响

通过探明A或B交配型因子对双核体回收核型的影响,探讨了香菇双核体经原生质体形成和再生后两个成员核偏分离现象的遗传基础。结果表明,B因子或某些假定的与B因子连锁的基因明显影响去双核化后核的存活力,而A因子对此无明显影响。B因子的特异性与分离自两类异核体（A≠B≠和A=B≠）回收的成员核的存活比例密切相关。可以根据这种功能将B因子排成一个分级的序列。两个参试菌株中的4个B因子的特异性顺序为B1〉B3〉B4〉B2。     

与偏分离位点连锁的QTL作图的统计方法

提出了一种统计方法,可以估计与偏分离位点连锁的QTL的位置和效应。该方法利用回交群体中呈现偏分离的分子标记,首先用最大似然法对偏分离位点与标记位点之间的重组率和配子存活率进行估计,然后用区间作图法估计加性-显性模型下QTL的位置和效应参数。该方法可用于对常规作图研究中表现偏分离的标记进行分析,以帮助我们发现新的偏分离基因（或不育基因）和数量性状位点。     

水稻日本晴与广陆矮4号杂交F2群体SSR标记偏分离原因探析

以全基因组测序已经完成的材料粳稻日本晴和完成了第4染色体全序列测序的籼稻广陆矮4号的杂交F2作为构图群体,共90个单株,构建了一张含148个微卫星标记的水稻分子遗传图谱。该F2群体显著偏分离非常高,发现有49个分子标记表现偏分离（P〈0．05）,占总标记数的33．11％,这些偏分离标记中有36个偏向广陆4号,13个偏向杂合体,没有偏向日本晴的偏分离标记。讨论了配子体基因和孢子体基因导致偏分离的原因,通过已经定位的配子体基因和杂种不育基因分布在偏分离集中的区域来进一步说明配子体基因和杂种不育基因确实是导致偏分离形成的原因,而且还通过未定位的标记分析了偏分离的原因。     

Alpha-1-antitrypsin-deficient phenotype is not maintained by segregation distortion

Recent reports have suggested that the alpha-1-antitrypsin allele PiZ, which in homozygotes results in severe deficiency of this important protease inhibitor, is maintained at a relatively high gene frequency through the mechanism of segregation distortion. We report here on 121 nuclear families selected because only one parent was segregating the Z allele. After correcting for ascertainment, no evidence of preferential transmission was observed in 278 informative offspring.     

T-DNA mediated disruption of essential gametophytic genes in Arabidopsis is unexpectedly rare and cannot be inferred from segregation distortion alone

Many genes are thought to be expressed during the haploid phase in plants, however, very few haploid-specific genes have been isolated so far. T-DNA insertion mutagenesis is a powerful tool for generating mutations that affect gametophyte viability and function, as disruption of a gene essential for these processes should lead to a defect in the transmission of the gametes. Mutants can therefore be screened on the basis of segregation distortion for a reporter resistance gene contained in the T-DNA. We have screened the Versailles collection of Arabidopsis transformants for 1:1 Kan^R:Kan^S segregation after selfing, focussing on gametophyte mutations which show normal transmission through one gametophyte and cause lethality or dysfunction of the other. Only 1.3% (207) of the 16,000 lines screened were scored as good candidates. Thorough genetic analysis of 38 putative T-DNA transmission defect lines (Ttd) identified 8 defective gametophyte mutants, which all showed 0 to 1% T-DNA transmission through the pollen. During the screen, we observed a high background of low-penetrance mutations, often affecting the function of both gametophytes, and many lines which were likely to carry chromosomal rearrangements. The reasons for the small number of retained lines (all male gametophytic) are discussed, as well as the finding that, for most of them, residual T-DNA transmission is obtained through the affected gametophyte. Received: 27 July 1998 / Accepted: 16 September 1998     

Segregation,linkage, and diversity of allozymes in knobcone pine

Summary Female gametophytes of knobcone pine were used to study genetic variation at 58 loci in 26 enzyme systems. Mendelian segregation and linkage were tested at 21 loci. Got1, Pgi2, Mnr3, Adh2, and Lap2 were linearly arrayed in a single linkage group. Est and Acp3, and Flest and Lap1, formed two independent linkage groups. Although Mendelian segregation was the rule, several cases of segregation distortion were observed. Pooled over trees, Lap1 and Aap1 showed significant distortion. Of 11 cases of distortion observed for individual trees, 10 showed an excess of common alleles. Pooled over both loci and trees, giving a total sample of 17,183 gametes, the common alleles were significantly overrepresented by 1.1%, and heterogeneity was highly significant. Our results, and others in the literature, suggest that segregation distortion may affect the genetic structure of conifer populations.     

A genetic linkage map for tef [Eragrostis tef (Zucc.) Trotter]

Tef [Eragrostis tef (Zucc.) Trotter] is the major cereal crop in Ethiopia. Tef is an allotetraploid with a base chromosome number of 10 (2n = 4x = 40) and a genome size of 730 Mbp. Ninety-four F₉ recombinant inbred lines (RIL) derived from the interspecific cross, Eragrostis tef cv. Kaye Murri × Eragrostis pilosa (accession 30-5), were mapped using restriction fragment length polymorphisms (RFLP), simple sequence repeats derived from expressed sequence tags (EST–SSR), single nucleotide polymorphism/insertion and deletion (SNP/INDEL), intron fragment length polymorphism (IFLP) and inter-simple sequence repeat amplification (ISSR). A total of 156 loci from 121 markers was grouped into 21 linkage groups at LOD 4, and the map covered 2,081.5 cM with a mean density of 12.3 cM per locus. Three putative homoeologous groups were identified based on multi-locus markers. Sixteen percent of the loci deviated from normal segregation with a predominance of E. tef alleles, and a majority of the distorted loci were clustered on three linkage groups. This map will be useful for further genetic studies in tef including mapping of loci controlling quantitative traits (QTL), and comparative analysis with other cereal crops.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Identification and characterization of segregation distortion loci along chromosome 5B in tetraploid wheat

Segregation distortion genes are widespread in plants and animals and function by their effect on competition among gametes for preferential fertilization. In this study, we evaluated the segregation distortion of molecular markers in multiple reciprocal backcross populations derived from unique cytogenetic stocks involving the durum cultivar Langdon (LDN) and wild emmer accessions that allowed us to study the effects of chromosome 5B in isolation. No segregation distortion of female gametes was observed, but three populations developed to analyze segregation of male gametes had genomic regions containing markers with skewed segregation ratios. One region of distortion was due to preferential transmission of LDN alleles over wild emmer alleles through male gametes. Another region required the presence of LDN 5B chromosomes in the female for preferential fertilization by male gametes harboring LDN alleles indicating that the corresponding genes in the female gametes can govern genes affecting segregation distortion of male gametes. A third region of distortion was the result of preferential transmission of wild emmer alleles over LDN alleles through male gametes. These results indicate the existence of different distorter/meiotic drive elements among different genotypes and show that distortion factors along wheat chromosome 5B differ in chromosomal location as well as underlying mechanisms.     

Development of an RFLP map in diploid alfalfa

Summary We have developed a restriction fragment length polymorphism (RFLP) linkage map in diploid alfalfa (Medicago sativa L.) to be used as a tool in alfalfa improvement programs. An F₂ mapping population of 86 individuals was produced from a cross between a plant of the W2xiso population (M. sativa ssp. sativa) and a plant from USDA PI440501 (M. sativa ssp. coerulea). The current map contains 108 cDNA markers covering 467.5 centimorgans. The short length of the map is probably due to low recombination in this cross. Marker order may be maintained in other populations even though the distance between clones may change. About 50% of the mapped loci showed segregation distortion, mostly toward excess heterozygotes. This is circumstantial evidence supporting the maximum heterozygote theory which states that relative vigor is dependent on maximizing the number of loci with multiple alleles. The application of the map to tetraploid populations is discussed.