Genetic rescue of inviable hybrids between Drosophila melanogaster and its sibling species

Post-mating mechanisms are central to the establishment of reproductive isolation between different, but closely related, species. Post-mating isolation mechanisms include hybrid breakdown, hybrid sterility and hybrid lethality and may, in some cases, be reinforced by pre-mating mechanisms such as ethological differentiation. In the Drosophila melanogaster species sub-group post-mating reproductive isolation is ensured by both the inviability and the sterility of hybrids. For example when D. melanogaster females are crossed to D. simulans males the hybrid progeny are normally all female; the hybrid males die as third instar larvae. The viable hybrid females are totally sterile. Little is known of the genetic basis for either hybrid sterility or hybrid inviability, although Coyne and others have begun a genetic analysis of the sterility of hybrids within this species sub-group. We have discovered a single gene difference that rescues the otherwise inviable male hybrids from the cross between D. melanogaster females and males of its three closest relatives. The study of this locus may shed light on the genetic control of both speciation and development.     

Wolbachia-induced incompatibility precedes other hybrid incompatibilities in Nasonia

Wolbachia are cytoplasmically inherited bacteria that cause a number of reproductive alterations in insects, including cytoplasmic incompatibility, an incompatibility between sperm and egg that results in loss of sperm chromosomes following fertilization. Wolbachia are estimated to infect 15-20% of all insect species, and also are common in arachnids, isopods and nematodes. Therefore, Wolbachia-induced cytoplasmic incompatibility could be an important factor promoting rapid speciation in invertebrates, although this contention is controversial. Here we show that high levels of bidirectional cytoplasmic incompatibility between two closely related species of insects (the parasitic wasps Nasonia giraulti and Nasonia longicornis) preceded the evolution of other postmating reproductive barriers. The presence of Wolbachia severely reduces the frequency of hybrid offspring in interspecies crosses. However, antibiotic curing of the insects results in production of hybrids. Furthermore, F1 and F2 hybrids are completely viable and fertile, indicating the absence of F1 and F2 hybrid breakdown. Partial interspecific sexual isolation occurs, yet it is asymmetric and incomplete. Our results indicate that Wolbachia-induced reproductive isolation occurred in the early stages of speciation in this system, before the evolution of other postmating isolating mechanisms (for example, hybrid inviability and hybrid sterility).     

Incipient speciation by divergent adaptation and antagonistic epistasis in yeast

Establishing the conditions that promote the evolution of reproductive isolation and speciation has long been a goal in evolutionary biology. In ecological speciation, reproductive isolation between populations evolves as a by-product of divergent selection and the resulting environment-specific adaptations. The leading genetic model of reproductive isolation predicts that hybrid inferiority is caused by antagonistic epistasis between incompatible alleles at interacting loci. The fundamental link between divergent adaptation and reproductive isolation through genetic incompatibilities has been predicted, but has not been directly demonstrated experimentally. Here we empirically tested key predictions of speciation theory by evolving the initial stages of speciation in experimental populations of the yeast Saccharomyces cerevisiae. After replicate populations adapted to two divergent environments, we consistently observed the evolution of two forms of postzygotic isolation in hybrids: reduced rate of mitotic reproduction and reduced efficiency of meiotic reproduction. This divergent selection resulted in greater reproductive isolation than parallel selection, as predicted by the ecological speciation theory. Our experimental system allowed controlled comparison of the relative importance of ecological and genetic isolation, and we demonstrated that hybrid inferiority can be ecological and/or genetic in basis. Overall, our results show that adaptation to divergent environments promotes the evolution of reproductive isolation through antagonistic epistasis, providing evidence of a plausible common avenue to speciation and adaptive radiation in nature.     

The genetic basis of Haldane's rule

'Haldane's rule', formulated by J. B. S. Haldane in 1922, states that: "When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex". His rule is now known to apply in mammals, lepidopterans, birds, orthopterans and dipterans. In Drosophila, for example, Bock cites 142 cases of interspecific hybridizations that produce one sterile and one fertile sex in the offspring, all but one of these crosses yielding sterile XY males and fertile XX females. Despite much speculation, however, the genetic basis of Haldane's rule remains unknown. Haldane himself rejected the simple explanation that males are innately more sensitive than females to the effects of hybridization because groups with heterogametic females (such as birds and butterflies) usually show female sterility in hybrids, so that heterogamety itself is the critical feature. He and others suggested that heterogametic infertility or inviability in hybrids arises by a genetic imbalance between X chromosomes and autosomes. An alternative explanation is that this syndrome is caused by a mismatch of X and Y chromosomes. Here I show that in the Drosophila melanogaster subgroup, Haldane's rule for fertility apparently arises from a genetic interaction between X and Y chromosomes and not from an imbalance between sex chromosomes and autosomes. This finding has important implications for understanding the evolution of interspecific reproductive isolation.     

Comparative analysis and evolutionary significance of the HMG1 gene in crucian carp,blunt snout bream,and their polyploid progeny

The full-length mRNA of the high mobility group protein 1 coding gene (HMG1) was obtained by RACE-PCR from red crucian carp (Carassius auratus red var.),blunt snout bream (Megalobrama amblycephala),and their triploid and tetraploid progeny.The sequence contained an open reading frame of 579 nucleotides coding for 193 amino acids.The nucleotide identity of HMG1 was higher between the tetraploid hybrid and the maternal red crucian carp (99%) than between the tetraploid hybrid and the paternal blunt snout bream (97%).The nucleotide identity between the triploid hybrids and each parent (95%) was lower than that between the parents (98%).The protein identity between the tetraploid hybrid and each parent (100%) was higher than that between the triploid hybrid and each parent (97%).Our results suggest that interspecific hybridization generates a shock to the HMG1 gene in triploid hybrids,causing divergence of nucleotides.The HMG1 protein of the tetraploid hybrids was consistent with that of its parents,which reduced the barrier of cross incompatibility between alleles,providing the basis for the bisexual fertile tetraploid hybrids forming a new polyploid species in nature.The secondary and tertiary structures of the HMG1 protein contain eight helices,three switches,two DNA-binding domains in the N-terminus,and a long acidic tail in the C-terminus.Together,these data suggest that the HMG1 protein plays a role of protein-DNA interactions,facilitating various DNA-dependent activities in the nucleus.We also investigated the phylogeny of fish,amphibian,reptilian,bird,and mammalian HMG1 proteins.Our results suggest that HMG1 is an ancestral protein that has been highly conserved.These data provide clues as to how interspecific hybridization may form polyploid hybrids.     

Comparative Studies on the Molecular Genetic Diversities among Haliotis discus hannai,H.discus discus and Their Hybrids

0　IntroductionAsagroupofthemostimportantmarineunivalvegastropods ,thegenusabalonehasmorethan 1 0speciescultivatedintheworldnowadays .However,themajorityofspeciesgrowslowly ,challengingtheircultivation .Manystudies ,includingtriploidizationofH .discushan nai (Zhang ,1 998) ,gynogenesisofH .discushannai(Fujino ,1 990 )andgrowthhormonegenetransformationofH .rufescens (Power,1 996 ) ,werecarriedouttowardthedevelopmentoffastgrowingvarieties .Unfortunately ,theseapproacheshavenotbroughtprofoundc…     

基于SSR标记的丛生竹杂种鉴定、遗传分析和指纹图谱构建

[目的]准确鉴定丛生竹杂交种,分析杂种及其亲本的遗传关系,开发可以利用的指纹图谱.[方法]以孝顺竹(Bambusa multiplex)×麻竹(Dendrocalamus latiflorus)、孝顺竹(B.multiplex)×粉单竹(B.chungii)两个杂交群体为材料,从已公布的竹子SSR引物中随机选取30对引...     

Ancestry of unisexual salamanders.

In eastern North America there are populations of all-female salamanders that incorporate the nuclear genomes of two or three of four sympatric bisexual species. The hybrids can be diploid, triploid, tetraploid or pentaploid, and 18 different combinations have been reported. All hybrids require sperm from a sympatric male of one of the bisexual species to reproduce, but the sperm may or may not be incorporated in the egg. Some of the hybrids are believed to represent separate, clonal species, but little is known of the origin of this hybrid complex. Vertebrate mitochondrial DNA is inherited maternally, allowing identification of the female parent that gave rise to hybrid lineages. A portion of the cytochrome b gene was sequenced from diploid and triploid hybrids that represent combinations of all four species. Nearly all hybrids had a similar mitochondrial genome sequence, independent of nuclear genome composition and ploidy, and the sequence was distinct from that of any of the four bisexual species. The hybrids maintain a mitochondrial lineage that has evolved independently of their nuclear genome and represent the most ancient known unisexual vertebrate lineage.     

Molecular and phenotypic variation in the achaete-scute region of Drosophila melanogaster

Variation in quantitative characters underlies much adaptive evolution and provides the basis for selective improvement of domestic species, yet the genetic nature of quantitative variation is poorly understood. Many loci affecting quantitative traits have been identified by the segregation of mutant alleles with major qualitative effects. These alleles may represent an extreme of a continuum of allelic effects, and most quantitative variation could result from the segregation of alleles with subtle effects at loci identified by alleles with major effects. The achaete-scute complex in Drosophila melanogaster plays a central part in bristle development and has been characterized at the molecular level. The hypothesis that naturally occurring quantitative variation in bristle number could be associated with wild-type alleles of achaete-scute was tested by correlating phenotypic variation in bristle number with molecular variation in restriction maps in this region among chromosomes extracted from natural populations. DNA insertion variation in the achaete-scute region was found to be strongly associated with variation in bristle number.     

Tracking the evolution of insecticide resistance in the mosquito Culex pipiens.

The evolution of pesticide resistance provides some of the most striking examples of darwinian evolution occurring over a human life span. Identification of resistance alleles opens an outstanding framework in which to study the evolution of adaptive mutations from the beginning of pesticide application, the evolution of interactions between alleles (dominance) or between loci (epistasis). Here we show that resistance alleles can also be used as markers to dissect population processes at a microevolutionary scale. We have focused on the antagonistic roles of selection and migration involved in the dynamics of local adaptation with reference to allelic frequencies at two resistance loci in the mosquito Culex pipiens. We find that their frequencies follow an annual cycle of large amplitude (25%), and we precisely unravel the seasonal variation of migration and selection underlying this cycle. Our results provide a firm basis on which to devise an insecticide treatment strategy that will better control the evolution of resistance genes and the growth of mosquito populations.     

Advances in the understanding of inter-subspecific hybrid sterility and wide-compatibility in rice

Hybrid sterility is a major form of postzygotic reproductive isolation and frequently occurs in hybrids between divergent populations, such as the indica and japonica subspecies of Asian cultivated rice （Oryza sativa L.）. It has been a major barrier for utilization of the strong heterosis expressed in hybrids between indica and japonica. A large number of loci for rice inter-subspecific hybrid sterility have been identified by genetic analysis. Cytological studies revealed that male and female gamete abortions and reduced affinity between the uniting gametes all occurred in indica-japonica hybrids, suggesting the complexity of the causes for inter-subspecific hybrid sterility. Two genes conditioning embryo-sac and pollen sterility respectively in indica-japonica hybrids have been cloned recently, providing opportunities for molecular characterization of the indica-japonica hybrid sterility and wide-compatibility. Future studies should aim at cloning more genes for indica-japonica hybrid sterility, characterizing the underlying molecular mechanism, and utilization of the findings for the development of inter-subspecific hybrids to increase rice productivity.     

MHC polymorphism pre-dating speciation

Two features distinguish the polymorphism of the major histocompatibility complex (MHC) loci from that of other loci: its high diversity and the large genetic distance between MHC alleles. More than 100 alleles exist in natural populations in the mouse at each of the functional class I and class II alleles, all alleles occurring at frequencies that cannot be explained by recurrent mutations. Some of the alleles differ by approximately 70 nucleotides in the coding region alone and some of the products of the allelic genes differ by more than 50 amino acids. It has generally been assumed that these differences accumulated after species inception. Here, we present evidence for an alternative explanation of the origin of MHC polymorphism: a large part of the MHC polymorphism pre-dates speciation and is passed on from species to species. We describe allelic differences that must have arisen before the separation of mice and rats from a common ancestor more than 10 million years ago.     

Morphological evolution through multiple cis-regulatory mutations at a single gene

One central, and yet unsolved, question in evolutionary biology is the relationship between the genetic variants segregating within species and the causes of morphological differences between species. The classic neo-darwinian view postulates that species differences result from the accumulation of small-effect changes at multiple loci. However, many examples support the possible role of larger abrupt changes in the expression of developmental genes in morphological evolution. Although this evidence might be considered a challenge to a neo-darwinian micromutationist view of evolution, there are currently few examples of the actual genes causing morphological differences between species. Here we examine the genetic basis of a trichome pattern difference between Drosophila species, previously shown to result from the evolution of a single gene, shavenbaby (svb), probably through cis-regulatory changes. We first identified three distinct svb enhancers from D. melanogaster driving reporter gene expression in partly overlapping patterns that together recapitulate endogenous svb expression. All three homologous enhancers from D. sechellia drive expression in modified patterns, in a direction consistent with the evolved svb expression pattern. To test the influence of these enhancers on the actual phenotypic difference, we conducted interspecific genetic mapping at a resolution sufficient to recover multiple intragenic recombinants. This functional analysis revealed that independent genetic regions upstream of svb that overlap the three identified enhancers are collectively required to generate the D. sechellia trichome pattern. Our results demonstrate that the accumulation of multiple small-effect changes at a single locus underlies the evolution of a morphological difference between species. These data support the view that alleles of large effect that distinguish species may sometimes reflect the accumulation of multiple mutations of small effect at select genes.     

利用SSR 标记正确判断小麦杂交种中的杂株

 正确判断杂交种中混有的杂株, 是准确鉴定小麦杂交种种子纯度的前提。在分析180 份杂交组合的基础上, 以“京麦1100”为例, 提出了利用SSR 标记正确判断小麦杂交种中杂株的方法:1)筛选双亲间有多态性的引物不少于5 对;2)利用该引物检测亲本及杂交种的基因型, 并按照杂交种基因型记录原则记录每个个体的基因型;3)正确判断真实杂交种基因型:不论亲本是否存在非纯合位点现象, 只要某个体同时具备父母本的基因型, 均视为真实杂交种的基因型;4)判断杂株:当某个体在多个SSR 位点上与真实杂交种基因型不同时, 判断为杂株。     

Fitness costs of R-gene-mediated resistance in Arabidopsis thaliana

Resistance genes (R-genes) act as an immune system in plants by recognizing pathogens and inducing defensive pathways. Many R-gene loci are present in plant genomes, presumably reflecting the need to maintain a large repertoire of resistance alleles. These loci also often segregate for resistance and susceptibility alleles that natural selection has maintained as polymorphisms within a species for millions of years. Given the obvious advantage to an individual of being disease resistant, what prevents these resistance alleles from being driven to fixation by natural selection? A cost of resistance is one potential explanation; most models require a lower fitness of resistant individuals in the absence of pathogens for long-term persistence of susceptibility alleles. Here we test for the presence of a cost of resistance at the RPM1 locus of Arabidopsis thaliana. Results of a field experiment comparing the fitness of isogenic strains that differ in the presence or absence of RPM1 and its natural promoter reveal a large cost of RPM1, providing the first evidence that costs contribute to the maintenance of an ancient R-gene polymorphism.     

Speciation by hybridization in Heliconius butterflies

Speciation is generally regarded to result from the splitting of a single lineage. An alternative is hybrid speciation, considered to be extremely rare, in which two distinct lineages contribute genes to a daughter species. Here we show that a hybrid trait in an animal species can directly cause reproductive isolation. The butterfly species Heliconius heurippa is known to have an intermediate morphology and a hybrid genome, and we have recreated its intermediate wing colour and pattern through laboratory crosses between H. melpomene, H. cydno and their F1 hybrids. We then used mate preference experiments to show that the phenotype of H. heurippa reproductively isolates it from both parental species. There is strong assortative mating between all three species, and in H. heurippa the wing pattern and colour elements derived from H. melpomene and H. cydno are both critical for mate recognition by males.     

节节麦和黑麦杂种胚再生植株的分子细胞遗传学分析

利用杂种幼胚愈伤组织诱导和再生技术产生了节节麦和黑麦杂种幼胚无性系.胚拯救直接成苗植株、继代培养60d的再生植株和继代培养420d的再生杂种植株PMCs减数分裂MI染色体构型分别为10.82I+1.57II+0.01III,9.96I+1.59II+0.25III+0.03IV,7.59Ⅰ+3.10Ⅱ+0.05Ⅲ+0.01IV.继代培养420d左右胚再生无性系其减数分裂MI染色体配对频率大幅度提高,利用原位杂交技术分析表明其染色体配对频率提高和染色体结构和数量变异无关.提示体细胞无性系变异可能是增加部分同源染色体配对,增加属间种基因交流的一个新途径.     

Bt/CpTI抗虫转基因水稻及其非转基因亲本与普通野生稻杂种F2群体的等位基因偏态分离

了解水稻转基因是否影响野生稻基因传递的频率,对于评价转基因逃逸及其生态影响有重要意义.本研究构建了不含转基因和含转基因的栽培稻与普通野生稻杂种F2群体,利用分子标记检测了F2群体各位点的基因型和基因频率.以连续卡方分析了F2群体相关位点的基因型和基因频率观察值是否符合理论分离比,同时对各位点进行了连锁不平衡分析,并对实验群体所需的最小样本量进行理论探索.结果表明两个F2群体分别有25.93%与33.33%的位点出现了显著的非随机分离,非转基因与转基因F2群体分别在偏态分离位点数与偏离亲本方向上出现一定差异,并观察到连锁不平衡位点.实验个体数应不少于280个才能保证实验结果准确.外源转基因在杂种后代群体中会因为选择而影响基因分离,进而影响杂种群体的进化潜力.     

蝗总科6种蝗虫6个种群的遗传分化

采用水平淀粉凝胶电泳技术研究东亚飞蝗、大垫尖翅蝗、中华稻蝗、中华蚱蜢、亚洲小车蝗和黄胫小车蝗6个种的种群遗传结构及其分化.在所检测的13个基因座位(Ak、Ck、G3pd、Gpi、Hk-1、Hk-2、Idh-1、Idh-2、Ldh、Mdh-1、Mdh-2、Mdph、Pgm)中,有2个基因座位(G3pdh和Mdh-2)在6个种群中均为单态(0.95标准);5个基因座位(Gpi、Hk-1、Hk-2、Mdph和Pgm)在6个种群中均为多态;其余的基因座位至少在一个种群内有两个以上的等位基因.除Gpi和Mdh-1在多数种群符合哈-温(H-W)平衡期望值,其余大多数基因座位的基因型频率显著偏离哈-温(H-W)平衡(P<0.05).从A、P、Ho和He值可知,中华蚱蜢的遗传多样性最低,其次是亚洲小车蝗和黄胫小车蝗,而大垫尖翅蝗、东亚飞蝗和中华稻蝗遗传多样性均较高.根据Roger's遗传距离用非加权算术平均法(UPGMA)进行的聚类分析符合传统形态学和细胞学研究结果.     

MHC antigens in urine as olfactory recognition cues

The classical class I antigens of the major histocompatibility complex (MHC) are cell-surface glycoproteins which were originally discovered because they cause rapid rejection of cells or tissues grafted between unrelated individuals. These molecules are encoded by the K, D and L loci of the mouse MHC (and analogous loci in other species) which show extreme species polymorphism and a large number of alleles. In an outbreeding population 3.6 X 10(9) unique MHC class I phenotypes can be encoded by the 100 alleles at each of the K and D loci and the 6 alleles at the L locus. This level of polymorphism ensures that the cells and tissues of each unrelated individual are uniquely identified by their class I membrane-bound antigens. Like other membrane bound proteins, these class I molecules are anchored in the lipid bilayer by a hydrophobic domain encoded by exon 5. However, there have been reports of the occurrence of classical class I molecules in true solution in the blood of humans, mice, and rats. We report here that classical polymorphic class I molecules in normal rats are constitutively excreted in the urine and that untrained rats can distinguish the smell of urine samples taken from normal donors that differ only at the class I MHC locus and therefore excrete different allelomorphs of class I molecules in their urine.