Purifying Selection Causes Widespread Distortions of Genealogical Structure on the Human X Chromosome

The extent to which selective forces shape patterns of genetic and genealogical variation is unknown in many species. Recent theoretical models have suggested that even relatively weak purifying selection may produce significant distortions in gene genealogies, but few studies have sought to quantify this effect in humans. Here, we employ a reconstruction method based on the ancestral recombination graph to infer genealogies across the length of the human X chromosome and to examine time to most recent common ancestor (TMRCA) and measures of tree imbalance at both broad and very fine scales. In agreement with theory, TMRCA is significantly reduced and genealogies are significantly more imbalanced in coding regions and introns when compared to intergenic regions, and these effects are increased in areas of greater evolutionary constraint. These distortions are present at multiple scales, and chromosomal regions as broad as 5 Mb show a significant negative correlation in TMRCA with exon density. We also show that areas of recent TMRCA are significantly associated with the disease-causing potential of site as measured by the MutationTaster prediction algorithm. Together, these findings suggest that purifying selection has significantly distorted human genealogical structure on both broad and fine scales and that few chromosomal regions escape selection-induced distortions.     

A Female Sterile Screen of the Drosophila Melanogaster X Chromosome Using Hybrid Dysgenesis: Identification and Characterization of Egg Morphology Mutants

We have conducted a hybrid dysgenic screen of the X chromosome for mutations affecting female fertility, with particular attention to those causing abnormal egg and eggshell morphology. In a screen of 4017 dysgenic strains, 398 mutants derived from 168 different germ lines were isolated and assigned to eight classes according to their diverse phenotypes. One interesting class consists of mutants that block oogenesis at specific stages. Our analysis has focused on mutations affecting eggshell formation, including mutants that lay morphologically abnormal sterile eggs as well as those that lay no eggs but exhibit blocks in the late stages of oogenesis. A subset of 48 mutants was assorted into 30 allelic groups by inter se complementation and genetically localized by interval mapping. Two multiallele complementation groups, de1 (7 alleles) and ne1 (8 alleles), were identified as well as five two-allele complementation groups. A search for alleles among mutants generated in other female sterile screens was unsuccessful, pointing to the distinctive nature of the dysgenic mutant collection. The single case of allelism determined in this study was one with a lethal allele of the Broad-Complex, l(1)npr, suggesting a possible involvement of ecdysone in choriogenesis. A subset of 18 dysgenic strains was analyzed for P element hybridization and 16 of these were found to have hybridization signals in the appropriate cytogenetic interval. By examining these signals in two or more alleles of the same complementation group, we have been able to tentatively localize two mutations. Light and electron microscopy of the eggshell in 43 different strains has revealed a variety of effects. The respiratory appendages were defective in 27 of these mutants. Effects on the ultrastructure of the main body of the endochorion were not strongly correlated with the appendage defects, and could be classified as minor (14 mutants) or major (16 mutants). Although 13 mutants showed no ultrastructural chorion defects, six of these had defective respiratory appendages.     

Hybrid Dysgenesis-Induced Quantitative Variation on the X Chromosome of Drosophila Melanogaster

To determine the ability of the P-M hybrid dysgenesis system of Drosophila melanogaster to generate mutations affecting quantitative traits, X chromosome lines were constructed in which replicates of isogenic M and P strain X chromosomes were exposed to a dysgenic cross, a nondysgenic cross, or a control cross, and recovered in common autosomal backgrounds. Mutational heritabilities of abdominal and sternopleural bristle score were in general exceptionally high-of the same magnitude as heritabilities of these traits in natural populations. P strain chromosomes were eight times more mutable than M strain chromosomes, and dysgenic crosses three times more effective than nondysgenic crosses in inducing polygenic variation. However, mutational heritabilities of the bristle traits were appreciable for P strain chromosomes passed through one nondysgenic cross, and for M strain chromosomes backcrossed for seven generations to inbred P strain females, a result consistent with previous observations on mutations affecting quantitative traits arising from nondysgenic crosses. The new variation resulting from one generation of mutagenesis was caused by a few lines with large effects on bristle score, and all mutations reduced bristle number.     

Hybrid Sterility Locus on Chromosome X Controls Meiotic Recombination Rate in Mouse

Meiotic recombination safeguards proper segregation of homologous chromosomes into gametes, affects genetic variation within species, and contributes to meiotic chromosome recognition, pairing and synapsis. The Prdm9 gene has a dual role, it controls meiotic recombination by determining the genomic position of crossover hotspots and, in infertile hybrids of house mouse subspecies Mus m. musculus (Mmm) and Mus m. domesticus (Mmd), it further functions as the major hybrid sterility gene. In the latter role Prdm9 interacts with the hybrid sterility X 2 (Hstx2) genomic locus on Chromosome X (Chr X) by a still unknown mechanism. Here we investigated the meiotic recombination rate at the genome-wide level and its possible relation to hybrid sterility. Using immunofluorescence microscopy we quantified the foci of MLH1 DNA mismatch repair protein, the cytological counterparts of reciprocal crossovers, in a panel of inter-subspecific chromosome substitution strains. Two autosomes, Chr 7 and Chr 11, significantly modified the meiotic recombination rate, yet the strongest modifier, designated meiotic recombination 1, Meir1, emerged in the 4.7 Mb Hstx2 genomic locus on Chr X. The male-limited transgressive effect of Meir1 on recombination rate parallels the male-limited transgressive role of Hstx2 in hybrid male sterility. Thus, both genetic factors, the Prdm9 gene and the Hstx2/Meir1 genomic locus, indicate a link between meiotic recombination and hybrid sterility. A strong female-specific modifier of meiotic recombination rate with the effect opposite to Meir1 was localized on Chr X, distally to Meir1. Mapping Meir1 to a narrow candidate interval on Chr X is an important first step towards positional cloning of the respective gene(s) responsible for variation in the global recombination rate between closely related mouse subspecies.     

Recombination between Small X Chromosome Duplications and the X Chromosome in Caenorhabditis Elegans

Twelve new X chromosome duplications were identified and characterized. Eight are translocated to autosomal sites near four different telomeres, and four are free. Ten include unc-1(+), which in wild type is near the left end of the X chromosome, and two of these, mnDp72(X;IV) and mnDp73(X;f), extend rightward past dpy-3. Both mnDp72 and mnDp73 recombined with the one X chromosome in males in the unc-1-dpy-3 interval at a frequency 15- to 30-fold higher than was observed for X-X recombination in hermaphrodites in the same interval. Recombinant duplications and recombinant X chromosomes were both recovered. Recombination with the X chromosome in the unc-1-dpy-3 interval was also detected for five other unc-1(+) duplications, even though their right breakpoints lie within the interval. In hermaphrodites, mnDp72 and mnDp73 promoted meiotic X nondisjunction and recombined with an X chromosome in the unc-1-dpy-3 interval at frequencies comparable to that found for X-X recombination; mnDp72(X;IV) also promoted trisomy for chromosome IV. A mutation in him-8 IV was identified that severely reduced recombination between the two X chromosomes in hermaphrodites and between mnDp73 and the X chromosome in males. Recombination between the X chromosome and duplications of either the right end of the X or a region near but not including the left end was rare. We suggest that the X chromosome has one or more elements near its left end that promote meiotic chromosome pairing.     

Widespread Alterations in Translation Elongation in the Brain of Juvenile Fmr1 Knockout Mice

1. Download high-res image (162KB)
2. Download full-size image

     

Widespread Chromosome Variation in the Endangered Grassland Forb Rutidosis leptorrhynchoides F. Muell. (Asteraceae: Gnaphalieae)

Rutidosis leptorrhynchoides is an endangered plant endemic tosoutheastern Australia. Chromosome analysis of 19 of the 24known populations of the species has identified 17 differentchromosome variants or cytotypes. The most common cytotypesare a diploid and a tetraploid based on x = 11, and triploidand hexaploid plants with this basic number were also observed.Diploids, triploids and tetraploids based on a second basicnumber ofx = 13 were also seen. Plants with 2 n = 24 were shownto be hybrids between diploids with the two different basicnumbers. Meiotic chromosome pairing analysis of the plants with2n = 24 showed a maximum of two trivalents indicating the presenceof extra copies of one pair of large and one pair of small chromosomesin the 2 n = 26 plants. In addition, a number of different aneuploidsof the 2 n = 22 and 2 n = 44 races were found and many of thesealso showed structural chromosomal variation. The distributionof the two main chromosome races is disjunct with the tetraploidsconfined to southern Victoria. To avoid dysgenic effects, futurere-establishment efforts for this species should avoid mixingseed from different chromosome races. Copyright 2001 Annalsof Botany Company Aneuploidy, conservation genetics, karyotypes, meiosis, polyploidy     

棉花转基因恢复系杂种F1小孢子发生的细胞学观察

对转gst基因棉花恢复系浙大强恢"配制的杂种F1(三系杂交棉)的成熟花药和花粉育性进行了研究.结果表明,在花药的长、宽、鲜重和成熟花粉粒的育性方面,浙大强恢"所配的F1和保持系DES-HAB277"接近,无显著性差异,但比受体恢复系DES-HAF277"所配的F1分别提高47.7%、61.8%、28.5%和39.6%.以不育系DES-HAMS277"和保持系DES-HAB277"的花药为对照,对浙大强恢"和受体恢复系所配的F1的小孢子发生进行了细胞学观察发现,不育系小孢子败育主要发生在造孢细胞增殖期和小孢子母细胞形成期,且在减数分裂期彻底败育,不能形成四分体;受体恢复系所配的F1在小孢子发生和雄配子形成的各个发育时期都有部分败育,平均败育率约为20%,且主要发生在小孢子母细胞减数分裂期和小孢子单核期;而浙大强恢"所配的F1与保持系一样,花药的发育、小孢子的发生以及雄配子的形成均正常.研究结果从细胞形态学方面证明gst基因对三系杂交棉具有防止部分小孢子败育和提高花粉育性的功能.     

哺乳动物X染色体失活机制

哺乳动物X染色体连锁基因的剂量平衡,是通过雌性胚胎发育早期随机或印记失活一条X染色体来实现的,这是一个复杂的过程,包括：启动、计数、选择、维持等一系列的步骤。X染色体失活中心是X染色体失活的主控开关座位,调节X失活的早期事件,失活发生后,X染色体的失活状态可稳定地存在并传递给后代,这一过程涉及基因组印记的形成。此外,在雄性动物,精原细胞减数分裂早期也存在着短暂的X染色体失活现象。现对哺乳动物X染色体失活机制的最新进展进行综述。     

Nonrandom X Chromosome Inactivation Is Influenced by Multiple Regions on the Murine X Chromosome

During the development of female mammals, one of the two X chromosomes is inactivated, serving as a dosage-compensation mechanism to equalize the expression of X-linked genes in females and males. While the choice of which X chromosome to inactivate is normally random, X chromosome inactivation can be skewed in F1 hybrid mice, as determined by alleles at the X chromosome controlling element (Xce), a locus defined genetically by Cattanach over 40 years ago. Four Xce alleles have been defined in inbred mice in order of the tendency of the X chromosome to remain active: Xce^a < Xce^b < Xce^c < Xce^d. While the identity of the Xce locus remains unknown, previous efforts to map sequences responsible for the Xce effect in hybrid mice have localized the Xce to candidate regions that overlap the X chromosome inactivation center (Xic), which includes the Xist and Tsix genes. Here, we have intercrossed 129S1/SvImJ, which carries the Xce^a allele, and Mus musculus castaneus EiJ, which carries the Xce^c allele, to generate recombinant lines with single or double recombinant breakpoints near or within the Xce candidate region. In female progeny of 129S1/SvImJ females mated to recombinant males, we have measured the X chromosome inactivation ratio using allele-specific expression assays of genes on the X chromosome. We have identified regions, both proximal and distal to Xist/Tsix, that contribute to the choice of which X chromosome to inactivate, indicating that multiple elements on the X chromosome contribute to the Xce.     

花药培养过程栽培稻花粉不育杂种F1与亲本台中65的细胞学研究

利用活体压片、半薄及超薄切片技术,对栽培稻(Oryza sativa L.)花粉不育杂种F1及育性正常的亲本台中65离体培养前后的小孢子及花药壁进行细胞学研究.结果表明:与台中65相比,杂种F1的花粉小孢子在发育至单核中-晚期,出现比例较高的胞质凝聚小孢子和少量星型小孢子,正常小孢子和淀粉化小孢子比例降低.在离体条件下,胞质凝聚小孢子、星型小孢子、正常小孢子、液泡化小孢子、淀粉化小孢子的发育主要沿着胞质凝聚败育过程、孢子体发育过程、配子体发育过程、液泡化过程和淀粉化过程进行;药壁组织在离体条件下,杂种F1比台中65的绒毡层降解速度快,中层膨大程度高.杂种F1与台中65在离体培养下小孢子发育及药壁细胞学的差异主要是受到S-a座位内等位基因互作及离体培养环境的影响.     

Masculinization of the X Chromosome in the Pea Aphid

Evolutionary theory predicts that sexually antagonistic mutations accumulate differentially on the X chromosome and autosomes in species with an XY sex-determination system, with effects (masculinization or feminization of the X) depending on the dominance of mutations. Organisms with alternative modes of inheritance of sex chromosomes offer interesting opportunities for studying sexual conflicts and their resolution, because expectations for the preferred genomic location of sexually antagonistic alleles may differ from standard systems. Aphids display an XX/X0 system and combine an unusual inheritance of the X chromosome with the alternation of sexual and asexual reproduction. In this study, we first investigated theoretically the accumulation of sexually antagonistic mutations on the aphid X chromosome. Our results show that i) the X is always more favourable to the spread of male-beneficial alleles than autosomes, and should thus be enriched in sexually antagonistic alleles beneficial for males, ii) sexually antagonistic mutations beneficial for asexual females accumulate preferentially on autosomes, iii) in contrast to predictions for standard systems, these qualitative results are not affected by the dominance of mutations. Under the assumption that sex-biased gene expression evolves to solve conflicts raised by the spread of sexually antagonistic alleles, one expects that male-biased genes should be enriched on the X while asexual female-biased genes should be enriched on autosomes. Using gene expression data (RNA-Seq) in males, sexual females and asexual females of the pea aphid, we confirm these theoretical predictions. Although other mechanisms than the resolution of sexual antagonism may lead to sex-biased gene expression, we argue that they could hardly explain the observed difference between X and autosomes. On top of reporting a strong masculinization of the aphid X chromosome, our study highlights the relevance of organisms displaying an alternative mode of sex chromosome inheritance to understanding the forces shaping chromosome evolution.     

ErbB2 Receptor Over-Expression Improves Post-Traumatic Peripheral Nerve Regeneration in Adult Mice

In a transgenic mice (BALB-neuT) over-expressing ErbB2 receptor, we investigated the adult mouse median nerve in physiological and pathological conditions. Results showed that, in physiological conditions, the grip function controlled by the median nerve in BALB-neuT mice was similar to wild-type (BALB/c). Stereological assessment of ErbB2-overexpressing intact nerves revealed no difference in number and size of myelinated fibers compared to wild-type mice. By contrast, after a nerve crush injury, the motor recovery was significantly faster in BALB-neuT compared to BALB/c mice. Moreover, stereological assessment revealed a significant higher number of regenerated myelinated fibers with a thinner axon and fiber diameter and myelin thickness in BALB-neuT mice. At day-2 post-injury, the level of the mRNAs coding for all the ErbB receptors and for the transmembrane (type III) Neuregulin 1 (NRG1) isoforms significantly decreased in both BALB/c and BALB-neuT mice, as shown by quantitative real time PCR. On the other hand, the level of the mRNAs coding for soluble NRG1 isoforms (type I/II, alpha and beta) increased at the same post-traumatic time point though, intriguingly, this response was significantly higher in BALB-neuT mice with respect to BALB/c mice. Altogether, these results suggest that constitutive ErbB2 receptor over-expression does not influence the physiological development of peripheral nerves, while it improves nerve regeneration following traumatic injury, possibly through the up-regulation of soluble NRG1 isoforms.     

X Chromosome Control of Meiotic Chromosome Synapsis in Mouse Inter-Subspecific Hybrids

Hybrid sterility (HS) belongs to reproductive isolation barriers that safeguard the integrity of species in statu nascendi. Although hybrid sterility occurs almost universally among animal and plant species, most of our current knowledge comes from the classical genetic studies on Drosophila interspecific crosses or introgressions. With the house mouse subspecies Mus m. musculus and Mus m. domesticus as a model, new research tools have become available for studies of the molecular mechanisms and genetic networks underlying HS. Here we used QTL analysis and intersubspecific chromosome substitution strains to identify a 4.7 Mb critical region on Chromosome X (Chr X) harboring the Hstx2 HS locus, which causes asymmetrical spermatogenic arrest in reciprocal intersubspecific F1 hybrids. Subsequently, we mapped autosomal loci on Chrs 3, 9 and 13 that can abolish this asymmetry. Combination of immunofluorescent visualization of the proteins of synaptonemal complexes with whole-chromosome DNA FISH on pachytene spreads revealed that heterosubspecific, unlike consubspecific, homologous chromosomes are predisposed to asynapsis in F1 hybrid male and female meiosis. The asynapsis is under the trans- control of Hstx2 and Hst1/Prdm9 hybrid sterility genes in pachynemas of male but not female hybrids. The finding concurred with the fertility of intersubpecific F1 hybrid females homozygous for the Hstx2^Mmm allele and resolved the apparent conflict with the dominance theory of Haldane''s rule. We propose that meiotic asynapsis in intersubspecific hybrids is a consequence of cis-acting mismatch between homologous chromosomes modulated by the trans-acting Hstx2 and Prdm9 hybrid male sterility genes.