Plant sex determination and sex chromosomes

Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.     

Plant evolution: modern sex chromosomes

The first detailed map has been produced of a plant chromosome carrying sex-determining genes. The new data show that, in papaya, these genes lie in a quite extensive non-recombining region. This region is nevertheless a small part of the papaya genome compared with other male-specific genome regions, such as mammalian Y chromosomes.     

Mammalian sex chromosomes: Evolution of organization and function

Comparisons of chromosome size, morphology and gene arrangements between mammals of different species permit us to deduce the genome characteristics of the common ancestor, and to chart the changes that have occurred during the divergence of the two lineages. The more distantly related are the species compared, the more remote the common ancestor whose characteristics can be deduced. This means that, providing there are sufficient similarities to warrant comparison, the more divergent the species compared, the more significant the contribution to our understanding of the organization of an ancestral mammalian genome and the process of mammalian genome evolution. One of the genetic surprises of the last decade was the discovery that, although gross karyotypes of distantly related orders of eutherian mammals (e.g. cat, cow, rabbit, man) have diverged extensively, gene mapping studies reveal the presence of large chromosome segments conserved across at least 60 million years (O'Brien et al. 1988). This finding makes it worthwhile to extend genetic comparisons to the two groups of mammals most distantly related to eutherian mammals--marsupials and monotremes. Here we will review comparisons of the sex chromosomes in these three major groups of extant mammals, and show how they have led us to a new view of the evolution of mammalian sex chromosome organization and function in sex determination and X chromosome inactivation.     

Plant cytosine-5 DNA methyltransferases: structure, function, and molecular evolution

A detailed analysis of the structure and function, along with evolutionary aspects, of the main plant cytosine-5 DNA methyltransferases (C5-MTases) is presented. The evolutionary relationships between the already known and four candidate plant C5-MTases identified in this work were investigated using the distance, maximum-parsimony, and maximum-likelihood approaches. The topologies of the trees were overall congruent: four monophyletic groups corresponding to the four plant C5-MTase families were clearly distinguished. In addition, sequence analyses of the plant C5-MTase target recognition domain sequences were performed and phylogenetic trees were reconstructed showing that there is good conservation among but not within the plant C5-MTase families. Furthermore, a conserved dipeptide that plays an important role in flipping the target base into the catalytic site of the C5-MTases was identified in all plant C5-MTases under study.     

Plant sulfite reductase: molecular structure, catalytic function and interaction with ferredoxin

Plant sulfite reductase contains the siroheme and the [4Fe-4S] cluster as catalytically active redox centers and catalyzes the six-electron reductions of sulfite and nitrite using electrons donated from ferredoxin. A heterologous expression of a cDNA for maize sulfite reductase in E. coli has enabled us to produce the wild-type and mutant enzymes. Putative substrate-binding basic residues, located at the siroheme distal side, have been substituted for other residues with neutral or negatively charged side chains. Kinetic studies of the resulting mutant enzymes have demonstrated that substrate specificity for the two anions is remarkably changed by amino acid substitutions at a single site. We have also produced two classes of ferredoxin mutants with less ability to donate electrons to sulfite reductase: one with a defect in the recognition of the partner enzyme and the other with an unfavorable redox property. This article summarizes our knowledge about the structure function relationships of plant sulfite reductase.     

On the molecular structure of chromosomes

Summary It is suggested that the construction of molecular models of the chromosome may prove to be a fruitful way of attacking the fundamental problems of cytology and genetics. Certain micelles consisting of molecules of protamine-nucleate are suggested for consideration. They are designed to interpret in molecular terms the postulates of genetics and the nature of the gene, and are constructed with special reference to the findings of the cytologists as to the capacity of the chromosomes to swell, to contract, to grow and to divide.Based upon a lecture given in the physics department of the University of Manchester on May 13, 1935. The Author is indebted to W. T.Astbuby, L. E.Bayliss, J. D.Bernal, D.Crowfoot, C.D.Darlington, P.& G.Eggleton, J.M.Gulland, L.J.Harris, D.Jordan Lloyd, M. F.Howson, W. O.Kermack, E. H.Neville, M.Polanyi and J. Z.Young for advice and criticism.     

性染色体与精子功能研究进展

最新研究表明：Y染色体上男性特有序列(MSY)是一个包含不同DNA序列的嵌合体,包含多个回文序列。回文序列上经常发生臂间基因交换,使Y染色体具有自我保护能力。女性失活X染色体上有15%的基因逃离失活进行表达,可能在男女性别不同和女性个体间差异中起决定作用。精子在受精时,是一个综合运输体,不仅仅只向卵子转移DNA,还有RNA和蛋白质。     

Plant Hsp70 molecular chaperones: Protein structure, gene family, expression and function

The Hsp70 molecular chaperones of plants are encoded by a multi-gene family whose members are developmentally regulated and differentially expressed in response to temperature stress and other conditions that interrupt normal protein folding or favor protein denaturation. Under non-stressful conditions, Hsp70 cognates function in concert with a variety of co-chaperones to facilitate folding of de novo synthesized proteins, assist in transport of precursor proteins into organelles and to help target damaged proteins for degradation. Stress-induced Hsp70s function to mitigate aggregation of stress-denatured proteins and to refold non-native proteins restoring their biological function through iterative cycles of adenine nucleotide hydrolysis-dependent peptide binding and release. Much of what is known about how plant Hsp70s function comes from the study of Hsp70s from other types of organisms. Owing to their unique biology, much remains to be learned about the many functions Hsp70s play in plants.     

Serpins: structure,function and molecular evolution

The superfamily of serine proteinase inhibitors (serpins) are involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and tumor suppression and are expressed in a cell-specific manner. The average protein size of a serpin family member is 350-400 amino acids, but gene structure varies in terms of number and size of exons and introns. Previous studies of all known serpins identified 16 clades and 10 orphan sequences. Vertebrate serpins can be conveniently classified into six sub-groups. We provide additional data that updates the phylogenetic analysis in the context of structural and functional properties of the proteins. From these, we can conclude that the functional classification of serpins relies on their protein structure and not on sequence similarity.     

Mammalian sex chromosomes

The X and Y chromosomes of the musk shrew are the two largest in the complement and they regularly form a single chiasma during meiosis. This chiasma is located in the short arms of the X and Y, both of which show partial C-banding at meiosis. The in vitro incorporation of 5-bromodeoxyuridine/tritiated thymidine during late S reveals that the non-C-band region of the Y finishes replication later than the C-band positive heterochromatin. During meiosis, the sex bivalent opens out early in pachytene to reveal a single chiasma which persists until late metaphase-I. In surface-spread, silver-stained meiocytes, the sex bivalent morphology changes from a phase of extensive pairing to one which includes a visible chiasma through a brief diffuse stage. Observations on C-banded meiocytes show a shift in the sex pair from a C-band positive to a negative state as compared to their corresponding somatic pattern. Comparable changes are also observed in the sex bivalents of other mammals which undergo a chiasmatic exchange. This suggests that in addition to pairing homology, an alteration in the chromatin configuration may be necessary for crossing over to occur between the sex chromosomes.     

Insect sex chromosomes

The two X chromosomes in tetraploid spermatogonial cells from Gryllotalpa fossor respond differentially to the production of chromatid aberrations by ³H-uridine (³H-U). As in diploid female somatic cells, only the euchromatic arm of one X shows such aberrations. The equivalent arm of the other X and the constitutive arms of both Xs are not affected. This differential response of the homologous arms of the two Xs appears to be due to a facultative heterochromatinization of one of them. It is suggested that an imprinting process, which has been assumed to occur during fertilization in other cases of X-inactivation, may not be necessary for the differential regulation of two X chromosomes in this case.     

Insect sex chromosomes

A presumptive mechanism of X inactivation has been investigated by using tritiated uridine-induced chromosome aberrations to distinguish active from inactive X chromosome arms in the insect Gryllotalpa fossor. Previous work on therian mammals has shown that constitutive and facultative heterochromatin are less susceptible to breakage by ³H-Urd than euchromatin (active). The present study indicates that, irrespective of the presence of two X chromosomes in females, only one of these is affected as in males and that the total number of aberrations induced by ³H-Urd in both male and female Gryllotalpa is the same. This suggests that in the female only one arm of one X chromosome is active and that a facultative heterochromatinization of the homologous arm of the other X is operative coupled with the presence of constitutive heterochromatin in the second arm of both X chromosomes.     

47,XXX females,sex chromosomes,and tooth crown structure

Summary Enamel thickness of the maxillary permanent central incisors and canines in seven Finnish 47,XXX females, their first-degree male and female relatives, and control males and females from the general population were determined from radiographs. The results showed that enamel in the teeth of 47,XXX females was clearly thicker than that of normal controls. On the other hand, the thickness of dentin (distance between mesial and distal dentinoenamel junctions) in 47,XXX females' teeth was about the same as that in normal control females, but clearly reduced as compared with that in control males. It is therefore obvious that in the triple-X chromosome complement the extra X chromosome is active in amelogenesis, whereas it has practically no influence on the growth of dentin. The calculations based on present and previous results in 45,X females and 47,XYY males indicate that the X chromosome increases metric enamel growth somewhat more effectively than the Y chromosome. Possibly, halfway states exist between active and repressed enamel genes on the X chromosome. The Y chromosome seems to promote dental growth in a holistic fashion.