Ooplasmic segregation and axis formation in the polychaeteNereis virens embryo

Ooplasmic segregation is of great importance in the development of Annelida. The mechanisms of this process are very diverse in different groups of polychaetes, oligochaetes, and leeches (Fernandezet al., 1998). Ooplasmic segregation inNereis virens is connected with the first meiotic spindle formation and animal-vegetative axis appearance. Spherical polyaxial symmetry of the oocyte transforms into radial stratified symmetry in the course of ooplasmic segregation. There are two main steps of ooplasmic segregation inNereis virens. The first step begins after the cortical reaction when the central clear cytoplasm reaches the surface of the oocyte. The movement of the cytoplasm is sensitive to nocodazole, colchicine, and cytochalasin B and appears to be mediated by microtubules and, partly, by microfilaments. The second step is not sensitive to the microtubule inhibitors and is mediated mainly by actin filaments. Ooplasmic segregation inNereis virens may be considered as a primitive form of ooplasmic segregation in Annelida.     

Discordance between phylogenetics and coalescent-based divergence modelling: exploring phylogeographic patterns of speciation in the Carex macrocephala species complex

We fit a molecular data set, consisting of the rpL16 cpDNA marker and eight microsatellite loci, to the isolation-with-migration model as implemented in IM a to test a well-supported phylogenetic hypothesis of relationships within the Carex macrocephala species complex (Cyperaceae). The phylogenetic hypothesis suggests C. macrocephala from North America is reciprocally monophyletic and is sister to a reciprocally monophyletic clade of C. kobomugi . The North American C. macrocephala and C. kobomugi clade form a sister clade with a lineage of Asian C. macrocephala , thereby forming a paraphyletic C. macrocephala species. Not only does the phylogenetic hypothesis suggest C. macrocephala is paraphyletic, but it also suggests that the two lineages which share a partially overlapping distribution, Asian C. macrocephala and C. kobomugi , are not the most closely related. To test these relationships, we used coalescent-based population genetic models to infer divergence time for each lineage pair within the species complex. The coalescent-based models account for the stochastic forces which drive population divergence, and can account for the lineage sorting that occurs prior to lineage divergence. A drawback to phylogenetic-based phylogeographical analyses is that they do not account for stochastic lineage sorting that occurs between gene divergence and lineage divergence. By comparing the relative divergence time of the three main lineages within this group, Asian C. macrocephala , North American C. macrocephala , and C. kobomugi , we concluded that the phylogenetic hypothesis is incorrect, and the divergence between these lineages occurred during the Late Pleistocene epoch.     

心脏的起源和细胞谱系

心脏是器官发生过程中最早形成的结构之一.心脏的原基分布图和心脏细胞谱系是心脏起源和形态发生研究的主要方面.总结了近几年来以鸡和小鼠为模型,在心脏发生和心脏前体细胞定位方面的研究情况.     

Boys will be boys: social affinity among males drives social segregation in western grey kangaroos

In many sexually dimorphic and polygynous species, individuals exhibit social segregation by grouping with others of their own sex. Several proximate mechanisms have been proposed to explain social segregation: female avoidance of males, male avoidance of females, avoidance of harassment, male social affinity, female social affinity and activity budget asynchrony. Some of these have been tested in ungulate species, but few previous studies have been able to rigorously test, or distinguish between, these mechanisms because they have failed to examine the fission/fusion dynamics of groups. We tested these proximate mechanisms simultaneously in western grey kangaroos Macropus fuliginosus , by examining whether females, small males or large males instigated segregation by leaving mixed-sex groups or joining individuals of their own sex or size class. Females, small males and large males left mixed-sex groups as if leaving were independent of sex–size class. In contrast, large males joined male-only groups more frequently than expected. These results suggest that the need for males to maintain contact with other males can contribute to the cohesion of male-only groups and promote segregation. As male–male competition occurs in many polygynous species that sexually segregate, a comparable mechanism might be operating in other taxa, and should be examined further.     

Food segregation in three species of portunid crabs

Gastric mills of 2,619 specimens of portunid crabs(Portunus pelagicus, P. sanguinolentus andCharybdis feriatus) in Tolo Harbour, Hong Kongwere examined. Of these, 1,498 containedrecognizable food items. A total of 34 food items wereidentified, 10 of which (29.4%) were common to thethree crab species. The principal food items werebivalves, decapods, fish and algae. However, thepercent frequency of occurrence of these food itemsdiffered significantly (ANOVA, p < 0.001) among thecrabs. In addition to these principalfood items, differences were noted in other dietarycategories: P. pelagicusand P. sanguinolentus fed on the brittle starOphiura kinbergi, whereas P.sanguinolentus and C. feriatus fed on detritusand gastropods. The diet of P. sanguinolentuswas found to be more similar to C. feriatus thanto P. pelagicus. Exploitation of different fooditems and selective feeding on common prey may reduceinter-specific competition for natural food resourcesand allow the three portunid crabs to co-exist in thesame habitat. This revised version was published online in July 2006 with corrections to the Cover Date.     

高精度时空转录组揭示小鼠早期胚胎三胚层细胞谱系发生过程

从受精卵发育成具有不同细胞类型个体的过程中,细胞命运受到多个层次的调控.在哺乳动物的早期胚胎发育过程中,原肠运动是外、中、内三个胚层的建立过程,为后续的器官发生和形态建成提供了发育蓝图.然而目前对于三胚层命运建立的分子机制认识并不清晰.该文通过对小鼠早期胚胎的时空转录组分析,从分子层面揭示了外、中、内三胚层谱系发生的整...     

Asymptotic Distributions of Coalescence Times and Ancestral Lineage Numbers for Populations with Temporally Varying Size

The distributions of coalescence times and ancestral lineage numbers play an essential role in coalescent modeling and ancestral inference. Both exact distributions of coalescence times and ancestral lineage numbers are expressed as the sum of alternating series, and the terms in the series become numerically intractable for large samples. More computationally attractive are their asymptotic distributions, which were derived in Griffiths (1984) for populations with constant size. In this article, we derive the asymptotic distributions of coalescence times and ancestral lineage numbers for populations with temporally varying size. For a sample of size n, denote by T_m the mth coalescent time, when m + 1 lineages coalesce into m lineages, and A_n(t) the number of ancestral lineages at time t back from the current generation. Similar to the results in Griffiths (1984), the number of ancestral lineages, A_n(t), and the coalescence times, T_m, are asymptotically normal, with the mean and variance of these distributions depending on the population size function, N(t). At the very early stage of the coalescent, when t → 0, the number of coalesced lineages n − A_n(t) follows a Poisson distribution, and as m → n, n(n ? 1)T_m/2N(0) follows a gamma distribution. We demonstrate the accuracy of the asymptotic approximations by comparing to both exact distributions and coalescent simulations. Several applications of the theoretical results are also shown: deriving statistics related to the properties of gene genealogies, such as the time to the most recent common ancestor (TMRCA) and the total branch length (TBL) of the genealogy, and deriving the allele frequency spectrum for large genealogies. With the advent of genomic-level sequencing data for large samples, the asymptotic distributions are expected to have wide applications in theoretical and methodological development for population genetic inference.     

Molecular mechanism by which the nucleoid occlusion factor, SlmA, keeps cytokinesis in check

In Escherichia coli, cytokinesis is orchestrated by FtsZ, which forms a Z-ring to drive septation. Spatial and temporal control of Z-ring formation is achieved by the Min and nucleoid occlusion (NO) systems. Unlike the well-studied Min system, less is known about the anti-DNA guillotining NO process. Here, we describe studies addressing the molecular mechanism of SlmA (synthetic lethal with a defective Min system)-mediated NO. SlmA contains a TetR-like DNA-binding fold, and chromatin immunoprecipitation analyses show that SlmA-binding sites are dispersed on the chromosome except the Ter region, which segregates immediately before septation. SlmA binds DNA and FtsZ simultaneously, and the SlmA-FtsZ structure reveals that two FtsZ molecules sandwich a SlmA dimer. In this complex, FtsZ can still bind GTP and form protofilaments, but the separated protofilaments are forced into an anti-parallel arrangement. This suggests that SlmA may alter FtsZ polymer assembly. Indeed, electron microscopy data, showing that SlmA-DNA disrupts the formation of normal FtsZ polymers and induces distinct spiral structures, supports this. Thus, the combined data reveal how SlmA derails Z-ring formation at the correct place and time to effect NO.     

Restricted expression of LUZP in neural lineage cells: A study in embryonic stem cells

A novel protein LUZP with 3 leucine zipper motifs at its amino terminus is predominantly expressed in the adult brain. A modified gene targeting approach was employed in an attempt to establish in vitro and in vivo models in which Luzp is knock-out (KO) for phenotype assessment and a reporter gene lacZ is knock-in (KI) for tracing its expression. We report in this study the molecular cloning of the Luzp gene, its targeting vector construction and Luzp-KO/lacZ-KI embryonic stem (ES) clone selection. Since LUZP is also expressed in ES cells, the possibility of embryonic lethality cannot be excluded when attempting to establish Luzp-null mutant mice. We have therefore examined the development of homozygous Luzp-KO/lacZ-KI clones in nude mice. Tissue types derived from all three embryonic germ layers were observed in teratomas developed in nude mice. In situ X-gal staining further revealed restricted expression of LUZP in neural lineage cells.