A primitive fish provides key characters bearing on deep osteichthyan phylogeny

Osteichthyans, or bony vertebrates, include actinopterygians (teleosts and their relatives) and sarcopterygians (coelacanths, lungfishes and tetrapods). Despite features found in basal actinopterygians (for example, Dialipina and Ligulalepis) and basal sarcopterygians (for example, Psarolepis and Achoania), the morphological gap between the two lineages remains wide and how sarcopterygians developed a dermal surface covering known as cosmine (composed of a pore-canal network and a single layer of odontodes and enamel) is still poorly known. Here we describe a primitive fossil fish, Meemannia eos gen. et sp. nov., that possesses an actinopterygian-like skull roof and a cosmine-like dermal surface combining a pore-canal network (found in various fossil sarcopterygians) with superimposed layers of odontodes and enamel (previously known in actinopterygians and some acanthodians). This 405-million-year-old fish from the Lower Devonian of Yunnan (China) demonstrates that cosmine in many fossil sarcopterygians arose step by step through the acquisition of a pore-canal network followed by the subsequently developed ability to resorb previous generations of odontodes and enamel. Meemannia provides key characters for studying deep osteichthyan phylogeny and indicates a possible morphotype for the common ancestor of actinopterygians and sarcopterygians.     

A primitive fish close to the common ancestor of tetrapods and lungfish

The relationship of the three living groups of sarcopterygians or lobe-finned fish (tetrapods, lungfish and coelacanths) has been a matter of debate. Although opinions still differ, most recent phylogenies suggest that tetrapods are more closely related to lungfish than to coelacanths. However, no previously known fossil taxon exhibits a concrete character combination approximating the condition expected in the last common ancestor of tetrapods and lungfish -- and it is still poorly understood how early sarcopterygians diverged into the tetrapod lineage (Tetrapodomorpha) and the lungfish lineage (Dipnomorpha). Here we describe a fossil sarcopterygian fish, Styloichthys changae gen. et sp. nov., that possesses an eyestalk and which exhibits the character combination expected in a stem group close to the last common ancestor of tetrapods and lungfish. Styloichthys from the Lower Devonian of China bridges the morphological gap between stem-group sarcopterygians (Psarolepis and Achoania) and basal tetrapodomorphs/basal dipnomorphs. It provides information that will help in the study of the relationship of early sarcopterygians, and which will also help to resolve the tetrapod-lungfish divergence into a documented sequence of character acquisition.     

对称本原矩阵本原指数的一个计算公式及其应用

引入了图的奇直径、偶直径的概念,给出了本原图的本原指数的一个计算公式,讨论了奇直径、偶直径的关系,并利用我们的结果给出了邵嘉裕教授在《中国科学》发表的一个著名结论的简捷证明.     

一个有同样指数集的更小本原矩阵类的简单证明

对对角元非零至少有一对非零对称元但非对称的n阶本原矩阵的指数集En^ ={2,3,…2n-2}的结论^[1]，本文给出其中一更小类本原矩阵已有此指数集，且证明更简洁而不引用任何结果，另外，还给出一著名定理的简短证明。     

A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah

Therizinosauroids are an enigmatic group of dinosaurs known mostly from the Cretaceous period of Asia, whose derived members are characterized by elongate necks, laterally expanded pelves, small, leaf-shaped teeth, edentulous rostra and mandibular symphyses that probably bore keratinized beaks. Although more than a dozen therizinosauroid taxa are known, their relationships within Dinosauria have remained controversial because of fragmentary remains and an unusual suite of characters. The recently discovered 'feathered' therizinosauroid Beipiaosaurus from the Early Cretaceous of China helped to clarify the theropod affinities of the group. However, Beipiaosaurus is also poorly represented. Here we describe a new, primitive therizinosauroid from an extensive paucispecific bonebed at the base of the Cedar Mountain Formation (Early Cretaceous) of east-central Utah. This new taxon represents the most complete and most basal therizinosauroid yet discovered. Phylogenetic analysis of coelurosaurian theropods incorporating this taxon places it at the base of the clade Therizinosauroiden, indicating that this species documents the earliest known stage in the poorly understood transition from carnivory to herbivory within Therizinosauroidea. The taxon provides the first documentation, to our knowledge, of therizinosauroids in North America during the Early Cretaceous.     

带环的本原不可幂反对称带号有向图的局部基

设S是一个带号有向图,如果S的底图D（S）对称,且每个2圈都是负圈,则称S是反对称带号有向图．设S是一个n阶带环的本原不可幂反对称带号有向图,本文证明了：1）S的局部基ls（k）≤n＋k,并刻划了其极图特征;2）{ls（k）：S为带环的本原不可幂反对称带号有向图}={2,3,…,n＋k}．     

阶为p~2qr群的局部本原图

称点传递图Γ是X-局部本原的,如果X是其自同构群Aut(Γ)的子群,且对Γ的任意顶点v,Xv都本原地作用在Γ(v)上。本文完全分类了当|X|=p2qr时的X-局部本原图。     

用递推法查找伪随机码和本原多项式

为适应扩频通信的广泛应用,寻找快速查找伪随机码和本原多项式的方法。通过分析伪随机码的产生过程,提出利用“递推法”查找任意长度的m系列伪髓机码和本原多项式的方法,并借助MATLAB语言将“递推法”实现。寻找到了全部小于19阶的共计48 793个伪随机码和本原多项式,并更正和补充了某些参考文献中的部分数据,建立了伪随机码和本原多项式库,可供科技工作者随时使用参考。     

A primitive Y chromosome in papaya marks incipient sex chromosome evolution

Many diverse systems for sex determination have evolved in plants and animals. One involves physically distinct (heteromorphic) sex chromosomes (X and Y, or Z and W) that are homozygous in one sex (usually female) and heterozygous in the other (usually male). Sex chromosome evolution is thought to involve suppression of recombination around the sex determination genes, rendering permanently heterozygous a chromosomal region that may then accumulate deleterious recessive mutations by Muller's ratchet, and fix deleterious mutations by hitchhiking as nearby favourable mutations are selected on the Y chromosome. Over time, these processes may cause the Y chromosome to degenerate and to diverge from the X chromosome over much of its length; for example, only 5% of the human Y chromosome still shows X-Y recombination. Here we show that papaya contains a primitive Y chromosome, with a male-specific region that accounts for only about 10% of the chromosome but has undergone severe recombination suppression and DNA sequence degeneration. This finding provides direct evidence for the origin of sex chromosomes from autosomes.     

恰含1个n圈和2个s圈的本原有向图的scrambling指数

在图D_(s,n)的基础上,增加1个s长圈.研究含有1个n长圈和2个s长圈(2个s长圈有公共顶点)的本原有向图.通过分析图中每个点经过t长途径所到达的点的集合及点的个数,得出此类本原有向图的scrambling指数.     

人工鱼群算法在孔群加工路径优化中的应用研究

将人工鱼群算法应用于孔群加工路径优化的研究,建立以最短加工路径为目标的路径优化数学模型,阐述算法实施的具体过程并进行算例分析.结果表明,该方法求最优解的性能优于Hopfield算法、进化蚁群算法、人工免疫算法以及改进的遗传算法,获得的最优路径可以节省71.47%的行走路程.     

迹非零的本原不可幂对称符号模式矩阵的基

研究迹非零的本原不可幂对称符号模式矩阵,完全解决了基的最大值、极矩阵和指数集问题.     

围长为2的本原不可幂带号有向图的Lewin指数集

设S是一个本原不可幂带号有向图,则存在正整数k,使得存在u,v∈V（S）,S中从u到v有长为k和k＋1的有向SSSD途径（SSSD途径是有相同起点、终点、长度和不同符号的一对有向途径）,这样的最小正整数k称为S的Lewin指数,记作l（S）．给出了围长为2的本原不可幂带号有向图的Lewin指数的最大值和指数集的部分元素．     

An extant cichlid fish radiation emerged in an extinct Pleistocene lake

The haplochromine cichlid fish of the East African Great Lakes represent some of the fastest and most species-rich adaptive radiations known, but rivers in most of Africa accommodate only a few morphologically similar species of haplochromine cichlid fish. This has been explained by the wealth of ecological opportunity in large lakes compared with rivers. It is therefore surprising that the rivers of southern Africa harbour many, ecologically diverse haplochromines. Here we present genetic, morphological and biogeographical evidence suggesting that these riverine cichlids are products of a recent adaptive radiation in a large lake that dried up in the Holocene. Haplochromine species richness peaks steeply in an area for which geological data reveal the historical existence of Lake palaeo-Makgadikgadi. The centre of this extinct lake is now a saltpan north of the Kalahari Desert, but it once hosted a rapidly evolving fish species radiation, comparable in morphological diversity to that in the extant African Great Lakes. Importantly, this lake seeded all major river systems of southern Africa with ecologically diverse cichlids. This discovery reveals how local evolutionary processes operating during a short window of ecological opportunity can have a major and lasting effect on biodiversity on a continental scale.