动物性别进化的经济学

经济学无处不在,包括动物从无性繁殖到有性繁殖的过程。在这个过程中,先是雌雄同体的自体受精和异体受精,然后是雌雄异体的性转变（包括性逆转和改变子代的性比）,最终为雌雄异体的异体交配。     

VA菌根共生的起源和进化

ＶＡ菌根共生的起源和进化赵之伟（云南大学生物学系,昆明６５００９１）ＯｒｉｇｉｎａｎｄＥｖｏｌｕｔｉｏｎｏｆｔｈｅＶＡＭｙｃｏｒｒｈｉｚａｌＳｙｍｂｉｏｓｉｓ．ＺｈａｏＺｈｉｗｅｉ（ＢｉｏｌｏｇｙＤｅｐａｒｔｍｅｎｔｏｆＹｕｎｎａｎＵｎｉｖｅｒｓｉｔ...     

蕨类植物和裸子植物的起源与进化

最近笔者拜读了由中山大学生物系与南京大学生物系合编的《植物学》(系统、分类部分),高等教育出版社,1984年3月第四次印刷本,发现在该书的157—188页,即于论述“蕨类植物的起源与进化”和“裸子植物的起源与进化”两节中,存在一些值得与作者商榷的问题,现评论如下,谨希同行专家广为讨论。     

细胞的起源和进化（一）

现代地球上绝大部分的生命都是以单个细胞或多个细胞的形式存在的。生命起源后细胞基本结构的形成和细胞结构在进化过程中的发展都是一些重大的生物进化事件。因此对细胞本身的起源和进化的研究是探索生物进化的重要组成部分。１最原始的细胞细胞是如何产生的呢？要解决这...     

犬细小病毒:从起源到进化

犬细小病毒(Canine parvovirus,CPV-2)首次分离于1978年,被认为是由遗传关系相近的猫泛白细胞减少症病毒(FPLV)或其他肉食兽细小病毒(FPLV-like virus)跨宿主感染犬产生的新病原.其感染能引起新生犬急性心肌炎或幼犬出血性肠炎,造成较高的发病率和死亡率.该病原爆发后短短一年时间内即广泛流行到世界各地.随着CPV-2对宿主的适应和变异,新抗原变异型(CPV-2a、CPV-2b和CPV-2e)不断产生并在世界各地逐步替代了CPV-2的流行.伴随CPV-2抗原变异的同时,其对宿主(犬、猫)嗜性、毒力等生物学特性也随之改变.本文综述了CPV-2过去30多年在世界流行和变异情况,并探讨了基因变异在病毒跨宿主传播中的意义.     

鲸类的起源和进化—兼论淡水豚类的进化

鲸类,无疑是最早,且最为成功地适应水体生活的哺乳动物。它们种类多,数量大,分布广。纺锤形体型、鳍状肢,由纤维软骨支持的水平尾裂,以及颅骨缩短、鼻孔后移、耳骨被气囊或脂肪体与颈椎隔开、缩短或合并的颈椎等,构成了它们适应于水环境而不同于别的哺乳动物的特点。然而,一些解剖学、胚胎学和生化分析的结果,如胎儿     

小麦的分类、起源与进化

小麦是世界历史上最古老的作物之一,约在一万年以前,在西南亚,人类首先成功地将小麦进行栽培生产粮食,因此小麦栽培历史和人类开化有着密切的相互关系。在现代世界农业中,小麦也是最重要的作物之一。它分布面积很广,从北纬67°的挪威、芬兰和苏联到南纬45°的阿根庭都有栽培。世界生产小麦的地区有苏联南部、美国中部平     

小麦的分类、起源与进化

小麦是世界历史上最古老的作物之一,约在一万年以前,在西南亚,人类首先成功地将小麦进行栽培生产粮食,因此小麦栽培历史和人类开化有着密切的相互关系。在现代世界农业中,小麦也是最重要的作物之一。它分布面积很广,从北纬67°的挪威、芬兰和苏联到南纬45°的阿根庭都有栽培。世界生产小麦的地区有苏联南部、美国中部平原和加拿大相邻地区、地中海地区、中国、印度、阿根廷北部和澳大利亚的西南部。这类作物在比较宽的环境 范围内都能获得高产。据统计,全世界有1/3的人口以小麦为主要粮食.消费的小麦食物卡值占全世界的20%以上。     

动物眼睛的起源与进化研究进展

动物眼睛的起源与进化过程是人类在探索自然奥秘过程中遇到的一个非常有趣且引人注目的研究热点,目前这方面的研究已取得了丰硕的成果。从眼睛出现的过程、眼睛进化所需的时间、眼睛的单起源论、眼睛的类型与进化、光感受器与眼睛进化的关系、光感受器和视蛋白的类型与进化这6个方面简要概述了动物眼睛起源与进化的主要研究进展。     

Evidence for maintenance of sex by pathogens in plants

The predominance of outcrossing despite the substantial transmission advantage of self-fertilization remains a paradox. Theory suggests that selection can favor outcrossing if it enables the production of offspring that are less susceptible to pathogen attack than offspring produced via self-fertilization. Thus, if pathogen pressure is contributing to the maintenance of outcrossing in plants, there may be a positive correlation between the number of pathogen species attacking plant species and the outcrossing rate of the plant species. We tested this hypothesis by examining the association between outcrossing rate and the number of fungal pathogen species that attack a large, taxonomically diverse set of seed plants. We show that plant species attacked by more fungal pathogen species have higher outcrossing rates than plants with fewer enemies. This relationship persists after correcting for study bias among natural and agricultural species of plants. We also accounted for the nested hierarchy of relationships among plant lineages by conducting phylogenetically independent contrasts (PICs) within genera and families that were adequately represented in our dataset. A meta-analysis of the correlation between pathogen and outcrossing PICs shows that there is a positive correlation between pathogen species number and outcrossing rates. This pattern is consistent with the hypothesis that pathogen-mediated selection may contribute to the maintenance of outcrossing in species of seed plants.     

The Ratchet and the Red Queen: the maintenance of sex in parasites

The adaptive significance of sexual reproduction remains as an unsolved problem in evolutionary biology. One promising hypothesis is that frequency‐dependent selection by parasites selects for sexual reproduction in hosts, but it is unclear whether such selection on hosts would feed back to select for sexual reproduction in parasites. Here we used individual‐based computer simulations to explore this possibility. Specifically, we tracked the dynamics of asexual parasites following their introduction into sexual parasite populations for different combinations of parasite virulence and transmission. Our results suggest that coevolutionary interactions with hosts would generally lead to a stable coexistence between sexual parasites and a single parasite clone. However, if multiple mutations to asexual reproduction were allowed, we found that the interaction led to the accumulation of clonal diversity in the asexual parasite population, which led to the eventual extinction of the sexual parasites. Thus, coevolution with sexual hosts may not be generally sufficient to select for sex in parasites. We then allowed for the stochastic accumulation of mutations in the finite parasite populations (Muller's Ratchet). We found that, for higher levels of parasite virulence and transmission, the population bottlenecks resulting from host–parasite coevolution led to the rapid accumulation of mutations in the clonal parasites and their elimination from the population. This result may explain the observation that sexual reproduction is more common in parasitic animals than in their free‐living relatives.     

Why is sex so rare in Lecane inermis (Rotifera: Monogononta) in wastewater treatment plants?

The monogonont rotifer Lecane inermis is commonly known as a facultative parthenogen. Unexpectedly, among numerous lineages we isolated from wastewater treatment plants (WWTP), only one was capable of sexual reproduction. We investigated why sex was so rare among L. inermis in WWTP. The reproductive modes of lineages derived from the sexual lineage were examined. Among all lineages, the fraction of those reproducing only asexually for 14 d was 0.39. In the subsequent round, the fraction of asexual lineages reached 0.61. The population growth rate of the parthenogenetic lineages was significantly higher than that of the sexual lineages. We simulated the fate of rotifer populations in WWTP by removing 10% of the experimental cultures each day. After 10 d of these conditions, the mean number of females was greater in parthenogenetic than in sexual lineages. After 20 d, only solitary dormant eggs remained in the sexual lineages. It therefore appears that the loss of sex may be attributed to the specific conditions of the WWTP. When there is no risk of desiccation and no need for dormancy, the fast‐growing parthenogens outcompete lineages investing in costly sex. Furthermore, even if some resting eggs were deposited, they would be washed out from the system with the excess sludge.     

Reproductive mode and genetic variation suggest a North American origin of European Letharia vulpina

Our data on the intercontinental population biology of Letharia vulpina show an unexpected shift from a recombining North American population with unique haplotypes to genetically depauperate Swedish and Italian populations, each with many representatives of a single repeated haplotype. Analysis of eight loci in 47 individuals supported recombination in North American populations and showed almost no variation among European populations. We infer that a genetic bottleneck caused by limited long-distance dispersal accounts for the lack of genetic variation found in marginal populations. This lack of variation in the European populations makes it impossible to use population genetic means to distinguish clonal reproduction from self-fertilization or even outcrossing, but phenotype indicates that reproduction in the marginal populations is by clonal spread, via soredia and isidioid soredia.     

Sex at the origin: an Asian population of the rice blast fungus Magnaporthe oryzae reproduces sexually

Sexual reproduction may be cryptic or facultative in fungi and therefore difficult to detect. Magnaporthe oryzae, which causes blast, the most damaging fungal disease of rice, is thought to originate from southeast Asia. It reproduces asexually in all rice‐growing regions. Sexual reproduction has been suspected in limited areas of southeast Asia, but has never been demonstrated in contemporary populations. We characterized several M. oryzae populations worldwide both biologically and genetically, to identify candidate populations for sexual reproduction. The sexual cycle of M. oryzae requires two strains of opposite mating types, at least one of which is female‐fertile, to come into contact. In one Chinese population, the two mating types were found to be present at similar frequencies and almost all strains were female‐fertile. Compatible strains from this population completed the sexual cycle in vitro and produced viable progenies. Genotypic richness and linkage disequilibrium data also supported the existence of sexual reproduction in this population. We resampled this population the following year, and the data obtained confirmed the presence of all the biological and genetic characteristics of sexual reproduction. In particular, a considerable genetic reshuffling of alleles was observed between the 2 years. Computer simulations confirmed that the observed genetic characteristics were unlikely to have arisen in the absence of recombination. We therefore concluded that a contemporary population of M. oryzae, pathogenic on rice, reproduces sexually in natura in southeast Asia. Our findings provide evidence for the loss of sexual reproduction by a fungal plant pathogen outside its centre of origin.     

Bloody‐minded parasites and sex: the effects of fluctuating virulence

Asexual lineages can grow at a faster rate than sexual lineages. Why then is sexual reproduction so widespread? Much empirical evidence supports the Red Queen hypothesis. Under this hypothesis, coevolving parasites favour sexual reproduction by adapting to infect common asexual clones and driving them down in frequency. One limitation, however, seems to challenge the generality of the Red Queen: in theoretical models, parasites must be very virulent to maintain sex. Moreover, experiments show virulence to be unstable, readily shifting in response to environmental conditions. Does variation in virulence further limit the ability of coevolving parasites to maintain sex? To address this question, we simulated temporal variation in virulence and evaluated the outcome of competition between sexual and asexual females. We found that variation in virulence did not limit the ability of coevolving parasites to maintain sex. In fact, relatively high variation in virulence promoted parasite‐mediated maintenance of sex. With sufficient variation, sexual females persisted even when mean virulence fell well below the threshold virulence required to maintain sex under constant conditions. We conclude that natural variation in virulence does not limit the relevance of the Red Queen hypothesis for natural populations; on the contrary, it could expand the range of conditions over which coevolving parasites can maintain sex.     

The Tangled Bank: The maintenance of sexual reproduction through competitive interactions

The maintenance of sexual reproduction is discussed using a model based on the familiar Lotka-Volterra competition equations. Both the equilibrium and the stability conditions that allow a sexual population to resist invasion by a single asexual clone are considered. The equilibrium conditions give results similar to previous models: When the cost of sex, within phenotype niche width, and environmental variance are low, the sexual population coexists with the asexual clone and remains at a high density. However, the asexual clone is never completely excluded. Analysis of the stability conditions shows a different picture: The introduction of an asexual clone considerably reduces the stability of the community. However, owing to its larger total niche width, the sexual population exists partly in a “competitor-free space” where the asexual clone has almost no influence on the outcome of the interactions. Therefore the asexual clone is less stable than the sexual population and has a higher probability of extinction. In contrast, the sexual population does not become extinct, since the extreme phenotypes remain at a stable, though low, density, and the central phenotypes, where stability is low, are recreated every generation through recombination. I therefore conclude that the ecological conditions under which sexual reproduction is favored over asexual reproduction are fairly easily attained and are more general than previous analyses had suggested.     

On sex allocation and selfing in higher plants

Summary Sex allocation (male allocation/female allocation) as a function of selfing rate is studied in the wild riceOryza perennis. Using dry weight measures, the male/female ratio is linearly related to the selfing rate. This linear relationship may have a fairly radical interpretation in terms of current sex allocation theory. It suggests that the intermediate selfing rates are themselves maintained by a form of frequency dependence. In particular, the linearity suggests: (i) the relative fitness of a selfed versus outcrossed offspring decreases with increased selfing; (ii) in equilibrium, a selfed offspring is approximately half as fit as an outcrossed offspring; (iii) the frequency dependence, being the opposite of that proposed in most selfing models, may result from the same forces thought to be involved in the maintenance of sex itself, and (iv) the position of the fitted line contains information about the plant's use of wind pollination for male reproduction. It suggests that wind shows much less mixing of pollen than previously assumed, and/or that there are severe morphological constraints on pollen presentation. The above interpretations are clearly speculative and tentative. Possible problems in the analysis, and some alternatives for data interpretation are discussed.     

Why anisogamy drives ancestral sex roles

There is a clear tendency in nature for males to compete more strongly for fertilizations than females, yet the ultimate reasons for this are still unclear. Many researchers—dating back to Darwin and Bateman—have argued that the difference is ultimately driven by the fact that males (by definition) produce smaller and more numerous gametes than females. However, this view has recently been challenged, and a formal validation of the link between anisogamy and sex roles has been lacking. Here, we develop mathematical models that validate the intuition of Darwin and Bateman, showing that there is a very simple and general reason why unequal gamete numbers result in unequal investment in sexually competitive traits. This asymmetry does not require multiple mating by either sex, and covers traits such as mate searching, where the male bias has been difficult to explain. Furthermore, our models show males and females are predicted to diverge more strongly when the fertilization probability of each female gamete is high. Sex roles thus ultimately trace back to anisogamy and the resulting consequences for the fertilization process.