Saltational evolution of trunk segment number in centipedes

SUMMARY Saltational changes in segment numbers have likely occurred in arthropod evolution, especially if mechanisms of segment formation involve a multiplicative phase, as recently suggested in the evo-devo literature. Here we provide for the first time evidence of major phenotypic saltation in the evolution of segment number in a lineage of centipedes, with a newly discovered species of scolopender having segment numbers duplicated with respect to its closest relatives, and to all the remaining 700+ species of Scolopendromorpha known to date.     

Saltational evolution: hopeful monsters are here to stay

Since 150 years it is hypothesized now that evolution always proceeds in a countless number of very small steps (Darwin in On the origin of species by means of natural selection or the preservation of favoured races in the struggle of life, Murray, London, 1859), a view termed “gradualism”. Few contemporary biologists will doubt that gradualism reflects the most frequent mode of evolution, but whether it is the only one remains controversial. It has been suggested that in some cases profound (“saltational”) changes may have occurred within one or a few generations of organisms. Organisms with a profound mutant phenotype that have the potential to establish a new evolutionary lineage have been termed “hopeful monsters”. Recently I have reviewed the concept of hopeful monsters in this journal mainly from a historical perspective, and provided some evidence for their past and present existence. Here I provide a brief update on data and discussions supporting the view that hopeful monsters and saltational evolution are valuable biological concepts. I suggest that far from being mutually exclusive scenarios, both gradual and saltational evolution are required to explain the complexity and diversity of life on earth. In my view, gradual changes represent the usual mode of evolution, but are unlikely to be able to explain all key innovations and changes in body plans. Saltational changes involving hopeful monsters are probably very exceptional events, but since they have the potential to establish profound novelties sometimes facilitating adaptive radiations, they are of quite some importance, even if they would occur in any evolutionary lineage less than once in a million years. From that point of view saltational changes are not more bizarre scenarios of evolutionary change than whole genome duplications, endosymbiosis or impacts of meteorites. In conclusion I argue that the complete dismissal of saltational evolution is a major historical error of evolutionary biology tracing back to Darwin that needs to be rectified.     

Parallel Saltational Evolution of Ultrafast Movements in Snapping Shrimp Claws

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从细胞内共生到多尺度共生：回顾与展望

基于共生概念的历史变化,目前人们普遍接受了广义共生概念。即共生是包含互利共生（mutualism）、偏利共生（commensalism）和拮抗/寄生（antagonism/parasitism）的共生连续体。本文简述了近20年间,全球9次国际共生学术大会取得的重要成果,对细胞内共生、时间、空间以及多种互作尺度共生关系的研究利用进展进行了评述。同时展望了一些活跃共生领域的研究概况,如共生失调 （dysbiosis）、植物-微生物-昆虫三角共生关系（plant-microbe-insect triangle）、细菌-真菌互作（bacterial- fungal interaction,BFI）、菌根菌-真菌内生细菌-植物多方共生联盟（multipartite symbiosis consortium）以及与共生相关微生物组的集合群落（metacommunity）研究及应用等。共生（symbiosis）正成为当代生物学的核心原则,正以一种与更宏大系统方法相一致的概念,从根本上改变了传统上的一些生物学概念,如孤立性的个体（individuality）概念。基因组测序和高通量RNA技术分析揭示,动、植物与共生微生物的重要互作,打破了迄今为止生物个体的特征边界,挑战了这些学科的定义;共生不仅是一对一的互利共生关系,共生实际是多种共生模式的连续共生体。此外,植物-昆虫-微生物互作的三角关系;菌根-真菌-真菌内生细菌-植物的多方联盟等新关系的发现,更把生命科学推向了快速发展的方向。这些科学进展不仅对生物科学的遗传学、免疫学、进化、发育、解剖学和生理学的研究至关重要,拓宽了新的视野,而且对农业中生物制剂研发,人类微生物组的管理及调控,以及对发酵食品及工业微生物生产的设计和管理将产生积极影响。     

生物的寄生与共生

寄生和共生是生物彼此交互中的2种关系,也是生物学上2个重要的概念,学生在生物学学习中易对这2个概念产生混淆。主要就寄生和共生的概念以及它们之间的协同进化论述。以期为中学教师和其他生物学工作者提供参考。     

Speciation by symbiosis

In the Origin of Species, Darwin struggled with how continuous changes within a species lead to the emergence of discrete species. Molecular analyses have since identified nuclear genes and organelles that underpin speciation. In this review, we explore the microbiota as a third genetic component that spurs species formation. We first recall Ivan Wallin's original conception from the early 20th century on the role that bacteria play in speciation. We then describe three fundamental observations that justify a prominent role for microbes in eukaryotic speciation, consolidate exemplar studies of microbe-assisted speciation and incorporate the microbiota into classic models of speciation.     

The Glycine-Glomus-Rhizobium symbiosis

Soybean [ Glycine max , (L.) Merr, cv. Lancer] plants were grown in a sterile rooting medium watered daily with a nutrient solution containing 4, 20, 100, or 500 μM, P. Plants were inoculated with Rhizobium japonicum , strain 61A118 and grown in the presence or absence of the endomycorrhizal fungus Glomus fasciculatus , Gerdemann et Trappe. Plants grown at the highest P regime had six times higher shoot dry weight than those grown in the lowest P regime. Nodulation did not occur at 4 μM P. Nodule dry weight increased 200-fold from the 20 to the 500 μM P treatment. Percentage P in shoots and nodules differed significantly among all treatment levels. Acetylene reduction by nitrogenase increased logarithmically with increasing amounts of P. Hydrogen evolution was not detectable at the 20 μM P level. The relative efficiency of nitrogen fixation increased with increasing P stress. Infection by Glomus fasciculatus , at the 500 μM P level was negligible and did not affect the parameters measured. At all other treatment levels the mycorrhizal plants had significantly higher rates of N₂ fixation, plant and nodule mass and P content.     

Immunity and symbiosis

The invertebrate immune system, which has become a major research focus, shares basic features of innate immunity with vertebrates and men. A special feature apparently found only in invertebrates is their close association with vertically heritable symbiotic microorganisms. The validity of the simple view of symbiosis as a mutually beneficial interaction between two uneven partners mainly improving the nutritional state of the two companions has been challenged, however, as symbiotic interactions might involve more partners, and symbiotic functions of the microorganisms are much more diverse than previously assumed. Likewise, microorganisms considered to be mostly harmful to their hosts have been shown to enhance host fitness under some circumstances. The role of a symbiont itself might change between environments or life stages of the host and symbionts might have features previously thought to be specific for pathogens. Understanding symbiotic interactions requires the comprehension of the cross-talk between the symbiotic companions, and the dissection of how long-lasting infections are established without eliminating the symbiont by host immune responses. Fascinating new findings in this field revealed that symbiosis might contribute to defence against pathogens or natural enemies. New symbiont-based approaches to defeat agricultural pests or pathogen transmission by arthropod vectors are becoming conceivable.     

Models of symbiosis

A tentative outline of concepts is proposed for the evolutionary genetics of symbiosis. There are three main topics. The first concerns the tension between the integrative and disruptive forces of kin selection. Kin selection can be disruptive because competition among close relatives favors dispersal and a reduction in relatedness among neighbors. Kin selection acts independently within each species of a symbiotic community but has important consequences for the integration of the community into a cooperative unit. The second topic describes the evolution of beneficial, synergistic effects between species. The evolution of mutual effects depends on various correlations between species. Genetic correlations are analogous to linkage disequilibrium in standard Mendelian genetics. Correlations in reproductive success between symbiotic partners arise from codispersal and reproductive synchrony. The third topic concerns the evolution of asymmetrical symbioses in which one species can dominate its partner. Dominance may explain the evolution of uniparental inheritance among cytoplasmic symbionts and a peculiar form of germ-soma separation in the symbionts of insects.     

Signaling in symbiosis

In recent years, the major focus in nodulation research has been on the genetic dissection of Nod-factor signaling. Components of this pathway appear to be shared with signaling processes that are induced during the formation of mycorrhiza. With the cloning of orthologs of the NIN and DMI2 genes from several legumes, the molecular characteristics of components of the Nod-factor-signaling pathway are now starting to be revealed. Orthologs of HAR1, a key player in the systemic autoregulatory mechanism controlling nodule numbers, have also been cloned recently. The mechanism by which nodulation is autoregulated is related to that by which fixed nitrogen inhibits nodulation. Genes that are involved in Nod-factor signaling may be targets for mechanisms that suppress nodulation. If this is the case, it would bring two fascinating areas of symbiosis together.     

The Rhizobium--legume symbiosis.

The rhizobia are soil microorganisms that can interact with leguminous plants to form root nodules within which conditions are favourable for bacterial nitrogen fixation. Legumes allow the development of very large rhizobial populations in the vicinity of their roots. Infections and nodule formation require the specific recognition of host and Rhizobium, probably mediated by plant lectins. Penetration of the host by a compatible Rhizobium species usually provokes host root cell division to form the nodule, and a process of differentiation by both partners then ensues. In most cases the rhizobia alter morphologically to form bacteroids, which are usually larger than the free-living bacteria and have altered cell walls. At all stages during infection, the bacteria are bounded by host cell plasmalemma. The enzyme nitrogenase is synthesized by the bacteria and, if leghaemoglobin is present, nitrogen fixation will occur. Leghaemoglobin is a product of the symbiotic interaction, since the globin is produced by the plant while the haem is synthesized by the bacteria. In the intracellular habitat the bacteria are dependent upon the plant for supplies of energy and the bacteroids, in particular, appear to differentiate so that they are no longer able to utilize the nitrogen that they fix. Regulation of the supply of carbohydrate and the use of the fixed nitrogen thus appear to be largely governed by the host.     

Osmoregulation in anthozoan-dinoflagellate symbiosis

Endosymbiosis creates a unique osmotic circumstance. Hosts are not only responsible for balancing their internal osmolarity with respect to the external environment, but they must also maintain a compatible osmotic environment for their endosymbionts, which may themselves contribute to the net osmolarity of the host cell through molecular fluxes and/or exchange. Cnidarian hosts that harbor intracellular dinoflagellates (zooxanthellae) are excellent examples of such a symbiosis. These associations are characterized by the exchange of osmotically active compounds, but they are temporally stable under normal environmental conditions indicating that these osmotically driven exchanges are effectively and rapidly regulated. Although we have some knowledge about how asymbiotic anthozoans and algae osmoregulate, our understanding of the physiological mechanisms involved in regulating an intact anthozoan-dinoflagellate symbiosis is poor. Large-scale expulsion of endosymbiotic zooxanthellae, or bleaching, is currently considered to be one of the greatest threats to coral reefs worldwide. To date, there has been little consideration of the osmotic scenarios that occur when these symbioses are exposed to the conditions that normally elicit bleaching, such as increased seawater temperatures and UV radiation. Here we review what is known about osmoregulation and osmotic stress in anthozoans and dinoflagellates and discuss the osmotic implications of exposure to environmental stress in these globally distributed and ecologically important symbioses.     

Microalgal symbiosis in biotechnology

This review provides an analysis of recent published work on interactions between microorganisms, especially the ones involving mainly nutrient exchanges and at least with one microalga species. Examples of microbial partners are given, with a remark to the potential application of cultures of an autotroph and a heterotroph, which grow simultaneously, taking advantage of the complementary metabolisms. These are particularly interesting, either due to economic or sustainable aspects, and some applications have already reached the commercial stage of development. The added advantages of these symbiotic cultures are biomass, lipid, and other products productivity enhancement a better utilization of resources and the reduction or even elimination of process residues (including carbon dioxide and other greenhouse gases) to conduct an increasingly greener biotechnology. Among the several symbiotic partners referred, the microalgae and yeast cultures are the most used. The interaction between these two microorganisms shows how to enhance the lipid production for biodiesel purposes compared with separated (stand-alone) cultures.     

Breeding for better symbiosis

The present review gives a critical assessment of the literature dealing with symbiosis between rhizobia and legumes and between AM fungi and most plants. Associative N₂ fixation (even though strictly speaking not a symbiotic relationship) does have some characteristics of symbiosis due to mutualistic dependence and usefulness of the relationship, and is therefore covered in this review. Nodulation in the rhizobia–legume symbiosis may be limited by an insufficient amount of the nod-gene inducers released from seed and/or roots. However, there is genotypic variation in the germplasm of legume species in all components of the signalling pathway, suggesting a prospect for improving nodulation by selecting and/or transforming legume genotypes for increased exudation of flavonoids and other signalling compounds. Deciphering chromosomal location as well as cloning nod, nif and other genes important in nodulation and N₂ fixation will allow manipulation of the presence and expression of these genes to enhance the symbiotic relationship. Increased efficacy of symbiotic N₂ fixation can be achieved by selecting not only the best host genotypes but by selecting the best combination of host genotype and nodule bacteria. As flavonoids exuded by legume seedlings may not only be nod-gene inducers, but also stimulants for hyphal growth of the AM fungi, selecting and/or transforming plants to increase exudation of these flavonoids may result in a double benefit for mycorrhizal legumes. Mutants unable to sustain mycorrhizal colonisation are instrumental in understanding the colonisation process, which may ultimately pay off in breeding for the more effective symbiosis. In conclusion, targeted efforts to breed genotypes for improved N₂ fixation and mycorrhizal symbiosis will bring benefits in increased yields of crops under a wide range of environmental conditions and will contribute toward sustainability of agricultural ecosystems in which soil-plant-microbe interactions will be better exploited.