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1.
The evolution of the scrotum: a new hypothesis   总被引:1,自引:0,他引:1  
The adaptive significance of the scrotum is unresolved after more than 60 years of debate and experimentation. The "training hypothesis" introduced here suggests that testicular descent is a mechanism for improving sperm quality. The hypothesis proposes that: (1) testicular descent decreases blood supply to maturing sperm cells, (2) sperm mitochondria respond to the resulting oxygen stress by enhancing their enzymatic machinery for oxidative metabolism, as do oxygen-stressed muscle cell mitochondria, and (3) the resulting increase in aerobic fitness of sperm cells is advantageous in inter-ejaculate competition. The hypothesis suggests that there is a quantity-quality trade-off in sperm production, where taxa with internal testes produce large volumes of low-quality sperm while taxa with scrotal testes produce smaller volumes of higher-quality sperm.  相似文献   

2.
The evolution of metamorphosis in amphibians   总被引:2,自引:0,他引:2  
A survey is provided of the external transformations that coincide with metamorphosis or a water-to-land transition, and of transformations during water-to-land transition in the retinal projection, the brain stem, the lateral-line system, and the inner ear of amphibians. Among the three orders of amphibians, the frogs are characterized by more pronounced transformations during the water-to-land transition than are the other two orders. Some of the progressive and regressive changes in the sensory and nervous system are presented and a scenario is suggested for the evolution of these transformations among amphibians. Suggestions that metamorphosis in frogs can recapitulate the water-to-land transition of ancestral amniotic vertebrates are refuted.  相似文献   

3.
Cladoendesis is a method of phylogeny analysis opposed to various matrix methods. In contrast to matrix methods, in cladoendesis phylogenetic trees are not built each time as new ones, but are reconstructed based on the previous results. Each character of each taxon is compared with its ancestral condition in the ground-plan of the higher taxon. The revealed part of the phylogeny is represented in a form of hierarchical classification. In addition to the principles of natural classification elaborated by C. Linnaeus, P.-A. Latreille, and others, evolutionary theory elaborated by Ch. Darwin and others, and principles of phylogenetic systematics elaborated by W. Hennig and others, cladoendesis includes rank-free dual nomenclature system (DualNom) and advanced method of text layout elaborated by the author. Important components of cladoendesis are the use of the term “plesiomorphon” and taking into account such important evolutionary events as conservation and deconservation of characters. Cladoendesis allowed the author to make comprehensive revision of the phylogeny of Ephemeroptera, find homology in the structure of the maxillae of adult Aphaniptera, larval maxillae of Neuropteroidea-Birostrata, larval legs of Nannomecoptera, and genital parts of some insect taxa. It also allows understanding the nature and evolution of insect metamorphosis. It is generally believed that initially insect ontogenesis proceeds the shortest way, while such phenomena as substitutions of organs by means of their atrophy and subsequent restoring are secondary. In contrast to this, the flagellum of the antenna of Amyocerata initially grows by addition of proximal segments and simultaneous loss of distal ones; in many taxa, including Metabola (insects with complete metamorphosis), distal segments are not aborted. Some authors tried to explain the origin of complete metamorphosis by various reasons: ecological, morphogenetic, or others. This approach is wrong because all insects with complete metamorphosis constitute the holophyletic taxon Metabola Burmeister 1832, i.e., originate from a single ancestral species which acquired this kind of metamorphosis. If complete metamorphosis could appear in response to some factor, it would appear many times in different species, in which case the taxon Metabola, characterized by complete metamorphosis, would be polyphyletic. The holophyly of Metabola is well proven by cladoendesis but cannot be revealed by any matrix method. Based on understanding of these facts, the author was able to discover the specific features which appeared in the common ancestor of Metabola and then became conserved and were inherited by all its descendants. These features include loss of the scape in the larval antenna (leading to a peculiar transformation of antennae in metamorphosis) and a peculiar mode of leg transformation during molt from larva to pupa. During the larval/pupal molt, the leg loses musculature and gets an immobile knee bend, so that the pupa is unable to use its legs. This conserved feature determines the inactive mode of life of the pupae of most insects. It is usually believed that male coccids (Gallinsecta De Geer 1776) have metamorphosis similar to the complete metamorphosis of Metabola. But the phylogenetic position of Gallinsecta and Metabola, as revealed by cladoendesis, does not allow one to assume common modifications in their metamorphoses. Now, when concrete autapomorphies of Metabola have been found, it becomes possible to compare metamorphoses of Metabola and Gallinsecta. Examination of Orthezia urticae and some other coccids shows that metamorphosis of their males shares no modifications with the true complete metamorphosis. The inactive mode of life of their nymphs is not connected with any anatomical reason, but purely with the fact that in the course of transformation from the feeding wingless larva to the non-feeding winged adult, the nymph has already lost its mouth apparatus and has not yet got functional wings, so it does not need to move. At the same time, actively feeding stages of Gallinsecta, both males and females, have an unusual mode of molt transformation of the legs and antennae; this is a unique autapomorphy of Gallinsecta not found in any other insect taxon.  相似文献   

4.
Summary Sex is one of the most creative of the major transitions in Evolution and its existence allows faster and wider mobility of species in the ‘History of Life’. We postulate that sex evolved from prokaryotes in the tail of the fitness distribution curve for a given environment. Once sex was established we have the potential for the evolution of species and the rich flowering of organisms in a relatively short period of time.  相似文献   

5.
Genetic capacitors moderate expression of heritable variation and provide a novel mechanism for rapid evolution. The prototypic genetic capacitor, Hsp90, interfaces stress responses, developmental networks, trait thresholds and expression of wide-ranging morphological changes in Drosophila and other organisms. The Hsp90 capacitor hypothesis, that stress-sensitive storage and release of genetic variation through Hsp90 facilitates adaptive evolution in unpredictable environments, has been challenged by the belief that Hsp90-buffered variation is unconditionally deleterious. Here we review recent results supporting the Hsp90 capacitor hypothesis, highlighting the heritability, selectability, and potential evolvability of Hsp90-buffered traits. Despite a surprising bias toward morphological novelty and typically invariable quantitative traits, Hsp90-buffered changes are remarkably modular, and can be selected to high frequency independent of the expected negative side-effects or obvious correlated changes in other, unselected traits. Recent dissection of cryptic signal transduction variation involved in one Hsp90-buffered trait reveals potentially dozens of normally silent polymorphisms embedded in cell cycle, differentiation and growth control networks. Reduced function of Hsp90 substrates during environmental stress would destabilize robust developmental processes, relieve developmental constraints and plausibly enables genetic network remodeling by abundant cryptic alleles. We speculate that morphological transitions controlled by Hsp90 may fuel the incredible evolutionary lability of metazoan life-cycles.  相似文献   

6.
7.
Signaling networks: the origins of cellular multitasking   总被引:6,自引:0,他引:6  
Jordan JD  Landau EM  Iyengar R 《Cell》2000,103(2):193-200
  相似文献   

8.
The kinetics of signaling endosome retrograde transport along axons is analyzed and offered as evidence that such transport is more efficient than diffusion or calcium wave-based signaling systems over even relatively small distances. Evidence is provided to support the signaling endosome hypothesis and to expand the hypothesis to include signaling in many cell types and many cellular dimensions. Finally, a saltatory, regenerating inositol 1,4,5-trisphosphate wave model is offered to reconcile current discrepancies in the literature regarding endosomal-based retrograde signaling.  相似文献   

9.
 Gene conversion is often invoked to explain the evolution of sequence patterns observed in major histocompatibility complex (MHC) genes and their alleles. This is the gene conversion hypothesis of MHC sequence evolution. These observations and their interpretation probably belong in a larger theoretical framework, namely the evolution of systems of resistance to rapidly evolving pathogens. This review looks critically at the evidence in favor of the gene conversion hypothesis in this context. We conclude that the case for the existence of an adaptive mechanism in the MHC favoring gene conversion mutations is not proven.  相似文献   

10.
A working hypothesis to explain the polyploid evolution of Acipenserids as a result of reticular speciation is presented. It is assumed that the process of polyploidization is a result of multiple origin of poly-chromosomal sturgeon species and different DNA content despite similar karyotypes and low rate of mtDNA evolution.  相似文献   

11.
Natural products, many of which are decorated with essential sugar residues, continue to serve as a key platform for drug development. Adding or changing sugars attached to such natural products can improve the parent compound's pharmacological properties, specificity at multiple levels, and/or even the molecular mechanism of action. Though some natural-product glycosyltransferases (GTs) are sufficiently promiscuous for use in altering these glycosylation patterns, the stringent specificity of others remains a limiting factor in natural-product diversification and highlights a need for general GT engineering and evolution platforms. Herein we report the use of a simple high-throughput screen based on a fluorescent surrogate acceptor substrate to expand the promiscuity of a natural-product GT via directed evolution. Cumulatively, this study presents variant GTs for the glycorandomization of a range of therapeutically important acceptors, including aminocoumarins, flavonoids and macrolides, and a potential template for engineering other natural-product GTs.  相似文献   

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Recent meta-analyses have provided a comprehensive overview of studies investigating Toxoplasma gondii antibodies in schizophrenic patients, thus attempting to clarify the potential role these infections might play in causing schizophrenia. Issues for further research have been suggested. Associations and theories that may enrich the current level of knowledge with regard to this significant subject deserve attention. Anti-parasitic agents as well as antipsychotics are effective in treating parasitosis. Both classes of drugs have been shown to exert dopaminergic activity. Parasites and human organisms have a long history of mutual contact. The effect of parasitosis on the host and the host's response to infection are undoubtedly the product of a long evolutionary process. The neurochemical background of delusions of parasitosis is potentially similar to ancient evolutionary traces of altered neurotransmission and neuropeptide gene expression caused by parasites; these include fungal and viral infections. This is very unique in medicine if a class of drugs is effective in the treatment of an illness but also cures the delusion of the same disorder as well. Furthermore, metabolic disturbances such as hyperglycemia and insulin resistance were reported several decades before the antipsychotic era. Toxoplasmosis may also be linked to insulin resistance. Schizophrenia research can benefit from understanding this evolutionary link. New chemical entities that are liable to alter neurochemical changes related to the brain's perception of the risk of predation secondary to parasites may result in new approaches for the treatment of psychosis. These findings suggest that further research is needed to clarify this evolutionary link between parasite infection and delusions of parasitosis. We believe this model may well open up new avenues of research in the discovery of drugs to counteract schizophrenia.  相似文献   

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16.
The three living monophyletic divisions of Class Mammalia are the Prototheria (monotremes), Metatheria (marsupials), and Eutheria (`placental' mammals). Determining the sister relationships among these three groups is the most fundamental question in mammalian evolution. Phylogenetic comparison of these mammals by either anatomy or mitochondrial DNA has resulted in two conflicting hypotheses, Theria and Marsupionta, and has fueled a ``genes versus morphology' controversy. We have cloned and analyzed a large nuclear gene, the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R), from representatives of all three mammalian groups, including platypus, echidna, opossum, wallaby, hedgehog, mouse, rat, rabbit, cow, pig, bat, tree shrew, colugo, ringtail lemur, and human. Statistical analysis of this nuclear gene unambiguously supports the morphology-based Theria hypothesis that excludes monotremes from a clade of marsupials and eutherians. The M6P/IGF2R was also able to resolve the finer structure of the eutherian mammalian family tree. In particular, our analyses support sister group relationships between lagomorphs and rodents, and between the primates and Dermoptera. Statistical support for the grouping of the hedgehog with Feruungulata and Chiroptera was also strong. Received: 8 December 2000 / Accepted: 01 February 2001  相似文献   

17.
The evolution of viviparity increases the potential for genomic conflicts between mothers and offspring over the level of maternal investment. The viviparity-driven-conflict hypothesis predicts that such conflicts will drive the evolution of asymmetrical reproductive isolation between populations with divergent mating systems. We tested this hypothesis using crosses between populations of a poeciliid fish that differ in their level of polyandry. Our results support the prediction of an asymmetry in the rate of spontaneous abortion in reciprocal crosses, with the highest rate occurring in crosses between females from a relatively monandrous population and males from a relatively polyandrous population. The patterns of offspring size were not consistent with the pattern predicted by the viviparity-driven-conflict hypothesis: crosses between a monandrous female and a polyandrous male did not produce larger offspring than the reciprocal cross. This discrepancy was due to the presence of an effect of the maternal population on offspring size: polyandrous females produced larger offspring than monandrous females. In addition, offspring size was positively correlated with maternal size in crosses involving a polyandrous male. We discuss these results in light of models for intra- and intergenomic epistasis and the rapid origin of asymmetric reproductive isolation in viviparous taxa.  相似文献   

18.
Mycobacteriophages are viruses that infect mycobacterial hosts such as Mycobacterium smegmatis and Mycobacterium tuberculosis. All mycobacteriophages characterized to date are dsDNA tailed phages, and have either siphoviral or myoviral morphotypes. However, their genetic diversity is considerable, and although sixty-two genomes have been sequenced and comparatively analyzed, these likely represent only a small portion of the diversity of the mycobacteriophage population at large. Here we report the isolation, sequencing and comparative genomic analysis of 18 new mycobacteriophages isolated from geographically distinct locations within the United States. Although no clear correlation between location and genome type can be discerned, these genomes expand our knowledge of mycobacteriophage diversity and enhance our understanding of the roles of mobile elements in viral evolution. Expansion of the number of mycobacteriophages grouped within Cluster A provides insights into the basis of immune specificity in these temperate phages, and we also describe a novel example of apparent immunity theft. The isolation and genomic analysis of bacteriophages by freshman college students provides an example of an authentic research experience for novice scientists.  相似文献   

19.
In the first part of the article, an account of moral judgment in terms of emotional dispositions is given. This account provides an expressivist explanation of three important features of moral demands: inescapability, authority independence and meriting. In the second part of the article, some ideas initially put forward by Christopher Boehm are developed and modified in order to provide a hypothesis about the evolution of the ability to token moral judgments. This hypothesis makes evolutionary sense of inescapability, authority independence and meriting. It does so by referring to the selection pressures generated in the Late Pleistocene by large-game hunting. If the hypothesis is correct, we can say that, in a sense, meat made us moral.  相似文献   

20.
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