水螅的无性生殖

淡水水螅的无性繁殖方式是出芽生殖。水螅胃区的皮肌细胞摄取的营养物质,经细胞间传递方式,转移到芽体的内、外胚层,为芽体的发育提供了能量。胃区的干细胞与增殖的细胞不断迁移到芽体,保障芽体发生时的细胞数。水螅的头部与基盘对芽体的发生,同时存在着激活与抑制2对位置信息素,共同控制并决定了芽体在体柱上的发生位置。芽体先发生垂唇与触手芽,其发生位置必需远离母体头部抑制素的作用。芽体发育后期,发生基盘时,必须远离母体基盘抑制素的作用。     

Asexual reproduction in the suctorianDiscophrya collini

Summary Discophrya collini reproduces asexually through the formation of a ciliated swarmer by evaginative budding. This process is initiated by the repeated replication of a single subcortical kinetosome to form a kinetosome field. The epiplasm of the multilayered cortex covering this field becomes reduced in thickness and the whole cortex invaginates to produce an internal embryonic cavity. The kinetosomes become organised into rows, and each produces a cilium which projects into the cavity. On completion of the embryonic cavity its walls are extruded through the cavity opening to form an external ciliated swarmer connected to the parent by a thin bridge of cytoplasm. It is suggested that this evagination is induced by a rapid breakdown of supporting microtubules in the cavity wall and the subsequent hydrostatic pressure exerted by the body cytoplasm. The connecting bridge shows no specialised ultrastructural features and separation of swarmer from parent probably is achieved by the active movement of the swarmer. The cytoplasm of the swarmer is similar in structure to that of the adult cell but contains a number of primordia of tentacle axonemes. The infraciliature resembles that of other suctorian swarmers. On settling, the cilia of the swarmer are lost, at least some by resorption, a stalk may be secreted and the axoneme primordia are extended to form functional tentacles.     

Asexual reproduction, habitat colonization and habitat maintenance in bryophytes

Asexual reproduction s. l. is widespread in plants and also a basic reproductive mechanism in bryophytes. Today, three types of asexual reproduction are recognized: (1) the asexual reproduction s. str. by regeneration from ± specialized caducous organs (leaves, leaf apices, shoots, branches, bulbils) and by production of specialized propagules (gemmae, protonemal brood cells, tubers), (2) fragmentation of plants, resp. part of plants into ± unspecialized fragments, and (3) clonal reproduction (cloning). The latter occurs in bryophytes by protonema decay, by disintegration of modules, resp. formation of ramets (dividuals, “daughter plants”) that leads to self-cloning or forced-cloning of parts of the gametophyte (shoots, stoloniferous and rhizomatous axes, rhizoid wicks, basiscopic innovation plants). Clonal reproduction (cloning), in former time scarcely noted, gained great interest within the last decade mainly in vascular plants showing clonal growth. This reproduction mechanism is thought to be a keystone factor for asexual reproduction, habitat colonization and habitat maintenance. Species which reproduce clonally are able to colonize and maintain habitats in an effective way by the so-called “consequent vegetative multiplication”. The review presents an overview of the current state of knowledge of asexual reproduction types in bryophytes, with a focus on fragmentation and clonal reproduction (cloning), the mechanisms of habitat colonization and habitat maintenance, which all are of important significance in the dynamic processes of development of bryophyte populations.     

Asexual reproduction of planarians: Metric studies

A relationship was studied between fission and restoration of body and its individual parts under different experimental conditions in planarians of the Dugesia tigrina asexual race. The body and its fragments were studied morphomterically. After fission, the growth of planarians demonstrated topographic differences. The separated tail fragments and postpharyngeal area, in which the zone of fission is formed, were growing at the highest rate. More active growth was also noted over the long body axis. Fission and growth were more active in isolated planarians, as compared to those kept in groups.     

Asexual reproduction of planarians: metric studies

A relationship was studied between fission and restoration of body and its individual parts under different experimental conditions in planarians of the Dugesia tigrina asexual race. The body and its fragments were studied morphomterically. After fission, the growth of planarians demonstrated topographic differences. The separated tail fragments and postpharyngeal area, in which the zone of fission is formed, were growing at the highest rate. More active growth was also noted over the long body axis. Fission and growth were more active in isolated planarians, as compared to those kept in groups.     

Asexual reproduction and regeneration ofCatenula (Turbellaria,Archoophora)

Summary Studies were made onCatenula, a turbellarian of the order Catenulida, which had been cultured for 6 years in our laboratory. Fission begins inCatenula when the animal exceeds a specifically defined length. Neoblasts accumulate where the body wall narrows, near the subepithelial nerve cell. These cells have a large nucleus of condensed chromatin and a large active nucleolus. They have little cytoplasm, which in addition to free ribosomes, contains a small number of rough endoplastic reticular cisternae and a few mitochondria. Stem cells of epithelium were also found. These cells are similar to neoblasts, having additionally a bundle of centrioles in the cytoplasm.Differentiation of tissues and cells during regeneration proceeds in a manner identical to that during paratomy. After injury the neoblasts collect in two primordia of the brain, but do not form blastemae, as occurs in Tricladida. It is likely that dedifferentiation plays some role in each of the processes examined. A theoretical model of the mechanisms controlling paratomy and regeration is presented. The factors controlling these processes include the inductor formed by the subepithelial nerve cells and the inhibitor blocking it, formed by the brain. The inductor is probably a neurosecretion that combines with a competent receptor on the surface of cells capable of dedifferentiation.     

Asexual reproduction by leaf fragmentation in Mnium stellare Hedw.

Abstract

Orthotrichum incurvomarginatum, O. armatum and O. oreophilum all belonging to subgenus Phaneroporum section Leiocarpa are described from South Africa. O. incurvomarginatum replaces O. afro-fastigiatum sensu Lewinsky, whereas O. afrofastigiatum C. Müll is regarded as a dubious name. O. armatum is characterized by aristate leaves with recurved margins and emergent to shortly exserted capsules which are ribbed when dry. O. oreophilum has longly exserted smooth capsules with eight exostome teeth and eight endostome segments, both with a coarse ornamentation of papillae and vermicular ridges. O. firmum is reported new to South Africa, O. rupestre new to the kingdom of Lesotho.     

Asexual and sexual reproduction in the rotifer Brachionus plicatilis cultured at different salinities

Two strains of the rotifer Brachionus plicatilis (L-type) differing in the levels of mictic female and male production, were grown in batch cultures with the alga Nannochloropsis gaditana as food, at two low (2.5 and 10), and two high (40 and 50) salinities. While both the low (strain S-1) and the high (strain S-3) sexual reproducing strains developed similar growth cycles at 2.5 and 10, the population growth response at 40 and 50 showed that; 1) in strain S-1, mixis can be suppressed in conditions that still allow asexual reproduction, and 2) in strain S-3 mictic female and male production are possible at nearly zero asexual population growth rates. In strain S-3, a double log linear relationship between the densities of males and females was found. These results show that mixis can occur over a wide ranges of female population density, and support the hypothesis that sexual reproduction is a strain dependent component of the general reproductive response.     

Asexual reproduction,regeneration, and somatic embryogenesis in the planarian Dugesia tigrina (Turbellaria)

In reviewing recent research published in Russian on regeneration and asexual reproduction, the following morphogenetic processes in the planarian Dugesia tigrina are considered: 1) regeneration of lost parts of the body; 2) regeneration of the whole worm from fragments of the body, either by normal regeneration when the inital polarity of the fragment is retained or by somatic embryogenesis when one or more new axes of polarity arise; 3) somatic embryogenesis, or development of individuals from somatic cells; 4) hypermorphosis, or the presence of more than the usual number of organs or body parts, a process that can be interpreted in terms of somatic embryogenesis; and 5) asexual reproduction. Some morphological, biochemical, and physiological studies of the division zone in D. tigrina demonstrate peculiarities of a local breakdown of integrative functions, a breakdown which in turn causes division of the individual to take place at this zone; timing of division is controlled by the organism as an integrated whole.     

The effects of climate change on the reproduction of coastal invertebrates

Environmental cues control or synchronize the reproductive cycle of many marine invertebrates. Of these environmental cues, photoperiod and temperature have been shown to moderate reproduction either individually or in combination. In addition, they may act directly or, in the case of photoperiod, set circannual clock mechanisms. These environmental cues may affect a number of reproductive parameters, including sex determination, gametogenesis and spawning. Gonadotrophic and spawning hormones appear to act as the transducers between the environment and the gamete, and limited evidence indicates that temperature and photoperiod can alter levels of these. Such processes occur in a range of estuarine invertebrates that constitute important components of the diets of overwintering birds. Global warming is likely to uncouple and alter the phase relationship between temperature and photoperiod and this is likely to have significant consequences for animals that develop gametes during the winter and spawn in the spring in temperate northern latitudes. Species that cue reproduction to photoperiod are likely to be particularly vulnerable. Although this is unlikely to lead to extinctions, it may cause local extirpations. However, this will depend on speed of adaptation to changing climate in relation to speed of climate change and the degree of mixing between populations across the range of the species. More likely will be significant impacts on fecundity, spawning success and recruitment, and this may have significant implications for overwintering birds of national and international importance, and, ultimately, on the conservation status of estuaries such as the Humber in the UK.     

Asexual reproduction of the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

The goal of this research is to enhance our knowledge of thecontributions of doliolids to the planktonic community as consumersand secondary producers. The objectives are to quantify asexualreproduction and carbon release rates of Dolioletta gegenbauriphorozooids at four food concentrations and four temperaturesin order to determine their impact as producers throughout thewater column. Although doliolids are abundant in numerous regionsof the coastal ocean, and are considered to be major planktonicgrazers, data on rates of asexual reproduction are scarce. Laboratoryexperiments were conducted at 20°C at food concentrationsof 7, 20, 60 and 160 µg C l^-1 of Thalassiosira weissflogiiand Rhodomonas sp., and at 60 µg C l^-1 at 16.5, 20, 23.5and 26.5°C, to quantify the phorozooid release rates ofgonozooids, and the amount of carbon released. Results fromthese experiments suggest that release rates increased at 20°Cas concentration increases from 7 to 160 µg C l^-1. Releaserates remained similar as the temperature increased from 16.5to 26.5°C at a phytoplankton concentration of 60 µgC l^-1. Food concentration and temperature has an effect on phorozooidreproductive longevity, size of gonozooids released, and theamount of carbon released asexually. Doliolid reproduction ratesare a function of environmental food concentrations and temperatures,and the results imply that doliolids can be important secondaryproducers in the neritic environment.     

Asexual reproduction and molecular systematics of the sea anemone Anthopleura krebsi (Actiniaria: Actiniidae)

In this paper we use allozyme analyses to demonstrate that individuals in Anthopleura krebsi aggregates are monoclonal. Additionally, sympatric samples of the red and the green colour-morphs of A. krebsi from Pernambuco, Brazil were genetically compared and no significant differences were observed between them (gene identity = 0.992), indicating that they do not belong to different biological species. All individuals within aggregates of the green colour-morph were found to be identical over the five polymorphic loci analysed. Such results would be extremely unlikely (P < 10(-11)) if the individuals analysed had been generated through sexual reproduction, thus confirming the presence of asexual reproduction in this species.     

Asexual reproduction and genetic determination of growth form in the coral Pavona cactus: biochemical genetic and immunogenic evidence

Summary Tissue grafting and electrophoresis were used to study the genotypic structure of a population of the scleractinian coral, Pavona cactus. Three growth forms were distinguished within one continuous population of this morphologically variable species. Both techniques provided evidence of localized asexual reproduction within each growth form, a result consistent with numerous field observations of naturally occurring fragments. A perfect association between clonal genotype and growth form was found in an electrophoretic survey of 80 colonies. 23 multi-locus genotypes were detected in the 80 colonies tested. All genotypically similar colonies had the same growth form, even where colonies were separated by 50 m. Although environmental gradients undoubtedly modify colony morphology, the high correlation between genotype and growth form suggests that major differences in colony morphology are genetically determined.Tissue grafting tests did not reliably identify all clones. Fusions developed between all electrophoretically indistinguishable colonies, consistent with the initial assumption that fusion between paired colonies would indicate selfrecognition. However, there was also one fusion in 20 pairings of electrophoretically different colonies. Although there was general agreement between the two techniques, the one inconsistent fusion suggests that caution should be exercised in the application of histocompatibility tests as bioassays for clonal population structure, and that electrophoresis is the more appropriate technique for this species.The ability of genotypes to dominate in intraspecific competitive interactions and to survive fragmentation was assessed. An intraspecific dominance hierarchy was identified among the 6 clones tested. Competition was highly asymmetrical between dominant and subordinate-ranking clones. Genotypes that were most successful in producing widespread clones were found to dominate intraspecific competitive interactions and had high rates of fragment survival. Offprint requests to: D.J. Ayre Contribution No 252 from the Australian Institute of Marine Science     

Morphofunctional organization of reserve stem cells providing for asexual and sexual reproduction of invertebrates

Published and original data indicating evolutionary conservation of the morphofunctional organization of reserve stem cells providing for asexual and sexual reproduction of invertebrates are reviewed. Stem cells were studied in representatives of five animal types: archeocytes in sponge Oscarella malakhovi (Porifera), large interstitial cells in colonial hydroid Obelia longissima (Cnidaria), neoblasts in an asexual race of planarian Girardia tigrina (Platyhelmintes), stem cells in colonial rhizocephalans Peltogasterella gracilis, Polyascus polygenea, and Thylacoplethus isaevae (Arthropoda), and colonial ascidian Botryllus tuberatus (Chordata). Stem cells in animals of such diverse taxa feature the presence of germinal granules, are positive for proliferating cell nuclear antigen, demonstrate alkaline phosphatase activity (a marker of embryonic stem cells and primary germ cells in vertebrates), and rhizocephalan stem cells express the vasa-like gene (such genes are expressed in germline cells of different metazoans). The self-renewing pool of stem cells is the cellular basis of the reproductive strategy including sexual and asexual reproduction.     

Asexual reproduction in scyphistomae of Aurelia sp.: Effects of temperature and salinity in an experimental study

The growth and survival of colonies and individuals within sedentary polyp colonies of moon jellyfish (Aurelia sp.) was investigated at three temperatures and three salinities in laboratory experiments. Growth rates of colonies (number of polyps and number of buds in the colony) and individuals (number of buds per active scyphistomae) significantly increased with temperature, but were not affected by salinity. Survival was high in all treatment combinations indicating a wide tolerance to environmental conditions. However, scyphistomae at the lowest temperature had a greater percentage of larger individuals and slower population growth rate than those at warmer temperatures. These results suggest that the reproductive strategy to maximise production of Aurelia sp. is to increase the size of scyphistomae colonies by asexual budding when conditions are good (warmer temperatures and abundant food generally during spring and summer). Budding activity slows, but the size of scyphistomae increases, during the colder winter period leading up to strobilation, resulting in the production of a greater number of ephyrae. The trigger for strobilation is possibly stressful conditions. However, if trigger conditions do not occur, the colony of scyphistomae can continue to grow and survive through a broad range of conditions spanning many seasons, thus ensuring survival of the population.