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 and the turbellarian archetype

The turbellarian archetype is widely believed to have been a hermaphrodite lacking asexual reproduction, and such asexual reproduction as is now seen in the Turbellaria (as paratomy and architomy) is generally assumed to have arisen secondarily several times independently. Asexual reproduction clearly prevails among the most primitive metazoans such as the placozoans, sponges, and radiates, however, and if the Platyhelminthes is indeed an early offshoot of bilaterian evolution, as some have claimed, then it is reasonable to expect asexual reproduction to be a primitive feature of the Turbellaria. Asexual reproduction by paratomy or architomy is found in all three main evolutionary lines of the Turbellaria and is most common among primitive groups such as the Catenulida and Macrostomida. The discovery of a new, apparently primitive marine genus of Macrostomida having paratomy widens the known incidence of asexual reproduction within that order. The presence of a muscle ring around the gut of several distantly related genera of the Macrostomida and similarities this ring shows with septa in the division plane of paratomizing species are evidence that paratomy was a feature of the stem species for this order — a feature only secondarily lost in most macrostomids — and suggest that asexual reproduction is a primitive feature for the Platyhelminthes as a whole.     

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.     

藓类植物的无性繁殖及其应用

藓类植物的无性繁殖方式复杂多样,在多变、极端或不可预测的环境中具有明显优势.按形态特点,藓类植物的无性繁殖方式分为营养繁殖和特化的无性繁殖方式2种,特化的无性繁殖体包括掉落枝尖部、鞭状枝、易掉落的小枝、珠芽、假根芽胞或块茎、芽胞、脆折的叶片和内生芽胞等8种类型.本文综述了藓类植物中无性繁殖体的形态及其发生过程,介绍了苔藓植物无性繁殖在组织培养、分子生物学研究、沙漠结皮形成过程以及园林建设上的应用.     

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 reproduction and protonemal development in vitro in Fontinalis antipyretica Hedw.

As sporophytes are very rare and asexual propagules unknown it is generally assumed that the moss Fontinalis antipyretica spreads almost exclusively via detached shoots carried in water currents. This study of regeneration in vitro reveals that F. antipyretica produces a variety of filament systems originating from virtually every part of the gametophyte, including: cortical cells in the bases of detached shoots, the margins and abaxial surfaces of leaves, stems following leaf removal and the laminae of detached leaves. The filaments vary from protonemata comprising short rectangular cells with transverse cross-walls to unbranched rhizoids. Filamentous gemmae, liberated by schizolysis, and spherical brood cells are produced in ageing and desiccating cultures. In nature these asexual propagules are probably produced in response to falling water levels and may have an important and hitherto unsuspected role in the spread and spatial genetic structure of F. antipyretica. In F. antipyretica the high level of phenotypic plasticity, that characterizes the mature gametophytes of aquatic bryophytes, also extends to the filamentous phase in the life cycle. Major differences between axenic and contaminated cultures are likely due to positive associations between the moss and bacterial and/or fungal contaminants.     

Asexual reproduction in introduced and native populations of the ant Cerapachys biroi

Asexual reproduction is particularly common among introduced species, probably because it helps to overcome the negative effects associated with low population densities during colonization. The ant Cerapachys biroi has been introduced to tropical and subtropical islands around the world since the beginning of the last century. In this species, workers can reproduce via thelytokous parthenogenesis. Here, we use genetic markers to reconstruct the history of anthropogenic introductions of C. biroi, and to address the prevalence of female parthenogenesis in introduced and native populations. We show that at least four genetically distinct lineages have been introduced from continental Asia and have led to the species' circumtropical establishment. Our analyses demonstrate that asexual reproduction dominates in the introduced range and is also common in the native range. Given that C. biroi is the only dorylomorph ant that has successfully become established outside of its native range, this unusual mode of reproduction probably facilitated the species' worldwide spread. On the other hand, the rare occurrence of haploid males and at least one clear case of sexual recombination in the introduced range show that C. biroi has not lost the potential for sex. Finally, we show that thelytoky in C. biroi probably has a genetic rather than an infectious origin, and that automixis with central fusion is the most likely underlying cytological mechanism. This is in accordance with what is known for other thelytokous eusocial Hymenoptera.     

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 and vegetative growth of Bionectria ochroleuca in response to temperature and photoperiod

Growth and reproduction are two essential life‐history traits for fungi. Understanding life‐history strategies provides insight into the environmental adaption of species. Here, we investigated the colonial morphology, vegetative growth, and asexual reproduction of the ascomycete fungus Bionectria ochroleuca in response to a variety of environmental conditions. We demonstrated that the increased temperature from 15 to 25°C induced mycelial growth and conidiation in B. ochroleuca. We also found that the optimal temperatures for mycelial growth and conidial formation in this fungus species were 25 and 30°C, respectively. However, as the temperature increased from 25 to 30°C, mycelial growth was suppressed, but the total number of conidia was significantly increased. The shift in light–dark cycles dramatically changed the morphological features of the colonies and affected both vegetative growth and asexual reproduction. Under incubation environments of alternating light and dark (16:8 and 8:16 light:dark cycles), conidiophores and conidia in the colonies formed dense‐sparse rings and displayed synchronous wave structures. When the light duration was prolonged in the sequence of 0, 8, 16, and 24 hr per day, mycelial growth was suppressed, but conidiation was promoted. Together, our results indicate that temperature and light period may trigger a trade‐off between vegetative growth and asexual reproduction in B. ochroleuca.     

Asexual reproduction in Microstomum lineare (Turbellaria). I. An autoradiographic and ultrastructural study

The distribution of cells preparing for proliferation during asexual reproduction by paratomy in Microstomum lineare (Turbellaria) was investigated using the technique of exogenous tritiated thymidine ([³H]T) labelling and routine electron microscopy. The subsequent fates of labelled cells in developing and mature zooids were followed by fixing tissues at various intervals. The only labelled cells are mesenchymal and gastrodermal neoblasts, occurring along the whole worm without any special growth zones or axial gradient. Organ primordia develop as a result of locally dividing as well as of migrating cells.     

Asexual reproduction and somatic growth of the fissiparous sea star Allostichaster polyplax in New Zealand

Allostichaster polyplax is a fissiparous (capable of reproducing asexually by fission) sea star found in intertidal and shallow subtidal habitats in New Zealand and Australia. A localized population of A. polyplax occurred at a density of 2.7?individuals?m^?2 in an intertidal boulder field at Otago Harbour, South Island, New Zealand. The percentage of sea stars undergoing fission, recorded from January 2010 to March 2011, peaked in January (midsummer) in both years. The frequency of fission decreased for larger sea stars (mean arm length, R?>?25?mm), suggesting an ontogenic shift from asexual to sexual reproduction. Effects of fission (i.e. arm regeneration) and food availability (small gastropods) on growth, storage and sexual reproduction were examined in a factorial laboratory experiment. There was no effect of arm regeneration or food on total arm growth. Storage capacity, measured as pyloric caeca index (PCI), was greater in full-armed than regenerating sea stars, and greater in fed than starved individuals. Fed individuals had a similar PCI as field-collected ones. Gonads were observed in two individuals (males) in the full-armed and fed treatment. Fission did not occur during the experiment. Feeding rate on snails was greater for full-armed than regenerating sea stars. Water temperature had a weak positive effect on the feeding rate of regenerating sea stars. Our findings suggest that fission and subsequent growth of ramets can sustain demographically stable and geographically isolated populations of A. polyplax. The role of sexual reproduction in population dynamics and connectivity of this species (and fissiparous sea stars in general) remains equivocal and requires further research on genetic differentiation within and among populations.     

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.