Diplospory and Parthenogenesis in Sexual x Agamospermous (Apomictic ) Erigeron (Asteraceae) Hybrids

Segregation for asexual seed production was evaluated for 130 experimental F1 hybrids resulting from a cross between diploid (2n=18) sexual Erigeron strigosus and triploid (2n=27) agamospermous Erigeron annuus. Paternity of hybrids was documented using 13 RAPD markers. The distribution of F1 chromosome numbers is bimodal, centering on diploid and triploid ploidal levels but with underrepresentation of diploids. Diplosporous versus meiotic megagametophyte development was ascertained microscopically for >/=100 ovules per plant. Diplospory ranges from 0% to 100% among all progeny but is uniformly low (0%-3%) for 17 diploid hybrids. The inheritance of diplospory in Erigeron appears to be best explained by a one-locus-two-allele polysomic model with selection against gametes homozygous for diplospory. Parthenogenesis, estimated via seed counts, ranges from 0% to 60% and apparently is contingent upon diplospory, as seed production was absent or very low in predominantly meiotic hybrids. However, the absence of parthenogenesis in many highly diplosporous hybrids indicates that these two aspects of agamospermous development are not strictly associated. The segregation of both diplospory and parthenogenesis in this population will permit further genetic dissection of these traits with molecular marker-based analyses.     

动物的孤雌生殖

孤雌生殖（parthenogenesis）也叫做“单性生殖或处女生殖”。孤雌生殖的定义是：“雌性个体产生的卵,无需受精．可发育成新雌性个体。”植物也可能有类似的情形,但植物受精作用的细节多少有些不同,所以这个定义只适用于动物。     

The Pattern of Genetic Variability in Apomictic Clones of Taraxacum officinale Indicates the Alternation of Asexual and Sexual Histories of Apomicts

Dandelions (genus Taraxacum) comprise a group of sexual diploids and apomictic polyploids with a complicated reticular evolution. Apomixis (clonal reproduction through seeds) in this genus is considered to be obligate, and therefore represent a good model for studying the role of asexual reproduction in microevolutionary processes of apomictic genera. In our study, a total of 187 apomictic individuals composing a set of nine microspecies (sampled across wide geographic area in Europe) were genotyped for six microsatellite loci and for 162 amplified fragment length polymorphism (AFLP) markers. Our results indicated that significant genetic similarity existed within accessions with low numbers of genotypes. Genotypic variability was high among accessions but low within accessions. Clustering methods discriminated individuals into nine groups corresponding to their phenotypes. Furthermore, two groups of apomictic genotypes were observed, which suggests that they had different asexual histories. A matrix compatibility test suggests that most of the variability within accession groups was mutational in origin. However, the presence of recombination was also detected. The accumulation of mutations in asexual clones leads to the establishment of a network of clone mates. However, this study suggests that the clones primarily originated from the hybridisation between sexual and apomicts.     

Parthenogenesis and Polyploidy in Beetles

SYNOPSIS. Most contemporary authorities on the taxonomy of Coleopteraagree that its three-quarter million described species and subspeciesshould be placed in three suborders: Archostemata, Adephaga,and Polyphaga. Judged by the absence of males or the occurrenceof variant sex ratios, parthenogenesis occurs in all three.Those that have been studied cytologically and a tew that cansafely be surmised are discussed with reference to their probableorigin and evolutionary significance.     

Systematic and Evolutionary Implications of Parthenogenesis in the Hymenoptera

SYNOPSIS. Two types of parthenogenesis, arrhenotoky and thelytoky,exist in the Hymenoptera. Arrhenotoky, the development of malesfrom unfertilized eggs, is present in all wasps and bees. Thelytoky,the development of diploid females from unfertilized eggs, ispresent in a few species. Two types of thelytoky, apomixis andautomixis, are known. Most thelytokous Hymenoptera are automictic.No meiosis, only mitosis, occurs in apomixis. Meiosis does occurin automixis, allowing crossing-over and segregation of genes.Advantages of thelytoky are that heterotic combinations becomefixed, gene loss is reduced, and reproduction requires onlya single individual. One advantage of arrhenotoky is that geneticload in males is eliminated. Both environmental and geneticfactors contribute to sex-determination in the haplodiploidsystem of Hymenoptera. Haplodiploidy can facilitate the evolutionof social behavior. Parthenogenesis creates some taxonomic problemssince thelytokous clones do not fit the generally accepted biologicalspecies concept. Some members of bisexual populations probablyacquirethelytoky, forming their own clones, races, or species.     

苹果无融合生殖矮生砧木的组织培养

1植物名称苹果属创新无融合生殖矮生砧木资源,即扎矮山定子(Malus baccata)与平邑甜茶(M.hupehensis)杂交后代优系93-10和93-24.     

Evolution and Ecology of Parthenogenesis in Earthworms

A model for the origin of parthenogenesis in hermaphroditesis developed. If a dominant mutation causing parthenogeneticdevelopment of eggs without affecting meiotic production ofsperm arises, the parthenogens will increase in frequency tofixation. Concomitantly, there is selection for reallocationof resources from male to female-related functions in both parthenogeneticand sexual individuals. Occasional fertilization of unreducedeggs may produce polyploid clones. Both the loss of male-relatedstructures and polyploidy are common in parthenogenetic earthworms.Parthenogenesis should be favored in patchy and temporally unstablehabitats, in which -selection may be expected, because it facilitatescolonization and rapid population growth, and because selectionby the biotic component of the environment presumably is reduced.Parthenogenetic earthworms commonly occur in decaying logs,leaf litter, and the upper, organic layers of the soil, whereassexual species more often inhabit the deeper, more stable soilhorizons. Long-term persistence of clones depends on their abilityto survive and reproduce under a variety of environmental conditions.It is proposed that successful clones possess "general purpose"genotypes that allow persistence in spite of temporal changesand facilitate active dispersal through heterogeneous environmentsbetween patches of prime habitat. Two common clones of the parthenogeneticearthworm Octolasion tyrtaeum seem to possess general purposegenotypes, as they occur in a wide variety of soil and habitattypes and are geographically widespread.     

哺乳动物孤雌干细胞与孤雌生殖

未受精的孤雌胚胎衍生的孤雌胚胎干细胞(parthenogenetic embryonic stem cells,pESCs),具有与胚胎干细胞(embryonic stem cells,ESCs)相似的多向分化和自我更新能力,且具备来.源广泛、获取高效及低致瘤性等优势,因此成为近年来的研究热点.该文概述了pESCs特殊...     

Genomic Imprinting and Problem of Parthenogenesis in Mammals

Genomic imprinting belongs by its nature to problems of epigenetics, which studies hereditary changes in gene expression not related to defective sequences of DNA nucleotides. Epigenetic mechanisms of control, including genomic imprinting, are involved in many processes of normal and pathological development of humans and animals. Disturbances of genomic imprinting may lead to various consequences, such as formation of developmental anomalies and syndromes in humans, appearance of the large offspring syndrome and increased mortality upon cloning of mammals, and death of parthenogenetic embryos soon after implantation and beginning of organogenesis. The death of diploid parthenogenetic or androgenetic mammalian embryos is determined by the absence of expression of the genes of imprinted loci of the maternal or paternal genome, which leads to significant defects in development of tissues and organs. A review is provided of the studies aimed at search of possible normalization of misbalanced gene activity and modulation of genomic imprinting effects during parthenogenetic development in mammals.__________Translated from Ontogenez, Vol. 36, No. 4, 2005, pp. 300–309.Original Russian Text Copyright © 2005 by Platonov.     

Cytological Investigation of the Mechanism of Parthenogenesis in Drosophila mercatorum

Thelytokous parthenogenesis (female progeny only) in animals is believed to arise initially in unfertilized eggs produced by bisexual females via the fusion of two haploid nuclei following meiosis, to produce diploid female progeny. The transition from sexual to parthenogenetic mechanisms of reproduction requires that the egg replace the paternal contributions of a haploid genetic complement and the basal body, which is thought to be essential for centrosome formation. The transitional facultative parthenogenetic stage is usually associated with a high rate of failed or abortive development, but the molecular and mechanistic reasons for this failure remain unclear. We show that a facultatively parthenogenetic strain of Drosophila mercatorum produces a high percentage of unfertilized eggs competent to restore diploidy and form centrosomes de novo following meiosis. The female meiotic products replicate and divide by an acentrosomal mechanism in most oocytes and cytoplasmic centrosomes form in 35% of the oocytes. However, after pronuclear replication the cytoplasmic centrosomes must "capture" two haploid nuclei in order to restore diploidy. In practice, this process frequently fails due to centrosome-mediated capture events of single or more than two haploid nuclei, as well as multiple nuclear capture events in a single embryo when excess free centrosomes are not inactivated following formation of the first zygotic nucleus. Additionally, as development proceeds, many of the centrosomes that initiate syncytial development do not remain functional, possibly due to centrosome maturation defects, and later stages of syncytial development fail. The combined effect of the high error rate associated with nuclear capture and the failure of centrosome maturation during later developmental prevents successful parthenogenesis in most of the eggs that initiate development. This shows that the high rate of failed development associated with the transition from sexual to parthenogenetic reproduction is limited by the low probability of the formation of a diploid zygotic nucleus with the correct complement of centrosomes in D. mercatorum.     

昆虫孤雌生殖中中心体的组装和意义

中心体是动物细胞主要的微管组织中心(microtubule organizing center, MTOC), 负责组织纺锤体.近年来, 关于昆虫卵母细胞减数分裂后中心体形成的深入研究对阐明昆虫孤雌生殖的过程和机制以及了解孤雌生殖的进化和形成具有重要意义.综述了第一次有丝分裂纺锤体的形成、中心体的装配以及中心体对于昆虫孤雌生殖的意义, 表明昆虫孤雌生殖的普遍模式就是在没有精子提供中心粒的情况下, 卵子通过新形成的中心体进行有丝分裂, 中心体自我组装限制的解除可能是进行孤雌生殖的转折点.     

Parthenogenesis of rabbit oocytes activated by different stimuli

Oocyte activation is one of the essential elements determining the success of nuclear transfer and the subsequent development of cloned embryos both in vitro and in vivo. Experiments were conducted to optimize the protocol for oocyte activation in a regular nuclear transfer study. In vivo derived oocytes were collected at 14–15 h from New Zealand white rabbits after ovulation treatment and were activated +18 h post-ovulation treatment. Single activation agents including calcium ionophore (A23187, 5 μM, 5 min), ethanol (Eth, 7%, 7 min), and thimerosal (200 μM, 10 min) were tested. Cleavage rates were highest in the ethanol-treated group (37%) compared to other treatments (19–25%). Very low blastocyst rates (2–3%) resulted which were not significantly different among treatments (P>0.05). Combined single agent treatment (calcium stimulators) with protein kinase inhibitor, 6-DMAP were used to achieve a full oocyte activation. Both pronuclear and blastocyst formation rates were significantly higher (P<0.05) in the Eth+6-DMAP treatment group (38 and 27%) than in the other groups (16–21 and 7–9%, respectively, P<0.05). Low (0.2 mM) and high (2.5 mM) concentrations of 6-DMAP treatments with different treatment lengths (1.5 and 3.5 h) in the combined groups were also compared. Blastocyst formation and cleavage rates were greater in the high concentration with less treatment time groups (36% versus 4–20%, P<0.05). In conclusion, single activation agents, either Ca²⁺ stimulators or protein kinase inhibitors, could not fully activate mature rabbit oocytes. The best activation procedure obtained in this study was the Eth+6-DMAP combined treatment, which may be incorporated into regular nuclear transfer or cloning protocols.     

RAPD analysis reveals genetic variability among sexual and Apomictic Paspalum dilatatum poiret biotypes

Paspalum dilatatum is a valuable forage grass in the subtropics. This species consists of several sexual (tetraploid) and apomict (penta- and hexaploid) biotypes. It has been proposed that the presence of a genome of unknown origin, the X genome, is responsible for apomixis in penta- and hexaploid biotypes. Here we evaluated the utility of random amplified polymorphic DNA (RAPD) markers for discriminating sexual and apomictic P. dilatatum biotypes. DNA samples from nine accessions, including P. intermedium, P. juergensii, and P. dilatatum (ssp. flavescens, and the common and Uruguayan biotypes) were analyzed with 86 RAPD primers. Three hundred sixty-two fragments were scored and genetic similarity estimates revealed that the penta- and hexaploid biotypes were highly similar (S(D) > or = 0.913). Forty RAPDs were unique to the penta- and hexaploid biotypes. Overall RAPD markers were useful for assessing genetic variation among closely related P. dilatatum genotypes as well as generating putative X genome markers.     

Parthenogenesis in Mites and Ticks (Arachnida: Acari)

SYNOPSIS. Parthenogenesis is discussed according to type (arrhenotoky,thelytoky, deuteroLoky, artificial parthenogenesis, gynogenesis)and presence in various acarine taxa. Evidence for parthenogenesisis based on rearing and/or cytological studies. Arrhenotokyoccurs in Mesostigmata, Prostigmata and Astigmata and usuallydoes not occur sporadically throughout these taxa; when presentit operates in many closely related species as the major typeof reproduction in certain genera, subfamilies or families.Thelytoky is present in Mesostigmata, Metastigmata, Prostigmata,Astigmata and Cryptostigmata, but in no case is the major modeof reproduction in higher taxa, i.e. subfamilies, families.Its sporadic occurrence may or may not be accompanied by polyploidy.Deuterotoky occurs in one acarine group (Listrophoridae) andartificial parthenogenesis is possible in ticks (Metastigmata).Gynogenesis is present in several species of haplo-diploid Mesostigmata(some Dermanyssidae, Phytoseiidae), although it is usually consideredin connection with thelytoky. Current studies of the effectsof ploidy level on morphological variation indicate greatercharacter variability in diploid females than in haploid males.In some species of ticks thelytoky is expressed differentiallyeven on an intraspecific level. One geographic strain of a speciesmay have the ability to reproduce thelytokously, but if matedproduces males and females.     

Parthenogenesis in non-rodent species: developmental competence and differentiation plasticity

An oocyte can activate its developmental process without the intervention of the male counterpart. This form of reproduction, known as parthenogenesis, occurs spontaneously in a variety of lower organisms, but not in mammals. However, it must be noted that mammalian oocytes can be activated in vitro, mimicking the intracellular calcium wave induced by the spermatozoon at fertilization, which triggers cleavage divisions and embryonic development. The resultant parthenotes are not capable of developing to term and arrest their growth at different stages, depending on the species. It is believed that this arrest is due to genomic imprinting, which causes the repression of genes normally expressed by the paternal allele. Human parthenogenetic embryos have recently been proposed as an alternative, less controversial source of embryonic stem cell lines, based on their inherent inability to form a new individual. However many aspects related to the biology of parthenogenetic embryos and parthenogenetically derived cell lines still need to be elucidated. Limited information is available in particular on the consequences of the lack of centrioles and on the parthenote's ability to assemble a new embryonic centrosome in the absence of the sperm centriole. Indeed, in lower species, successful parthenogenesis largely depends upon the oocyte's ability to regenerate complete and functional centrosomes in the absence of the material supplied by a male gamete, while the control of this event appears to be less stringent in mammalian cells. In an attempt to better elucidate some of these aspects, parthenogenetic cell lines, recently derived in our laboratory, have been characterized for their pluripotency. In vitro and in vivo differentiation plasticity have been assessed, demonstrating the ability of these cells to differentiate into cell types derived from the three germ layers. These results confirmed common features between uni- and bi-parental embryonic stem cells. However data obtained with parthenogenetic cells indicate the presence of an intrinsic deregulation of the mechanisms controlling proliferation vs. differentiation and suggest their uni-parental origin as a possible cause.     

Effects of the Environment on the Symptom Pattern of Nickel Toxicity in the Oat Plant

Under conditions of nickel toxicity in oats (Avena byzantina),a predisposing condition for the development of chlorosis isinduced in areas of the leaf before emergence from the coleoptileor the enclosing leaf sheath. These areas give rise to chloroticbands which develop on the emerged leaf a little over 24 h afteremergence of the tissue. Alternating light and dark are essentialfor the development of chlorotic bands. The evidence indicatesthat the potentially green tissue is that which is developingunder the coleoptile during the day and emerges later in theday or early in the night, while the potentially chlorotic tissueis that which develops under the coleoptile during the nightand emerges from the top of the coleoptile later in the nightor during the following daylight hours before midday. Plants containing high levels of nickel contain higher levelsof protochlorophyll and lower levels of of chlorophyll thancontrol plants. The visual symptom of nickel toxicity is influenced by the lengthof the light and dark periods. The nature of these effects isdiscussed. Avena byzantina, oat, nickel toxicity symptoms, chlorosis