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PubMed2006

Two different evolutionary origins of stem cell systems and their molecular basis.

We propose two major evolutionary origins of stem cell systems in the animal kingdom. Adult pluripotent stem cell systems are found in many invertebrates and probably evolved as components of asexual reproduction. Lineage-specific stem cell systems probably evolved later and include neural and hematopoietic stem cell types. We propose that these two types of stem cell systems evolved independently. The vasa-like genes regulate reproductive stem cells, but not lineage-specific stem cells, which may be regulated by gcm genes. Here, we review the evidence for the molecular basis for the evolutionary origin of these two different stem cell systems.

Animals↗

PubMed2001

The asexual enchytraeid worm Cognettia sphagnetorum (Oligochaeta) has increased Cu resistance in polluted soil.

We studied Cu resistance in the asexual (reproduction through fragmentation) enchytraeid worms (Cognettia sphagnetorum, Oligochaeta) originating from two sites: one uncontaminated, and another contaminated by heavy metals. Adult worms were smaller and population density was lower at the polluted site. However, adults from the contaminated site had better survival in Cu-contaminated soil, but lower survival as juveniles (fragments). As we do not know the genetic basis of Cu resistance of the worms, it may have been reached by acclimatization via induced Cu regulation. Because fragmentation is the only mode of reproduction, all phenotypic properties (including resistance) of a parental generation could be transferred to filial generations.

Adaptation, Physiological↗

PubMed1975

Cephalic mechanism for social control of fissioning in planarians: III. Central nervous system centers of facilitation and inhibition.

Previous studies have indicated that asexual reproduction (fissioning) in the planarian Dugesia dorotocephala is socially controlled through a cephalic mechanism: Isolation releases fissioning; grouping inhibits it; decapitation, at the level of the auricles, releases it even in grouped subjects. The brain is not necessary for programming the actual events of fissioning; these are orchestrated by the segmental plexus fissioning (SPF) system. Various surgical cuts were made to ablate selected portions of the central nervous system of isolated and grouped planarians in order to ascertain the inhibitory or facilitatory effects of these in the physiological mediation of such control on the SPF system. These results were synthesized into a model of this control system; the anterior lobes and optic regions of the brain inhibit the SPF system, and the anterior and caudal segmental plexuses facilitate it. These influences are partially tonic and partially contingent upon social stimulation.

Animals↗

PubMed2004

Apomixis technology development-virgin births in farmers' fields?

Apomixis is the process of asexual reproduction through seed, in the absence of meiosis and fertilization, generating clonal progeny of maternal origin. Major benefits to agriculture could result from harnessing apomixis in crop plants. Although >400 apomictic plant species are known, apomixis is rare among crop plants, and the transfer of apomixis to crop varieties by conventional breeding has been largely unsuccessful. Because apomictic and sexual pathways are closely related, de novo engineering of apomixis might be achieved in sexually reproducing crops. Early consideration of issues relating to biosafety and intellectual property (IP) management can facilitate the acceptance and deployment of apomixis technology in agriculture.

Agriculture↗

PubMed2002

Multilocus population structure of Tapesia yallundae in Washington State.

Population genetic structure of the fungal wheat pathogen Tapesia yallundae in Washington State was determined using genetically characterized amplified fragment length polymorphic (AFLP) markers and mating-type (MAT1-1 or MAT1-2). Segregation and linkage relationships among 164 AFLP markers and MAT were analysed using 59 progeny derived from an in vitro cross. Alleles at 158 AFLP loci and the mating-type locus segregated in a 1:1 ratio. Ten unlinked markers were chosen to determine genetic and genotypic diversity and to test the hypothesis of random mating and population differentiation among five subpopulations of T. yallundae representative of the geographical distribution of wheat production in eastern Washington. Among 228 isolates collected, overall gene diversity was high (h = 0.425) and a total of 91 unique multilocus genotypes (MLG) were identified, with 32 MLG occurring at least twice. The overall population genetic structure was consistent with random mating based on the segregation of mating-type, index of association (IA), parsimony tree length permutation test (PTLPT) and genotypic diversity analyses. However, clonal genotypes were found within each subpopulation and were also distributed among the five subpopulations. No significant differences in allele frequencies were found among the five subpopulations for all 10 loci based on contingency table analysis (G2) and Wier & Cockerham's population differentiation statistic theta (theta = -0.008, P = 0.722). T. yallundae appears to consist of a large homogeneous population throughout eastern Washington with both sexual and asexual reproduction contributing to the observed population genetic structure despite no report of sexual fruiting bodies of T. yallundae occurring under natural field conditions.

Ascomycota↗

PubMed2004

Genetic structure of the deep-sea coral Lophelia pertusa in the northeast Atlantic revealed by microsatellites and internal transcribed spacer sequences.

The azooxanthellate scleractinian coral Lophelia pertusa has a near-cosmopolitan distribution, with a main depth distribution between 200 and 1000 m. In the northeast Atlantic it is the main framework-building species, forming deep-sea reefs in the bathyal zone on the continental margin, offshore banks and in Scandinavian fjords. Recent studies have shown that deep-sea reefs are associated with a highly diverse fauna. Such deep-sea communities are subject to increasing impact from deep-water fisheries, against a background of poor knowledge concerning these ecosystems, including the biology and population structure of L. pertusa. To resolve the population structure and to assess the dispersal potential of this deep-sea coral, specific microsatellites markers and ribosomal internal transcribed spacer (ITS) sequences ITS1 and ITS2 were used to investigate 10 different sampling sites, distributed along the European margin and in Scandinavian fjords. Both microsatellite and gene sequence data showed that L. pertusa should not be considered as one panmictic population in the northeast Atlantic but instead forms distinct, offshore and fjord populations. Results also suggest that, if some gene flow is occurring along the continental slope, the recruitment of sexually produced larvae is likely to be strongly local. The microsatellites showed significant levels of inbreeding and revealed that the level of genetic diversity and the contribution of asexual reproduction to the maintenance of the subpopulations were highly variable from site to site. These results are of major importance in the generation of a sustainable management strategy for these diversity-rich deep-sea ecosystems.

Animals↗

PubMed2001

Multidimensional epistasis and the disadvantage of sex.

Sex is thought to facilitate accumulation of initially rare beneficial mutations by allowing simultaneous allele replacements at many loci. However, this advantage of sex depends on a restrictive assumption that the fitness of a genotype is determined by fitness potential, a single intermediate variable to which all loci contribute additively, so that new alleles can accumulate in any order. Individual-based simulations of sexual and asexual populations reveal that under generic selection, sex often retards adaptive evolution. When new alleles are beneficial only if they accumulate in a prescribed order, a sexual population may evolve two or more times slower than an asexual population because only asexual reproduction allows some overlap of successive allele replacements. Many other fitness surfaces lead to an even greater disadvantage of sex. Thus, either sex exists in spite of its impact on the rate of adaptive allele replacements, or natural fitness surfaces have rather specific properties, at least at the scale of intrapopulation genetic variability.

Adaptation, Physiological↗

PubMed1999

Mice cloned from embryonic stem cells.

Cloning allows the asexual reproduction of selected individuals such that the offspring have an essentially identical nuclear genome. Cloning by nuclear transfer thus far has been reported only with freshly isolated cells and cells from primary cultures. We previously reported a method of cloning mice from adult somatic cells after nuclear transfer by microinjection. Here, we apply this method to clone mice from widely available, established embryonic stem (ES) cell lines at late passage. With the ES cell line R1, 29% of reconstructed oocytes developed in vitro to the morula/blastocyst stage, and 8% of these embryos developed to live-born pups when transferred to surrogate mothers. We thus cloned 26 mice from R1 cells. Nuclei from the ES cell line E14 also were shown to direct development to term. We present evidence that the nuclei of ES cells at G(1)- or G(2)/M-phases are efficiently able to support full development. Our findings demonstrate that late-passage ES cells can be used to produce viable cloned mice and provide a link between the technologies of ES cells and animal cloning. It thus may be possible to clone from a single cell a large number of individuals over an extended period.

Animals↗

PubMed2006

Pennisetum squamulatum: is the predominant cytotype hexaploid or octaploid?

Apomixis is a mode of asexual reproduction where maternal clones are produced through seeds. Consequently, genetic segregation is prevented in hybrid progenies. Pennisetum squamulatum has been used to transfer apomixis into the related sexual species Pennisetum glaucum by the introgression of an apospory-specific genomic region (ASGR)-carrier chromosome. Crosses between P. glaucum and P. squamulatum or Pennisetum purpureum have been relatively easy to make even though P. squamulatum has been reported to have a different basic chromosome number than the other 2 species (9 vs. 7) and to be hexaploid (2n = 6x = 54). Our extensive examination of one accession had shown a chromosome number of 2n = 56. In order to determine if there was a variation among accessions, we counted the number of chromosomes in 5 accessions of P. squamulatum using centromeric and 18S-5.8S-26S rDNA probes as molecular cytological markers. Our results showed that P. squamulatum is most likely octaploid with a basic chromosome number of 7 (2n = 8x = 56) and may belong to the secondary gene pool of Pennisetum. Moreover, a morphologically similar ASGR-carrier chromosome that confers apomixis was observed in all accessions.

Chromosomes, Artificial, Bacterial↗

PubMed2003

Evolution of clonality and polyploidy in a weevil system.

The increased interest in asexual organisms calls for in-depth studies of asexual complexes that actively give rise to new clones. We present an extensive molecular study of the Otiorhynchus scaber (Coleoptera, Curculionidae) weevil system. Three forms have traditionally been recognized: diploid sexuals, triploid, and tetraploid parthenogens. All forms coexist in a small central area, but only the polyploid parthenogens have colonized marginal areas. Analyzing the phylogenetic relationship, based on three partial mitochondrial genes, of 95 individuals from 19 populations, we find that parthenogenesis and polyploidy have originated at least three times from different diploid lineages. We observe two major mitochondrial lineages, with over 2.5% sequence divergence between the most basal groups within them, and find that current distribution and phylogenetic relationships are weakly correlated. Quite unexpectedly, we also discover diploid clones that coexist with, and are morphologically indistinguishable from, the diploid sexual females. Our results support that these diploid clones are derived directly from the diploid sexuals. We also find that it is mainly an increase in ploidy level and not the benefits of asexual reproduction that confers to polyploid parthenogens the advantage over their diploid sexual relatives.

Animals↗

PubMedDate not supplied

The evolution of sex and recombination in a varying environment.

Theoretical models of the evolutionary advantages of sex and genetic recombination in a temporally varying environment are analyzed. The models assume a quantitative trait controlled by many additive genes and subject to nor-optimal selection. The optimal value of the trait is assumed either to move steadily in one direction or to follow an autocorrelated linear Markov process. The consequences for population mean fitness of a reduction in genetic variance, due to a shift from sexual to asexual reproduction, are examined. It is shown that a large reduction in mean fitness can result from such a shift in the case of a steadily moving optimum, under light conditions. The conditions are much more stringent with a randomly varying environment, especially if the autocorrelation is low. The conditions for spread of a rare modifier affecting the rate of genetic recombination are also examined, and the strength of selection on such a modifier is determined. Again, the case of a steadily moving optimum is more favorable than a random optimum for the evolution of increased recombination.

Biological Evolution↗

PubMed2005

Paradoxical polyembryony? Embryonic cloning in an ancient order of marine bryozoans.

Prolific polyembryony is reported in few major taxa, but its occurrence has generated theoretical debate on potential conflict between sexual and asexual reproduction. It is, therefore, important to genetically confirm a widely cited inference, based on microscopy, that polyembryony characterizes marine bryozoans of the order Cyclostomata. Microsatellite genotyping of brooded embryos and maternal colonies conclusively demonstrated polyembryony, while genetic variation among broods within colonies indicated outcrossing via water-borne sperm, in the rocky-shore species Crisia denticulata. The characteristically voluminous brood chamber of cyclostomes is judged to be an adaptation linked to larval cloning and hence an indicator of polyembryony. We speculate that although the almost universal occurrence of polyembryony among crown-group Cyclostomata is probably attributable to phylogenetic constraint, adaptive consequences are likely to be significant.

Animals↗

PubMed2006

Temporal habitat variability and the maintenance of sex in host populations of the pea aphid.

The evolutionary maintenance of sex, despite competition from asexual reproduction, has long intrigued the evolutionary biologists owing to its numerous apparent short-term costs. In aphids, winter climate is expected to determine the maintenance of sexual lineages in the high latitude zones owing to their exclusive ability to produce frost-resistant eggs. However, diverse reproductive modes may coexist at a local scale where climatic influence is counteracted by microgeographical factors. In this study, we tested the influence of local habitat characteristics on regional coexistence of reproductive modes in the pea aphid, Acyrthosiphon pisum. In the laboratory, the induction of sexual morph production of many pea aphid genotypes from the local fields of annual (pea and faba bean) and perennial (alfalfa and red clover) crops in Western France indicated that A. pisum lineages from annual crops had a significantly higher investment in sexual reproduction than A. pisum lineages from the perennial hosts. We propose that temporal habitat variability exerts a selective pressure to maintain the sexual reproduction in A. pisum. The ecological and evolutionary consequences of the association between the mode of reproduction and the host population on gene flow restriction and on ecological specialization are discussed.

Animals↗

PubMed2004

Loss of complementation and the logic of two-step meiosis.

Meiosis is usually a two-step process: two divisions preceded by a duplication. One-step meiosis, a single division without prior replication, is a more logical way to produce haploid gametes; moreover, one-step meiosis leads to higher variabilty in the progeny than two-step meiosis. Yet one-step meiosis is very rare in nature, and may not even exist at all. I suggest that this is because one-step meiosis, in contrast to two-step meiosis, can be easily invaded and replaced by asexual reproduction. I discuss why other existing peculiar forms of division leading to the production of haploid gametes, but not one-step meiosis, have the same effect as two-step meiosis.

Animals↗

PubMed1977

A light and electron microscope study of the interaction of soil bacteria with Phytophthora cinnamomi Rands.

Light- and electron-microscopic examination showed that bacteria became associated with the hyphae and asexual reproductive structures of P. cinnamomi in soil. In suppressive soils this association appears to be correlated with hyphal lysis, inhibition of zoospore production, and sporangial breakdown. One notable feature of the microbial association between P. cinnamomi and soil bacteria is the formation of extensive slime material. Many of the bacteria isolated from the fungal hyphosphere display antagonism to the growth of P. cinnamomi in vitro. The bacteria are morphologically varied and include Pseudomonas, Bacillus, and Streptomyces spp. These observations suggest that the appropriate manipulation of the antagonistic bacteria may provide a means of biological control of P. cinnamomi.

Fungi↗

PubMed2003

Genetic variation and gene flow of broadcast spawning and planula brooding coral, Goniastrea aspera (Scleractinia) in the Ryukyu Archipelago, southern Japan.

The scleractinian coral Goniastrea aspera (Verrill) undergoes both broadcast spawning and planulae brooding in the Ryukyu Archipelago of southern Japan. Genetic variation and gene flow in G. aspera were studied using allozyme electrophoresis. We tested the hypothesis that gene flow is determined by the competency period of the planulae. We also assessed the relative contributions of sexual and asexual reproduction to recruitment. For the five staining systems surveyed, G. aspera encoded five polymorphic loci and one monomorphic locus. The genotype frequencies in each population significantly differed from the expected Hardy-Weinberg equilibrium (HWE), indicating that the local populations of G. aspera are not fully panmictic. The high ratio of the observed number of genotypes to the number of individuals (0.90 +/- 0.07, mean NG:N +/- SD) and the observed to expected genotypic diversity (0.84 +/- 0.11, mean GO:GE +/- SD) suggested that each population is likely maintained by sexual reproduction. The genetic differentiation (FST) and value of average number of migrants per generation (Nem) among and within regions ranged from 0.025 to 0.104 and 2.2 to 9.6, respectively. Comparisons with other species demonstrated that larva survival rates also influence gene flow. In addition, gene flow on distant reefs by planulae originating from spawning might prevent divergence by planulae originating from brooding for short-distant dispersal among and within populations of G. aspera in the Ryukyu Archipelago.

Animals↗

PubMed2000

Effects of food concentration and availability on the incidence of cloning in planktotrophic larvae of the sea star Pisaster ochraceus.

A decade ago, cloning was first observed in the planktotrophic larvae of sea stars obtained from plankton tows. However, no controlled experimental studies have investigated what factors may regulate this remarkable phenomenon. In the present study we offer the first documentation of cloning in the planktotrophic larvae of Pisaster ochraceus from the northern Pacific coast. This species was used as a model system to investigate three factors that may influence the incidence of asexual reproduction (cloning) in planktotrophic sea star larvae. In an initial experiment, larvae were reared under nine combinations of three temperatures and three food (phytoplankton) concentrations. Larvae reared at 12-15 degrees C and fed the highest food concentrations grew larger than the other larvae and produced significantly more clones. In a second experiment, qualitatively different algal diets were fed to larvae reared under the conditions found to be optimal in the initial experiment. Up to 24% of the larvae consuming a mixed phytoplankton diet of Isochrysis galbana, Chaetocerous calcitrans, and Dunaliella tertiolecta cloned, and significantly more clones were produced by these larvae than by those fed monospecific diets. Our experiments indicate that cloning generally occurs after larvae have attained asymptotic body length and only when food is abundant and of high quality. Since larval mortality is considered to be extremely high for marine invertebrates with planktotrophic larvae, production of clones under optimal conditions of temperature and food may serve to increase larval populations when the environment is most conducive to larval growth.

Animals↗

PubMed2001

Escape hatches for the clonal offspring of serpulid polychaetes.

Serpulid polychaetes in the genera Filograna and Salmacina reproduce asexually by releasing a single bud at a time from their posterior ends into their calcareous tubes. Here I show that buds of Salmacina amphidentata gain access to the exterior of these tubes via escape hatches built into the tubes by the parent worms. Each escape hatch consists of a hole in the tube blocked by a calcareous disc that is supported in place by an organic membrane. After buds detach from their parents, the calcareous discs are dislodged, and buds begin to form their own tubes from the resulting openings. Repeated bouts of asexual reproduction result in the formation of aggregations of branched tubes. A survey of Filograna and Salmacina spp. from the Atlantic, Indian, and Pacific oceans suggests that the formation of escape hatches for clonal offspring is common to many members of these genera.

Animals↗