Sexual Generation Research Articles

Remembrance of things past retrieved from the Paramecium genome

Paramecium and other ciliates are the only unicellular eukaryotes that separate germinal and somatic functions. A germline micronucleus transmits the genetic information to sexual progeny, while a somatic macronucleus expresses the genetic information during vegetative growth to determine the phenotype. At each sexual generation, a new macronucleus develops from the zygotic nucleus through programmed rearrangements of the germline genome. Paramecium tetraurelia somatic genome sequencing, reviewed here, has provided insight into the organization and evolution of the genome. A series of at least 3 whole genome duplications was detected in the Paramecium lineage and selective pressures that determine the fate of the gene duplicates analyzed. Variability in the somatic DNA was characterized and could be attributed to the genome rearrangement processes. Since, in Paramecium, alternative genome rearrangement patterns can be inherited across sexual generations by homology-dependent epigenetic mechanisms and can affect phenotype, I discuss the possibility that ciliate nuclear dimorphism buffers genetic variation hidden in the germline.

Enlightenment of yeast mitochondrial homoplasmy: diversified roles of gene conversion.

Mitochondria have their own genomic DNA. Unlike the nuclear genome, each cell contains hundreds to thousands of copies of mitochondrial DNA (mtDNA). The copies of mtDNA tend to have heterogeneous sequences, due to the high frequency of mutagenesis, but are quickly homogenized within a cell (“homoplasmy”) during vegetative cell growth or through a few sexual generations. Heteroplasmy is strongly associated with mitochondrial diseases, diabetes and aging. Recent studies revealed that the yeast cell has the machinery to homogenize mtDNA, using a common DNA processing pathway with gene conversion; i.e., both genetic events are initiated by a double-stranded break, which is processed into 3′ single-stranded tails. One of the tails is base-paired with the complementary sequence of the recipient double-stranded DNA to form a D-loop (homologous pairing), in which repair DNA synthesis is initiated to restore the sequence lost by the breakage. Gene conversion generates sequence diversity, depending on the divergence between the donor and recipient sequences, especially when it occurs among a number of copies of a DNA sequence family with some sequence variations, such as in immunoglobulin diversification in chicken. MtDNA can be regarded as a sequence family, in which the members tend to be diversified by a high frequency of spontaneous mutagenesis. Thus, it would be interesting to determine why and how double-stranded breakage and D-loop formation induce sequence homogenization in mitochondria and sequence diversification in nuclear DNA. We will review the mechanisms and roles of mtDNA homoplasmy, in contrast to nuclear gene conversion, which diversifies gene and genome sequences, to provide clues toward understanding how the common DNA processing pathway results in such divergent outcomes.

Telomerase deficiency in a colonial ascidian after prolonged asexual propagation

In organisms that propagate by agametic cloning, the parental body is the reproductive unit and fitness increases with clonal size, so that colonial metazoans, despite lack of experimental data, have been considered potentially immortal. Using asexual propagation rate as a measure of somatic performance, and telomerase activity and relative telomere length as molecular markers of senescence, old (7-12 years) asexual strains of a colonial ascidian, Diplosoma listerianum, were compared with their recent sexually produced progeny. We report for the first time evidence for long-term molecular senescence in asexual lineages of a metazoan, and that only passage between sexual generations provides total rejuvenation permitting indefinite propagation and growth. Thus, this colonial ascidian has not fully escaped ageing. The possibility of somatic replicative senescence also potentially helps to explain why metazoans, with the capacity for asexual propagation through agametic cloning, commonly undergo cycles of sexual reproduction in the wild.

Genetics or epigenetics? A peculiar case from the ciliate life

Inheritance of three mating types (MTs) in the ciliate Dileptus anser is described. When reproduced vegetatively, clones of dilepti retain their mating type invariably. In sexual generations (at crosses) the character behaves as controlled by one locus with three alleles in it, manifesting serial dominance. In other words, the character seems to be under the direct genic control. However, after treatment with Actinomycin D, ciliates from clones which stably express this or that MT, become destabilized and start to express in turn all three MTs. It suggests that actually MT of such a clone results from stable epigenetic differentiation of a complex, multipotential locus.

Discovery of a New Plagiotrochus Species (Hymenoptera: Cynipidae) Inducing Galls on the Evergreen Oak in Japan

Abstract Plagiotrochus masudai Ide, Wachi & Abe sp. nov. (Hymenoptera: Cynipidae), a gall inducer on Quercus (Cyclohalanopsis) glauca Thunberg in Japan, is described. This is the first description of an oak gall wasp species (tribe Cynipini) inducing galls on Cyclohalanopsis. The unique life cycle of P. masudai takes 3 yr to complete. The adult wasps of the sexual generation emerge from bud galls on Q. glauca in spring of the first year, and those of the asexual generation emerge from twig galls in spring of the third year.

Sex Determination: A Worm Does It by Elimination

Parasitic nematode worms of the genus Strongyloides have an alternation of many asexual, all-female generations with a sexual generation composed of males and females. Males of S. papillosus have now been shown to be produced by elimination of chromosomal material that constitutes the X chromosome in its close relatives.

Taxonomic Status of the Oak Gall WaspCallirhytis hakonensis(Hymenoptera: Cynipidae), with Description of the Sexual Generation

AbstractComparisons of the external morphology of the holotype (female adult) of Callirhytis hakonensis Ashmead (Hymenoptera: Cynipidae) and the female adults reared and identified as the asexual generation of this species by H. Masuda corroborated the accuracy of his identification. Masuda previously demonstrated the heterogony in this gall wasp by rearing experiments, The holotype (the asexual generation) of this cynipid is redescribed, and female and male adults of the sexual generation are described on the basis of specimens reared by him, Because the adults of the asexual and sexual generations had the diagnostic features of the genus Andricus and not Callirhytis, this gall wasp is transferred to the genus Andricus. Judging from the descriptions of the wasps and galls, Andricus symbioticus Kovalev and Andricus attractus Kovalev are synonymized with A. hakonensis.

Life cycle of Early Cambrian microalgae from theSkiagia-plexus acritarchs

Light microscopy studies on new materials and museum collections of early Cambrian organic-walled microfossils, informally called acritarchs, provide the observations on phenetic features that permit a comparison to certain Modern microalgae and the recognition of various developmental stages in their life cycle. the microfossils derive from various depositional settings in Estonia, Australia, Greenland, Sweden, and Poland. the exceptionally preserved microfossils reveal the internal body within the vesicle, the endocyst, and the process of releasing the endocyst from the cyst. Vegetative cells, cysts, and endocysts are distinguished, and the hypothetical reconstruction of a complex life cycle with the alternation of sexual and asexual generations is proposed. Acritarchs from theSkiagia-plexus are cysts, and likely zygotes in the sexual generation, which periodically rested as “benthic plankton.” Some microfossils of theLeiosphaeridia-plexus that are inferred to be vegetative cells were planktonic and probably haplobiontic. These form-taxa may belong to a single biological species, or a few closely related species, and represent the developmental stages and alternating generations in a complex life cycle that is expressed by polymorphic, sphaero- and acanthomorphic acritarchs. the morphological resemblance and diagnostic cell wall ultrastructure with the trilaminar sheath structure known from earlier studies suggest that the early Cambrian microfossils are the ancestral representatives and/or early lineages to the Modern class Chlorophyceae and the orders Volvocales and Chlorococcales.

Comparative myoanatomy of cycliophoran life cycle stages

The metazoan phylum Cycliophora includes small cryptic epibionts that live attached to the mouthparts of clawed lobsters. The life cycle is complex, with alternating sexual and asexual generations, and involves several sessile and free-living stages. So far, the morphological and genetic characterization of cycliophorans has been unable to clarify the phylogenetic position of the phylum. In this study, we add new details on the muscular anatomy of the feeding stage, the attached Prometheus larva, the dwarf male, and the female of one of the two hitherto described species, Symbion pandora. The musculature of the feeding stage is composed of myofibers that run longitudinally in the buccal funnel (two fibers) and in the trunk (variable number of fibers). The mouth opening is lined by a myoepithelial ring musculature. A complex myoepithelial sphincter is situated proximal to the anus. In the attached Prometheus larva, three longitudinal sets of myofilaments run dorsally, laterally, and ventrally along the entire anterior-posterior body axis. The muscular architecture of the dwarf male is complex, especially close to the penis, in the posterior part of the body. An X-shaped muscle structure is found on the dorsal side, whereas on the ventral side, longitudinal muscles and a V-shaped muscle structure are present. These muscles are complemented by additional dorsoventral muscles. The mesodermal muscle fibers attach to the cuticle via the epidermis in all life cycle stages studied herein. The musculature of the female is similar to that of the Pandora larva of Symbion americanus and includes dorsoventral muscles and longitudinal muscles that run in the dorsal and ventral body region. Overall, our results reveal striking similarities in the muscular arrangement of the life cycle stages of both Symbion species.

Genetic analysis of cry1Ab gene in segregating populations of rice.

Three transgenic Bt lines (Tarom Molaii, Neda and Nemat), all containing a synthetic cry1Ab gene were crossed with conventional rice varieties. The mode of resistance to striped stem borer (Chilo suppressalis) in these lines was investigated through field studies and PCR analysis of progenies in F2 and BC1populations. The monogenic segregation of 3:1 and 1:1 were observed in segregating populations of F2 and BC1, respectively, either phenotypically under natural infestation of SSB as well as at molecular level by means of multiplexing PCR analysis using cry1Ab and RG100 primers. These results acquired from phenotypic and genotypic analyses of 1346 and 243 individual plants at field and laboratory conditions imply stable transmission of cry1Ab gene through sexual generations in indica/indica genetic backgrounds. Therefore, rapid development of new elite Bt rice cultivars with sufficient insect resistance and ideal agronomic properties can be achieved through combination of conventional breeding with marker assisted selection. Key words: Rice, cry1Ab, segregation, insect resistance.

Haploid technology allows for the efficient and rapid generation of homozygous antibody‐accumulating transgenic tobacco plants

The large-scale production of plant-derived recombinant proteins requires the breeding of lines homozygous for the transgene(s). These can be selected by progeny testing over multiple sexual generations, but a more efficient means is to fix homozygosity in a single generation using doubled haploid technology. In this study, transgenic tobacco plants, hemizygous for both of the independently inherited genes encoding the light and heavy chains of the anti-human immunodeficiency virus monoclonal antibody 2F5, were used to establish embryogenic pollen cultures. The improved protocol employed in this study guaranteed a very high regeneration efficiency, with more than 50% of the regenerants being spontaneously doubled haploids. Hence, there was no requirement to chemically induce chromosome doubling to recover sufficient entirely homozygous recombinants. As expected, approximately 25% of the regenerants were homozygous for both transgenes. Thus, the employment of haploid technology allowed for the efficient and rapid generation of true-breeding tobacco lines accumulating functional immunoglobulins.

Microarray analysis of vegetative phase change in maize

Vegetative phase change is the developmental transition from the juvenile phase to the adult phase in which a plant becomes competent for sexual reproduction. The gain of ability to flower is often accompanied by changes in patterns of differentiation in newly forming vegetative organs. In maize, juvenile leaves differ from adult leaves in morphology, anatomy and cell wall composition. Whereas the normal sequence of juvenile followed by adult is repeated with every sexual generation, this sequence can be altered in maize by the isolation and culture of the shoot apex from an adult phase plant: an 'adult' meristem so treated reverts to forming juvenile vegetative organs. To begin to unravel the as-yet poorly understood molecular mechanisms underlying phase change in maize, we compared gene expression in two juvenile sample types, leaf 4 and culture-derived leaves 3 or 4, with an adult sample type (leaf 9) using cDNA microarrays. All samples were leaf primordia at plastochron 6. A gene was scored as 'phase induced' if it was up- or downregulated in both juvenile sample types, compared with the adult sample type, with at least a twofold change in gene expression at a P-value of < or =0.005. Some 221 expressed sequence tags (ESTs) were upregulated in juveniles, and 28 ESTs were upregulated in adults. The largest class of juvenile-induced genes was comprised of those involved in photosynthesis, suggesting that maize plants are primed for energy production early in vegetative growth by the developmental induction of photosynthetic genes.

Both Costs and Benefits of Sex Correlate With Relative Frequency of Asexual Reproduction in Cyclically ParthenogenicDaphnia pulicariaPopulations

Sexual reproduction is generally believed to yield beneficial effects via the expansion of expressed genetic variation, which increases the efficiency of selection and the adaptive potential of a population. However, when nonadditive gene action is involved, sex can actually impede the adaptive progress of a population. If selection promotes coupling disequilibria between genes of similar effect, recombination and segregation can result in a decrease in expressed genetic variance in the offspring population. In addition, when nonadditive gene action underlies a quantitative trait, sex can produce a change in trait means in a direction opposite to that favored by selection. In this study we measured the change in genotypic trait means and genetic variances across a sexual generation in four populations of the cyclical parthenogen Daphnia pulicaria, which vary predictably in their incidence of sexual reproduction. We show that both the costs and benefits of sex, as measured by changes in means and variances in life-history traits, increase substantially with decreasing frequency of sex.

Analysis of sequence variability in the macronuclear DNA of Paramecium tetraurelia: A somatic view of the germline

Ciliates are the only unicellular eukaryotes known to separate germinal and somatic functions. Diploid but silent micronuclei transmit the genetic information to the next sexual generation. Polyploid macronuclei express the genetic information from a streamlined version of the genome but are replaced at each sexual generation. The macronuclear genome of Paramecium tetraurelia was recently sequenced by a shotgun approach, providing access to the gene repertoire. The 72-Mb assembly represents a consensus sequence for the somatic DNA, which is produced after sexual events by reproducible rearrangements of the zygotic genome involving elimination of repeated sequences, precise excision of unique-copy internal eliminated sequences (IES), and amplification of the cellular genes to high copy number. We report use of the shotgun sequencing data (>10(6) reads representing 13 x coverage of a completely homozygous clone) to evaluate variability in the somatic DNA produced by these developmental genome rearrangements. Although DNA amplification appears uniform, both of the DNA elimination processes produce sequence heterogeneity. The variability that arises from IES excision allowed identification of hundreds of putative new IESs, compared to 42 that were previously known, and revealed cases of erroneous excision of segments of coding sequences. We demonstrate that IESs in coding regions are under selective pressure to introduce premature termination of translation in case of excision failure.

Evidence for widespread cryptic sexual generations in apparently purely asexual Andricus gallwasps

Oak gallwasps (Hymenoptera, Cynipidae, Cynipini) are one of seven major animal taxa that commonly reproduce by cyclical parthenogenesis (CP). A major question in research on CP taxa is the frequency with which lineages lose their sexual generations, and diversify as purely asexual radiations. Most oak gallwasp species are only known from an asexual generation, and secondary loss of sex has been conclusively demonstrated in several species, particularly members of the holarctic genus Andricus. This raises the possibility of widespread secondary loss of sex in the Cynipini, and of diversification within purely parthenogenetic lineages. We use two approaches based on analyses of allele frequency data to test for cryptic sexual generations in eight apparently asexual European species distributed through a major western palaearctic lineage of the gallwasp genus Andricus. All species showing adequate levels of polymorphism (7/8) showed signatures of sex compatible with cyclical parthenogenesis. We also use DNA sequence data to test the hypothesis that ignorance of these sexual generations (despite extensive study on this group) results from failure to discriminate among known but morphologically indistinguishable sexual generations. This hypothesis is supported: 35 sequences attributed by leading cynipid taxonomists to a single sexual adult morphospecies, Andricus burgundus, were found to represent the sexual generations of at least six Andricus species. We confirm cryptic sexual generations in a total of 11 Andricus species, suggesting that secondary loss of sex is rare in Andricus.

Transgene-promoted epigenetic switches of chalcone synthase activity in petunia plants.

Epigenetic variation affecting pigment pattern formation in petunia flowers due to the insertion of transgenes encoding chalcone synthase is described. The loss of pigment formation in petals or parts of petals is due to the post-transcriptional degradation of chalcone synthase RNA, from both the endogenous petunia chalcone synthase genes and from the chalcone synthase transgenes. The RNA cleavage pathway and its control are described. Different epigenetic states of RNA breakdown are correlated with specific cytosine methylation changes in the coding sequences of the genes. The probability, extent and developmental location of chalcone synthase RNA breakdown are related to the number and organization of transgenes in the genome but epigenetic switches that affect RNA turnover probably occur in meristems and between sexual generations. Hypotheses to explain how the transgenes influence the levels of chalcone synthase RNA breakdown and how different epigenetic states are created are discussed.

Plant remains from the Kreftenheye Formation (Eemian) at Raalte, The Netherlands

The Kreftenheye Formation is a fluviatile deposit occurring in the central part of the Netherlands. The present study deals with the plant remains found in suction-dredged bulk samples from its largely Eemian Zutphen Member, a clayey sediment formerly known as the clay–peat bed or the Salvinia bed. It reports the discovery of a large number of galls induced by the asexual generation of the currently southeast European oak gall wasp Andricus hungaricus. The associated plant remains include seeds, fruits, conifer cones, wood, spines, leaf fragments, buds, bud scales, stem fragments, bryophytes, sporangia, oospores, palynomorphs, galls, jet and amber. The palynomorph samples indicate the Late Eemian pollen zone E5b of the Carpinus phase as the origin of the galls, but the other macrofossils may have a wider stratigraphic range, including possibly also material from (a part) of the zones E3–E5a. Several elements, such as Celtis, Chamaecyparis, Juglans, Magnolia and Pterocarya, date almost certainly from earlier interglacials. The list of plant remains is dominated by fresh water plants, riparian elements and ancient woodland taxa, which reflects the ecological range of the IJssel valley and adjacent ice-pushed ridges very well. Thermophilous elements include Brasenia schreberi, Buxus sempervirens, Cornus mas, Najas minor, Salvinia natans, Trapa natans and possibly also Andricus hungaricus. Quercus cerris, the probable host of the sexual generation of A. hungaricus was not found.

Elimination of a specialised facultative symbiont does not affect the reproductive mode of its aphid host

Abstract 1. Microorganisms that manipulate the reproduction of their hosts through diverse mechanisms including the induction of parthenogenesis are widespread among arthropods.2. The pea aphid, Acyrthosiphon pisum, shows a variation in its reproductive mode, with lineages reproducing by cyclical parthenogenesis (obligate alternation of parthenogenetic and sexual generations each year) and others by obligate parthenogenesis (continuous asexual reproduction all year round). In addition, the pea aphid harbours, along with Buchnera the primary aphid endosymbiont, several facultative symbionts whose prevalence differs among host populations.3. The possible influence of a Rickettsia facultative symbiont on the reproductive mode of its host was tested on two pea aphid clones by comparing the response of infected and uninfected individuals with the same genetic background to conditions that typically induce the production of sexual morphs.4. No significant effect of the Rickettsia infection was found on the type of reproductive morphs produced (sexual vs. asexual) or on their quantities for the two clones.5. However, the Rickettsia had a detrimental effect on the fitness of its aphid host, in apparent contradiction to the high prevalence of this symbiont in some host populations. It is suggested that this negative impact may disappear under specific environmental conditions, transforming a parasitic association into a mutualistic one.

Nuclear differentiation for mating types in the ciliate Dileptus anser: A hypothesis

We have obtained the first data on inheritance and genetic determination of mating types (MTs) in the lower ciliate Dileptus anser (=D. margaritifer). Observation of MT instability in young, just matured exconjugant clones obtained from some (not all) crosses appears to be the key finding in our study. In a clone of this kind, the states of maturity and immaturity (or adolescence) often alternate and/or one MT changes to another, sometimes repeatedly, during the period of several weeks after the clone's maturation. On occasion, all three MTs found in this species can be expressed consecutively. All cells of the culture are synchronously involved in such changes of their sexual state--no spontaneous selfing (intraclonal conjugation) was ever observed in such cultures. These observations suggest that the mat locus in dilepti is a compound integral one; it is inherited as a whole and can specify expression of any one of possible MTs (much as it occurs in Tetrahymena thermophila). Some other mechanisms, supposedly epigenetic ones among them, control what MT will be expressed in a given exconjugant clone in particular. Steady functioning of these mechanisms provides stable, unambiguous differentiation of the compound mat locus to one and only one MT and subsequent Mendelian behavior of the character over sexual generations. If, for some still unknown reasons the control of such differentiation is disturbed, MT expressed in a given exconjugant clone becomes unstable and its Mendelian behavior can be violated.

Sexual Generation and the “two-bodied” shapes of creatures in Edmund Spenser’s Faerie Queene

This article follows an aspect of Spenser’s structure of the The Faerie Queene, namely that of duality, to discuss how the notion of the body is treated. This representation gives way to doubleness, if not to say problematic expression of bipolarity, someties as both male and female figures, in the process of “sexual” regeneration or creation. The article, thus, mostly highlights the role of Britomart, the heroine of Book III, and Artegall, hero of Book V, as the one is presented in an armour, brandishing a “phallic” lance, and the other is introduced in a woman’s attire. In both of them, therefore, one finds the principle of the duality of bodily representation, or as Pico della Mirandola has it, that the male and female representation can be seen as “two powers in the same substance.”