Role of microtubules and microtubule organizing centers on meiotic chromosome elimination in Sciara ocellaris

A cyclical switch of gametogenic pathways in hybrids depends on the ploidy level

The cellular and molecular mechanisms governing sexual reproduction are conserved across eukaryotes. Nevertheless, hybridization can disrupt these mechanisms, leading to asexual reproduction, often accompanied by polyploidy. In this study, we investigate how ploidy level and ratio of parental genomes in hybrids affect their reproductive mode. We analyze the gametogenesis of sexual species and their diploid and triploid hybrids from the freshwater fish family Cobitidae, using newly developed cytogenetic markers. We find that diploid hybrid females possess oogonia and oocytes with original (diploid) and duplicated (tetraploid) ploidy. Diploid oocytes cannot progress beyond pachytene due to aberrant pairing. However, tetraploid oocytes, which emerge after premeiotic genome endoreplication, exhibit normal pairing and result in diploid gametes. Triploid hybrid females possess diploid, triploid, and haploid oogonia and oocytes. Triploid and haploid oocytes cannot progress beyond pachytene checkpoint due to aberrant chromosome pairing, while diploid oocytes have normal pairing in meiosis, resulting in haploid gametes. Diploid oocytes emerge after premeiotic elimination of a single-copied genome. Triploid hybrid males are sterile due to aberrant pairing and the failure of chromosomal segregation during meiotic divisions. Thus, changes in ploidy and genome dosage may lead to cyclical alteration of gametogenic pathways in hybrids.

Non-random chromosome segregation and chromosome eliminations in the fly Bradysia (Sciara)

Mendelian inheritance is based upon random segregation of homologous chromosomes during meiosis and perfect duplication and division of chromosomes in mitosis so that the entire genomic content is passed down to the daughter cells. The unusual chromosome mechanics of the fly Bradysia (previously called Sciara) presents many exceptions to the canonical processes. In male meiosis I, there is a monopolar spindle and non-random segregation such that all the paternal homologs move away from the single pole and are eliminated. In male meiosis II, there is a bipolar spindle and segregation of the sister chromatids except for the X dyad that undergoes non-disjunction. The daughter cell that is nullo-X degenerates, whereas the sperm has two copies of the X. Fertilization restores the diploid state, but there are three copies of the X chromosome, of which one or two of the paternally derived X chromosomes will be eliminated in an early cleavage division. Bradysia (Sciara) coprophila also has germ line limited L chromosomes that are eliminated from the soma. Current information and the molecular mechanisms for chromosome imprinting and eliminations, which are just beginning to be studied, will be reviewed here.

Delete and survive: strategies of programmed genetic material elimination in eukaryotes

Genome stability is a crucial feature of eukaryotic organisms because its alteration drastically affects the normal development and survival of cells and the organism as a whole. Nevertheless, some organisms can selectively eliminate part of their genomes from certain cell types during specific stages of ontogenesis. This review aims to describe the phenomenon of programmed DNA elimination, which includes chromatin diminution (together with programmed genome rearrangement or DNA rearrangements), B and sex chromosome elimination, paternal genome elimination, parasitically induced genome elimination, and genome elimination in animal and plant hybrids. During programmed DNA elimination, individual chromosomal fragments, whole chromosomes, and even entire parental genomes can be selectively removed. Programmed DNA elimination occurs independently in different organisms, ranging from ciliate protozoa to mammals. Depending on the sequences destined for exclusion, programmed DNA elimination may serve as a radical mechanism of dosage compensation and inactivation of unnecessary or dangerous genetic entities. In hybrids, genome elimination results from competition between parental genomes. Despite the different consequences of DNA elimination, all genetic material destined for elimination must be first recognised, epigenetically marked, separated, and then removed and degraded.

The unusual reproductive system of head and body lice (Pediculus humanus)

Insect reproduction is extremely variable, but the implications of alternative genetic systems are often overlooked in studies on the evolution of insecticide resistance. Both ecotypes of Pediculus humanus (Phthiraptera: Pediculidae), the human head and body lice, are human ectoparasites, the control of which is challenged by the recent spread of resistance alleles. The present study conclusively establishes for the first time that both head and body lice reproduce through paternal genome elimination (PGE), an unusual genetic system in which males transmit only their maternally derived chromosomes. Here, we investigate inheritance patterns of parental genomes using a genotyping approach across families of both ecotypes and show that heterozygous males exclusively or preferentially pass on one allele only, whereas females transmit both in a Mendelian fashion. We do however observe occasional transmission of paternal chromosomes through males, representing the first known case of PGE in which whole-genome meiotic drive is incomplete. Finally, we discuss the potential implications of this finding for the evolution of resistance and invite the development of new theoretical models of how this knowledge might contribute to increasing the success of pediculicide-based management schemes.

Histone H3 phosphorylation and non-disjunction of the maternal X chromosome during male meiosis in sciarid flies

An extremely unorthodox method of chromosome segregation is found in sciarid flies (Diptera, Sciaridae), where at male meiosis, the whole paternal complement is eliminated and the maternal X chromosome undergoes non-disjunction. At meiosis I, a monopolar spindle directs the segregation of maternal chromosomes to the single pole, whereas paternal chromosomes are discarded. At meiosis II, although maternal autosomes segregate normally, the X chromosome remains undivided. A cis-acting locus within the heterochromatin proximal to the centromere is known to regulate X centromere activity. By immunofluorescence analysis in spermatocytes from Sciara ocellaris and Sciara coprophila, we investigated histone H3 phosphorylation at Ser10, Ser28, Thr3 and Thr11 during male meiosis. We found that chromosome condensation and H3 phosphorylation patterns differ between chromosomes of different parental origin at the time of paternal set elimination. Importantly, at meiosis II, the maternal X chromosome differs from the rest of the chromosomes in that its centromeric region does not become phosphorylated at the four histone H3 sites. We provide here the first evidence linking the under-phosphorylated H3 status of the X chromosome centromeric region with its meiotic non-disjunction in sciarid flies. Our findings strongly support the idea that the deficiency in local H3 phosphorylation inactivates the X centromere at the transition from meiosis I to meiosis II.

Methylation of histone H3 at Lys4 differs between paternal and maternal chromosomes in Sciara ocellaris germline development

An outstanding example of programmed chromosome elimination and genomic imprinting is found in sciarid flies (Diptera, Sciaridae), where whole chromosomes of paternal origin are selectively discarded from the genome during development. In early germ cells a single paternal X chromosome is eliminated in embryos of both sexes and in male meiotic cells the whole paternal complement is discarded. In sciarids, differential acetylation of histones H3 and H4 occurs between chromosomes of different parental origin, both in early germ nuclei and in male meiotic cells (Goday and Ruiz, 2002). We here investigated histone methylation modifications between chromosomes in germline cells of Sciara ocellaris. In early germ nuclei, maternal chromosomes show high levels of di- and trimethylated histone H3 at Lys4, whereas this histone modification is not detected in paternal chromosomes. In male meiosis, only the eliminated paternal chromosomes exhibit high levels of di- and trimethylated histones H3 at Lys4 and dimethylated H4 at Lys20. In early germ nuclei, RNA polymerase II associates to maternally-derived chromosomes but lacks phosphorylation of the C-terminal domain on Ser2. We found that histone H3 methylation at Lys4 does not correlate with transcriptional activity in early Sciara germline nuclei. The results support the conclusion that specific covalent chromatin modifications are involved in the imprinted behaviour of germline chromosomes in Sciara.

Differential acetylation of histones H3 and H4 in paternal and maternal germline chromosomes during development of sciarid flies

A classic example of chromosome elimination and genomic imprinting is found in sciarid flies (Diptera. Sciaridae), where whole chromosomes of exclusively paternal origin are discarded from the genome at different developmental stages. Two types of chromosome elimination event occur in the germline. In embryos of both sexes, the extrusion of a single paternal X chromosome occurs in early germ nuclei and in male meiotic cells the whole paternal complement is discarded. In sciarids, early germ nuclei remain undivided for a long time and exhibit a high degree of chromatin compaction, so that chromosomes are cytologically individualized. We investigated chromatin differences between parental chromosomes in Sciara ocellaris and S. coprophila by analyzing histone acetylation modifications in early germ nuclei. We examined germ nuclei from early embryonic stages to premeiotic larval stages, male meiotic cell and early somatic nuclei following fertilization. In early germ cells, only half of the regular chromosome complement is highly acetylated for histones H4 and H3. The chromosomes that are highly acetylated are paternally derived. An exception is the paternal X chromosome that is eliminated from germ nuclei. At later stages preceding the initiation of mitotic gonial divisions, all chromosomes of the germline complement show similar high levels of histone H4/H3 acetylation. In male meiosis, maternal chromosomes are highly acetylated for histones H4 and H3, whereas the entire paternal chromosome set undergoing elimination appears under-acetylated. The results suggest that histone acetylation contributes towards specifying the imprinted behavior of germline chromosomes in sciarids.

Centrosome dynamics and inheritance in related sexual and parthenogenetic Bacillus (Insecta Phasmatodea)

In animals, some general features of centrosome dynamics and inheritance have been widely recognized. The most acknowledged model assigns to sperm the contribution of a centriole to the fertilized egg, which in turn provides the pericentriolar materials, including gamma-tubulin, recruiting them from the cytoplasm: the main zygote microtubule organizing center (MTOC) is thus reconstituted to organize first the spermaster and then the full first embryonic spindle. Obviously the model cannot apply to parthenogenetic systems, which actually rely on egg components alone. In stick insects of the Bacillus genus, the spindle of both somatic and germ cells is clearly anastral, therefore we have been investigating their centrosome in sexual and parthenogenetic taxa by analyzing its component dynamics and transmission through the use of monoclonal beta- and gamma-tubulin antibodies and transmission electron microscopy (TEM). It has been shown that in sexually reproducing species the spermatozoon does not contribute the centriole, so that the egg wholly provides the MTOC and the ensuing anastral spindle of the embryo: MTs appear to derive from pronuclear chromatin surroundings and no asters are observed. The parthenogenetic embryo development is the same as the sexual one if syngamy is excepted. The parthenogenetic mechanism realized by these panoistic insects appears to differ from that observed in the meroistic hymenopteran and drosophilid species, where the embryo spindle derives from asters formed in the egg cortex. In stick insects, the lack of sperm contribution to embryonic centrosome appears to be a major trait accounting for the widespread occurrence of facultative and obligate parthenogenesis within the order.

Meics, a novel zinc-finger protein which relocates from nuclei to the central meiotic spindle during Drosophila spermatogenesis

A Drosophila gene encoding a novel zinc-finger protein, Meics, was cloned using a monoclonal antibody. The predicted amino acid sequence contains 12 zinc-finger motifs of the C2H2-type. During spermatogenesis, Meics distributes intranuclearly at pre- and post-meiotic stages whereas it relocates to central-spindle microtubules at both meiotic divisions.

Chromosome elimination in sciarid flies

The programmed elimination of part of the genome through chromosome loss or chromatin diminution constitutes an exceptional biological process found to be present in several diverse groups of organisms. The occurrence of this phenomenon during early embryogenesis is generally correlated to somatic versus germ-line differentiation. A most outstanding example of chromosome elimination and genomic imprinting is found in sciarid flies, where whole chromosomes of exclusive parental origin are selectively eliminated at different developmental stages. Three types of tissue-specific chromosome elimination events occur in sciarids. During early cleavages, one or two X paternal chromosomes is/are discarded from somatic cells of embryos which then develop as females or males respectively. Thus, the sex of the embryo is determined by the number of eliminated paternal X chromosomes. In germ cells, instead, a single paternal X chromosome is eliminated in embryos of both sexes. In addition, while female meiosis is orthodox, male meiosis is highly unusual as the whole paternal chromosome set is discarded from spermatocytes. As a consequence, only maternally derived chromosomes are included in the functional sperm. This paper reviews current cytological and molecular knowledge on the tissue-specific cell mechanisms evolved to achieve chromosome elimination in sciarids.

Monastral bipolar spindles in meiosis II of male Trichosia pubescens (Sciaridae): early stages of spindle formation and chromosome orientation

The metaphase spindle of male meiosis II in fungus gnats (Sciaridae) is one example of naturally occurring monastral bipolar spindles. To gain further insights into how the bipolar spindle is formed in the presence of only one polar center, prometaphase of male meiosis II was investigated in the sciarid Trichosia pubescens by means of anti-tubulin immunofluorescence, DAPI chromosome staining, and electron microscopy of ultrathin serial sections. The first step in spindle formation after interkinesis seems to be the organization of an astral half-spindle, probably by MTOC activity of the astral region. With the exception of the non-disjunctional X chromosome, which always lies close to the aster, the chromosomes are found to occupy various positions with respect to the astral region, revealing different orientations of their chromatid kinetochores. It was observed that some of the mal-oriented kinetochores are associated with microtubules that, due to their orientation perpendicular to the spindle axis, are unlikely to originate from the astral region. Therefore, these mal-oriented microtubules are taken as an indication of a dispersed MTOC activity near the chromosomes or at kinetochores. According to recent models of chromosome-induced spindle self-organization [e.g., Merdes et al., 1997: J. Cell Biol. 138:953-956], they could be responsible for the formation of the other (anastral) half-spindle and for amphitelic (bipolar) orientation of the chromosomes.