Clara Goday

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. BioEssays 23:242–250, 2001.

Unusual kinetochores and chromatin diminution in Parascaris

The first description of the cytogenetics of the horse parasitic nematode Ascaris megalocephala (Parascaris equorum) represented an important step in the history of genetics. The chromosomes of this organism possess a particular centromeric organization and undergo a process known as chromatin diminution in all presomatic cells during early development. Both these unusual features make Parascaris a good model to study general aspects of chromosome biology.

Centromere organization in meiotic chromosomes of Parascaris univalens

The chromosomes of Parascaris univalens possess a continuous centromeric structure spanning their entire length in gonial cells. A cytological and ultrastructural analysis of P. univalens meiotic chromosomes was performed. The results show that during meiosis the holocentric germline chromosomes of male P. univalens undergo restriction of kinetic activity to the heterochromatic terminal regions. These regions lack kinetochore structures and interact directly with spindle microtubules.

Cytological characterization of sex chromosomes and ribosomal DNA location in Anastrepha species (Diptera, Tephritidae).

This paper reports a comparative analysis of heterochromatin organization in the sex chromosomes of the fruit fly Anastrepha. Mitotic chromosomes of eight Anastrepha species from different taxonomic groups were stained with DAPI and chromomycin A3 fluorochromes followed by C-banding. A specific sex-chromosome banding pattern was obtained for each of the analyzed species. Fluorescence in situ hybridization (FISH) was performed to investigate the chromosomal location of rDNA loci. In all cases the rDNA sequences were found to localize exclusively to the sex chromosomes. The results further extend the chromosomal knowledge of Anastrepha and allow a precise species identification.

Cytological analysis of chromosomes in the two species Parascaris univalens and P. equorum

Chromosome organization and the phenomenon of “chromatin diminution” in the two species Parascaris univalens (2n=2) and P. equorum (2n=4) were cytologically analysed by a variety of staining techniques (Quinacrine, Hoechst 33258, Chromomycin A3 and C-banding). The results show that: (1) the chromosomes of the two species differ markedly in both the location and the type of heterochromatin they contain; (2) in both species there is a strong chromosome polymorphism which, however, ranges within a basic species-specific phenotype; (3) the heterochromatin can be eliminated in presomatic cells during early embryogenesis at two different stages and in both cases the consequence of this process is the generation of somatic cells with a 2n=60 karyotype. Moreover, evidence suggesting the sterility of hybrids between the two species is provided.

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.

Chromosome Organization and Heterochromatin Elimination in Parascaris

A cytological analysis by modern banding techniques of gonial metaphases in two Parascaris forms that have been considered varieties but now seem to be two species [P. univalens (karyotype 2n = 2) and P. equorum (karyotype 2n = 4)] reveals a different chromosome organization in each. Parascaris univalens chromosomes contain only terminal heterochromatin, while P. equorum chromosomes also contain intercalary heterochromatin. In the somatic cells of both species during early embryogenesis, chromatin diminution occurs in and consists of the elimination of all heterochromatin independent of its localization in the chromosomes.

Banding patterns of the chromosomes of man and the chimpanzee

SummaryA comparison of the banding patterns of the chromosomes of man (Homo sapiens) and the chimpanzee (Pan troglodytes) using G (Giemsa) and Q (quinacrine) banding techniques shows that the differences between the karyotypes of the two species are the result of a few simple structural rearrangements, which include one centric fusion, five pericentric inversions, one paracentric inversion and the absence of one secondary constriction.ZusammenfassungEin Vergleich der Bandenmuster der Chromosomen des Menschen (Homo sapiens) und des Schimpansen (Pan troglodytes) bei Anwendung der G(Giemsa)- und Q(Quinacrin)-Bandentechnik zeigt, daß die Unterschiede zwischen den Karyotypen beider Species das Ergebnis einiger weniger einfacher Umstrukturierungen sind, nämlich einer zentrischen Fusion, 5 perizentrischen Inversionen, einer parazentrischen Inversion und des Fehlens einer sekundären Konstriktion.

Molecular analysis and developmental expression of the Sex-lethal gene of Sciara ocellaris (Diptera order, Nematocera suborder)

This paper reports the cloning and characterization in Sciara ocellaris of the gene homologous to Sex-lethal (Sxl) of Drosophila melanogaster. This gene plays the key role controlling sex determination and dosage compensation in the latter species. The Sciara Sxl gene produces a single transcript encoding a single protein in both males and females. This protein, found inside the nucleus, is expressed in all tissues. Both Sciara and Drosophila Sxl proteins are highly conserved at their two RNA-binding domains. In both Sciara sexes, the Sxl protein co-localizes with transcription-active regions dependent on RNA polymerase II but not on RNA polymerase I. It would appear that in Sciara, Sxl does not appear to play the key discriminative role in controlling sex determination and dosage compensation that it plays in Drosophila.

Heterochromatin and histone modifications in the germline-restricted chromosome of the zebra finch undergoing elimination during spermatogenesis

In the zebra finch (Taeniopygia guttata) a germline-restricted chromosome (GRC) is regularly present in males and females. While the GRC is euchromatic in oocytes, in spermatocytes this chromosome is cytologically seen as entirely heterochromatic and presumably inactive. At the end of male meiosis, the GRC is eliminated from the nucleus. By immunofluorescence on microspreads, we investigated HP1 proteins and histone modifications throughout male meiotic prophase, as well as in young spermatid stages after the GRC elimination. We found that in prophase spermatocytes the GRC chromatin differs from that of the regular chromosome complement. The GRC is highly enriched in HP1β and exhibits high levels of di- and tri-methylated histone H3 at lysine 9 and tri- and di-methylated histone H4 at lysine 20. The GRC does not exhibit neither detectable levels of di- and tri-methylated histone H3 at lysine 4 nor acetylated histone H4 at lysine 5 and 8. The results prove the heterochromatic organization of the GRC in male germline and strongly suggest its transcriptional inactive state during male prophase. Following elimination, in young spermatids the GRC lacks HP1β signals but maintains high levels of methylated histone H3 at lysine 9 and methylated histone H4 at lysine 20. The release of HP1 from the GRC with respect to its elimination is discussed.