An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches. This large, germline-restricted chromosome (GRC) is eliminated from somatic cells and spermatids and transmitted via oocytes only. Its origin, distribution among avian lineages, and function were mostly unknown so far. Using immunolocalization of key meiotic proteins, we found that GRCs of varying size and genetic content are present in all 16 songbird species investigated and absent from germline genomes of all eight examined bird species from other avian orders. Results of fluorescent in situ hybridization of microdissected GRC probes and their sequencing indicate that GRCs show little homology between songbird species and contain a variety of repetitive elements and unique sequences with paralogs in the somatic genome. Our data suggest that the GRC evolved in the common ancestor of all songbirds and underwent significant changes in the extant descendant lineages.
Over the past decade, the free-living flatworm Macrostomum lignano has been successfully used in many areas of biology, including embryology, stem cells, sexual selection, bioadhesion and aging. The increased use of this powerful laboratory model, including the establishment of genomic resources and tools, makes it essential to have a detailed description of the chromosome organization of this species, previously suggested to have a karyotype with 2n = 8 and one pair of large and three pairs of small metacentric chromosomes. We performed cytogenetic analyses for chromosomes of one commonly used inbred line of M. lignano (called DV1) and uncovered unexpected chromosome number variation in the form of aneuploidies of the largest chromosomes. These results prompted us to perform karyotypic studies in individual specimens of this and other lines of M. lignano reared under laboratory conditions, as well as in freshly field-collected specimens from different natural populations. Our analyses revealed a high frequency of aneuploids and in some cases other numerical and structural chromosome abnormalities in laboratory-reared lines of M. lignano, and some cases of aneuploidy were also found in freshly field-collected specimens. Moreover, karyological analyses were performed in specimens of three further species: Macrostomum sp. 8 (a close relative of M. lignano), M. spirale and M. hystrix. Macrostomum sp. 8 showed a karyotype that was similar to that of M. lignano, with tetrasomy for its largest chromosome being the most common karyotype, while the other two species showed a simpler karyotype that is more typical of the genus Macrostomum. These findings suggest that M. lignano and Macrostomum sp. 8 can be used as new models for studying processes of partial genome duplication in genome evolution.
Guppy X and Y chromosomes represent an early stage in sex chromosome divergence. Synapsis and recombination between X and Y chromosomes attract special attention because recombination suppression promotes their differentiation, but previous studies have given contradictory results. Linkage analysis indicated that recombination between X and Y was extremely rare (<10%) and occurred in the medial part of the Y chromosome, while cytological analysis demonstrated regular association between the distal ends of the X and Y at diakinesis. In this study, we examine pairing and recombination between X and Y chromosomes using immunolocalization of MLH1 to mark recombination nodules, and genomic in situ hybridization with a male DNA probe to identify the Y-specific heterochromatic region. Pairing between X and Y is initiated distally. Single crossovers were detected in 87% of XY synaptonemal complexes, most often in the distal region and less frequently in a median position indicating that end-to-end associations between X and Y are chiasmatic. Thus, we suggest that the very low frequency of recombination detected by linkage analysis in a previous study resulted from a lack of informative markers in distal regions.
GmbH, Germany). The location of each captured immunostained spread was recorded so that it could be relocated on the slide after FISH. Electron microscopy was carried out using a JEM-1400 electron microscope (JEOL, Tokyo, Japan) at 80 kV. All microscopy studies were carried out at the Center for Microscopic Analysis of Biological Objects of SD RAS (Novosibirsk, Russia). Corel PaintShop Pro X6 (Corel) was used for a correction of image brightness and contrast. Chromosome measurements and generation of recombination maps of GRCs. Centromeres were identified by ACA foci. MLH1 signals were only scored if they were localized on SCs. The length of the SC was measured in micrometers and the positions of MLH1 foci in relation to the centromere were recorded using MicroMeasure 3.3 30. SCs of GRC and macrochromosomes were identified by their relative lengths and centromeric indexes. SC1 is the largest submetacentric. SC2 and SC3 are large subacrocentrics of similar sizes but different centromeric indexes. SC4 is middle size metacentric, SC5 and SC6 are subacrocentrics of the same size, which differ from each other in the centromeric indexes. On bone marrow metaphase chromosome spreads, Z and W are identified as a pair of non-matching macrochromosomes: metacentric and submetacentric (correspondingly). At SC spreads, ZW is identified as macrobivalent with misaligned centromeres and/or asynapsed ends of the axial elements. GRC is identified as the only acrocentric macrobivalent or univalent. To generate recombination maps, we divided the length of the SC into equal intervals approximately equal to 1 µm and plotted the proportion of MLH1 foci located in each interval. STATISTICA 6.0 software package (StatSoft, Tulsa, OK, USA) was used for descriptive statistics. All results were expressed as mean ± SD; p < 0.05 was considered as statistically significant.
The free-living flatworm Macrostomum lignano is a model organism for evolutionary and developmental biology studies. Recently, an unusual karyotypic diversity was revealed in this species. Specifically, worms are either ‘normal’ 2n = 8, or they are aneuploid with one or two additional large chromosome(s) (i.e. 2n = 9 or 2n = 10, respectively). Aneuploid worms did not show visible behavioral or morphological abnormalities and were successful in reproduction. In this study, we generated microdissected DNA probes from chromosome 1 (further called MLI1), chromosome 2 (MLI2), and a pair of similar-sized smaller chromosomes (MLI3, MLI4). FISH using these probes revealed that MLI1 consists of contiguous regions homologous to MLI2-MLI4, suggesting that MLI1 arose due to the whole genome duplication and subsequent fusion of one full chromosome set into one large metacentric chromosome. Therefore, one presumably full haploid genome was packed into MLI1, leading to hidden tetraploidy in the M. lignano genome. The study of Macrostomum sp. 8 — a sibling species of M. lignano — revealed that it usually has one additional pair of large chromosomes (2n = 10) showing a high homology to MLI1, thus suggesting hidden hexaploidy in its genome. Possible evolutionary scenarios for the emergence of the M. lignano and Macrostomum sp. 8 genomes are discussed.
The genus Macrostomum represents a diverse group of rhabditophoran flatworms with >200 species occurring around the world. Earlier we uncovered karyotype instability linked to hidden polyploidy in both M. lignano (2n = 8) and its sibling species M. janickei (2n = 10), prompting interest in the karyotype organization of close relatives. In this study, we investigated chromosome organization in two recently described and closely related Macrostomum species, M. mirumnovem and M. cliftonensis, and explored karyotype instability in laboratory lines and cultures of M. lignano (DV1/10, 2n = 10) and M. janickei in more detail. We revealed that three of the four studied species are characterized by karyotype instability, while M. cliftonensis showed a stable 2n = 6 karyotype. Next, we performed comparative cytogenetics of these species using fluorescent in situ hybridization (FISH) with a set of DNA probes (including microdissected DNA probes generated from M. lignano chromosomes, rDNA, and telomeric DNA). To explore the chromosome organization of the unusual 2n = 9 karyotype discovered in M. mirumnovem, we then generated chromosome-specific DNA probes for all chromosomes of this species. Similar to M. lignano and M. janickei, our findings suggest that M. mirumnovem arose via whole genome duplication (WGD) followed by considerable chromosome reshuffling. We discuss possible evolutionary scenarios for the emergence and reorganization of the karyotypes of these Macrostomum species and consider their suitability as promising animal models for studying the mechanisms and regularities of karyotype and genome evolution after a recent WGD.
The free‐living flatworm Macrostomum lignano is used as a model in a range of research fields—including aging, bioadhesion, stem cells, and sexual selection—culminating in the establishment of genome assemblies and transgenics. However, the Macrostomum community has run into a roadblock following the discovery of an unusual genome organization in M. lignano, which could now impair the development of additional resources and tools. Briefly, M. lignano has undergone a whole‐genome duplication, followed by rediploidization into a 2n = 8 karyotype (distinct from the canonical 2n = 6 karyotype in the genus). Although this karyotype appears visually diploid, it is in fact a hidden tetraploid (with rarer 2n = 9 and 2n = 10 individuals being pentaploid and hexaploid, respectively). Here, we report on a phylogenetically informed search for close relatives of M. lignano, aimed at uncovering alternative Macrostomum models with the canonical karyotype and a simple genome organization. We taxonomically describe three new species: the first, Macrostomum janickei n. sp., is the closest known relative of M. lignano and shares its derived genome organization; the second, Macrostomum mirumnovem n. sp., has an even more unusual genome organization, with a highly variable karyotype based on a 2n = 9 base pattern; and the third, Macrostomum cliftonensis n. sp., does not only show the canonical 2n = 6 karyotype, but also performs well under standard laboratory culture conditions and fulfills many other requirements. M. cliftonensis is a viable candidate for replacing M. lignano as the primary Macrostomum model, being outcrossing and having an estimated haploid genome size of only 231 Mbp.
Macrostomum lignano is a free-living flatworm that is emerging as an attractive experimental animal for research on a broad range of biological questions. One feature setting it apart from other flatworms is the successful establishment of transgenesis methods, facilitated by a steady supply of eggs in the form of single-cell zygotes that can be readily manipulated. This, in combination with the transparency of the animal and its small size, creates practical advantages for imaging and fluorescence-activated cell sorting in studies related to stem cell biology and regeneration. M. lignano can regenerate most of its body parts, including the germline, thanks to the neoblasts, which represent the flatworm stem cell system. Interestingly, neoblasts seem to have a high capacity of cellular maintenance, as M. lignano can survive up to 210 Gy of γ-irradiation, and partially offset the negative consequence of ageing. As a non-self-fertilizing simultaneous hermaphrodite that reproduces in a sexual manner, M. lignano is also used to study sexual selection and other evolutionary aspects of sexual reproduction. Work over the past several years has led to the development of molecular resources and tools, including high-quality genome and transcriptome assemblies, transcriptional profiling of the germline and somatic neoblasts, gene knockdown, and in situ hybridization. The increasingly detailed characterization of this animal has also resulted in novel research questions, such as bio-adhesion based on its adhesionrelease glands and genome evolution due to its recent whole-genome duplication.
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