Identification of sex chromosomes in Eremias velox (Lacertidae, Reptilia) using lampbrush chromosome analysis

Reptiles are good objects for studying the evolution of sex determination, since they have different sex determination systems in different lineages. Lacertid lizards have been long-known for possessing ZZ/ZW type sex chromosomes. However, due to morphological uniformity of lacertid chromosomes, the Z chromosome has been only putatively cytologically identified. We used lampbrush chromosome (LBC) analysis and FISH with a W-specific probe in Eremias velox (Pallas, 1771) to unequivocally identify the ZW bivalent and investigate its meiotic behavior. The heterochromatic W chromosome is decondensed at the lampbrush stage, indicating active transcription, contrast with the highly condensed condition of the lampbrush W chromosomes in birds. We identified the Z chromosome by its chiasmatic association with the W chromosome as chromosome XIII of the 19 chromosomes in the LBC karyotype. Our findings agree with previous genetic and genomic studies, which suggested that the lacertid Z chromosome should be one of the smaller macrochromosomes

Germline-restricted chromosome (GRC) is widespread among songbirds

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

New high copy tandem repeat in the content of the chicken W chromosome

The content of repetitive DNA in avian genomes is considerably less than in other investigated vertebrates. The first descriptions of tandem repeats were based on the results of routine biochemical and molecular biological experiments. Both satellite DNA and interspersed repetitive elements were annotated using library-based approach and de novo repeat identification in assembled genome. The development of deep-sequencing methods provides datasets of high quality without preassembly allowing one to annotate repetitive elements from unassembled part of genomes. In this work, we search the chicken assembly and annotate high copy number tandem repeats from unassembled short raw reads. Tandem repeat (GGAAA) n has been identified and found to be the second after telomeric repeat (TTAGGG)n most abundant in the chicken genome. Furthermore, (GGAAA) n repeat forms expanded arrays on the both arms of the chicken W chromosome. Our results highlight the complexity of repetitive sequences and update data about organization of sex W chromosome in chicken

Chicken rRNA Gene Cluster Structure

<div><p>Ribosomal RNA (rRNA) genes, whose activity results in nucleolus formation, constitute an extremely important part of genome. Despite the extensive exploration into avian genomes, no complete description of avian rRNA gene primary structure has been offered so far. We publish a complete chicken rRNA gene cluster sequence here, including <i>5’ETS</i> (1836 bp), <i>18S rRNA</i> gene (1823 bp), <i>ITS1</i> (2530 bp), <i>5</i>.<i>8S rRNA</i> gene (157 bp), <i>ITS2</i> (733 bp), <i>28S rRNA</i> gene (4441 bp) and <i>3’ETS</i> (343 bp). The rRNA gene cluster sequence of 11863 bp was assembled from raw reads and deposited to GenBank under KT445934 accession number. The assembly was validated through <i>in situ</i> fluorescent hybridization analysis on chicken metaphase chromosomes using computed and synthesized specific probes, as well as through the reference assembly against <i>de novo</i> assembled rRNA gene cluster sequence using sequenced fragments of BAC-clone containing chicken NOR (nucleolus organizer region). The results have confirmed the chicken rRNA gene cluster validity.</p></div

GenBank representation of <i>ITS1</i> and <i>ITS2</i> sequences.

<p>GenBank representation of <i>ITS1</i> and <i>ITS2</i> sequences.</p