Molecular cytogenetic characterization of chromosome site-specific repetitive sequences in the Arctic lamprey (Lethenteron camtschaticum, Petromyzontidae)

Analyses of the Updated "Animal rDNA Loci Database" with an Emphasis on Its New Features

We report on a major update to the animal rDNA loci database, which now contains cytogenetic information for 45S and 5S rDNA loci in more than 2600 and 1000 species, respectively. The data analyses show the following: (i) A high variability in 5S and 45S loci numbers, with both showing 50-fold or higher variability. However, karyotypes with an extremely high number of loci were rare, and medians generally converged to two 5S sites and two 45S rDNA sites per diploid genome. No relationship was observed between the number of 5S and 45S loci. (ii) The position of 45S rDNA on sex chromosomes was relatively frequent in some groups, particularly in arthropods (14% of karyotypes). Furthermore, 45S rDNA was almost exclusively located in microchromosomes when these were present (in birds and reptiles). (iii) The proportion of active NORs (positively stained with silver staining methods) progressively decreased with an increasing number of 45S rDNA loci, and karyotypes with more than 12 loci showed, on average, less than 40% of active loci. In conclusion, the updated version of the database provides some new insights into the organization of rRNA genes in chromosomes. We expect that its updated content will be useful for taxonomists, comparative cytogeneticists, and evolutionary biologists.

Why Do Some Vertebrates Have Microchromosomes?

With more than 70,000 living species, vertebrates have a huge impact on the field of biology and research, including karyotype evolution. One prominent aspect of many vertebrate karyotypes is the enigmatic occurrence of tiny and often cytogenetically indistinguishable microchromosomes, which possess distinctive features compared to macrochromosomes. Why certain vertebrate species carry these microchromosomes in some lineages while others do not, and how they evolve remain open questions. New studies have shown that microchromosomes exhibit certain unique characteristics of genome structure and organization, such as high gene densities, low heterochromatin levels, and high rates of recombination. Our review focuses on recent concepts to expand current knowledge on the dynamic nature of karyotype evolution in vertebrates, raising important questions regarding the evolutionary origins and ramifications of microchromosomes. We introduce the basic karyotypic features to clarify the size, shape, and morphology of macro- and microchromosomes and report their distribution across different lineages. Finally, we characterize the mechanisms of different evolutionary forces underlying the origin and evolution of microchromosomes.

Comparative mapping of a new repetitive DNA sequence and chromosome region-specific probes unveiling rearrangements in an Amazonian frog complex

The frog species Physalaemus ephippifer exists in the Amazonian region and harbors heteromorphic Z and W chromosomes. A genetic lineage closely related to this species was recognized based on its mitochondrial DNA and RADseq-style markers, but its taxonomic status is still unclear and has been referred to as Lineage 1 of "P. cuvieri". The heteromorphic sex chromosomes found in P. ephippifer are not present in this lineage and which of its chromosome pairs is homologous to the sex chromosomes of P. ephippifer remain to be elucidated as well as the role of such a karyotypic divergence in the evolution of these frogs. Here, we described a new family of repetitive DNA and used its chromosomal sites along with the markers detected by a probe constructed from the microdissected segment of the Z chromosome of P. ephippifer to infer chromosomal homology. We also analyzed an unnamed species that is considered to be the sister group of the clade composed of Lineage 1 of "P. cuvieri" and P. ephippifer. Our results suggest that complex rearrangements involving the chromosomes that were inferred to be homeologous to the sex chromosomes of P. ephippifer have occurred during the divergence of this group of frogs.

Cell culture-based karyotyping of orectolobiform sharks for chromosome-scale genome analysis

Karyotyping, traditionally performed using cytogenetic techniques, is indispensable for validating genome assemblies whose sequence lengths can be scaled up to chromosome sizes using modern methods. Karyotype reports of chondrichthyans are scarce because of the difficulty in cell culture. Here, we focused on carpet shark species and the culture conditions for fibroblasts and lymphocytes. The utility of the cultured cells enabled the high-fidelity characterization of their karyotypes, namely 2n = 102 for the whale shark (Rhincodon typus) and zebra shark (Stegostoma fasciatum), and 2n = 106 for the brownbanded bamboo shark (Chiloscyllium punctatum) and whitespotted bamboo shark (C. plagiosum). We identified heteromorphic XX/XY sex chromosomes for the two latter species and demonstrated the first-ever fluorescence in situ hybridization of shark chromosomes prepared from cultured cells. Our protocols are applicable to diverse chondrichthyan species and will deepen the understanding of early vertebrate evolution at the molecular level.

Characterization and Evolution of Germ1, an Element that Undergoes Diminution in Lampreys (Cyclostomata: Petromyzontidae)

The sea lamprey (Petromyzon marinus) undergoes substantial genomic alterations during embryogenesis in which specific sequences are deleted from the genome of somatic cells yet retained in cells of the germ line. One element that undergoes diminution in P. marinus is Germ1, which consists of a somatically rare (SR) region and a fragment of 28S rDNA. Although the SR-region has been used as a marker for genomic alterations in lampreys, the evolutionary significance of its diminution is unknown. We examined the Germ1 element in five additional species of lamprey to better understand its evolutionary significance. Each representative species contained sequences similar enough to the Germ1 element of P. marinus to be detected via PCR and Southern hybridizations, although the SR-regions of Lampetra aepyptera and Lethenteron appendix are quite divergent from the homologous sequences of Petromyzon and three species of Ichthyomyzon. Lamprey Germ1 sequences have a number of features characteristic of the R2 retrotransposon, a mobile element that specifically targets 28S rDNA. Phylogenetic analyses of the SR-regions revealed patterns generally consistent with relationships among the species included in our study, although the 28S-fragments of each species/genus were most closely related to its own functional rDNA, suggesting that the two components of Germ1 were assembled independently in each lineage. Southern hybridizations showed evidence of genomic alterations involving Germ1 in each species. Our results suggest that Germ1 is a R2 retroelement that occurs in the genome of P. marinus and other petromyzontid lampreys, and that its diminution is incidental to the reduction in rDNA copies during embryogenesis.