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All songbirds studied so far have a germline-restricted chromosome (GRC), which is present in the germ cells and absent in the somatic cells. It shows a wide variation in size, morphology, and genetic content between the songbird species. In this paper, we analyzed GRC behavior in female and male meiosis of the great tit, using immunolocalization of meiotic proteins and FISH with GRC-derived DNA probes. We found that, despite dozens of million years of independent evolution, the great tit GRC displays a striking similarity with the GRCs of two species of martins and two species of estrildid finches examined earlier. It was usually present in two copies in females forming recombining bivalent and in one copy in males forming a condensed heterochromatic body with dotted-like axial elements of the synaptonemal complex. We observed mosaicism for the GRC copy number in the female and male great tit. This indicates that one of the GRC copies might be passively lost during premeiotic germ cell divisions. After the meiotic prophase, the GRC was ejected from most male germ cells. The reverse and interspecies FISH with GRC-specific microdissected DNA probes indicates that GRCs of the great tit, pale martin, and zebra finch differ substantially in their genetic content despite similarities in the meiotic behavior.

Abstract Background Meiotic recombination is an important source of genetic variability. Studies on mammals demonstrate a substantial interspecies variation in overall recombination rate, which is dependent mainly on chromosome (2n) and chromosome arm number (FN). Bird karyotypes are very conservative with 2n being about 78–82 and FN being 80–90 in most species. However, some families such as Apodidae (swifts) and Falconidae (falcons) show a substantial karyotypic variation. In this study, we describe the somatic and pachytene karyotypes of the male Common Swift (Apus apus) and the pachytene karyotype of the male Eurasian Hobby (Falco subbuteo) and estimate the overall number and distribution of recombination events along the chromosomes of these species. Methods The somatic karyotype was examined in bone marrow cells. Pachytene chromosome spreads were prepared from spermatocytes of adult males. Synaptonemal complexes and mature recombination nodules were visualized with antibodies to SYCP3 and MLH1 proteins correspondingly. Results The karyotype of the Common Swift consists of three metacentric, three submetacentric and two telocentric macrochromosomes and 31 telocentric microchromosomes (2n = 78; FN = 90). It differs from the karyotypes of related Apodidae species described previously. The karyotype of the Eurasian Hobby contains one metacentric and 13 telocentric macrochromosomes and one metacentric and ten telocentric microchromosomes (2n = 50; FN = 54) and is similar to that described previously in 2n, but differs for macrochromosome morphology. Despite an about 40% difference in 2n and FN, these species have almost the same number of recombination nodules per genome: 51.4 ± 4.3 in the swift and 51.1 ± 6.7 in the hobby. The distribution of the recombination nodules along the macrochromosomes was extremely polarized in the Common Swift and was rather even in the Eurasian Hobby. Conclusions This study adds two more species to the short list of birds in which the number and distribution of recombination nodules have been examined. Our data confirm that recombination rate in birds is substantially higher than that in mammals, but shows rather a low interspecies variability. Even a substantial reduction in chromosome number does not lead to any substantial decrease in recombination rate. More data from different taxa are required to draw statistically supported conclusions about the evolution of recombination in birds.

Chromosomal locations and amounts of 5-methylcytosinerich chromosome regions were detected in the karyotypes of 13 bird species by indirect immunofluorescence using a monoclonal anti-5-methylcytosine antibody. These species belong to 7 orders and 10 families of modern (Neognathae) and primitive (Palaeognathae) birds and are characterized by macro- and microchromosomes as well as ZW sex chromosomes. In all 13 species, the hypermethylated chromosome segments are confined to constitutive heterochromatin. The chromosomal locations of hypermethylated DNA regions in the karyotypes are constant and species-specific. There is no general rule with regard to the distribution of these hypermethylated chromosome regions in the genomes of birds. In most instances, hypermethylated segments are located in the centromeric regions of chromosomes, but in the sex chromosomes, these can also be found in telomeric and interstitial postitions. In most of the species studied, the centromeric heterochromatin in many, if not all, of the microchromosomes is hypermethylated. However, in one species, the only detectable hypermethylated heterochromatic regions are located in one pair of macroautosomes and in the Z sex chromosome, but none of the microchromosomes contains visible quantities of 5-methylcytosine. The analysis of 5-methylcytosine-rich chromosome regions can be very helpful for the comparative cytogenetics of closely related species or subspecies. It also reflects the dynamic evolutionary process operating in the highly repetitive DNA of eukaryotic chromosomes. [ABSTRACT FROM AUTHOR]

Studies on mammals demonstrate wide interspecific variation in the number and distribution of recombination events along chromosomes. Birds represent an interesting model group for comparative analysis of cytological and ecological drivers of recombination rate evolution. Yet, data on variation in recombination rates in birds are limited to a dozen of species. In this study, we used immunolocalization of MLH1, a mismatch repair protein marking mature recombination nodules, to estimate the overall recombination rate and distribution of crossovers along macrochromosomes in female and male meiosis of the gray goose (Anser anser). The average number of MLH1 foci was significantly higher in oocytes than in spermatocytes (73.6 ± 7.8 and 58.9 ± 7.6, respectively). MLH1 foci distribution along individual macrobivalents showed subtelomeric peaks, which were more pronounced in males. Analysis of distances between neighboring MLH1 foci on macrobivalents revealed stronger crossover interference in male meiosis. These data create a framework for future genetic and physical mapping of the gray goose. [ABSTRACT FROM AUTHOR]

Among birds, Tyrannidae comprises one of the most diverse and species-rich families. Although cytogenetic data have shown interesting results in this family, such as variations in the macrochromosome morphology and diploid number, only a few species have had their karyotypes described. In the present study, we report the characterization of the karyotype of Elaenia spectabilis ( Passeriformes, Tyrannidae) by means of classical and molecular cytogenetics. The results show that syntenic groups of Gallus gallus ( GGA) were conserved, except GGA1 and GGA4, which were divided into two different pairs each. However, the results obtained with Leucopternis albicollis probes revealed the occurrence of inversions in segments homologous to GGA1q, similar to those observed in other Passerifomes ( Turdus), and one inversion in GGA1p. These results suggest that the centric fission in GGA1, as well as the inversions observed in segments homologous to GGA1q, appeared in the early history of Passeriformes because they could be detected in Oscine and Suboscine species. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 391-398. [ABSTRACT FROM AUTHOR]

The Coscoroba ( Coscoroba coscoroba), endemic to southern South America, is traditionally considered as an early branch from the common ancestor leading to true geese and swans. Recently, an interesting association between the Coscoroba and Cape Barren goose ( Cereopsis novaehollandiae) as sister groups has been proposed. We present here the characterization of the karyotype of C. coscoroba using whole chromosome probes derived from Gallus gallus macrochromosomes. Our data showed that C. coscoroba has the highest diploid number among Anseriformes (2 n = 98), and the conservation of macrochromosome pairs 1-10 indicates that the increase in diploid number has occurred by fission events involving only the microchromosomes. Moreover, the similarity between the diploid numbers of C. coscoroba (2 n = 98) and Cereopsis novaehollandiae (2 n = 92) reinforces the phylogenetic position of these two species as sister groups, considering that other species of geese and swans have diploid numbers close to 2 n = 80. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 274-279. [ABSTRACT FROM AUTHOR]

Laser microdissection of individual mammalian chromosomes (> 2 µm) has been achieved though the use of a microscope slide coated with a polyethylene naphthalate (PEN) membrane. Although these slides have proved sufficient for larger chromosomes, they are insufficient for small chromosomes (< 1 µm). We have developed a new type of slide which allows laser microdissection of single Japanese quail microchromosomes (0.5 µm) and macrochromosomes (3–4 µm). To test the usefulness of these slides, a Japanese quail single nucleus, a macrochromosome, and a microchromosome were collected with Laser pressure catapulting, the B-actin gene was PCR amplified, and sequenced. The resulting PCR product was confirmed by nucleotide sequencing to be B-actin. These newly developed slides were shown to facilitate the laser microdissection of both Japanese quail macrochromosomes and microchromosomes. [ABSTRACT FROM AUTHOR]

All songbirds studied so far have a germline-restricted chromosome (GRC), which is present in the germ cells and absent in the somatic cells. It shows a wide variation in size, morphology, and genetic content between the songbird species. In this paper, we analyzed GRC behavior in female and male meiosis of the great tit, using immunolocalization of meiotic proteins and FISH with GRC-derived DNA probes. We found that, despite dozens of million years of independent evolution, the great tit GRC displays a striking similarity with the GRCs of two species of martins and two species of estrildid finches examined earlier. It was usually present in two copies in females forming recombining bivalent and in one copy in males forming a condensed heterochromatic body with dotted-like axial elements of the synaptonemal complex. We observed mosaicism for the GRC copy number in the female and male great tit. This indicates that one of the GRC copies might be passively lost during premeiotic germ cell divisions. After the meiotic prophase, the GRC was ejected from most male germ cells. The reverse and interspecies FISH with GRC-specific microdissected DNA probes indicates that GRCs of the great tit, pale martin, and zebra finch differ substantially in their genetic content despite similarities in the meiotic behavior., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Torgasheva, Malinovskaya, Zadesenets, Shnaider, Rubtsov and Borodin.)

Background: Meiotic recombination is an important source of genetic variability. Studies on mammals demonstrate a substantial interspecies variation in overall recombination rate, which is dependent mainly on chromosome (2n) and chromosome arm number (FN). Bird karyotypes are very conservative with 2n being about 78–82 and FN being 80–90 in most species. However, some families such as Apodidae (swifts) and Falconidae (falcons) show a substantial karyotypic variation. In this study, we describe the somatic and pachytene karyotypes of the male Common Swift (Apus apus) and the pachytene karyotype of the male Eurasian Hobby (Falco subbuteo) and estimate the overall number and distribution of recombination events along the chromosomes of these species. Methods: The somatic karyotype was examined in bone marrow cells. Pachytene chromosome spreads were prepared from spermatocytes of adult males. Synaptonemal complexes and mature recombination nodules were visualized with antibodies to SYCP3 and MLH1 proteins correspondingly. Results: The karyotype of the Common Swift consists of three metacentric, three submetacentric and two telocentric macrochromosomes and 31 telocentric microchromosomes (2n = 78; FN = 90). It differs from the karyotypes of related Apodidae species described previously. The karyotype of the Eurasian Hobby contains one metacentric and 13 telocentric macrochromosomes and one metacentric and ten telocentric microchromosomes (2n = 50; FN = 54) and is similar to that described previously in 2n, but differs for macrochromosome morphology. Despite an about 40% difference in 2n and FN, these species have almost the same number of recombination nodules per genome: 51.4 ± 4.3 in the swift and 51.1 ± 6.7 in the hobby. The distribution of the recombination nodules along the macrochromosomes was extremely polarized in the Common Swift and was rather even in the Eurasian Hobby. Conclusions: This study adds two more species to the short list of birds in which the number and distribution of recombination nodules have been examined. Our data confirm that recombination rate in birds is substantially higher than that in mammals, but shows rather a low interspecies variability. Even a substantial reduction in chromosome number does not lead to any substantial decrease in recombination rate. More data from different taxa are required to draw statistically supported conclusions about the evolution of recombination in birds. [ABSTRACT FROM AUTHOR]