Cytogenetic Characterization of Two <b><i>Metynnis</i></b> Species (Characiformes, Serrasalmidae) Reveals B Chromosomes Restricted to the Females

Abstract

Karyotypes and chromosomal characteristics with focus on B chromosomes of 2 species of the serrasalmid genus Metynnis, namely M. lippincottianus and M. maculatus, were examined using conventional (C-banding) and molecular (FISH mapping of minor and major rDNAs and Rex1, Rex3, and Rex6 retrotransposable elements) protocols. Both species possessed a diploid chromosome number of 2n = 62 and karyotypes composed of 32 metacentric + 28 submetacentric + 2 subtelocentric and 32 metacentric + 26 submetacentric + 4 subtelocentric, respectively; one small B element was found in the female genome of M. lippincottianus. C-banding revealed heterochromatin in the pericentromeric and terminal portions of all chromosomes of both species; the B chromosome was entirely heterochromatic. FISH showed 18S rDNA sites in 2 chromosome pairs in both species (pairs 19 and 22), and a large block in the B chromosome, while 5S rDNA signals were detected in the first pair of subtelocentric chromosomes in both species, moreover in M. maculatus an additional labeled pair 4 was observed. Mapping of the Rex1, Rex3, and Rex6 retrotransposable elements in the genomes of M. lippincottianus and M. maculatus indicated that they were dispersed throughout nearly all the chromosomes of the complement, except for the B chromosome of M. lippincottianus.

A supernumerary “B-sex” chromosome drives male sex determination in the Pachón cavefish, Astyanax mexicanus

“…21,24,26 Female or male sex-restricted or sex-predominant Bs have been described in many fishes-for instance, in some cichlids [12][13][14] and characiforms. 8,9,27,28 In A. mexicanus, Bs were recently described as being restricted only in some, but not all, males, 19 suggesting population differences in the frequency and sex linkage of Bs that have also been reported in many species. [4][5][6]29 In addition to metaphase spreads (Figures 1E and 1F), we also detected Bs in pachytene chromosome spreads from testes of two Pachó n males (n = 52 and n = 25).…”

Comparative cytogenetics of Serrasalmidae (Teleostei: Characiformes): The relationship between chromosomal evolution and molecular phylogenies

“…Subsequently, the chromosomes were counterstained with DAPI, analyzed in an Olympus BX51 fluorescence microscope and classified according to [ 51 ]. The 5S and 18S rDNA sequences were obtained from the eight species present in Table 1 , with addition to published data from [ 35 ].…”

“…Significant advances in cytogenetic studies in Serrasalmidae utilizing different approaches also occurred, ranging from chromosomal characterization of species, such as Pygocentrus cariba Humboldt and Valenciennes 1821 [ 31 ] and Myleus micans Lütken 1875 [ 32 ], to the use of cytogenetic markers to identify hybrids between Colossoma and Piaractus [ 33 , 34 ]. Recently, the presence of a B chromosome restricted to females was described to Metynnis lippincottianus (Cope 1870) [ 35 ].…”

“…The 2n in the family varies from 54 chromosomes in Colossoma [ 36 , 37 ], Mylossoma [ 38 ], and Piaractus [ 33 , 38 ] to 62—in Metynnis [ 35 , 39 , 40 ]. Intermediate 2n, such as 58 and 60 chromosomes, were already reported in Pygocentrus and Serrasalmus [ 26 , 28 , 29 , 37 , 41 – 43 ].…”

A New Variant B Chromosome in Auchenipteridae: The Role of (GATA)_n and (TTAGGG)_n Sequences in Understanding the Evolution of Supernumeraries in <b><i>Trachelyopterus</i></b>

“…However, in the last decades this scenario has changed [Navarro-Domínguez et al, 2017]. Several studies reported the presence of protein-coding genes in Bs, for instance, ribosomal DNA sequences [Rajičić et al, 2015;Soares et al, 2018;Dhar et al, 2019;Favarato et al, 2019], histone genes [Teruel et al, 2010;Bueno et al, 2013], proto-oncogenes and tumor-suppressor genes [Graphodatsky et al, 2005;Ruban et al, 2017;Makunin et al, 2018], mitochondrial ribosome assembly and translational activity regulatory genes [Navarro-Domínguez et al, 2017], DNA repair genes [Valente et al, 2014], and homologs of genes involved in cell proliferation, differentiation, neuron synapse, and cell junction [Makunin et al, 2018]. Recently, analyses of the fine-scale sequence composition and expression of a paternal sex ratio-linked gene in Nasonia vitripennis (Walker, 1836) Bs, named haploidizer, also showed activity of the gene transcript, which causes female-to-male conversion by destroying the sperm's hereditary material in young embryos [Benetta et al, 2020].…”