Abstract
B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)15 microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.
Highlights
Meiosis is a highly regulated process of reduced cell division occurring in germ cells, which allows for genetic recombination (Ma et al, 2014)
In addition to 25 fully synaptized synaptonemal complexes (SC), SC-fluorescence in situ hybridization (FISH) in 2B individual preparations showed a SC association marked on only one end by the (CA)15 probe, corresponding to two self-paired and close B chromosomes (Figure 3E-G)
In some pachytene preparations of 2B individuals, 25 normal bivalents plus two SCs marked by the (CA)15 probe were seen at only one of the ends, corresponding to two independently self-paired B chromosomes (Figure 3I-K)
Summary
Meiosis is a highly regulated process of reduced cell division occurring in germ cells, which allows for genetic recombination (Ma et al, 2014). Iso-B chromosomes, reported in several species (Fletcher and Hewitt, 1988; Dias et al, 1998; Mestriner et al, 2000; Poletto et al, 2010; Valente et al, 2014; Pires et al, 2015), have the advantage of being composed of highly similar arms, allowing their perfect selfpairing, decreasing possible damage to cell division and increasing their chances of perpetuation (Battaglia, 1964; Mestriner et al, 2000; Serrano-Freitas et al, 2020)
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