Abstract
In the present study, we analyzed individuals of Hypostomus soniae (Loricariidae) collected from the Teles Pires River, southern Amazon basin, Brazil. Hypostomus soniae has a diploid chromosome number of 2n = 64 and a karyotype composed of 12 metacentric (m), 22 submetacentric (sm), 14 subtelocentric (st), and 16 acrocentric (a) chromosomes, with a structural difference between the chromosomes of the two sexes: the presence of a block of heterochromatin in sm pair No. 26, which appears to represent a putative initial stage of the differentiation of an XX/XY sex chromosome system. This chromosome, which had a heterochromatin block, and was designated proto-Y (pY), varied in the length of the long arm (q) in comparison with its homolog, resulting from the addition of constitutive heterochromatin. It is further distinguished by the presence of major ribosomal cistrons in a subterminal position of the long arm (q). The Nucleolus Organizer Region (NOR) had different phenotypes among the H. soniae individuals in terms of the number of Ag-NORs and 18S rDNA sites. The origin, distribution and maintenance of the chromosomal polymorphism found in H. soniae reinforced the hypothesis of the existence of a proto-Y chromosome, demonstrating the rise of an XX/XY sex chromosome system.
Highlights
The Teles Pires River, in the southern Amazon basin, is the home of at least 36 species of Loricariidae, and five species of Hypostomus Lacépède, 1803 (Siluriformes, Loricariidae) (Ohara et al 2017)
The present study describes a karyotype with a putative initial stage of the differentiation of sex chromosomes in a population of H. soniae from the basin of the Teles Pires River, in southern Amazonia
Hypostomus soniae has the diploid chromosome number of 2n = 64, fundamental number (FN) equal to 112, and a karyotype composed of 12m + 22sm + 14st + 16a chromosomes, in both males and females (Fig. 1A)
Summary
The Teles Pires River, in the southern Amazon basin, is the home of at least 36 species of Loricariidae, and five species of Hypostomus Lacépède, 1803 (Siluriformes, Loricariidae) (Ohara et al 2017). Hypostomus is considered to be one of the taxonomically most complex genera of Neotropical fish due to its enormous diversity of morphology and body pigmentation patterns, with a total of 203 recognized species (Froese and Pauly 2019). The diversification of this genus appears to be closely related to changes in the chromosome complement, which include diploid numbers (2n) ranging from 52 in H. emarginatus (Artoni and Bertollo 2001) to 84 in Hypostomus sp. G (Artoni et al 1998), H. cf. plecostomus (Oliveira et al 2015) and H. ancistroides (Kamei et al 2017)
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