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Genetica (2013) 141:381387 DOI 10.1007/s10709-013-9737-2
XX/XO, a rare sex chromosome system in Potamotrygon freshwater stingray from the Amazon Basin, Brazil
Francisco Carlos de Souza Valentim
Jorge Ivan Rebelo Porto Luiz Antonio Carlos Bertollo
Maria Claudia Gross Eliana Feldberg
Received: 22 January 2013 / Accepted: 5 September 2013 / Published online: 26 September 2013 Springer Science+Business Media Dordrecht 2013
Abstract Potamotrygonidae is a representative family of South American freshwater elasmobranchs. Cytogenetic studies were performed in a Potamotrygon species from the middle Negro River, Amazonas, Brazil, here named as Potamotrygon sp. C. Mitotic and meiotic chromosomes were analyzed using conventional staining techniques, C-banding, and detection of the nucleolus organizing regions (NOR) with Silver nitrate (Ag-NOR). The diploid number was distinct between sexes, with males having 2n = 67 chromosomes, karyotype formula 19m ? 8sm ? 10st ? 30a, and fundamental number (FN) = 104, and females having 2n = 68 chromosomes, karyotype formula 20m ? 8sm ? 10st ? 30a, and FN = 106. A large chromosome, corresponding to pair number two in the female karyotype, was missing in the male complement.
Male meiotic cells had 33 bivalents plus a large univalent chromosome in metaphase I, and n = 33 and n = 34 chromosomes in metaphase II. These characteristics are consistent with a sex chromosome system of the XX/XO type. Several Ag-NOR sites were identied in both male and female karyotypes. Positive C-banding was located only in the centromeric regions of the chromosomes. This sex chromosome system, which rarely occurs in sh, is now being described for the rst time among the freshwater rays of the Amazon basin.
Keywords Elasmobranchs Cytogenetic Endemic
stingray Ag-NOR Meiosis
Introduction
While most sh do not have differentiated sex chromosomes, approximately eight different systems of sex chromosomes (both simple and multiple ones) have already been described for this group (Oliveira et al. 2009). However, this number may actually be an underestimate, as there are sex systems that lack differentiated chromosomes and can only be detected by chromosomal banding or molecular techniques (Harvey et al. 2002; Centofante et al. 2003). Indeed, recent studies using chromosomal mapping of repetitive DNA showed a nascent XX/XY sex chromo-some system in one of the seven karyomorphs of the sh group Hoplias malabaricus. In this case, the...