Molecular cloning and characterization of the repetitive DNA sequences that comprise the constitutive heterochromatin of the W chromosomes of medaka fishes

Abstract

Among the medaka fishes of the genus Oryzias, most species have homomorphic sex chromosomes, while some species, such as Oryzias hubbsi and Oryzias javanicus, have heteromorphic ZW sex chromosomes. In this study, a novel family of repetitive sequence was molecularly cloned from O. hubbsi and characterized by chromosome in situ and filter hybridization, respectively. This repetitive element, which we designated as a BstNI family element, localized at heterochromatin regions on the W chromosome, as well as on two pairs of autosomes. Homologous sequences to this element were found only in O. javanicus, which is a sister species of O. hubbsi, suggesting that this repeated element originated in the common ancestor of these two species. However, the intensity of the hybridization signals was lower in O. javanicus than in O. hubbsi, and the chromosomal location of this element in O. javanicus was confined to heterochromatin regions on one pair of autosomes. Thus, we hypothesize that this repetitive element was extensively amplified in the O. hubbsi lineage, especially on its W chromosome, after the separation of the O. javanicus lineage. In addition, we also found the W chromosomal location of the 18S-28S ribosomal RNA genes in both O. hubbsi and O. javanicus. Our previous studies showed no linkage Chromosome Res (2012) 20:71-81

Chromosome mapping of 18S rDNA and 5S rDNA by dual-color fluorescence in situ hybridization in the half-smooth tongue sole (Cynoglossus semilaevis)

“…It was interesting that the telomeric regions of the W chromosome long arm showed an explicit signal in all analyzed metaphases of females. The same phenomenon has been reported in Oryzias hubbsi and O. javanicus (Takehana et al, 2012), with 3 18S rDNA sites located on a pair of autosomes and the W chromosome in females. In Triportheus auritus, 5 18S rDNA fluorescence hybridization signals are present on 2 autosomal pairs and the W chromosome (Cioffi et al, 2012).…”

“…Therefore, it is rational to speculate that chromosome fusions may also occur in C. semilaevis. It has also been reported that sex chromosomes are labile and that their formation in fish is often associated with the accumulation of repetitive sequences or long interspersed repeat elements (Carvalho et al, 2005;Ellegren, 2011;Takehana et al, 2012) and the silencing of genes (Zhang et al, 2001). This study of the C. semilaevis W chromosome showed that a number of repetitive sequences accumulated in heterochromatic regions (data not shown), which was also found in mammals, birds, and other fishes (Mank et al, 2006;Ellegren, 2011).…”

Sex chromosome composition revealed in Characidium fishes (Characiformes: Crenuchidae) by molecular cytogenetic methods

“…gomesi did not reveal any similarity among themselves or with any sequences deposited in GenBank to date (Table 3). This kind of dead-end result has been reported before when studying sequences present in the sex chromosomes of Leporinus elongatus (Nakayama et al 1994;Parisi-Maltempi et al 2007), Parodon hilarii Reinhardt, 1866 (Vicente et al 2003), Cydia pomonella L., 1758 (Fuková et al 2007) and Oryzias hubbsi Roberts, 1998 (Takehana et al 2012), for example. Of the seven distinct fragments analysed, only CgW9 evidenced similarity (∼73%) to a DNA transposon (Helitron-2 Dr) isolated from the genome of Danio rerio.…”

“…Despite the differences in sex chromosome determination, these chromosomes share some features: Y and W are mostly heterochromatic while X and Z usually have more euchromatic regions. The build-up of heterochromatin and gene inactivation in Y and W chromosomes are considered converging properties in the process of sex chromosome differentiation (Takehana et al 2012).…”

“…gomesi Travassos, 1956 has clearly differentiated Z and W chromosomes, with heterochromatin occupying most of the W chromosome but only the pericentromeric region of the Z chromosome, becoming easily distinguishable from the autosomes (Vicari et al 2008;Pansonato-Alves et al 2011;Machado et al 2011). While heterochromatin is known to be rich in repetitive DNA (e.g., Charlesworth et al 2005;Volff et al 2007), the identification and characterization of repetitive DNAs specific to sex chromosomes has also been performed in fishes (Parise-Maltempi et al 2007;Takehana et al 2012). However, no known function has been attributed to these sequences in fishes or in any other group of organisms where they were described.…”