Hexavalents in spermatocytes of Robertsonian heterozygotes between Mus m. domesticus 2n=26 from the Vulcano and Lipari Islands (Aeolian Archipelago, Italy)

Abstract

The size and shape of the chromosomes, as well as the chromosomal domains that compose them, are determinants in the distribution and interaction between the bivalents within the nucleus of spermatocytes in prophase I of meiosis. Thus the nuclear architecture characteristic of the karyotype of a species can be modified by chromosomal changes such as Rb chromosomes. In this study we analysed the meiotic prophase nuclear organization of the heterozygous spermatocytes from Mus musculus domesticus 2n=26, and the synaptic configuration of the hexavalent formed by the dependent Rb chromosomes Rbs 6.16, 16.10, 10.15, 15.17 and the telocentric chromosomes 6 and 17. Spreads of 88 pachytene spermatocytes from two males were studied and in all of them five metacentric bivalents, four telocentric bivalents, one hexavalent and the XY bivalent were observed. About 48% of the hexavalents formed a chain or a ring of synapsed chromosomes, the latter closed by synapsis between the short arms of telocentric chromosomes 6 and 17. About 52% of hexavalents formed an open chain of 10 synapsed chromosomal arms belonging to 6 chromosomes. In about half of the unsynapsed hexavalents one of the telocentric chromosome short arms appears associated with the X chromosome single axis, which was otherwise normally paired with the Y chromosome. The cluster of pericentromeric heterochromatin mostly determines the hexavalent's nuclear configuration, dragging the centromeric regions and all the chromosomes towards the nuclear envelope similar to an association of five telocentric bivalents. These reiterated encounters between these chromosomes restrict the interactions with other chromosomal domains and might favour eventual rearrangements within the metacentric, telocentric or hexavalent chromosome subsets. The unsynapsed short arms of telocentric chromosomes frequently bound to the single axis of the X chromosome could further complicate the already complex segregation of hexavalent chromosomes.

Keywords: Hexavalents; Robertsonian chromosomes; mouse spermatocytes; nuclear architecture..

Figure 1.

Mitotic and meiotic chromosomes of the hybrid male 2n=26 resulting from the cross between Vulcano and Lipari mice. a) Mitotic karyotype: five pairs of metacentric Rb chromosomes, four pairs of telocentric chromosomes, the sex chromosomes X and Y, and the remaining 6 chromosomes, 6, 6.16, 16.10, 10.15, 15.17 and 17, that present monobrachial homology. b) Meiotic chromosomes: five metacentric bivalents, one hexavalent, four telocentric bivalents and the sex bivalent. c) Nuclear spread of a pachytene spermatocyte: five metacentric bivalents (M), four telocentric bivalents (T), one hexavalent (H) and a complete synapsed XY bivalent (XY); the synaptonemal complexes were labelled with FITC anti-SYCP3 antibodies (green), the centromeres with Texas Red anti CENP-A antibodies (red) and the nuclear DNA was stained with DAPI (blue); scale bar: 2 μm.

Figure 2.

Nuclear spreads of pachytene spermatocytes. In the three nuclei the length of the synaptonemal complexes (green) and the centromere position within them (red) reveals five metacentric bivalents, four telocentric bivalents, one hexavalent and the XY bivalent (XY). a) Hexavalent in a closed chain of six synapsed chromosomes (arrow); the XY bivalent is not bound to the hexavalent. b) Hexavalent in an open elongated chain of six synapsed chromosomes (arrow); the XY bivalent is not bound to the hexavalent. c) Hexavalent in an open elongated chain of six synapsed chromosomes (arrow); the XY bivalent is bound to the hexavalent through the single and thickened end of the X chromosome. The synaptonemal complexes were labelled with FITC anti-SYCP3 antibodies (green), and the centromeres with Texas Red anti CENP-A antibodies (red). Scale bars: 2 μm.

Figure 3.

Synaptic configuration of the hexavalent in the meiotic prophase nucleus. a) Closed chain of six synapsed chromosomes: 6 (blue), 6.16 (blue-red), 16.10 (red-purple), 10.15 (purple-green, 15.17 (green-yellow) and 17 (yellow); the telocentric chromosomes 6 and 17 are bound to each other through their short arms; the sex bivalent is not bound to the hexavalent; all synapsed chromosomes describe arcs whose proximal telomeres and centromeres are immersed in a central block of pericentromeric heterochromatin coming from the six chromosomes (dotted area), which in turn is broadly attached to the nuclear envelope; the distal telomeres are attached to the nuclear envelope circularly at the surrounding perimeter of this heterochromatin cluster. b) Open chain of six synapsed chromosomes: 6 (blue), 6.16 (blue-red), 16.10 (red-purple), 10.15 (purple and green, 15.17 (green-yellow) and 17 (yellow); the short arms of the telocentric chromosomes 6 and 17 are not bound to each other and one of them is bound to the single axis of the X chromosome; each synapsed chromosome describes an arc whose proximal telomeres and centromeres are forming part of an elongated block of pericentromeric heterochromatin and the distal telomeres are attached in the neighboring surface of the nuclear envelope.