Achiasmatic sex chromosomes in <i>Pitymys duodecimcostatus</i>: mechanisms of association and segregation

Abstract

The meiotic behavior of the sex chromosomes of Pitymys duodecimcostatus was studied by electron microscopy of whole-mount synaptonemal complex preparations. The results established that the sex chromosomes of this species are achiasmatic and remain unassociated throughout meiotic prophase I in most spermatocytes. In other cells, nonspecific association of the X and Y occurred by means of filamentous bridges. Pitymys duodecimcostatus represents an additional example of a mammalian species lacking a homologous pairing segment in its sex chromosomes and extends current knowledge about this controversial subject. In this regard, we suggest that sex-chromosome association is a characteristic that probably followed different evolutionary paths in different mammals, leading to loss of the homologous segment in some species and its conservation in others. It is also suggested that in P. duodecimcostatus, and probably in many other species as well, three mechanisms may act in concert to permit joining of the X and Y chromosomes during meiotic prophase, and, consequently, to ensure proper segregation during anaphase I: (1) joining of the sex-chromosome axes at their ends to the nuclear membrane, (2) formation of fibrillar structures to hold the sex chromosomes together, and (3) cohesiveness due to sex-vesicle formation.

Synaptonemal complex analysis of Nellore and Gyr breeds of Bos taurus indicus

“…A similar mechanism of chromosome synapsis was observed in bulls (DOLLIN et al 1989) and deer mouse (GREENBAUM et al 1986). The morphological analyses of SC showed that the development of the zygotene and pachytene were identical to that observed for other mammals such as the Chinese hamster (MOSES 1977a, b;CARNEIRO et al 1991), deer mouse (GREENBAUM et al 1986) and rodents (FAGUNDES and YONENAGA-YASSUDA 1996).…”

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae

“…One of them stated that large heterochromatic regions on sex chromosomes could interfere with synapsis (Solari and Ashley, 1977). However, asynapsis also occurs in species without heterochromatic regions on sex chromosomes (Ashley et al, 1989;Carnero et al, 1991;Borodin et al, 1995;Megías-Nogales et al, 2003). Also, the loss or malfunction of the pairing regions in sex chromosomes has been proposed as the cause of asynapsis Jiménez et al, 1991).…”

“…Hence, several hypotheses have been proposed to explain the correct disjunction of sex chromosomes in asynaptic species. These hypotheses include: (a) attachment of sex chromosome ends to the nuclear envelope (Ashley et al, 1989;Carnero et al, 1991;Jiménez et al, 1991); (b) existence of fibrillar structures that attach the sex chromosomes Jiménez et al, 1991) and (c) the sex vesicle could help to keep the X and Y chromosomes together .…”

Sex chromosomes, synapsis, and cohesins: a complex affair

“…Conversely, sex chromosomes in almost all eutherian mammals only preserve a small region of homology. The extreme situation is found in some eutherian and in almost all marsupial mammals, in which sex chromosomes do not share any homologous region (Solari and Ashley 1977;Wolf et al 1988;Carnero et al 1991;Hayman 1990;Graves 1996). Moreover, in the species with XX/X0 sex determining system, many insects for instance, the heterogametic sex only bears a copy of the X chromosome.…”