Sex Chromosomes and Sex-Linked Genes

Evidence is accumulating that meiosis is subject to 'checkpoints' that monitor the quality of this complex process. In yeast, unresolved double strand breaks (DSBs) in DNA are thought to trigger a 'recombination checkpoint' that leads to pachytene arrest. In higher eukaryotes, there is evidence for a checkpoint that monitors chromosome synapsis and in mammals the most compelling evidence relates to the sex chromosomes. In normal male mice, there is synapsis between the X and Y pseudoautosomal regions; in XSxr(a)O mice, with a single asynaptic sex chromosome, there is arrest at the first meiotic metaphase, the arrested cells being eliminated by apoptosis (our unpublished data). Satisfying the requirement for pseudoautosomal synapsis by providing a pairing partner for the XSxr(a) chromosome avoids this arrest. We have considered that this 'synapsis checkpoint' may be a modification of the yeast 'recombination checkpoint' with unresolved DSBs (a corollary of asynapsis) providing the trigger for apoptosis. DSBs induced by irradiation are known to trigger apoptosis in a number of cell types via a p53-dependent pathway, and we now show that irradiation-induced spermatogonial apoptosis is also p53-dependent. In contrast, the apoptotic elimination of spermatocytes with synaptic errors proved to be p53-independent.

The meiotic checkpoint monitoring synapsis eliminates spermatocytes via p53-independent apoptosis

Meiotic silencing of unsynapsed chromatin (MSUC) is a key mechanism in spermatogenesis and a model system to study the dynamics of gene silencing. Here we show that MAEL, the ortholog of Drosophila's high mobility group box protein Maelstrom, is associated not only with the silenced XY body, but also with unsynapsed autosomes. Characterization of MAEL revealed that it interacts directly with the chromatin remodeler SNF5/INI1 and chromatin-associated protein SIN3B, which we also find localized to the XY body. This is the first time that a chromatin remodeler has been shown to associate with whole chromosomes. In addition, we show that MAEL is a component of the mouse meiotic nuage and its haploid cell counterpart, the chromatoid body. This is a site of accumulation of RNA and RNA processing enzymes, including proteins involved in the microRNA (miRNA) pathway. Furthermore, in the nuage, MAEL is present in a complex with germ cell specific MVH, an RNA helicase and Argonaute family members, MILI and MIWI. The presence of MAEL in these critical compartments of male germ cells and its interactions provide a link suggesting the involvement of the miRNA pathway in MSUC.

/pdf/mouse-maelstrom-the-link-between-meiotic-silencing-of-kb7trrq9q4.pdf

Mouse MAELSTROM: the link between meiotic silencing of unsynapsed chromatin and microRNA pathway?

Publisher Summary This chapter reviews that the use of data from the X-A translocations in the mouse allowed hypothetical answers to be formulated regarding the selection of one active X chromosome, physical linkage of autosomal genes to the active or inactive X chromosomal material regarding inactivation or activation, complete inactivation of an intact X chromosome versus incomplete inactivation of the X portion of an X-A segment, and random versus nonrandom inactivation with respect to primary and secondary causes. The chapter also discusses an interesting model for a gene regulation control system, which is proposed in higher organisms. The inactivation of the X chromosome would indicate that there are hierarchies of gene regulation. The X chromosome would be expected to have all the control systems. This latter system is hypothesized to be the determination mechanism for which X chromosome will be the inactive X chromosome.

X-Autosome Translocations in The Mouse: Total Inactivation Versus Partial Inactivation Of The X Chromosome

Biological systems express and reveal their specificity in different ways and at different levels. This specificity may be manifest in the choosing of a mate or a molecule and in the compatibility of a graft or a pollen grain. It is seen also in nutritional requirement and excretion product, in disease resistance and host range, in ecological preference and geographical distribution. It is expressed in chromosome pairing and the clotting of blood, in gametic fusion and in mating-call. All these properties express the distinctiveness of living systems and must ultimately rest on the shapes and surfaces of the macromolecules which contribute to their organisation. These chemical contours, in turn, are determined by the kind, the order and the degree of polymerisation of the monomer components.

The chromosome complement

Preparations were obtained from the testes of six sterile males of the T(X; ?) 16H stock. 81.7% of 694 primary spermatocytes in stages from diakinesis to first metaphase contained chain quadrivalents

A reciprocal translocation in the mouse between the x chromosome and a short autosome.

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A reciprocal translocation in the mouse between the x chromosome and a short autosome.