DMC1 is a meiosis-specific gene first discovered in yeast that encodes a protein with homology to RecA and may be component of recombination nodules. Yeast dmc1 mutants are defective in crossing over and synaptonemal complex (SC) formation, and arrest in late prophase of meiosis I. We have generated a null mutation in the Dmc1 gene in mice and show that homozygous mutant males and females are sterile with arrest of gametogenesis in the first meiotic prophase. Chromosomes in mutant spermatocytes fail to synapse, despite the formation of axial elements that are the precursor to the SC. The strong similarity of phenotypes in Dmc1-deficient mice and yeast suggests that meiotic mechanisms have been highly conserved through evolution.

Deficiencies or mutations in the human pseudoautosomal SHOX gene are associated with a series of short-stature conditions, including Turner syndrome, Leri-Weill dyschondrosteosis, and Langer mesomelic dysplasia. Although this gene is absent from the mouse genome, the closely related paralogous gene Shox2 displays a similar expression pattern in developing limbs. Here, we report that the conditional inactivation of Shox2 in developing appendages leads to a strong phenotype, similar to the human conditions, although it affects a different proximodistal limb segment. Furthermore, using this mouse model, we establish the cellular etiology of these defects and show that Shox2 acts upstream the Runx2 gene, a key regulator of chondrogenesis.limb ͉ SHOX

Little is known about the timing of meiotic prophase events during spermatogenesis in the mouse or how these events are related to cell-cycle progression. This work was designed to test hypotheses about the timing and biochemical correlates of developmental acquisition of competence to condense bivalent pairs of homologous chromosomes held together by chiasmata. The experimental approach takes advantage of the fact that okadaic acid (OA) treatment of pachytene spermatocytes causes precocious entry into metaphase I (MI) of meiosis. Leptotene and zygotene (L/Z) spermatocytes are not competent to respond to OA with condensation of chiasmate bivalent chromosomes. Competence for MI condensation of chiasmate bivalents is acquired by the middle of the pachytene stage of meiotic prophase, several days after homologous chromosomes become fully synapsed. The acquisition of MI competence is paralleled by the accumulation of histone H1t in the nuclei of mid-pachytene spermatocytes. Biochemical differences also exist between the incompetent L/Z spermatocytes and the competent pachytene spermatocytes. Both have the molecular components of metaphase promoting factor, CDC2 and CYCLIN B1; however, the histone H1 kinase activity of metaphase promoting factor of incompetent L/Z spermatocytes is not activated by OA, as it is in competent pachytene spermatocytes. Additionally, the CDC25C protein phosphatase is present in competent pachytene spermatocytes, but not in incompetent L/Z or early pachytene spermatocytes. Both incompetent and competent spermatocytes accumulate MPM-2 phosphoepitopes and phosphorylated histone H3 in response to OA treatment, indicating that presence of these antigens is not sufficient to promote condensation of meiotic chromosomes. These data demonstrate that meiotic competence of spermatocytes is acquired after homologous chromosome pairing is established and is coincident with first appearance of histone H1t and CDC25C protein phosphatase in spermatocytes.

Mechanisms of chromosome condensation and segregation during the first meiotic division are not well understood. Resolution of recombination events to form chiasmata is important, for it is chiasmata that hold homologous chromosomes together for their oppositional orientation on the meiotic metaphase spindle, thus ensuring their accurate segregation during anaphase I. Events at the centromere are also important in bringing about proper attachment to the spindle apparatus. This study was designed to correlate the presence and activity of two proteins at the centromeric heterochromatin, topoisomerase II alpha (TOP2A) and histone H3, with the processes of chromosome condensation and individualization of chiasmate bivalents in murine spermatocytes. We tested the hypothesis that phosphorylation of histone H3 is a key event instigating localization of TOP2A to the centromeric heterochromatin and condensation of chromosomes as spermatocytes exit prophase and progress to metaphase. Activity of topoisomerase II is required for condensation of chromatin at the end of meiotic prophase. Histone H3 becomes phosphorylated at the end of prophase, beginning with its phosphorylation at the centromeric heterochromatin in the diplotene stage. However, it cannot be involved in localization of TOP2A, since TOP2A is localized to the centromeric heterochromatin throughout most of meiotic prophase. This observation suggests a meiotic function for TOP2A in addition to its role in chromatin condensation. The use of kinase inhibitors demonstrates that phosphorylation of histone H3 can be uncoupled from meiotic chromosome condensation; therefore other proteins, such as those constituting metaphase-promoting factor, must be involved. These results define the timing of important meiotic events at the centromeric heterochromatin and provide insight into mechanisms of chromosome condensation for meiotic metaphase.

The MLH1 protein is required for normal meiosis in mice and its absence leads to failure in maintenance of pairing between bivalent chromosomes, abnormal meiotic division, and ensuing sterility in both sexes. In this study, we investigated whether failure to develop foci of MLH1 protein on chromosomes in prophase would lead to elimination of prophase spermatocytes, and, if not, whether univalent chromosomes could align normally on the meiotic spindle and whether metaphase spermatocytes would be delayed and/or eliminated. In spite of the absence of MLH1 foci, no apoptosis of spermatocytes in prophase was detected. In fact, chromosomes of pachytene spermatocytes from Mlh1(-/-) mice were competent to condense metaphase chromosomes, both in vivo and in vitro. Most condensed chromosomes were univalents with spatially distinct FISH signals. Typical metaphase events, such as synaptonemal complex breakdown and the phosphorylation of Ser10 on histone H3, occurred in Mlh1(-/-) spermatocytes, suggesting that there is no inhibition of onset of meiotic metaphase in the face of massive chromosomal abnormalities. However, the condensed univalent chromosomes did not align correctly onto the spindle apparatus in the majority of Mlh1(-/-) spermatocytes. Most meiotic metaphase spermatocytes were characterized with bipolar spindles, but chromosomes radiated away from the microtubule-organizing centers in a prometaphase-like pattern rather than achieving a bipolar orientation. Apoptosis was not observed until after the onset of meiotic metaphase. Thus, spermatocytes are not eliminated in direct response to the initial meiotic defect, but are eliminated later. Taken together, these observations suggest that a spindle assembly checkpoint, rather than a recombination or chiasmata checkpoint, may be activated in response to meiotic errors, thereby ensuring elimination of chromosomally abnormal gamete precursors.