HORMAD1 Is Essential for Male and Female Meiosis Figure 13. Hormad1–/– fertilized oocytes show widespread hypo- and hyperdiploidy. (A-B) In GV stage oocytes, parental chromosomes have replicated to produce identical sister chromatids, which are visualized as closely located double signals for examined chromosomes 18, 19 and X. (C) Histogram of individual chromosomes, does not show significant difference between the wild type and mutants. Fifty eight knockout GV oocytes and 112 wild type GV oocytes were scored. Abbreviations stand for the following: tetra-Tetrasomy (blue), tri-Trisomy (red), di-Disomy (green), mono-Monosomy (purple), zero-Zerosomy (grey). (D-F) E1.5 day (2-cell stage embryo) FISH assay using chromosomes 6 (green) and X-specific DNA probes (red). The wild type embryos were diploid for 6 and X (D). Trisomy for chromosome 6 (arrows) is noted in the knockout (E). In addition, chromosomes 6 and X demonstrate double hybridization signals, the same FISH pattern as observed during the GV stage, and indicative of sister chromatids. (F) Histograms for individual chromosomes show high prevalence of chromosomal aneuploidies in embryos derived from mutant oocytes. We scored 63 knockout and 46 wild type cells. (G-I) E2.5 day (8-cell stage embryo) FISH assay with chromosomes X (red), 11 (red) and 2 (yellow) specific probes. (G) A hybridization pattern of a normal diploid number of chromosomes in the wild type embryo. (H) Trisomy for chromosome X (arrow), as well as double signals likely produced by sister chromatids on chromosome 2 and 11 were observed in embryos derived from knockout eggs. (I) Histogram shows substantial number of trisomies and monosomies in the knockout as compared to the wild type. We scored 84 wild type and 96 knockout cells. (J and K) Immunofluorescence with b-tublin antibody (red) in wild-type and Hormad 2/2 MII stage oocytes. Hormad 2/2 oocytes are grossly disrupted with mis-orientation of the chromatid attachment to the spindle. DNA was stained with DAPI (Blue). doi:10.1371/journal.pgen.1001190.g013 plays an important role in mammalian meiosis. Our results indicate that HORMAD1 is the mammalian homologue to the Hop1 protein in yeast. Our current studies, and those of others, have shown that HORMAD1 protein is confined to germ cells and co-localizes with synaptonemal complex proteins SYCP2 and SYCP3 on the chromatin as part of the axial core [27,48]. Synaptonemal complex is a complex structure composed of multiple germ cell– specific and ubiquitously expressed proteins that connect paired homologous chromosomes. Similar to railroad tracks, the PLoS Genetics | www.plosgenetics.org 14 November 2010 | Volume 6 | Issue 11 | e1001190