synaptonemal complex axial lateral elements (SYCP2, SYCP3) are connected to each other by proteins known as transverse filaments (SYCP1, TEX12) [1]. The axial/lateral elements play critical roles in chromosome condensation, pairing, and repress recombination pathways that involve sister chromatids [1]. Synaptonemal complex formation is associated with HORMAD1 depletion from the axes [17–18]. Our data show that HORMAD1 phosphory- lation peaks at the time when HORMAD1 localization shifts to the sex body. ATM and ATR do not appear to be involved in HORMAD1 phosphorylation and binding to chromosomal axes. Previous study showed that Trip13, a homologue of yeast Pch2 kinase, may be involved in HORMAD1 depletion from synapsed chromosome axes [17]. Kinases, such as Mec1/Tel1 in yeast are important to effect inter-homologous recombination [21] by phosphorylating Hop1, but not much is known regarding HORMAD1. Future studies are necessary to understand HOR- MAD1 phosphorylation sites and responsible kinases. Although HORMAD1 is not essential for the binding of well- characterized germ cell–specific synaptonemal complex proteins such as SYCP1, SYCP2 and SYCP3, HORMAD1 deficient spermatocytes are defective in synapsis and do not form recognizable synaptonemal complexes. Hormad1 deficiency causes a male meiotic arrest, that is similar to male meiotic arrests observed in other components of the synaptonemal complex and recombination including: Atm, Spo11, Sycp1, Sycp2, Sycp3 and Dmc1 [27,30–31,48–50]. These findings are not surprising as HOR- MAD1 co-immunoprecipitates with known axial proteins, SYCP2, SYCP3, REC8, and SMC1b [18]. Whether these interactions are biologically significant, and whether HORMAD1 interacts with other members of the synaptonemal complex, including newly discovered HORMAD 2 protein, is unclear. Interestingly, Hop1 interacts with another phosphoprotein, Red1 [35,51], and whether HORMAD2 is the mammalian counterpart of Red1 remains to be seen. Like its counterpart in yeast, Hop1, Hormad1 deficiency disrupts early and later stages of recombination as shown by the drastic diminution in the formation of DMC1, RAD51, RPA and MLH1 foci. These findings differ from the Sycp3 mutant, which affects later stages of recombination [29]. DMC1 is a meiotic specific recombinase that together with ubiquitously expressed RAD51, catalyzes homologous pairing and DNA strand exchange, and marks earlier stages of recombination than MLH1 foci [30]. HORMAD1 expression is not significantly affected in Dmc1 mutant spermatocytes [17]. These results indicate that unrepaired DSBs and synaptonemal complexes between non-homologous chromosomes seen in Dmc1 mutants, do not affect HORMAD1 localization. In Hop1 mutants, DMC1 foci are only faintly detected when compared to controls [19]. These findings have led to a suggestion that Hop1 binds at or near the sites of DNA double strand break and modulates the action and perhaps recruitment of recombinases such as DMC1 and RAD51 [19]. Moreover, Hop1 appears to be part of the meiosis specific surveillance system that monitors meiotic double stranded break repair [21]. Our observations support the possibility that HORMAD1 affects early recombination and may therefore perform similar functions in the mammalian meiosis. During mammalian meiosis, X and Y chromosomes are inactivated and transcriptionally silenced (meiotic sex chromosome inactivation, MSCI) and chromatin condenses to form a sex (XY) body. The epigenetics of MSCI involves cH2AX, BRCA1 and ATR at a minimum. The most intriguing finding regarding HORMAD1 is its co-localization with BRCA1, ATR and cH2AX in the sex chromosomes during pachytene [18]. Hormad1 deficiency, similar to H2ax deficiency, abrogates formation of the HORMAD1 Is Essential for Male and Female Meiosis sex body, as well as cH2AX localization to the sex chromosomes. Hormad1 deficiency also abrogates BRCA1 and ATR localization to the sex chromosomes. Interestingly, H2ax deficiency does not affect BRCA1 localization to the sex chromosome, despite anomalous sex body, while BRCA1 deficiency does affect both ATR and cH2AX localization to the sex body. These findings indicate that HORMAD1 is involved in the initial recruitment of BRCA1, ATR and cH2AX to the sex body. The mechanism whereby HORMAD1 affects ATR, cH2AX and BRCA1 localization is unclear, but our results are consistent with the conclusion that HORMAD1 is upstream of ATR, cH2AX and BRCA1, and establishes HORMAD1 as an essential component of the MSCI complex. Moreover, we have shown here that Hormad1 deficient testes preferentially over-express X-linked genes as observed in mice deficient for other components of the MSCI complex such as cH2AX and BRCA1 [17]. Almost 20% of genes preferentially up-regulated from the Hormad12/2 testes are derived from the X chromosome. Hormad1 is therefore the first germ cell– specific component known to play a role in MSCI. Unlike males, meiosis I in females arrests during the embryonic development at the diplotene stage and is completed upon ovulation. The lack of dramatic germ cell apoptosis during ovarian follicle development has also been observed for Sycp2 and Sycp3 deficient mice [48,52], and reiterates the laxity of oocytes to early meiotic errors, as opposed to spermatogenesis, where meiotic check point causes the dramatic phenotype observed in male gonads. Hormad1 deficiency does not affect folliculogenesis nor ovulation. These findings are unusual, given that Hormad1 affects recombination, and recombination associated proteins DMC1 and MLH1 are important in early oogenesis. Unrepaired recombina- tion intermediates or defects in homologous chromosome pairing and synapsis are likely causes of early oocyte loss [53]. Spo11 deficient animals can ameliorate observed oocyte losses in recombination defective mutants, presumably due to the failure of Spo112/2 deficient mice to form DSBs that initiate meiotic recombination. Spo11 deficiency does not affect HORMAD1 association with the chromosome axes [17], a finding similarly observed for Hop1 localization in Spo112/2 yeast [9]. Moreover, Hop1 mutants are required for full levels of DSBs formation [37,54]. It is therefore possible that early loss of oocytes in Hormad1 deficient animals does not occur in part because unrepaired DSBs do not form. Reduced staining of cH2AX in Hormad1 mutant leptotene stage oocytes argues that unrepaired DSB formation is indeed reduced. However, unlike Spo112/2 ovaries that have abnormal folliculogenesis, Hormad12/2 ovaries are not defective in folliculogenesis and other mechanisms must protect Hormad1 mutant oocytes from early loss. Infertility in Hormad12/2 females is due to early embryo demise. This is in stark contrast to males, where spermatocytes are eliminated due to the pachytene defect. The greater ability of oocytes to tolerate meiosis I errors is well known [55], and exemplified by Sycp3 and Smc1b knockouts [52,56], which have defects in synapsis and recombination. Mechanisms that underlie oocyte’s insensitivity to such errors are not well understood [57]. Embryos derived from Hormad12/2 oocytes fertilized with wild- type sperm, cannot implant. Examination of post-fertilization events in fertilized Hormad12/2 oocytes, showed significant abnormalities in early embryo development, including growth lag, and severe disruption in development prior to the blastocyst stage. Very few blastocysts were produced in fertilized Hormad12/2 oocytes, and most were abnormal in morphology. We utilized FISH hybridization against specific chromosomes to show wide- spread hypo and hyperdiploidy during early embryonic development. These results are reminiscent of Sycp3 and Smc1b PLoS Genetics | www.plosgenetics.org 15 November 2010 | Volume 6 | Issue 11 | e1001190