We study the foundations of mammalian reproduction, with particular focus on sex chromosome biology and evolution, the fetal origins of gametes, and infertility. Most eukaryotes reproduce sexually: two haploid cells, or gametes, fuse to form a diploid which, through meiosis, gives rise to another generation of haploid cells, completing the life cycle. In most animals, including all mammals and birds, the gametes are of two types: large gametes, or oocytes, produced by diploids called females; and small gametes, or sperm, produced by diploids called males. In many animal species, the two diploid forms, or sexes, differ by one chromosome. In mammals, females are XX and males are XY. In birds, which are the closest living relatives of mammals, females are ZW and males are ZZ.
Sex chromosome genomics, evolution, and biology
Our goal is to better understand vertebrate sex chromosomes, including the male-specific Y chromosome of mammals, the female-specific W chromosome of birds, and their meiotic partners, the X and Z chromosomes, respectively. In 2003, our lab completed the sequencing of the human MSY (male-specific region of the Y chromosome), the first sex-specific chromosome to be sequenced from any organism. Because of the complex nature of the MSY, we developed a novel strategy, which we termed single-haplotype iterative mapping and sequencing (SHIMS), to assemble such intricately repetitive genomic regions. This exacting approach enabled us to produce an accurate and complete picture of the MSY's repetitive regions, which has yielded tremendous biological and medical insights. However, we recognized that in order to more fully understand the nature of sex chromosomes, the analysis of the human MSY must be accompanied by similarly methodical study of the sex chromosomes of other mammals and birds. Therefore, we set out to sequence the Y chromosomes of seven other mammals (chimpanzee, rhesus macaque, marmoset, mouse, rat, bull and opossum) and Z and W chromosomes of chicken. Our ultimate goal is to systematically and comprehensively compare the human X and Y chromosomes with the sex chromosomes of these eight other species, enabling us to reconstruct the course of sex chromosome evolution over hundreds of millions of years and gain further insight into the biological functions of Y chromosomes.
Germ cell development and sexual differentiation
Our goal is to learn how germ cells initiate the meiotic program and acquire a female or male identity (ultimately oocyte or sperm) through genetic and molecular analysis of germ cell differentiation in the mammalian ovary and testis. Whether germ cells develop as oocytes or sperm appears to be dependent on the timing of meiotic initiation, which occurs embryonically in females and during puberty in males. Starting in the late 1990's we have used the mouse as an experimental model to genetically dissect the process of meiotic initiation, which is a critical juncture in mammalian develoment, both female and male. Our initial genetic insight came from the discovery that the Stra8 gene is required for meiotic initiation in germ cells of fetal ovaries. Rapid progress followed, including 1) the identication of retinoic acid (RA) as an extrinsic inducer of Stra8, 2) the discovery that RA and Stra8 govern meiotic initiation in postnatal testes, and 3) the discovery that the Dazl gene encodes a meiotic competence factor, enabling fetal germ cells to respond the RA signal, express Stra8, and enter meiosis. Now that we have identified the critical players and a genetic regulatory network is beginning to emerge, we will probe meiotic initiation more deeply and test its connection to germ cell sexual identity.
Human reproductive disorders
Our goal is to translate basic knowledge gained through our analyses of sex chromosomes and germ cell development into new understandings of human reproductive disorders, including male infertility, Turner syndrome, and sex reversal. Our analysis of the human MSY, together with our ongoing studies of human MSY variation, has brought a reassessment of the chromosome's architecture and genetic content, and of its biological and medical significance. Knowledge of the MSY's elaborately mirrored structures, including eight massive palindromes, enabled the identification of five different interstitial MSY deletions, each recurring at different frequencies, that cause spermatogenic failure. We have also found that a high degree of MSY structural polymorphism exists among men, and one of these polymorphisms reduces sperm production and increases the risk of spermatogenic failure. At the most extreme end, aberrant recombination within the MSY's massive palindromes can generate mirror-image "isodicentric" Y chromosomes that contribute to not only spermatogenic failure, but also Turner syndrome and sex reversal.
Hughes JF, Skaletsky H, Brown LG, Pyntikova T, Graves TA, Fulton RS, Dugan S, Ding Y, Buhay CJ, Kremitzki C, Wang Q, Shen H, Holder M, Villasana D, Nazareth LV, Cree A, Courtney L, Veizer J, Kotkiewicz H, Cho T, Koutseva N, Rozen S, Muzny DM, Warren WC, Gibbs RA, Wilson RK, Page DC
(2012) Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes. Nature 483: 82-6
Gill ME, Hu Y, Lin Y, Page DC (2011) Licensing of gametogenesis, dependent on RNA binding protein DAZL, as a gateway to sexual differentiation of fetal germ cells. Proc Natl Acad Sci 108: 7443-8
Bellott DW, Skaletsky H, Pyntikova T, Mardis ER, Graves T, Kremitzki C, Brown LG, Rozen S, Warren WC, Wilson RK, Page DC (2010) Convergent evolution of chicken Z and human X chromosomes by expansion and gene acquisition. Nature 466: 612-16
Hughes JF, Skaletsky H, Pyntikova T, Graves TA, van Daalen SK, Minx PJ, Fulton RS, McGrath SD, Locke DP, Friedman C, Trask BJ, Mardis ER, Warren WC, Repping S, Rozen S, Wilson RK, Page DC (2010) Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content. Nature 463: 536-9
Rozen S, Marszalek JD, Alagappan RK, Skaletsky H, Page DC (2009) Remarkably little variation in proteins encoded by the Y chromosome's single-copy genes, implying effective purifying selection. Am J Hum Genet 85: 923-8
Lange J, Skaletsky H, van Daalen SK, Embry SL, Korver CM, Brown LG, Oates RD, Silber S, Repping S, Page DC (2009) Isodicentric Y chromosomes and sex disorders as byproducts of homologous recombination that maintains palindromes. Cell 138: 855-69
Lin Y, Gill ME, Koubova J, Page DC (2008) Germ cell–intrinsic and –extrinsic factors govern meiotic initiation in mouse embryos. Science 322:1685-7
Anderson E, Baltus AE, Roepers-Gajadien HL, Hassold TJ, de Rooij DG, van Pelt AMM, Page DC (2008) Stra8 and its inducer, retinoic acid, regulate meiotic initiation in both spermatogenesis and oogenesis in mice. Proc Natl Acad Sci 105:14876-80
Baltus AE, Menke DB, Hu Y, Goodheart ML, Carpenter AE, de Rooij DG, Page DC (2006) In germ cells of mouse embryonic ovaries, the decision to enter meiosis precedes premeiotic DNA replication. Nat Genet 38: 1430-4
Repping S, van Daalen SKM, Brown LG, Korver CM, Lange J, Marszalek JD, Pyntikova T, van der Veen F, Skaletsky H, Page DC, Rozen S (2006) High mutation rates have driven widespread architectural polymorphism among human Y chromosomes. Nat Genet 38: 463-7
Hughes JF, Skaletsky H, Pyntikova T, Minx PJ, Graves T, Rozen S, Wilson RK, Page DC (2005) Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee. Nature 437: 100-3
Bradley J, Baltus A, Skaletsky H, Royce-Tolland M, Dewar K, Page DC (2004)An X-to-autosome retrogene is required for spermatogenesis in mice. Nat Genet 36: 872-76
Menke DB, Koubova J, Page DC (2003) Sexual differentiation of germ cells in XX mouse gonads occurs in an anterior-to-posterior wave. Dev Biol 262: 303-12