Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.

Agate RJ, Grisham W, Wade J, Mann S, Wingfield J, et al: Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch. Proc Natl Acad Sci USA 100:4873–4878 (2003).
Andrews JEA, Smith CA, Sinclair AH: Sites of estrogen receptor and aromatase expression in the chicken embryo. Gen Comp Endocrinol 108:182–190 (1997).
Arit D, Bensch S, Hansson B, Hasselquist D, Westerdahl H: Observation of a ZZW female in a natural population: implications for avian sex determination. Proc Biol Sci 271 Suppl 4:S249–251 (2004).
Arnold A: Sexual differentiation of the zebra finch song system: positive evidence, negative evidence, null hypotheses, and a paradigm shift. J Neurobiol 33:572–584 (1997).
Backström N, Ceplitis H, Berlin S, Ellegren H: Gene conversion drives the evolution of HINTW, an ampliconic gene on the female-specific avian W chromosome. Mol Biol Evol 22:1992–1999 (2005).
Berlin S, Ellegren H: Fast accumulation of nonsynonymous mutations on the female-specific W chromosome in birds. J Mol Evol 62:66–72 (2006).
Bisoni L, Batlle-Morera, Birds AP, Suzuki M, McQueen HA: Female-specific hyperacetylation of histone H4 in the chicken Z chromosome. Chromosome Res 13:205–214 (2005).
Brennan J, Capel B: One tissue, two fates: molecular genetic events that underlie testis versus ovary development. Nat Rev 5:509–521 (2004).
Ceplitis H, Ellegren H: Adaptive molecular evolution of HINTW, a female-specific gene in birds. Mol Biol Evol 21:249–254 (2004).
Chapman SC, Lawson A, Macarthur WC, Wiese RJ, Loechel RH, et al: Ubiquitous GFP expression in transgenic chickens using a lentiviral vector. Development 132:935–940 (2005).
Cheng HH, Ying M, Tian YH, Guo Y, McElreavey K, Zhou RJ: Transcriptional diversity of DMRT1 (dsx- and mab3-related transcription factor 1) in human testis. Cell Res 16:389–393 (2006).
Dewing P, Shi T, Horvath S, Vilain E: Sexually dimorphic gene expression in mouse brain precedes gonadal differentiation. Brain Res Mol Brain Res 118:82–90 (2003).
Ebensperger C, Drews U, Mayerova A, Wolf U: Serological H-Y antigen in the female chicken occurs during gonadal differentiation. Differentiation 37:186–191 (1988).
Ellegren H: Dosage compensation: do birds do it as well? Trends Genet 18:25–28 (2002).
Fridolfsson AK, Cheng H, Copeland NG, et al: Evolution of avian sex chromosomes from an ancestral pair of autosomes. Proc Natl Acad Sci USA 95:8147–8152 (1998).
Gahr M: Male Japanese quails with female brains do not show male sexual behaviours. Proc Natl Acad Sci USA 100:7959–7964 (2003).
Govoroun MS, Pannetier M, Pailhoux E, Cocquet J, Brillard JP, et al: Isolation of chicken homolog of the FOXL2 gene and comparison of its expression patterns with those of aromatase during ovarian development. Dev Dyn 231:859–870 (2004).
Graves JAM: Sex and death in birds: a model of dosage compensation that predicts lethality of sex chromosome aneuploids. Cytogenet Genome Res 101:278–282 (2003).
Guo Y, Cheng H, Huang X, Gao S, Yu H, Zhou R: Gene structure, multiple alternative splicing, and expression in gonads of zebrafish Dmrt1. Biochem Biophys Res Commun 330:950–957 (2005).
Hamburger V, Hamilton HL: A series of normal stages in the development of the chicken embryo. J Morphol 88:49–92 (1951).
Harpavat S, Cepko CL: RCAS-RNAi: a loss-of-function method for the developing chick retina. BMC Dev Biol 6:2 (2006).
Hillier LW, Miller W, Birney E, et al: Sequence and comparative analysis of the chicken genome provide unique perspectives on vertebrate evolution. Nature 432:695–716 (2004).
Hori T, Asakawa S, Itoh Y, Shimizu N, Mizuno S: Wpkci, encoding an altered form of PKCI, is conserved widely on the avian W chromosome and expressed in early female embryos: implication of its role in female sex determination. Mol Biol Cell 11:3645–3660 (2000).
Hudson QJ, Smith CA, Sinclair AH: Aromatase inhibition reduces expression of FOXL2 in the embryonic chicken ovary. Dev Dyn 233:1052–1055 (2005).
Itoh Y, Melamed E, Yang X, Kampf K, Wang S, et al: Dosage compensation is less effective in birds than in mammals. J Biol 6:2 (2007).
Kent J, Wheatley SC, Andrews JE, Sinclair AH, Koopman P: A male-specific role for SOX9 in vertebrate sex determination. Development 122:2813–2822 (1996).
Kuroda Y, Arai N, Arita M, Teranishi M, Hori T, et al: Absence of Z-chromosome inactivation for five genes in male chickens. Chromosome Res 9:457–468 (2001).
Lin M, Thorne MH, Martin ICA, Sheldon BL, Jones RC: Development of the gonads in the triploid (ZZW and ZZZ) fowl, Gallus domesticus, and comparison with normal diploid males (ZZ) and females (ZW). Reprod Fert Dev 7:1185–1197 (1995).
Loffler K, Zarkower D, Koopman PA: Etiology of ovarian failure in blepharophimosis ptosis epicanthus inversus syndrome: FOXL2 is a conserved, early-acting gene in vertebrate ovarian development. Endocrinology 144:3237–3243 (2003).
Matsuda M, Nagahama Y, Shinomiya A, Sato T, Matsuda C, et al: DMY is a Y-specific DM-domain gene required for male development in the medaka fish. Nature 417:559–563 (2002).
McCarrey JR, Abbott UK: Chicken gonad differentiation following excision of primordial germ cells. Dev Biol 66:256–265 (1978).
McCarrey JR, Abbott UK: Functional differentiation of chick gonads following depletion of primordial germ cells. J Embryol Exp Morphol 68:161–174 (1982).
McQueen H, McBride D, Miele G, Bird AP, Clinton M: Dosage compensation in birds. Curr Biol 11:253–257 (2001).
Mizuno S, Kunita R, Nakabayashi O, Kuroda Y, et al: Z and W chromosomes of chickens: studies on their gene functions in sex determination and sex differentiation. Cytogenet Geome Res 99:236–244 (2002).
Moriyama S, Ogihara J, Kato J, Hori T, Mizuno S: PKCI-W forms a heterodimer with PKCI-Z and inhibits the biological activities of PKCI-Z in vitro, supporting the predicted role of PCKI-W in sex determination in birds. J Biochem 139:91–97 (2006).
Nakabayashi O, Kikuchi H, Kikuchi T, Mizuno S: Differential expression of genes for aromatase and estrogen receptor during gonadal development in chicken embryos. J Mol Endocrinol 20:193–202 (1998).
Nanda I, Schmid M: Conservation of avian Z chromosomes as revealed by comparative mapping of the Z-linked aldolase B gene. Cytogenet Genome Res 96:176–178 (2002).
Nanda I, Shan Z, Schartl M, Burt DW, Koehler M, et al: 300 million years of conserved synteny between chicken Z and human chromosome 9. Nat Genet 21:258–259 (1999).
Nanda I, Zend-Ajusch E, Shan Z, Grutzner F, Schartl M, et al: Conserved synteny between the chicken Z sex chromosome and human chromosome 9 includes the male regulatory gene DMRT1: a comparative (re)view on avian sex determination. Cytogenet Cell Genet 89:67–78 (2000).
Nanda I, Kondo M, Hornung U, Asakawa S, Winkler C, et al: A duplicated copy of DMRT1 in the sex-determining region of the Y chromosome of the medaka, Oryzias latipes. Proc Natl Acad Sci USA 99:11778–11783 (2002).
Nishikimi H, Kansaku N, Saito N, Usami M, Ohno Y, Shimada K: Sex differentiation and mRNA expression of P450c17, P450arom and AMH in gonads of the chicken. Mol Reprod Dev 55:2– 30 (2000).
O WS, Short RV, Renfree MB, Shaw G: Primary genetic control of somatic sexual differentiation in a mammal. Nature 331:716–717 (1988).
Ogawa A, Murata K, Mizuno S: The location of Z- and W-linked marker genes and sequence on the homomorphic sex chromosomes of the ostrich and the emu. Proc Natl Acad Sci USA 95: 4415–4418 (1998).
O’Neill M, Binder M, Smith CA, Andrews J, Reed K, et al: ASW: a gene with conserved avian W-linkage and female specific expression in chick embryonic gonad. Dev Gen Evol 210:243–249 (2000).
Oréal E, Pieau C, Mattei M-G, Josso N, Picard J-Y, et al: Early expression of AMH in chicken embryonic gonads precedes testicular SOX9 expression. Dev Dyn 212:522–532 (1998).
Oréal E, Mazaud S, Picard J-Y, Magre S, Carré-Eusebe D: Different patterns of Anti-Müllerian Hormone expression as related to DMRT1, SF-1, GATA-4, Wnt-4 and Lhx9 expression, in the chick differentiating gonads. Dev Dyn 225:221–232 (2002).
Pace HC, Brenner C: Feminizing chicks: a model for avian sex determination based on titration of Hint enzyme activity and the predicted structure of an Asw-Hint heterodimer. Genome Biol 4:R18 (2003).
Raudsepp T, Houck ML, O’Brien PC, Ferguson-Smith MA, Ryder OA, Chowdhary BP: Cytogenetic analysis of California condor (Gymnogyps californianus) chromosomes: comparison with chicken (Gallus gallus) macrochromosomes. Cytogenet Genome Res 98:54–60 (2002).
Raymond CS, Shamu CE, Shen MM, Seifert KJ, Hirsch B, et al: Evidence for evolutionary conservation of sex-determining genes. Nature 391:691–695 (1998).
Raymond CS, Parker ED, Kettlewell JR, Brown LG, Page DC, et al: A region of human chromosome 9p required for testis development contains two genes related to known sexual regulators. Hum Mol Genet 8:989–996 (1999a).
Raymond CS, Kettlewell J, Hirsch B, Bardwell V, Zarkower D: Expression of Dmrt1 in the genital ridge of mouse and chicken embryos suggests a role in vertebrate sexual development. Dev Biol 215:208–220 (1999b).
Raymond CS, Murphy MW, O’Sullivan MG, Bardwell VJ, Zarkower D: Dmrt1, a gene related to worm and fly sexual regulators, is required for mammalian testis differentiation. Genes Dev 14:2587–2595 (2000).
Reed KJ, Sinclair AH: FET-1: a novel W-linked, female specific gene up-regulated in the embryonic chicken ovary. Mech Dev 119:Suppl 1:S87–90 (2002).
Romanoff A: The Avian Embryo (Macmillan, New York 1960).
Scheib D: Effects and role of estrogens in avian gonadal differentiation. Differentiation 23:Suppl S87–92 (1983).
Schmid M, Nanda I, Hoehn H, et al: Second report on chicken genes and chromosomes 2005. Cytogenet Genome Res 109:415–479 (2005).
Scholz B, Kultima K, Mattsson A, Axelsson J, Brunstrom B, et al: Sex-dependent gene expression in early brain development of chicken embryos. BMC Neuroscience 15:7–12 (2006).
Shan Z, Nanda I, Wang Y, Schmid M, Vortkamp A, Haaf T: Sex-specific expression of an evolutionarily conserved male regulatory gene, DMRT1, in birds. Cytogenet Cell Genet 89:252–257 (2000).
Shetty S, Griffin DK, Graves JAM: Comparative painting reveals strong chromosome homology over 80 million years of bird evolution. Chromosome Res 7:289–295 (1999).
Shetty S, Kirby P, Zarkower D, Graves JAM: DMRT1 in a ratite bird; evidence for a role in sex determination and discovery of a putative regulatory element. Cytogenet Genome Res 99:245–251 (2002).
Smith C, Sinclair AH: Sex determination: Insights from the chicken. Bioessays 26:120–132 (2004).
Smith CA, Andrews JEA, Sinclair AH: Gonadal sex differentiation in chicken embryos: expression of estrogen receptor and aromatase genes. J Steroid Biochem Mol Biol 60:295–302 (1997).
Smith CA, Smith MJ, Sinclair AH: Expression of chicken steroidogenic factor-1 during gonadal sex differentiation. Gen Comp Endocrinol 113:187–196 (1999a).
Smith CA, Smith MJ, Sinclair AH: Gene expression during gonadogenesis in the chicken embryo. Gene 234:395–402 (1999b).
Smith CA, McClive PJ, Western PS, Reed KJ, Sinclair AH: Conservation of a sex-determining gene. Nature 402:601–602 (1999c).
Smith CA, Clifford V, Western PS, Wilcox SA, Bell KS, Sinclair AH: Cloning and expression of a DAX1 homologue in the chicken embryo. J Mol Endocrinol 24:23–32 (2000).
Smith CA, Katz M, Sinclair AH: DMRT1 is upregulated in the gonads during female-to-male sex reversal in ZW chicken embryos. Biol Reprod 68:560–570 (2003).
Smith CA, McClive PJ, Hudson Q, Sinclair AH: Male-specific cell migration into the developing gonad is a conserved process involving PDGF signaling. Dev Biol 284:337–350 (2005).
Solari AJ: Sex Chromosomes and Sex Determination in Vertebrates (CRC Press, Boca Raton 1993).
Sreenivasulu K, Ganesh S, Raman R: Evolutionarily conserved DMRT1 encodes alternatively spliced transcripts and shows dimorphic expression during gonadal differentiation in the lizard, Calotes versicolor. Mech Dev 119: Suppl 1:S55–64 (2002).
Stiglec R, Ezaz T, Graves JAM: Re-assignment of chicken W chromosome sequences to the Z chromosome by fluorescence in situ hybridisation (FISH). Cytogenet Genome Res 116:132–134 (2007).
Takada S, Wada T, Kaneda R, Choi YL, Yamashita Y, Mano H: Evidence for activation of the Amh gene expression by steroidogenic factor 1. Mech Dev 123:472–480 (2006).
Takagi N, Sasaki M: A phylogenetic study of bird karyotypes. Chromosoma 46:91–120 (1974).
Takagi N, Itoh M, Sasaki M: Chromosome studies in four species of ratites (Aves). Chromosoma 36:281–291 (1972).
Teranishi M, Shimada Y, Hori T, Nakabayashi O, Kikuchi T, et al: Transcripts of the MHM region on the chicken Z chromosome accumulate as non-coding RNA in the nucleus of female cells adjacent to the DMRT1 locus. Chromosome Res 9:147–165 (2001).
Thorne M, Sheldon B: Triploid intersex and chimeric chickens: useful models for studies of avian sex determination, in Reed KC, Graves JAM (eds): Sex Chromosomes and Sex Determining Genes, pp 201–208 (Harwood Academic, Chur 1993).
van de Lavoir MC, Diamond JH, Leighton PA, Mather-Love C, Heyer BS, et al: Germline transmission of genetically modified primordial germ cells. Nature 441:766–769 (2006).
van Limbough J: Morphogenese des cordons sexuels et signification de la zone medullaire de la gonade chez l’embryon de Poulet. Acta Anat 85:248–274 (1973).
Yamada D, Koyama Y, Komatsubara M, Urabe M, Mori M, et al: Comprehensive search for chicken W chromosome-linked genes expressed in early female embryos from the female-minus-male subtracted cDNA macroarray. Chromosome Res 12:741–754 (2004).
Yoshioka H, Ishimaru Y, Sugiyama N, Tsunekawa N, Noce T, et al: Mesonephric FGF signaling is associated with the development of sexually indifferent gonadal primordium in chick embryos. Dev Biol 280:150–161 (2005).
Zaccanti F, Vallisneri M, Quaglia A: Early aspects of differentiation in the gonads of chick embryos. Differentiation 43:71–80 (1990).
Zarkower D: Establishing sexual dimorphism: conservation amidst diversity? Nat Rev Genet 2:175–185 (2001).
Zhao Y, Lu H, Yu H, Cheng H, Zhou R: Multiple alternative splicing in gonads of chicken DMRT1. Dev Gene Evol 217:119–126 (2007).
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