New observations over the last 25 years of hormone-independent sexual dimorphisms have gradually and unequivocally overturned the dogma, arising from Jost's elegant experiments in the mid-1900s, that all somatic sex dimorphisms in vertebrates arise from the action of gonadal hormones. Although we know that Sry, a Y-linked gene, is the primary gonadal sex determinant in mammals, more recent analysis in marsupials, mice, and finches has highlighted numerous sexual dimorphisms that are evident well before the differentiation of the testis and which cannot be explained by a sexually dimorphic hormonal environment. In marsupials, scrotal bulges and mammary primordia are visible before the testis has differentiated due to the expression of a gene(s) on the X chromosome. ZZ and ZW gynandromorph finches have brains that develop in a sexually dimorphic way dependent on their sex chromosome content. In genetically manipulated mice, it is the X chromosomes, not the gonads, that determine many characters including rate of early development, adiposity, and neural circuits. Even spotted hyenas have sexual dimorphisms that cannot be simply explained by hormonal exposure. This review discusses the recent findings that confirm that there are hormone-independent sexual dimorphisms well before the gonads begin to produce their hormones.

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).
Al Nadaf S, Waters PD, Koina E, Deakin JE, Jordan KS, Graves JA: Activity map of the tammar X chromosome shows that marsupial X inactivation is incomplete and escape is stochastic. Genome Biol 11:R122 (2010).
Arnold AP: Experimental analysis of sexual differentiation of the zebra finch brain. Brain Res Bull 44:503-507 (1997a).
Arnold AP: Sexual differentiation of the zebra finch song system: positive evidence, negative evidence, null hypotheses, and a paradigm shift. J Neurobiol 33:572-584 (1997b).
Arnold AP: Sex chromosomes and brain gender. Nat Rev Neurosci 5:701-708 (2004).
Arnold AP: Mouse models for evaluating sex chromosome effects that cause sex differences in non-gonadal tissues. J Neuroendocrinol 21:377-386 (2009).
Arnold AP: The end of gonad-centric sex determination in mammals. Trends Genet 28:55-61 (2012).
Arnold AP, Burgoyne PS: Are XX and XY brain cells intrinsically different? Trends Endocrinol Metab 15:6-11 (2004).
Arnold AP, Chen X: What does the ‘four core genotypes' mouse model tell us about sex differences in the brain and other tissues? Front Neuroendocrinol 30:1-9 (2009).
Arnold AP, Chen X, Link JC, Itoh Y, Reue K: Cell-autonomous sex determination outside of the gonad. Dev Dyn 242:371-379 (2013).
Avery B, Madison V, Greve T: Sex and development in bovine in-vitro fertilized embryos. Theriogenology 35:953-963 (1991).
Avery B, Jørgensen CB, Madison V, Greve T: Morphological development and sex of bovine in vitro-fertilized embryos. Mol Reprod Dev 32:265-270 (1992).
Bermejo-Alvarez P, Rizos D, Rath D, Lonergan P, Gutierrez-Adan A: Sex determines the expression level of one third of the actively expressed genes in bovine blastocysts. Proc Natl Acad Sci USA 107:3394-3399 (2010).
Bolliger A, Canny AJ: The effect of synthetic oestrogens, stilboestrol and hexoestrol on the pouch and scrotum of Trichosurus vulpecula. J Proc R Soc New South Wales 75:21-36 (1941).
Brown CJ, Ballabio A, Rupert JL, Lafreniere RG, Grompe M, et al: A gene from the region of the human X inactivation center is expressed exclusively from the inactive X chromosome. Nature 349:38-44 (1991).
Brown CJ, Hendrich BD, Rupert JL, Lafreniere RG, Xing Y, et al: The human XIST gene: analysis of a 17 kb inactive X-specific RNA that contains conserved repeats and is highly localized within the nucleus. Cell 71:527-542 (1992).
Burgoyne PS: A Y-chromosomal effect on blastocyst cell number in mice. Development 117:341-345 (1993).
Burgoyne PS, Thornhill A: The genetic basis of XX-XY differences present before gonadal sex differentiation in mice, in Reed KC, Graves JAM (eds): Sex Chromosomes and Sex Determining Genes, pp 369-372 (Harwood Academic, Chur, Switzerland; Langhorne, USA 1993).
Burgoyne PS, Thornhill AR, Boudrean SK, Darling SM, Bishop CE, Evans EP: The genetic basis of XX-XY differences present before gonadal sex differentiation in the mouse. Philos Trans R Soc Lond B Biol Sci 350:253-260 discussion 260-251 (1995).
Burgoyne PS, Lovell-Badge R, Rattigan A: Evidence that the testis determination pathway interacts with a non-dosage compensated, X-linked gene. Int J Dev Biol 45:509-512 (2001).
Burns RK: The differentiation of sex in the opossum (Didelphis virginiana) and its modification by the male hormone testosterone propionate. J Morphol 65:79-119 (1939a).
Burns RK: Sex differentiation during the early pouch stages of the opossum (Didelphys virginiana) and a comparison of the anatomical changes induced by male and female sex hormones. J Morphol 65:497-547 (1939b).
Burns RK: Hormones and experimental modification of sex in the opossum. Biol Symp 9:125-146 (1942).
Burns RK: Role of hormones in the differentiation of sex, in Young WC (ed): Sex and Internal Secretions, Vol 1, pp 76-158 (Williams and Wilkins, Baltimore 1961).
Calatayud NE, Pask AJ, Shaw G, Richings NM, Osborn S, Renfree MB: Ontogeny of the oestrogen receptors ESR1 and ESR2 during gonadal development in the tammar wallaby, Macropus eugenii. Reproduction 139:599-611 (2010).
Chen X, Grisham W, Arnold AP: X chromosome number causes sex differences in gene expression in adult mouse striatum. Eur J Neurosci 29:768-776 (2009).
Chen X, McClusky R, Chen J, Beaven SW, Tontonoz P, et al: The number of X chromosomes causes sex differences in adiposity in mice. PLoS Genet 8:e1002709 (2012).
Chew KY, Pask AJ, Hickford D, Shaw G, Renfree MB: A dual role for SHH in phallic development. Sex Dev (in press) (2013a).
Chew KY, Shaw G, Yu H, Pask AJ, Renfree MB: Heterochrony in the regulation of the developing marsupial limb. Dev Dyn [Epub ahead of print] (2013b).
Cooper DW: The evolution of sex determination, sex chromosome dimorphism, and X-inactivation in therian mammals: a comparison of metatherians (marsupials) and eutherians (‘placentals'), in Reed KC, Graves JA (eds): Sex Chromosomes and Sex Determining Genes, pp 183-198 (Harwood Academic, Chur, Switzerland; Langhorne, USA 1993).
Coveney D, Shaw G, Renfree MB: Estrogen-induced gonadal sex reversal in the tammar wallaby. Biol Reprod 65:613-621 (2001).
Cunha GR, Wang Y, Place NJ, Liu W, Baskin L, Glickman SE: Urogenital system of the spotted hyena (Crocuta crocuta Erxleben): a functional histological study. J Morphol 256:205-218 (2003).
Cunha GR, Place NJ, Baskin L, Conley A, Weldele M, et al: The ontogeny of the urogenital system of the spotted hyena (Crocuta crocuta Erxleben). Biol Reprod 73:554-564 (2005).
Davies W, Isles A, Smith R, Karunadasa D, Burrmann D, et al: Xlr3b is a new imprinted candidate for X-linked parent-of-origin effects on cognitive function in mice. Nat Genet 37:625-629 (2005).
Deakin JE: Marsupial X chromosome inactivation: past, present and future. Aust J Zool 61:13-23 (2013).
De Vries GJ, Rissman EF, Simerly RB, Yang LY, Scordalakes EM, et al: A model system for study of sex chromosome effects on sexually dimorphic neural and behavioral traits. J Neurosci 22:9005-9014 (2002).
Dewing P, Chiang CW, Sinchak K, Sim H, Fernagut PO, et al: Direct regulation of adult brain function by the male-specific factor SRY. Curr Biol 16:415-420 (2006).
Drea CM, Weldele ML, Forger NG, Coscia EM, Frank LG, et al: Androgens and masculinization of genitalia in the spotted hyaena (Crocuta crocuta). 2. Effects of prenatal anti-androgens. J Reprod Fertil 113:117-127 (1998).
Fadem BH, Harder JD: Evidence for high levels of androgen in peripheral plasma during postnatal development in a marsupial: the gray short-tailed opossum (Monodelphis domestica). Biol Reprod 46:105-108 (1992).
Fadem BH, Tesoriero JV: Inhibition of testicular development and feminization of the male genitalia by neonatal estrogen treatment in a marsupial. Biol Reprod 34:771-776 (1986).
Foster JW, Brennan FE, Hampikian GK, Goodfellow PN, Sinclair AH, et al: Evolution of sex determination and the Y chromosome: SRY-related sequences in marsupials. Nature 359:531-533 (1992).
Frank LG, Glickman SE, Powch I: Sexual dimorphism in the spotted hyaena. J Zool Lond 221:308-313 (1990).
Frankham GJ, Temple-Smith PD: Absence of mammary development in male Dromiciops gliroides: another link to the Australian marsupial fauna. J Mammalogy 93:572-578 (2012).
Gardner DK, Larman MG, Thouas GA: Sex-related physiology of the preimplantation embryo. Mol Hum Reprod 16:539-547 (2010).
Gardner DK, Leese HJ: Assessment of embryo viability prior to transfer by the noninvasive measurement of glucose uptake. J Exp Zool 242:103-105 (1987).
Gatewood JD, Wills A, Shetty S, Xu J, Arnold AP, et al: Sex chromosome complement and gonadal sex influence aggressive and parental behaviors in mice. J Neurosci 26:2335-2342 (2006).
George FW, Hodgins MB, Wilson JD: The synthesis and metabolism of gonadal steroids in pouch young of the opossum, Didelphis virginiana. Endocrinology 116:1145-1150 (1985).
Glickman SE, Short RV, Renfree MB: Sexual differentiation in three unconventional mammals: spotted hyenas, elephants and tammar wallabies. Horm Behav 48:403-417 (2005).
Grant J, Mahadevaiah SK, Khil P, Sangrithi MN, Royo H, et al: Rsx is a metatherian RNA with Xist-like properties in X-chromosome inactivation. Nature 487:254-258 (2012).
Hughes RL, Pearse AM, Cooper DW, Joss JMP, Johnston PG, Jones MK: The genetic basis of marsupial gonadogenesis and sexual phenotype, in Reed KC, Graves JA (eds): Sex Chromosomes and Sex Determining Genes, pp 17-48 (Harwood Academic, Chur, Switzerland; Langhorne, USA 1993).
Hutson JM, Shaw G, O WS, Short RV, Renfree MB: Mullerian inhibiting substance production and testicular migration and descent in the pouch young of a marsupial. Development 104:549-556 (1988).
Johnston PG, Robinson ES: Glucose-6-phosphate dehydrogenase expression in heterozygous kangaroo embryos and extra-embryonic membranes. Genet Res 45:205-208 (1985).
Jost A: Recherches sur la différenciation sexuelle de l'embryon de lapin. III. Rôle des gonades foetales dans la différenciation sexuelle somatique. Arch Anat Microsc Morphol Exp 36:271-315 (1947).
Jost A: Recherches sur la différenciation sexuelle de l'embryon de lapin IV. Organogenèse sexuelle masculine après décapitation du foetus. Arch Anat Microsc Morph Exp 40:247-281 (1951).
Jost A: Problems of fetal endocrinology: the gonadal and hypophyseal hormones. Rec Progr Horm Res 8:379-418 (1953).
Jost A: The role of fetal hormones in prenatal development. Harvey Lect 55:201-226 (1961).
Jost A: Hormonal factors in the sex differentiation of the mammalian foetus. Philos Trans R Soc Lond B Biol Sci 259:119-130 (1970).
Koopman P: The molecular biology of SRY and its role in sex determination in mammals. Reprod Fertil Dev 7:713-722 (1995).
Koopman P, Munsterberg A, Capel B, Vivian N, Lovell-Badge R: Expression of a candidate sex-determining gene during mouse testis differentiation. Nature 348:450-452 (1990).
Leihy MW, Shaw G, Renfree MB, Wilson JD: Administration of 5α-androstane-3α,17β-diol to female tammar wallaby pouch young causes development of a mature prostate and male urethra. Endocrinology 143:2643-2651 (2002).
Leihy MW, Shaw G, Wilson JD, Renfree MB: Penile development is initiated in the tammar wallaby pouch young during the period when 5α-androstane-3α,17β-diol is secreted by the testes. Endocrinology 145:3346-3352 (2004).
McCrady E: The embryology of the opossum. Am Anat Mem 16:14-226 (1938).
Moore CR: Modification of sexual development in the opossum by sex hormones. Proc Soc Exp Biol 40:544-546 (1939).
Morgan CF: The normal development of the ovary of the opossum from birth to maturity and its reactions to sex hormones. J Morphol 73:27-85 (1943).
O WS, Short RV, Renfree MB, Shaw G: Primary genetic control of somatic sexual differentiation in a mammal. Nature 331:716-717 (1988).
Pask AJ, Calatayud NE, Shaw G, Wood WM, Renfree MB: Oestrogen blocks the nuclear entry of SOX9 in the developing gonad of a marsupial mammal. BMC Biol 8:113 (2010).
Renfree MB: Endocrinology of pregnancy, parturition and lactation in marsupials, in Lamming GE (ed): Marshall's Physiology of Reproduction, Vol 3: Pregnancy and Lactation, pp 677-766 (Chapman & Hall, London 1994).
Renfree MB, Short RV: Sex determination in marsupials: evidence for a marsupial-eutherian dichotomy. Philos Trans R Soc Lond B Biol Sci 322:41-53 (1988).
Renfree MB, Robinson ES, Short RV, Vandeberg JL: Mammary glands in male marsupials. I. Primordia in neonatal opossums Didelphis virginiana and Monodelphis domestica. Development 110:385-390 (1990).
Renfree MB, Wilson JD, Short RV, Shaw G, George FW: Steroid hormone content of the gonads of the tammar wallaby during sexual differentiation. Biol Reprod 47:644-647 (1992).
Renfree MB, O WS, Short RV, Shaw G: Sexual differentiation of the urogenital system of the fetal and neonatal tammar wallaby, Macropus eugenii. Anat Embryol (Berl) 194:111-134 (1996).
Renfree MB, Fenelon J, Wijiyanti G, Wilson JD, Shaw G: Wolffian duct differentiation by physiological concentrations of androgen delivered systemically. Dev Biol 334:429-436 (2009).
Robinson ES, Renfree MB, Short RV, VandeBerg JL: Mammary glands in male marsupials. 2. Development of teat primordia in Didelphis virginiana and Monodelphis domestica. Reprod Fertil Dev 3:295-301 (1991).
Ryhorchuk AR, Shaw G, Butler CM, Renfree MB: Effects of a 5α-reductase inhibitor, finasteride, on the developing prostate and testis of a marsupial. J Androl 18:123-130 (1997).
Schlinger BA, Arnold AP: Circulating estrogens in a male songbird originate in the brain. Proc Natl Acad Sci USA 89:7650-7653 (1992).
Scott WJ, Holson JF: Weight differences in rat embryos prior to sexual differentiation. J Embryol Exp Morphol 40:259-263 (1977).
Seller MJ, Perkins-Cole KJ: Sex difference in mouse embryonic development at neurulation. J Reprod Fert 79:159-161 (1987).
Sharman GB: Reproductive physiology of marsupials. Science 167:1221-1228 (1970).
Sharman GB, Robinson ES, Walton SM, Berger PJ: Sex chromosomes and reproductive anatomy of some intersexual marsupials. J Reprod Fert 21:57-68 (1970).
Sharman GB, Hughes RL, Cooper DW: The chromosomal basis of sex-differentiation in marsupials. Aust J Zool 37:451-466 (1990).
Shaw G, Renfree MB, Short RV, O WS: Experimental manipulation of sexual differentiation in wallaby pouch young treated with exogenous steroids. Development 104:689-701 (1988).
Shaw G, Harry JL, Whitworth DJ, Renfree MB: Sexual determination and differentiation in the marsupial Macropus eugenii, in Saunders NR, Hinds L (eds): Marsupial Biology Recent Research, New Perspectives, pp 132-141 (University of New South Wales Press, Sydney 1997).
Shaw G, Renfree MB, Leihy MW, Shackleton CH, Roitman E, Wilson JD: Prostate formation in a marsupial is mediated by the testicular androgen 5α-androstane-3α,17β-diol. Proc Natl Acad Sci U S A 97:12256-12259 (2000).
Shaw G, Fenelon J, Sichlau M, Auchus RJ, Wilson JD, Renfree MB: Role of the alternate pathway of dihydrotestosterone formation in virilization of the Wolffian ducts of the tammar wallaby, Macropus eugenii. Endocrinology 147:2368-2373 (2006).
Simpson HB, Vicario DS: Early estrogen treatment alone causes female zebra finches to produce learned, male-like vocalizations. J Neurobiol 22:755-776 (1991a).
Simpson HB, Vicario DS: Early estrogen treatment of female zebra finches masculinizes the brain pathway for learned vocalizations. J Neurobiol 22:777-793 (1991b).
Sinclair AH, Berta P, Palmer MS, Hawkins JR, Griffiths BL, et al: A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif. Nature 346:240-244 (1990).
Tyndale-Biscoe CH, Renfree MB: Reproductive Physiology of Marsupials (Cambridge University Press, Cambridge 1987).
Ullmann SL, Shaw G, Alcorn GT, Renfree MB: Migration of primordial germ cells to the developing gonadal ridges in the tammar wallaby Macropus eugenii. J Reprod Fertil 110:135-143 (1997).
Wade J, Arnold AP: Functional testicular tissue does not masculinize development of the zebra finch song system. Proc Natl Acad Sci USA 93:5264-5268 (1996).
Wilson JD, George FW, Shaw G, Renfree MB: Virilization of the male pouch young of the tammar wallaby does not appear to be mediated by plasma testosterone or dihydrotestosterone. Biol Reprod 61:471-475 (1999).
Wilson JD, Auchus RJ, Leihy MW, Guryev OL, Estabrook RW, et al: 5α-androstane-3α,17β-diol is formed in tammar wallaby pouch young testes by a pathway involving 5α-pregnane-3α,17α-diol-20-one as a key intermediate. Endocrinology 144:575-580 (2003).
Xie Q, Mackay S, Ullmann SL, Gilmore DP, Payne AP, Gray C: Postnatal development of Leydig cells in the opossum (Monodelphis domestica): an immunocytochemical and endocrinological study. Biol Reprod 58:664-669 (1998).
Zhao D, McBride D, Nandi S, McQueen HA, McGrew MJ, et al: Somatic sex identity is cell autonomous in the chicken. Nature 464:237-242 (2010).
Copyright / Drug Dosage / Disclaimer
Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher.
Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug.
Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.
You do not currently have access to this content.