The duck-billed platypus and short-beaked echidna are iconic species in Australia. Their morphology and physiology have puzzled scientists all over the world for more than 200 years. Recent genetic studies, particularly the platypus whole-genome sequencing project, have revealed the molecular basis of some of the extraordinary characteristics of monotremes. This and other works demonstrate the great value of research on our most distantly related mammalian relatives for comparative genomics and developmental biology. In this review we focus on the reproductive biology of monotremes and discuss works that unravel genes involved in lactation, testicular descent, gamete biology and fertilization, and early development. In addition we discuss works on the evolution of the complex sex chromosome system in platypus and echidna, which has also significant impact on our general understanding of mammalian sex chromosomes and sex determination.

Acharya KR, Stuart DI, Phillips DC, McKenzie HA, Teahan CG: Models of the three-dimensional structures of echidna, horse, and pigeon lysozymes: calcium-binding lysozymes and their relationship with alpha-lactalbumins. J Protein Chem 13:569–584 (1994).
Augee M, Gooden B, Musser AM: Echidna: Extraordinary Egg-laying Mammal. (CSIRO Publishing, Collingwood 2006).
Babin PJ: Conservation of a vitellogenin gene cluster in oviparous vertebrates and identification of its traces in the platypus genome. Gene 413:76–82 (2008).
Beard LA, Grigg GC: Reproduction in the short-beaked Echidna, Tachyglossus aculeatus: field observations at an elevated site in south-east Queensland. Proc Linn Soc NSW 122:89–99 (2000).
Beard LA, Grigg GC, Augee ML: Reproduction by Echidnas in a cold climate, in Augee ML (ed): Platypus and Echidnas, pp 93–100 (Royal Zoological Society of New South Wales, Sydney 1992).
Bedford JM: The coevolution of mammalian gametes; in Dunbar BS, O’Rand MG (eds): A Comparative Overview of Mammalian Fertilization, pp 3–35 (Plenum Press, New York 1991).
Bedford JM, Calvin HI: The occurrence and possible functional significance of -S-S crosslinks in sperm heads. J Exp Zool188:137–156 (1974a).
Bedford JM, Calvin HI: Changes in -S-S- linked structures of the sperm tail during epididymal maturation. J Exp Zool 187:181–204 (1974b).
Bedford JM, Rifkin JM: An evolutionary view of the male reproductive tract and sperm maturation in a monotreme mammal – the echidna, Tachyglossus aculeatus. Am J Anat 156:207–230 (1979).
Benda C: Die Spermiogenese der Monotremen. Jen Z Med Naturwiss 6:413–438 (1906).
Bick Y: The Meiotic Chain of Chromosomes of Monotremata (Royal Zoological Society of New South Wales, Sydney 1992).
Biggers JD: Reproduction in male marsupials. Symp Zool Soc Lond 15:251–310 (1966).
Bininda-Emonds OR, Cardillo M, Jones KE, MacPhee RD, Beck RM, et al: The delayed rise of present-day mammals. Nature 446:507–512 (2007).
Bogatcheva NV, Truong A, Feng S, Engel W, Adham IM, Agoulnik AI: GREAT/LGR8 is the only receptor for insulin-like 3 peptide. Mol Endocrinol 17:2639–2646 (2003).
Brice PH, Grigg GC, Beard LA, Donovan JA: Heat tolerance of short-beaked echidnas (Tachyglossus aculeatus) in the field. J Therm Biol 27:449–457 (2002a).
Brice PH, Grigg GC, Beard LA, Donovan JA: Patterns of inactivity in echidnas (Tachyglossus aculeatus) free-ranging in a hot dry climate: correlates with ambient temperature, time of day and season. Aust J Zool 50:461–475 (2002b).
Burrell H: The Platypus. Its Discovery, Zoological Position, Form and Characteristics, Habits, Life History, etc. (Angus and Robertson Ltd, Sydney 1927).
Carrick FN, Hughes RL: Reproduction in male monotremes. Austr Zool 20:211–231 (1978).
Carrick FN, Hughes RL: Aspects of the structure and development of monotreme spermatozoa and their relevance to the evolution of mammalian sperm morphology. Cell Tissue Res 222:127–141 (1982).
Coveney D, Shaw D, Hutson JM, Renfree MB: The development of the gubernaculum and inguinal closure in the marsupial Macropus eugenii. J Anat 201:239–256 (2002).
Davis BK: Timing of fertilization in mammals: cholesterol/phospholipid ratio as a determinant of the capacitation interval. Proc Natl Acad Sci USA 78:7560–7564 (1981).
Djakiew D: Studies of the male reproductive tract of the echidna (Tachyglossus aculeatus) with particular emphasis on the epididymis. Honours thesis, Department of Biological Sciences, University of Newcastle, NSW, Australia (1978).
Djakiew D: Reproduction in the Male Echidna (Tachyglossus aculateus) with Particular Emphasis on the Epididymis, pp 158 (Department of Biological Sciences, University of Newcastle, Newcastle 1982).
Djakiew D, Jones RC: Structural differentiation of the male genital ducts of the echidna (Tachyglossus aculeatus). J Anat 132:187–202 (1981).
Djakiew D, Jones RC: Ultrastructure of the ductus epididymidis of the echidna, Tachyglossus aculeatus. J Anat 135:625–634 (1982).
Djakiew D, Jones RC: Sperm maturation, fluid transport, and secretion and absorption of protein in the epididymis of the echidna, Tachyglossus aculeatus. J Reprod Fertil 68:445–456 (1983).
Dohm JC, Tsend-Ayush E, Reinhardt R, Grutzner F, Himmelbauer H: Disruption and pseudoautosomal localization of the major histocompatibility complex in monotremes. Genome Biol 8:R175 (2007).
Fawcett DW: The mammalian spermatozoon. Dev Biol 44:395–436 (1975).
Flannery TF, Groves CP: A revision of the genus Zaglossus (Monotremata, Tachyglossidae) with description of new species and subspecies. Mammalia 62:367–396 (1998).
Foster HA, Abeydeera LR, Griffin DK, Bridger JM: Non-random chromosome positioning in mammalian sperm nuclei, with migration of the sex chromosomes during late spermatogenesis. J Cell Sci 118:1811–1820 (2005).
Freyer C, Zeller U, Renfree MB: The marsupial placenta: a phylogenetic analysis. J Exp Zoolog A Comp Exp Biol 299:59–77 (2003).
Grafodatskaya D, Rens W, Wallis MC, Trifonov V, O’Brien P C, et al: Search for the sex-determining switch in monotremes: mapping WT1, SF1, LHX1, LHX2, FGF9, WNT4, RSPO1 and GATA4 in platypus. Chromosome Res 15:777–785 (2007).
Greaves IK, Rens W, Ferguson-Smith MA, Griffin D, Marshall Graves JA: Conservation of chromosome arrangement and position of the X in mammalian sperm suggests functional significance. Chromosome Res 11:503–512 (2003).
Green B, Griffiths M, Newgrain K: Intake of milk by suckling echidnas (Tachyglossus aculeatus). Comp Biochem Physiol A 81:441–444 (1985).
Griffiths AL, Renfree MB, Shaw G, Watts LM, Hutson JM: The tammar wallaby (Macropus eugenii) and the Sprague-Dawley rat: comparative anatomy and physiology of inguinoscrotal testicular descent. J Anat 183:441–450 (1993).
Griffiths M: Echidnas (Permagon Press, Oxford, London, New York 1968).
Griffiths M: The Biology of the Monotremes (Academic Press, New York, San Fransisco, London 1978).
Griffiths M: Monotremes, in Neill EKaJ (ed): Encyclopedia of Reproduction, pp 295–302 (Academic Press, New York 1999).
Gruetzner F, Ashley T, Rowell DM, Marshall Graves JA: How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals. Chromosoma 115:75–88 (2006).
Grützner F, Graves JAM: A platypus’ eye view of the mammalian genome. Curr Opin Genet Dev 14:642–649 (2004).
Grützner F, Rens W, Tsend-Ayush E, El-Mogharbel N, O’Brien PC, et al: In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature 432:913–917 (2004).
Haaf T, Ward DC: Higher order nuclear structure in mammalian sperm revealed by in situ hybridization and extended chromatin fibers. Exp Cell Res 219:604–611 (1995).
Hazzouri M, Rousseaux S, Mongelard F, Usson Y, Pelletier R, et al: Genome organization in the human sperm nucleus studied by FISH and confocal microscopy. Mol Reprod Dev 55:307–315 (2000).
Holland N, Jackson SM: Reproductive behaviour and food consumption associated with the captive breeding of platypus (Ornithorhynchus anatinus). J Zool (Lond) 256:279–288 (2002).
Home E: A description of the anatomy of the Ornithorhyncus paradoxus. Phil Trans R Soc 1802:67–84 (1802a).
Home E: A description of the anatomy of Ornithorhynchus hystrix. Phil Trans R Soc 1802:348–364 (1802b).
Hughes RL, Hall LS: Early development and embryology of the platypus. Phil Trans R Soc Lond B Biol Sci 353:1101–1114 (1998).
Johnson SD, Smith D, Pyne M, Stenzel D, Holt WV: One-sided ejaculation of echidna sperm bundles. Am Nat 170:E162–164 (2007).
Jones RC: Studies of the structure of the head of boar spermatozoa from the epididymis. J Reprod Fertil Suppl 13:51–64 (1971).
Jones RC: Evolution of the vertebrate epididymis. J Reprod Fertil Suppl 53:163–182 (1998).
Jones RC: Evolution of the epididymis, in Robaire B, Hinton BT (eds): The Epididymis. From Molecules to Clinical Practice: A comprehensive Survey of the Efferent Ducts, the Epididymis and the Vas Deferens, pp 11–33 (Kluwer Academic/Plenum Publishers, New York, London 2002).
Jones RC, Brosnan MF: Studies of the deferent ducts from the testis of the African elephant Loxodonta africana. I. Structural differentiation. J Anat 132:371–386 (1981).
Jones RC, Lin M: Spermatogenic cycle in birds and mammals, in Milligan SR (ed): The Oxford Reviews of Reproductive Biology, pp 233–264 (Oxford University Press, Oxford, New York, Tokyo 1993).
Jones RC, Stone GM, Zupp J: Reproduction in the male echidna, in Augee ML (ed): Platypus and Echidnas, pp 115–126 (Royal Society of New South Wales, Sydney 1992).
Jones RC, Djakiew D, Dacheux JL: Adaptations of the echidna (Tachyglossus aculateus) for sperm production, particularly in an arid environment. Austr Mammal 26:199–204 (2004).
Jones RC, Dacheux JL, Nixon B, Ecroyd HW: The role of the epididymis in sperm competition. Asian J Androl 9:493–499 (2007).
Jovine L, Qi H, Williams Z, Litscher ES, Wassarman PM: Features that affect secretion and assembly of zona pellucida glycoproteins during mammalian oogenesis. Soc Reprod Fertil Suppl 63:187–201 (2007).
Kamat AA, Feng S, Bogatcheva NV, Truong A, Bishop CE, Agoulnik AI: Genetic targeting of relaxin and insulin-like factor 3 receptors in mice. Endocrinology 145:4712–4720 (2004).
Kawasaki K, Weiss KM: Mineralized tissue and vertebrate evolution: the secretory calcium-binding phosphoprotein gene cluster. Proc Natl Acad Sci USA 100:4060–4065 (2003).
Kikuchi M, Kawano K, Nitta K: Calcium-binding and structural stability of echidna and canine milk lysozymes. Protein Sci 7:2150–2155 (1998).
Lester KS, Boyde A, Gilkeson C, Archer M: Marsupial and monotreme enamel structure. Scanning Microsc 1:401–420 (1987).
Lin M, Jones RC: Spermiogenesis and spermiation in the Japanese quail (Coturnix coturnix japonica). J Anat 183:525–535 (1993).
Lin M, Jones RC: Spermiogenesis and spermiation in a monotreme mammal, the platypus, Ornithorhynchus anatinus. J Anat 196:217–232 (2000).
Mackay S, Xie Q, Ullmann SL, Gilmore DP, Payne AP: Postnatal development of the reproductive system in the grey short-tailed opossum, Monodelphis domestica. Anat Embryol (Berl) 208:121–133 (2004).
Meckelio IF: Ornithorhynchi paradoxi Descriptio Anatomica. Gerhardum Fleischerum Lipsiae (1826).
Messer M, Kerry KR: Milk carbohydrates of the echidna and the platypus. Science 180:201–203 (1973).
Messer M, Gadiel PA, Ralston G, Griffiths M: Carbohydrates of the milk of the platypus. Aust J Biol Sci 36:129–137 (1983).
Messer M, Griffiths M, Rismiller PD, Shaw DC: Lactose synthesis in a monotreme, the echidna (Tachyglossus aculeatus): isolation and amino acid sequence of echidna alpha-lactalbumin. Comp Biochem Physiol B Biochem Mol Biol 118:403–410 (1997).
Meyer-Ficca M, Muller-Navia J, Scherthan H: Clustering of pericentromeres initiates in step 9 of spermiogenesis of the rat (Rattus norvegicus) and contributes to a well defined genome architecture in the sperm nucleus. J Cell Sci 111:1363–1370 (1998).
Morel L, Dufaure JP, Depeiges A: The lipocalin sperm coating lizard epididymal secretory protein family: mRNA structural analysis and sequential expression during the annual cycle of the lizard, Lacerta vivipara. J Mol Endocrinol 24:127–133 (2000).
Murtagh C: A unique cytogenetics system in monotremes. Chromosoma 65:37–57 (1977).
Nef S, Parada LF: Cryptorchidism in mice mutant for Insl3. Nat Genet 22:295–299 (1999).
Niwa H, Toyooka Y, Shimosato D, Strumpf D, Takahashi K, Yagi R, Rossant J: Interaction between Oct3/4 and Cdx2 determines trophectoderm differentiation. Cell 123:917–929 (2005).
Niwa H, Sekita Y, Tsend-Ayush E, Grützner F: Platypus Pou5f1 reveals the first steps in the evolution of trophectoderm differentiation and pluripotency in mammals. Evol Dev, in press (2008).
Oftedal OT: The mammary gland and its origin during synapsid evolution. J Mammary Gland Biol Neoplasia 7:225–252 (2002).
Ono R, Nakamura K, Inoue K, Naruse M, Usami T, et al: Deletion of Peg10, an imprinted gene acquired from a retrotransposon, causes early embryonic lethality. Nat Genet 38:101–106 (2006).
Park JL, Semyonov J, Chang CL, Yi W, Warren W, Hsu SYT: Origin of INSL3- mediated testicular descent in therian mammals. Genome Res 18:974–985 (2008).
Raymond CS, Kettlewell JR, Hirsch B, Bardwell VJ, 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 (1999).
Rens W, Grützner F, O’Brien P C, Fairclough H, Graves JA, Ferguson-Smith MA: Resolution and evolution of the duck-billed platypus karyotype with an X1Y1X2Y2X3Y3X4Y4 X5Y5 male sex chromosome constitution. Proc Natl Acad Sci USA 101:16257–16261 (2004).
Rens W, O’Brien PC, Grützner F, Clarke O, Graphodatskaya D, et al: The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z. Genome Biol 8:R243 (2007).
Retief JD, Winkfein RJ, Dixon GH: Evolution of the monotremes. The sequences of the protamine P1 genes of platypus and echidna. Eur J Biochem 218:457–461 (1993).
Rismiller PD: Field observations on Kangaroo Island echidnas (Tachyglossus aculateus multiaculateus) during the breeding season, in Augee ML (ed): Platypus and Echidnas, pp 101–105 (Royal Zoological Society of New South Wales, Sydney 1992).
Rismiller PD: Overcoming a prickly problem. Aust Nat Hist 4:22–29 (1993).
Rismiller PD: The Echidna – Australia’s Enigma. (Hugh Lauter Levin, Southport 1999).
Rismiller PD, McKelvey MW: Frequency of breeding and recruitment in the short-beaked echidna, Tachyglossus aculeatus. J Mammal 81:1–17 (2000).
Rowe T, Rich TH, Vickers-Rich P, Springer M, Woodburne MO: The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proc Natl Acad Sci USA 105:1238–1242 (2008).
Selwood L, Johnson MH: Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins. Bioessays 28:128–145 (2006).
Setchell BP: The Mammalian Testis (Elek Books, London Cornell University Press, London 1978).
Sharp JA, Lefèvre C, Nicholas KR: Molecular evolution of monotreme and marsupial whey acidic protein genes. Evol Dev 9:378–392 (2007).
Solovei IV, Joffe BI, Hori T, Thomson P, Mizuno S, Macgregor HC: Unordered arrangement of chromosomes in the nuclei of chicken spermatozoa. Chromosoma 107:184–188 (1998).
Stewart S, Bausek N, Wohlrab F, Schneider WJ, Horrocks A, Wishart G: Species specificity in avian sperm:perivitelline interaction. Comp Biochem Physiol A Mol Integr Physiol 137:657–663 (2004).
Suzuki S, Ono R, Narita T, Pask AJ, Shaw G, et al: Retrotransposon silencing by DNA methylation can drive mammalian genomic imprinting. PLoS Genet 3:e55 (2007).
Teahan CG, McKenzie HA, Shaw DC, Griffiths M: The isolation and amino acid sequences of echidna (Tachyglossus aculeatus) milk lysozyme I and II. Biochem Int 24:85–95 (1991).
Temple-Smith PD: Seasonal breeding biology of the platypus, Ornithorhynchus anatinus (Shaw, 1799). PhD thesis submitted to Aust Nat Univ (1973).
Temple-Smith P, Grant T: Uncertain breeding: a short history of reproduction in monotremes. Reprod Fertil Dev 13:487–497 (2001).
van der Schoot P: Foetal genital development in Hyrax capensis, a species with primary testicondia: proposal for the evolution of Hunter’s gubernaculum. Anat Rec 244:386–401 (1996).
van Rheede T, Bastiaans T, Boone DN, Hedges SB, de Jong WW, Madsen O: The platypus is in its place: nuclear genes and indels confirm the sister group relation of monotremes and Therians. Mol Biol Evol 23:587–597 (2006).
Veyrunes F, Waters PD, Miethke P, Rens W, McMillan D, et al: Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes. Genome Res 18:965–973 (2008).
Wallis MC, Waters PD, Delbridge ML, Kirby PJ, Pask AJ, et al: Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes. Chromosome Res 15:949–959 (2007).
Warren WC, Hillier LW, Marshall Graves JA, Birney E, Ponting CP, et al: Genome analysis of the platypus reveals unique signatures of evolution. Nature 453:175–183 (2008).
Waters PD, Delbridge ML, Deakin JE, El-Mogharbel N, Kirby PJ, et al: Autosomal location of genes from the conserved mammalian X in the platypus (Ornithorhynchus anatinus): implications for mammalian sex chromosome evolution. Chromosome Res 13:401–410 (2005).
Watson JM, Meyne J, Graves JAM: Chromosomal composition and position in the echidna meiotic translocation chain, in Augee ML (ed): Platypus and Echidnas, pp 53–63 (Royal Zoological Society of New South Wales, Sydney 1992).
Watson JM, Meyne J, Graves JA: Ordered tandem arrangement of chromosomes in the sperm heads of monotreme mammals. Proc Natl Acad Sci USA 93:10200–10205 (1996).
Werdelin L, Nilsonne A: The evolution of the scrotum and testicular descent in mammals: a phylogenetic view. J Theor Biol 196:61–72 (1999).
Wilhelm D, Koopman P: The makings of maleness: towards an integrated view of male sexual development. Nat Rev Genet 7:620–631 (2006).
Woodburne MO, Rich TH, Springer MS: The evolution of tribospheny and the antiquity of mammalian clades. Mol Phylogenet Evol 28:360–385 (2003).
Zalensky A, Zalenskaya I: Organization of chromosomes in spermatozoa: an additional layer of epigenetic information? Biochem Soc Trans 35:609–611 (2007).
Zalensky AO, Allen MJ, Kobayashi A, Zalenskaya IA, Balhorn R, Bradbury EM: Well-defined genome architecture in the human sperm nucleus. Chromosoma 103:577–590 (1995).
Zimmermann S, Steding G, Emmen JM, Brinkmann O, Nayernia K, et al: Targeted disruption of the Insl3 gene causes bilateral cryptorchidism. Mol Endocrinol 13:681–691 (1999).
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.