title: Embryonic gonadal and sexual organ development in a small viviparous skink, Niveoscincus ocellatus.
abstract: The majority of research into the timing of gonad differentiation (and sex determination) in reptiles has focused on oviparous species. This is largely because: (1) most reptiles are oviparous; (2) it is easier to manipulate embryonic developmental conditions (e.g., temperature) of eggs than oviductal embryos and (3) modes of sex determination in oviparous taxa were thought to be more diverse since viviparity and environmental sex determination (ESD)/temperature-dependent sex determination (TSD) were considered incompatible. However, recent evidence suggests the two may well be compatible biological attributes, opening potential new lines of enquiry into the evolution and maintenance of sex determination. Unfortunately, the baseline information on embryonic development in viviparous species is lacking and information on gonad differentiation and sexual organ development is almost non-existent. Here we present an embryonic morphological development table (10 stages), the sequence of gonad differentiation and sexual organ development for the viviparous spotted snow skink (Niveoscincus ocellatus). Gonad differentiation in this species is similar to other reptilian species. Initially, the gonads are indifferent and both male and female accessory ducts are present. During stage 2, in the middle third of development, differentiation begins as the inner medulla regresses and the cortex thickens signaling ovary development, while the opposite occurs in testis formation. At this point, the Müllerian (female reproductive) duct regresses in males until it is lost (stage 6), while females retain both ducts until after birth. In the later stages of testis development, interstitial tissue forms in the medulla corresponding to maximum development of the hemipenes in males and the corresponding regression in the females.
authors.affiliation: Department of Biological Sciences, Macquarie University, New South Wales 2109, Australia. lneaves@rna.bio.mq.edu.au
authors.forename: Linda
authors.initials: L
authors.lastname: Neaves
authors.forename: Erik
authors.initials: E
authors.lastname: Wapstra
authors.forename: Debra
authors.initials: D
authors.lastname: Birch
authors.forename: Jane E
authors.initials: JE
authors.lastname: Girling
authors.forename: Jean M P
authors.initials: JM
authors.lastname: Joss
mesh_terms: D000818:Animals; D005260:Female; D006066:Gonads / Q000196:embryology*; D008116:Lizards / Q000033:anatomy & histology / Q000196:embryology*; D008297:Male; D019849:Sex Determination Processes; D012733:Sex Differentiation / Q000502:physiology*; D014566:Urogenital System / Q000196:embryology*; D052286:Viviparity, Nonmammalian