title: Variation in the ontogeny of sex steroid levels between latitudinal populations of the medaka.
abstract: BACKGROUND
  Sex steroids mediate the expression of sexual dimorphism during ontogeny, and populations that differ in the magnitudes of sexual dimorphism may accordingly differ in the ontogenetic patterns of their sex steroid levels. The medaka, Oryzias latipes species complex, shows geographic variation in the magnitude of sexual dimorphism with respect to the lengths of their anal and dorsal fins; dimorphism is greater in low-latitude populations than in high-latitude populations. However, sexual differences in the ontogenetic dynamics of sex steroids, and its interpopulation variation, have not been examined.
  
  
  RESULTS
  We measured testosterone (T), estradiol-17β (E2), and 11-ketotestosterone (11-KT) concentrations throughout ontogeny of laboratory-reared fish from two latitudinal populations: Aomori (northern) and Okinawa (southern). In both populations, the levels of all three steroids were high during early ontogenetic stages and decreased with growth. After reaching about 15 mm in standard length, when sexual dimorphisms in fin lengths became apparent, steroid levels increased and tended to plateau. Sexual differences in the steroid levels were observed only in the later ontogenetic stages; T and 11-KT levels were higher in males, while E2 levels were higher in females. Accordingly, interpopulation differences also became clearer; the southern fish tended to show higher T levels and lower E2 levels than the northern fish.
  
  
  CONCLUSIONS
  The ontogenetic patterns of sex steroid levels paralleled the ontogeny of anal and dorsal fins in the two latitudinal populations, suggesting that interpopulation variation in the degree of sexual dimorphisms in fin lengths is mediated by sex steroid-dependent regulation of fin elongation.
authors.affiliation: Tropical Biosphere Research Center, University of the Ryukyus, Okinawa, 903-0213 Japan.
authors.forename: Maiko
authors.initials: M
authors.lastname: Kawajiri
authors.affiliation: Faculty of Agriculture, University of Miyazaki, Miyazaki, 889-2192 Japan.
authors.forename: Katsuhisa
authors.initials: K
authors.lastname: Uchida
authors.affiliation: School of Marine Biosciences, Kitasato University, Kanagawa, 252-0373 Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Chiba
authors.affiliation: School of Marine Biosciences, Kitasato University, Kanagawa, 252-0373 Japan.
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Moriyama
authors.affiliation: Tropical Biosphere Research Center, University of the Ryukyus, Okinawa, 903-0213 Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Yamahira
keywords: 11-ketotestosterone; Estradiol-17β; Latitude; Oryzias latipes; Sexual dimorphism; Sexual selection; TR-FIA; Testosterone