Variation in the ontogeny of sex steroid levels between latitudinal populations of the medaka.

Kazunori Yamahira,Maiko Kawajiri,Hiroaki Chiba,Shunsuke Moriyama,Katsuhisa Uchida

doi:10.1186/s40851-015-0032-1

Kazunori Yamahira, Maiko Kawajiri + Show 3 more

Open Access

https://doi.org/10.1186/s40851-015-0032-1

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Abstract

IntroductionSex 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.ResultsWe 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.ConclusionsThe 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.

Highlights

Sex steroids mediate the expression of sexual dimorphism during ontogeny, and populations that differ in the magnitudes of sexual dimorphism may differ in the ontogenetic patterns of their sex steroid levels
The acceleration of fin elongation rates was greater in Okinawa males than in Aomori males, resulting in greater sexual dimorphism in anal fin length in the Okinawa population (Fig. 2)
Sex-specific ontogenetic dynamics of sex steroid levels We found that T, E2, and 11-KT levels are all high in early ontogenetic stages and decreased with their growth

Summary

Introduction

Sex steroids mediate the expression of sexual dimorphism during ontogeny, and populations that differ in the magnitudes of sexual dimorphism may differ in the ontogenetic patterns of their sex steroid levels. The medaka, Oryzias latipes species complex, shows sexual dimorphism in the length of median fins; i.e., the anal and dorsal fins are longer in adult males than in adult females [12, 13]. Developmental stage-specific expression of sexual dimorphism is well documented in the medaka; in males, Kawajiri et al Zoological Letters (2015) 1:31 the elongation rate of the median fins with respect to standard length (SL) increases when the fish reaches about 15 mm SL, indicating development of secondary sexual characteristics [14, 15]. In contrast, there is no change in the rate of fin elongation throughout ontogeny, resulting in the sexual dimorphism in adult fin lengths It is unclear how the sexspecific ontogenetic patterns of fin elongation are developmentally regulated by sex steroids

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