Abstract
BackgroundAndrogens induce male characters by activating androgen receptors (AR). Previous quantitative studies on AR in fishes have been limited to few tissues and/or a single season/reproductive state. The aim of this investigation was to study the possible role of AR-beta expression levels in the control of male traits in the three-spined stickleback. To that end, AR-beta expression levels in major tissues in breeding and post-breeding male and female sticklebacks were examined.MethodsAR-beta mRNA levels were quantified in ten tissues; eye, liver, axial muscle, heart, brain, intestine, ovary, testis, kidney and pectoral muscle in six breeding and post-breeding males and females using reverse transcription quantitative PCR.ResultsBreeding in contrast to post-breeding males built nests and showed secondary sexual characters (e.g. kidney hypertrophy) and elevated androgen levels. Post-breeding females had lower ovarian weights and testosterone levels than breeding females. AR-beta was expressed in all studied tissues in both sexes and reproductive states with the highest expression in the gonads and in the kidneys. The kidney is an androgen target organ in sticklebacks, from which breeding males produce the protein spiggin, which is used in nest-building. There was also high AR-beta expression in the intestine, an organ that appears to take over hyperosmo-regulation in fresh water when the kidney hypertrophies in mature males and largely loses this function. The only tissue that showed effects of sex or reproductive state on AR-beta mRNA levels was the kidneys, where post-breeding males displayed higher AR-beta mRNA levels than breeding males.ConclusionThe results indicate that changes in AR-beta mRNA levels play no or little role in changes in androgen dependent traits in the male stickleback.
Highlights
Androgens induce male characters by activating androgen receptors (AR)
Breeding males had higher KSI than the other groups (Tukey’s post-hoc test, p < 0.001 in each of six comparisons), but there were no differences between the latter (Table 1)
We found no significant differences in AR-beta mRNA levels in the kidneys between breeding male and female sticklebacks whereas in another study a 1.4-fold higher kidney AR-beta expression in mature males than in mature female sticklebacks was reported [14]
Summary
Previous quantitative studies on AR in fishes have been limited to few tissues and/or a single season/reproductive state The aim of this investigation was to study the possible role of AR-beta expression levels in the control of male traits in the threespined stickleback. An AR-beta with two splicing variants was cloned from the kidney of the three-spined stickleback Gasterosteus aculeatus [7] In this species the kidney is an androgen target organ, which hypertrophies in the breeding male and produces glue used in nest-building. This stickleback AR-beta showed high affinity binding (Kd in the nM range) for androgens and low number of binding sites (Bmax in the pmol/g tissue range), which is in consistent with nuclear receptors [7]. The trans-activation function of stickleback AR was verified in transfected human and zebra fish liver epithelial cell lines upon stimulation by androgen, preferentially by 11-ketotestosterone (11KT)
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