Evol Ecol Res 17: 583-601 (2016)     Full PDF if your library subscribes.

Overripening of eggs and changes in reproductive hormones
in the threespine stickleback, Gasterosteus aculeatus

Chrysoula Roufidou1, Marion Sebire2, Ioanna Katsiadaki2, Arshi Mustafa3, Monika Schmitz3, Ian Mayer4, Yi Ta Shao5 and Bertil Borg1

1Department of Zoology, Stockholm University, Stockholm, Sweden,  2Centre for Environment, Fisheries & Aquaculture Science (Cefas), Weymouth, UK,  3Department of Organismal Biology, Comparative Physiology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden,  4Faculty of Veterinary Medicine and Biosciences, Norwegian University of Life Sciences, Oslo, Norway and  5Institute of Marine Biology, National Taiwan Ocean University, Keelung, Taiwan

Correspondence: C. Roufidou, Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden.
e-mail: chrysoula.roufidou@zoologi.su.se


Background: Female threespine sticklebacks, Gasterosteus aculeatus, are batch spawners. As in most teleosts, the ovulated eggs are kept in the ovarian cavity until spawning. If spawning or spontaneous release of the eggs does not take place, they can become overripe and harden, and in most cases remain in the ovary. The overripe eggs are lost for reproduction and also block further spawnings. Reproductive hormones regulate egg production and may be involved in the mechanism of overripening.

Question: What are the reproductive endocrinological parameters characterizing overripening of ovulated eggs in the threespine stickleback?

Organism: Wild-caught adult threespine sticklebacks from the southern Baltic at Skåre in southern Sweden and the island of Askö in northwestern Baltic Proper in Sweden.

Experiments: We collected blood samples for hormone measurements, as well as pituitaries and brains for measurement of mRNA from both sexually mature non-overripe (non-ovulated and/or ovulated) and overripe (egg-bound) females. For the Skåre fish, sexual maturation was induced under laboratory conditions by exposure to a long photoperiod and we compared the non-overripe (including non-ovulated, with oocytes in different maturing or ripening stages, and ovulated females) with the overripe females. The Askö fish were sampled directly from nature, during the natural summer breeding season and we compared the non-overripe (including non-ovulated, with oocytes in different maturing or ripening stages, and ovulated females) with the overripe females.

Methods: In the fish collected from Skåre, we used radioimmunoassay to measure the plasma levels of four steroids: testosterone, estradiol, 17,20β-dihydroxypregn-4-en-3-one (17,20β-P), and 17,20β,21-trihydroxypregn-4-en-3-one (17,20β,21-P). We also measured the mRNA levels of gonadotropins [GTHs: follicle-stimulating hormone ( fsh-β) and luteinizing hormone (lh-β)] in the pituitary, and of gonadotropin-releasing hormones (GnRHs: gnrh2, gnrh3) and kisspeptin (kiss2) and its G protein-coupled receptor (gpr54) in the brain by real-time quantitative PCR. In the fish collected from Askö, we measured only progestogens (17,20β-P and 17,20β,21-P).

Results: In the fish from Skåre, overripe female sticklebacks had significantly lower levels of circulating plasma steroid hormones (testosterone, estradiol, 17,20β-P), as well as of pituitary lh-β and brain kiss2 and gpr54 mRNA than the non-overripe females. In the fish caught from Askö, overripe females had lower 17,20β-P levels than the non-overripe non-ovulated females, but there was no difference between the non-overripe ovulated and the overripe females. The 17,20β,21-P plasma levels were under the limit of detection in all groups.

Keywords: testosterone, estradiol, 17,20β-dihydroxypregn-4-en-3-one, 17,20β,21-trihydroxypregn-4-en-3-one, fsh-β, lh-β, gnrh2, gnrh3, kiss2, gpr54, ovulation, spawning, overripe, reproduction, stickleback, teleosts.

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