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Vitellogenesis in the three-spined stickleback, Gasterosteus aculeatus, over the spawning cycle and at egg overripening
Stockholm University, Faculty of Science, Department of Zoology.ORCID iD: 0000-0003-0022-0698
Stockholm University, Faculty of Science, Department of Zoology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Vitellogenesis was studied in the female three-spined stickleback, Gasterosteus aculeatus, a multiple-spawning fish producing several egg batches during the breeding season, over the spawning cycle and at overripening of eggs. For the spawning cycle, the females were sampled when they were ’’ready to spawn’’ and 6, 24, 48, and 72 hours post-spawning (hps). For the overripening of eggs, non-overripe (mature non-ovulated and ovulated) and overripe females were sampled from nature in the breeding season. Liver tissues were collected for relative vitellogenin mRNA levels measurements by real-time qPCR. vitellogenin mRNA was expressed at all time-points over the 3-day spawning cycle and the levels were significantly increased at 24 and 48 hps, a day before ovulation, compared to the ’’ready to spawn’’ and 6 hps females. Changes over the cycle were, however, small. The highest sample point was only 1.4 times higher than the lowest. The vitellogenin mRNA levels were positively correlated with both circulating estradiol and testosterone levels over the cycle. Overripe females had 1.5 times lower vitellogenin mRNA levels than the non-overripe non-ovulated but not than the non-overripe ovulated females. These findings suggest that vitellogenesis is rather continuous over the stickleback spawning cycle, which could be an advantage for a multiple spawner, and that overripening condition reduces vitellogenin production but does not eliminate it.

Keyword [en]
stickleback, vitellogenin, sex steroids, spawning cycle, egg overripening, vitellogenesis
National Category
Biological Sciences
Research subject
Functional Zoomorphology
Identifiers
URN: urn:nbn:se:su:diva-143275OAI: oai:DiVA.org:su-143275DiVA: diva2:1097527
Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-08-03Bibliographically approved
In thesis
1. Reproductive physiology of the female three-spined stickleback, Gasterosteus aculeatus
Open this publication in new window or tab >>Reproductive physiology of the female three-spined stickleback, Gasterosteus aculeatus
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reproduction in vertebrates, including fishes, is under control of the brain-pituitary-gonad (BPG) axis. The female three-spined stickleback, Gasterosteus aculeatus, produces egg clutches at intervals of a few days and spawns them in a nest built by male. Following ovulation, eggs are stored in the ovarian cavity surrounded by the ovarian fluid (OF). If spawning or spontaneous release do not occur, the eggs can undergo overripening, a phenomenon occurring both in nature and captivity.

In this PhD thesis, the changes of reproductive hormones and vitellogenesis were studied at overripening of eggs and over the natural spawning cycle. OF properties were also examined at overripening of eggs and after treatment with sex steroids.

Plasma levels of steroids: testosterone (T), estradiol (E2), 17,20β-dihydroxypregn-4-en-3-one (17,20β-P) and 17,20β,21-trihydroxypregn-4-en-3-one (17,20β,21-P) were measured by radioimmunoassay, and relative mRNA levels of the pituitary gonadotropins (fsh-β/lh-β), brain gonadotropin-releasing hormones (gnrh2/gnrh3) and kisspeptin and its receptor (kiss2/gpr54) by qPCR. Overripening of eggs was accompanied with a significant reduction in most of endocrine parameters of BPG axis (T, E2, 17,20β-P; lh-β; kiss2, gpr54). Low level of hormones could be advantageous for the overripe egg-bound females, since this would reduce further ovulations giving higher chances to survive and reproduce again. Over the 3-day spawning cycle, T and E2 were highly correlated, showed cyclicity with low levels at ovulation and increasing from 24 and 6 hours post-spawning (hps), respectively. Spawning may give rise to this increase as these rises did not occur if release of the eggs does not happen (overripe females). A peak at pituitary lh-β mRNA levels appeared 48 hps, a day before the next ovulation. No significant changes were found for the other studied hormones.

Vitellogenesis was studied by measurement of the vitellogenin mRNA levels in the liver by qPCR. The levels were highest at 24 and 48 hps and were positively correlated to both E2 and T over the cycle. However, changes were small suggesting a rather continuous vitellogenesis over the stickleback spawning cycle which could be an advantage for a multiple spawner with a limited spawning season. Overripening reduced vitellogenin mRNA levels but did not abolish it.

OF amount was diminished in overripe females and had a lower viscocity but higher dry weight and protein levels than in non-overripe ovulated females, suggesting that changes in OF properties are related to the egg overripening. The effects of steroids were studied using Silastic capsules. T and 17,20β-P induced an increase of OF amount, but protein levels were only increased in 17,20β-P-treated females, proposing a role of this steroid in the control of OF secretion. 1-D SDS-PAGE showed that OF contained several proteins, some of them came from eggs, but no consistent differences between groups.

Concluding, the knowledge of the reproductive physiological changes is important for understanding their essential roles in the production of viable eggs in this species but also in the reproductive physiology of female fishes in general.

Place, publisher, year, edition, pages
Stockholm: Department of Zoology, Stockholm University, 2017. 65 p.
Keyword
overripening of eggs, spawning cycle, brain-pituitary-gonad (BPG) axis, vitellogenin, ovarian fluid, sex steroids, reproduction, three-spined stickleback, Gasterosteus aculeatus, fish
National Category
Biological Sciences Zoology Other Biological Topics
Research subject
Functional Zoomorphology
Identifiers
urn:nbn:se:su:diva-143279 (URN)978-91-7649-854-5 (ISBN)978-91-7649-855-2 (ISBN)
Public defence
2017-09-08, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2017-08-16 Created: 2017-05-22 Last updated: 2017-08-14Bibliographically approved

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