PARENT SESSION
DEVELOPMENT OF THE EMBRYO
Harborside C
7:30 AM-10:00 AM

(126) DEVELOPMENTAL EXPRESSION OF THREE FORMS OF GnRH AND THE ONTOGENIC ALIGNMENT OF THE HYPOTHALAMUS-PITUITARY-GONADAL AXIS IN GILTHEAD SEABREAM, Sparus aurata.

Wong, Ten-Tsao¹, Gothilf, Yoav², Meiri, Iris³, Zmora, Nilli³, Elizur, Abigail⁴, Zohar, Yonathan¹, ¹ Center of Marine Biotechnology, Baltimore, MD² Department of Zoology, George S. Wise Faculty of Life Sciences, Tel-Aviv, Israel³ Israel Oceanographic and Limnological Research, Eilat, Israel⁴ Bribie Island Aquaculture Research Center, Queensland, Australia

ABSTRACT- Three forms of gonadotropin-releasing hormone (GnRH), seabream (sb) GnRH, salmon (s) GnRH and chicken (c) GnRH-II, exist in gilthead seabream. In order to investigate the developmental expression of multiple GnRHs and the ontogenic alignment of the hypothalamus-pituitary-gonadal (HPG) axis, we have utilized the whole-mount in situ hybridization and real-time fluorescence-based quantitative RT-PCR assays to study the developmental expression of marker genes at each level of the HPG axis, GnRHs in brain/hypothalamus, FSH and LH in pituitary and FSH receptor (FSH-R), LH receptor (LH-R) and VASA in gonad. Using whole-mount in situ hybridization, cGnRH-II was first detected at 36 hours post-fertilization (hpf) with a strong signal in the hindbrain and a weak signal in the midbrain. The midbrain signal increased continuously as the hindbrain signal decreased. sGnRH was first detected in the olfactory placode at 3 days post-fertilization (dpf) and migrated toward the forebrain. No sbGnRH was detected. Using real-time fluorescence-based quantitative RT-PCR, higher sbGnRH mRNA levels were detected during 8 to 14 hpf, but levels remained low in comparison to the transcript levels of cGnRH-II and sGnRH. cGnRH-II mRNA was first detected at 22 hpf and sGnRH mRNA was first detected at 36 hpf. Expression of both cGnRH-II and sGnRH increased continuously and maintained high levels, peaking at 14 dpf for cGnRH-II and at 16 dpf for sGnRH. Our results also indicated four ontogenic alignments of the HPG axis occurred during the first 36 days of development. The first alignment was apparent during 22 to 36 hpf, at which time the initial detection of both cGnRH-II and FSH coincided with higher expression of FSH-R and LH-R. The second alignment occurred at 5 dpf, at which time significantly higher levels of cGnRH-II, sGnRH, FSH, FSH-R and VASA, but not LH and LH-R were detected. The third alignment was shown at 8 dpf with a similar expression pattern as the second one. The fourth alignment occurred at 14 dpf with the highest expression levels of cGnRH-II, FSH and FSH-R during the first 36 days of embryonic development. Our data suggest that cGnRH-II and sGnRH in the brain/hypothalamus, FSH in the pituitary and FSH-R in the gonad, play major roles in the ontogenic alignment of the HPG axis.

KEY WORDS: GnRH