Program and Abstracts(PDF)

Poster presentation 2
CAPTIVE REPRODUCTION OF CHUB MACKEREL: EXPERIMENTAL SYSTEM
AND ANALYSIS OF NEUROENDOCRINE HORMONES
Sethu Selvaraj 1 , Tetsuro Shiraishi 1 , Hajime Kitano 1 , Masafumi Amano 2 , Mitsuo Nyuji 1 , Hirofumi
Ohga 1 , Akihiko Yamaguchi 1 , Michio Yoneda 3 , Akio Shimizu 3 , and Michiya Matsuyama 1*
1 Faculty of Agriculture, Kyushu University, Fukuoka, Japan
2
School of Marine Biosciences, Kitasato University, Iwate, Japan
3 Fisheries Research Agency, Yokohama, Japan
* E-mail: rinya_m at agr.kyushu-u.ac.jp
The chub mackerel (Scomber japonicus) belongs to the family Scombridae and is a key multiple
batch-spawning pelagic fish contributing to global marine fish production. Due to unreliable and
unpredictable wild catches, aquaculture of this species commenced recently in southwestern
Japan. Young or adult fish are caught from the wild using purse-seine and reared for an
appropriate duration in sea pens. This captive maintained male fish undergo normal
spermatogenesis but female fish after the completion of vitellogenesis fails to undergo final
oocyte maturation (FOM) and ovulation during spawning season (April-June).
The brain-pituitary-gonad (BPG) axis is a key neuroendocrine system involved in the
reproductive processes, and gonadotropin-releasing hormone (GnRH) represents a key upstream
signaling molecule in this system. Brain GnRH stimulates the synthesis and release of the
pituitary gonadotropins (GtHs), follicle-stimulating hormone (FSH) and luteinizing hormone
(LH), which regulate the synthesis of sex steroids that are responsible for gonadal growth and
maturation. In recent years, kisspeptins, encoded by the Kiss1 gene have emerged as novel
upstream regulator of GnRH secretion. Reproductive studies in chub mackerel is focused on
understanding the role of different components of the BPG axis.
Our previous studies revealed the role of pituitary GtHs in reproductive cycle and steroidogenic
pathways in the ovarian follicles of chub mackerel. However, the role of other upstream
regulators such as brain kisspeptins and GnRHs has not yet been elucidated. As a first step, we
isolated cDNAs encoding Kiss and GnRH forms and studied their quantitative changes in the
expression of Kiss and GnRH mRNA in the brain and corresponding GnRH peptides in the brain
and pituitary of females during different ovarian stages. Chub mackerel express two Kiss (Kiss1,
Kiss2) and three GnRH forms (seabream GnRH, sbGnRH; chicken GnRH-II, cGnRH-II; salmon
GnRH, sGnRH). Kiss1 levels in brain did not vary significantly among the different ovarian
stages. However, Kiss2 expression declined from the immature stage (November) to the
post-spawning (PS) season (August). Brain sbGnRH mRNA and corresponding peptide levels
showed significant changes only during PS season; however, pituitary peptide levels increased
gradually from the immature stage onward, peaking during late vitellogenesis (April) and the PS
season. No significant differences in cGnRH-II mRNA or corresponding peptide levels were
found. Likewise, brain sGnRH mRNA levels did not show significant changes; however,
corresponding peptide levels in the brain and pituitary rose significantly during late vitellogenesis
and the PS season, respectively. In conclusion, expression profiles of Kiss2 mRNA in the brain
and sbGnRH peptide in the pituitary suggest their possible involvement in the seasonal ovarian
development of chub mackerel in captivity.
65