Endocrine changes associated with the growth of pre-vitellogenic oocytes in coho salmon, Oncorhynchus kisutch

Abstract

It has been hypothesized that in salmonids there are two periods when growth can alter the decision to mature; thus, for a fall spawning species, an ‘initiation’ period in the fall is followed by a ‘permissive’ period for continuing maturation in the spring. Research in mammalian and avian models have shown that follicle-stimulating hormone (FSH), steroidogenic acute regulatory protein (StAR) and 17β-estradiol are key components controlling oocyte growth. In the present study we sampled two-year-old female coho salmon during the proposed fall ‘initiation’ period. Histological analyses of the ovaries indicated that from August to December (11–15 months prior to spawning) oocytes developed from the late perinucleolar to mid-cortical alveoli stage. The accumulation of cortical alveoli during this period was accompanied by significant increases in GSI, pituitary and plasma FSH, and plasma 17β-estradiol. In addition, small but significant increases in transcript levels for FSH receptor and StAR occurred during the fall. Such changes in FSH signaling are consistent with a proposed role for FSH stimulation of oocyte growth via steroid production during the fall initiation period.