title: Unilateral ovariectomy increases egg size and reduces follicular atresia in the semelparous coho salmon, Oncorhynchus kisutch.
abstract: Unilateral ovariectomy (ULO, removal of one ovary) is a powerful technique for studying aspects of reproductive physiology, including follicular recruitment and growth. To examine effects of ULO for the first time in a semelparous species, coho salmon (Oncorhynchus kisutch) were unilaterally ovariectomized during mid-vitellogenesis approximately 3 months before spawning. At termination of the study (79 days post-surgery), single ovaries of ULO fish were gravimetrically equivalent to paired ovaries of sham surgery, control fish. There was no evidence of recruitment of new vitellogenic follicles. Instead, the dramatic increase in ovary mass was attributable to hypertrophy of existing vitellogenic follicles (33% increase in volume) and increased fecundity achieved through a greater than two-fold reduction in follicular atresia. The composition of whole ovaries on a dry weight basis from ULO fish was greater in protein, but lower in lipid than that of control fish. Expressing the data on a per follicle basis, however, showed that follicles of ULO fish contained more protein, ash, water, and lipid. The results indicate that ULO of coho salmon induces compensatory hypertrophy of existing vitellogenic follicles, while maximizing fecundity through reduction of atresia. Thus, 3 months before spawning, coho salmon exhibit the ability to adjust final egg size and number when faced with significant depletion of ovarian follicles. This in vivo system provides a platform for further study of physiological mechanisms regulating follicular growth and atresia, and the trade-off between egg size and egg number.
authors.affiliation: School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195-5020, USA. jalucken@u.washington.edu
authors.forename: John Adam
authors.initials: JA
authors.lastname: Luckenbach
authors.forename: Makoto
authors.initials: M
authors.lastname: Kusakabe
authors.forename: Penny
authors.initials: P
authors.lastname: Swanson
authors.forename: Graham
authors.initials: G
authors.lastname: Young
mesh_terms: D000818:Animals; D049628:Body Size / Q000502:physiology; D005260:Female; D005298:Fertility / Q000502:physiology; D005496:Follicular Atresia / Q000502:physiology*; D017684:Oncorhynchus kisutch / Q000502:physiology*; D009865:Oocytes / Q000502:physiology*; D009929:Organ Size / Q000502:physiology; D006080:Ovarian Follicle / Q000502:physiology*; D010052:Ovariectomy