The reproductive physiology of female diploid and triploid brook trout (Salvelinus fontinalis)

Triploid female salmonids are unlike normal diploids in that they show impaired oocyte growth and cannot produce viable offspring. Typically, only a small number of oocytes develop past the meiotic block in triploids, and these oocytes show a slow rate of development. Triploid females have significantly lower plasma vitellogenin (VTG), testosterone (T) and estradiol-17β (E 2) levels and lower gonadosomatic indices (GSI), particularly during maturation in diploids. In spite of the growing commercial use of triploid females for the purposes of exploiting their sterility, the reproductive physiology of triploids is not completely understood. The main objective of this thesis was to investigate possible causes for delayed oocyte development in triploid females, using brook trout (Salvelinus fontinalis) as the model species. The specific objectives were to: (1) purify and characterize brook trout VTG (bt-VTG) and develop an enzyme linked immunosorbent assay (ELISA) for quantifying plasma VTG levels; (2) compare the reproductive endocrinology and ovarian growth of three age classes of diploids and triploids, in order to establish profiles of sexual development; (3) compare in vitro E2 production by diploid and triploid follicles of similar diameter, to determine whether triploid oocytes have a reduced capacity for E2 synthesis; and (4) determine whether long-term in vivo E2-treatments of triploid females would increase plasma VTG levels and accelerate rates of triploid oocyte growth. A bt-VTG ELISA was successfully developed, with performance characteristics comparable with other published protocols. For the three age classes, triploids showed lower GSI, VTG and steroid hormone levels than diploids, with VTG showing seasonal fluctuations resembling those of the diploids. Some 3+ triploids showed asynchronous ovarian growth, with a few producing mature oocytes. Under in vitro conditions, triploid and diploid follicles of similar diameter produced similar amounts of E2 with gonadotropin hormone treatment. The long-term administration of E2 had no effect on triploid oocyte growth but did have other physiological effects. Taken together, these findings indicate that the reproductive physiology of female triploid brook trout is similar to that of other triploid salmonids, and that delayed oocyte growth in triploids is not due to a reduced ability of triploid oocytes to synthesize E2.