Physiological aspects of oogenesis in two species of sticklebacks, Gasterosteus aculeatus L. and Apeltes quadracus (Mitchill)

During May at Woods Hole, female Gasterosteus aculeatus periodically produce clutches of 112±19 eggs with an average diameter of 1·31±0·05 mm. The fish generally have primordial follicles ranging up to 0·56 mm diameter and a clutch of larger follicles undergoing synchronous growth. The size of oocytes within a growing clutch appears to be random within a population, which indicates that recruitment of clutches is not induced by a local environmental event. The largest oocytes within the population of primordial follicles have just begun vitellogenesis but are temporarily arrested. All oocytes within follicles larger than 1·1 mm diameter undergo spontaneous maturation and enlarge to preovulatory size when incubated at 16° C in a simple saline medium. Added deoxycorticosterone can induce similar events in somewhat smaller follicles placed in culture. Thus, in vivo , follicles grow from 0·56 to 1·1 mm diameter by vitellogenesis, and further enlargement is achieved by hydration during steroid‐induced maturation. Females carrying follicles in maturational stages also have a new clutch of follicles entering vitellogenic growth from the population of primordial follicles. Injection of human chlorionic gonado‐tropin causes a recruitment of follicles into vitellogenesis regardless of the stage of follicles within the growing clutch. One interpretation of these results is that when vitellogenic follicles reach a diameter of 1·1 mm a surge of gonadotropin(s) induces the follicle cells to release steroid, which results in oocyte maturation; the same surge also recruits a new clutch of vitellogenic oocytes. Qualitatively similar results were obtained for Apeltes quadracus ; thus the recruitment phenomenon observed for G. aculeatus may be a general feature among sticklebacks.