Some endocrine aspects of precocious sexual maturation in the amago salmon Oncorhynchus rhodurus.

Abstract

Changes in plasma androgen and gonadotropin (GTH) levels were correlated with changes in morphology of testes and pituitaries during precocious sexual maturation in the male amago salmon Oncorhynchus rhodurus parr. Although testicular development in precocious male parr began in early summer (June), dramatic increases in gonadosomatic indices occurred during the month of September. Active spermiation began in early October and continued into early November. Plasma androgen levels began to increase sharply in late August, coinciding with the onset of rapid testicular development. High circulating levels of androgen (15-18ng/ml) were maintained during September, followed by a distinct drop in early October. Plasma GTH levels were relatively low during the major part of the spermatogenesis period and increased during the spawning period, followed by a sharp decline in spent males from mid-November. There were two types of pituitary cells, globular cells and vesicular cells, which were found to be closely associated with testicular development. The globular cells were characterized by having a few large globular inclusions as well as numerous fine granules. These cells increased remarkably in number during the active spermiation period, coincident with the rise in plasma GTH levels, suggesting that the globular cells are the major source of GTH. The chromophobic vesicular cells appeared most active during the later stage of testicular development. It is concluded from these observations that pituitary GTH plays a major role in precocious sexual maturation of the male amago salmon parr. This action of GTH may be mediated by the steroidogenic tissue of the testes.