The influence of brain extirpation on oogenesis in the polychaete Nereis diversicolor

Abstract

In the normal course of events, the oocytes of Nereis diversicolor are shed into the celom when they are about 18 μ in diameter. They grow slowly until they are about 70 μ and then enlarge rapidly to a diameter of about 150 μ, when they resume slow growth and mature at a size of 180–200 μ. They are fertile after the diameter exceeds 180 μ. After the oocytes have reached a size of 30 μ, the rapid growth phase can be precipitated by a sudden reduction in the level of juvenile hormone secreted by the supra-esophageal ganglion, effected by extirpating the brain. Even oocytes up to a size of about 140 μ that have already begun to grow rapidly can be further accelerated by brain extirpation. Such accelerated eggs never reach full size and invariably suffer from abnormal yolk deposition, in that yolk platelets are almost entirely lacking. But some of them can be fertilized when they are only 120–130 μ in diameter, although cleavage is irregular and the eggs fail to develop beyond three or four cleavages. It is concluded that a high level of juvenile hormone inhibits rapid oocyte growth, but that even after this inhibition has been lifted, continued secretion of juvenile hormone is necessary for vitellogenesis. In this respect, N. diversicolor resembles Platynereis dumerilii, although the two species may differ in matters of detail. From consideration of the course of oogenesis in a natural population of N. diversicolor, it is evident that the stage at which the rapid growth of oocytes begins varies throughout the population. The adults breed in early spring, and by late spring young worms contain oocytes. Initially they differ considerably in the size of their oocytes, but during the summer the population becomes very much more homogeneous in this respect, so that in late autumn and early winter, when rapid growth of oocytes is most marked, nearly all the worms are at the same stage of development. This suggests that worms containing small oocytes with respect to the rest of the population start rapid growth of oocytes at an earlier stage than the more advanced worms, and so catch up with them. This conclusion is consistent with the view that removal of the inhibition on maturation imposed by a high concentration of juvenile hormone is dependent upon environmental rather than purely endogenous factors.