Among the "primitive" chitons of the suborder Lepidopleurina, the eggs have a smooth jelly coat, and the sperm-equipped with a typical, prominent acrosome-probably can enter an egg at any point on its surface. All other chitons have eggs with more elaborate spinous or cupulous hulls that focus sperm to specific regions on the surface. Moreover, these sperm have 'evolved a long nuclear filament tipped by a minute acrosome which interacts with the egg in specific ways. Differences in the form of the egg hull and in the mechanism of fertilization among chitons.are providing insights into the evolution of this ancient molluscan taxon. In this issue (pp. 59-67), John Buckland-Nicks and Alan Hodgson describe fertilization in Callochiton castaneus from South Africa. This chiton retains a mixture of primitive and derived characters that together produce a novel mechanism of fertilization, which is represented on the cover. In the background of the cover is an unfertilized egg of C. castaneus, from which the jelly coat and part of the vitelline layer have been stripped to reveal a honeycomb of egg membrane cups. In the intact egg, these cups coincide with the bases of regularly spaced pores in the jelly coat. Fertile sperm seeking the egg locate one of these external pores and swim down it to the vitelline layer. The minute acrosome digests a pore, and the needle-like nuclear filament bridges the distance to the egg membrane cup. The micrograph superimposed on the background shows a fertilizing sperm in the process of injecting chromatin into the egg cortex. The unusual aspect of fertilization in these chitons is that the sperm organelles are apparently abandoned in a membrane bag on the egg surface. If this is indeed the case, then inheritance, not only of mitochondria, but also of centrioles and other cytoplasmic components, would be maternal.   Main | Research | Collaborations | Courses | Contact St.FX Biology | St.FX Home