Egg envelopes of Armadillidium vulgare (Latreille, 1804) (Crustacea, Isopoda Oniscidea): Ultrastructure and lectins binding.

The ultrastructural study carried out on (a) oocytes of Armadillidium vulgare during vitellogenesis, (b) mature eggs taken from the ovaries during the parturial moult of the posterior half of the body, and (c) fertilized eggs collected within a few hours of their release into the brood pouch, has clearly demonstrated that before the fertilization the chorion is the only envelope present in the egg of oniscidean isopods. In the mature eggs, the chorion appears as a uniformly electron-dense lamina, about 0.4-0.5 µm thick, which does not show any specialized area. A second envelope, described by other authors as vitelline envelope, is formed above the oolemma only right after fertilization and appears separated from the chorion by a space full of liquid. The ways in which the genesis of this envelope is realized are not yet clear; it could be interpreted rather as a fertilization membrane. The investigations carried out with the aid of a battery of FITC-lectins have highlighted the presence at the chorion surface of unfertilized eggs of various saccharide residues distributed in uniform way. No significant change was observed in the pattern of lectins binding to the chorion of eggs taken from the brood pouch, thus demonstrating how, after the fertilization, no significant rearrangement in the distribution of saccharide residues present on the egg surface occurs in A. vulgare. The ways in which, therefore, the recognition, the binding and the entry of the peculiar sperm of oniscidean isopods into the egg occur, still remain all to be deciphered. Microsc. Res. Tech. 79:792-798, 2016.