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Gametogenesis in Chitonida is highly unusual, giving rise to eggs with elaborate, often spiny hulls and sperm with needle-like nuclear extensions and tiny acrosomes. My studies of oogenesis in a variety of chitonid families have revealed unique mechanisms of egg hull formation. Three-dimensional reconstruction of serial sections of oogenesis in the chitonidAcanthochitona garnoti(Blainville) confirms that the egg hull is formed largely by microapocrine secretions, rather than by exocytosis of Golgi vesicles. This novel process, detailed for the first time in Mollusca, bypasses the endomembrane system and releases compounds formed in the cytoplasm directly from the cell surface via budding of ‘microaposomes’. Fertilization is no less unusual. In virtually all Metazoa, except chitonids, the entire sperm enters the egg, and sperm mitochondria are digested. Typically, the centrosome that derives from the sperm centriole acts as a microtubule organizing centre, moving the two pronuclei together to form the zygote nucleus and later initiating cleavage of the embryo. However, in chitonids the sperm digest a minute pore in the egg hull that allows injection of DNA but blocks entry of sperm organelles, including the nuclear membrane. Serial sections of fertilized eggs ofMopalia muscosa(Gould) confirm that a membrane bag, abandoned on the egg surface, contains the sperm organelles. Thus, not only is there maternal inheritance of mitochondria and centrioles in Chitonida but also other sperm proteins normally associated with fertilization may be blocked from entering the egg. [ABSTRACT FROM AUTHOR]

Egg hull formation during oogenesis in the chiton Callochiton dentatus does not follow the typical model of merocrine secretion involving Golgi vesicle exocytosis. Instead, microapocrine secretions are primarily responsible for egg hull formation, although merocrine secretions contribute 'areolae' and the vitelline layer. Microapocrine secretion mechanisms are poorly understood, involving a different cellular pathway than is typical. Egg hull formation in C. dentatus involves two types of microapocrine secretions released by the oocyte, one of which is described here for the first time. The plesiomorphic jelly-like egg hull of chitons, as exemplified by the eggs of members of the basal order Lepidopleurida and present also in eggs of C. dentatus (Chitonida: Callichitonidae), may have evolved solely as an oocyte secretion, whereas members of some other families in the order Chitonida form their egg hulls with considerable secretory input from the follicle cells as well. [ABSTRACT FROM AUTHOR]