Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

John W. Lynn


The morphological and functional mechanisms involved in the early fertilization events in the freshwater zebra mussel Dreissena polymorpha (Bivalvia:Veneroida:Dreissenidae) are presented. Mature adults release gametes in response to external applications of 5-hydroxytryptamine. Sperm retain a primitive morphology consisting of a head, midpiece and tail The head is approximately 1.6 $\mu$m in diameter and 4 $\mu$m in length. An acrosomal vesicle is highly compartmentalized with a central axial core containing actin. The midpiece is composed of 4 mitochondria and a pair of centrioles. A single flagellum is elaborated from the distal centriole. The eggs of D. polymorpha are 50-60 $\mu$m in diameter and are surrounded by a surface envelope. Eggs are isolecithal and spawned in first meiotic metaphase arrest. At insemination, sperm bind randomly to the egg surface and sperm enter into the egg cytoplasm in a two step process. First, the sperm gradually (10 $\mu$m/min) enters the egg cortex through an actin rich egg-derived insemination cone. Once in the egg, the sperm nucleus with attached axoneme and mitochondria rotate 180$\sp\circ$ and is laterally displaced along the cortex. The first stage is inhibited by cytochalasins and is microfilament dependent. During the second stage of sperm entry, the sperm axoneme is incorporated into the egg cytoplasm and the sperm head could be translocated rapidly (1 $\mu$m/sec) through the egg cytoplasm. The sperm axoneme extends deeper into the egg cytoplasm and remains active for several min following incorporation. Cytoplasmic particle flow is associated with the axoneme during this active phase. Following quiescence of the axoneme, the sperm nucleus decondenses and the mitochondria separate from the midpiece. The second step is microtubule dependent and inhibited by colchicine, colcemid, nocodazole, and taxol. Meiotic maturation resumes with sperm entry and the egg forms two polar bodies. Male and female pronuclear formation are synchronous and occur at similar rates. Cleavage occurs by 70 min and forms unequal blastomeres. Sperm entry occurs in the larger (CD) of the two blastomeres and sperm components brought into the cytoplasm are generally retained in the CD blastomere.