DAVENPORT, I.R.; WOURMS, J.P.; Clemson University: A novel actin-based framework aids in the evolution of extreme egg size in chondrichthyan fishes.

Of the 1200 species of chondrichthyan fishes, approximately 60% are viviparous. Typically they have low fecundity but compensate with large precocial offspring. Large offspring can be produced by: 1. lecithotrophy, where the developing embryo solely depends on its yolk reserves; or 2. matrotrophy, where the maternal organism supplies nutrients to the developing embryo throughout gestation. Many chondrichthyan fish that are lecithotrophic, produce eggs similar in size to those of other egg layers, but some produce extremely large eggs, 10 cm in diameter. Production of huge eggs presents physical and physiological challenges, namely, providing the egg with massive amounts of yolk and other materials required by the developing embryo, and that of maintaining its integrity before and after ovulation. In most eggs, simple villar projections from the follicle cells traverse the egg envelope and come in close proximity to the egg, producing the striated appearance of the zona radiata. In large shark eggs, however, there are extremely long, (50-60 �m) actin-based, branched, villar processes that physically connect the follicle cells directly to the egg. These radial processes appear to be a complex transport network. As the egg increases in size, the egg envelope narrows considerably and the actin-based processes form a tangential meshwork around the egg, similar to the radial framework in a car tire. At the time of ovulation, the actin network in the egg envelope would serve as a flexible supporting structure to maintain cellular integrity.