HOOGE, M.D.*; TYLER, S.: Non-muscle actin in sclerotic components of the female reproductive system of acoel turbellarians

The seminal bursa of many acoel turbellarians is associated with an insemination canal, called a mouthpiece or nozzle, that appears sclerotized–that is, stiff and refractile by light microscopy. The mouthpiece is composed of stacks of ring-shaped cells and the stiffening element is intracellular actin, as can be seen by fluorescence microscopy of phalloidin-stained whole mounts. The function of the mouthpiece has long been assumed to be sorting of sperm stored in the bursa for delivery to mature oocytes situated at the distal end of the mouthpiece. In species of Pseudmecynostomum (Acoelomorpha Mecynostomidae), a similar actin-rich structure is situated at the functionally opposite end of the bursa, that is between it and the vagina, which is surprisingly complex in this genus. It has been identified as a vaginal sphincter in descriptions of Pseudmecynostomum species, but we find, using electron microscopy, that it is not muscular and consists, as do bursa mouthpieces, of stacks of ring-shaped actin-rich cells. We propose that the bursa mouthpiece has assumed a new function in Pseudmecynostomum, perhaps by development of a secondary female pore near the distal end of the mouthpiece from an ancestor lacking the female pore.