Evidence for the mitochondrial origin of the eye lenses in embryos of Entobdella soleae (Plathelminthes, Monogenea).

The lens associated with each of the four pigmented eyes of the oncomiracidium of Entobdella soleae (Plathelminthes, Monogenea, Capsalidae) develops in a special region of the pigment cup cell of the eye. It is confirmed that the inner of the two membranes enclosing each lens bears short, inwardly projecting, membranous profiles identical to mitochondrial cristae. Studies of embryos incubated for 19 days at 12 degrees C (hatching begins at 28 days at this temperature) revealed that the matrix of the developing lens of each anterior eye contains many mitochondrial membrane compartments, some having the configuration of separate, small mitochondria. The implication is that the lens is derived from many fused mitochondria, rather than from a single large one. The anterior eyes of 19-day-old embryos are less well developed than the posterior eyes. Pigment granules in the anterior eyes appear to be at the premelanosome stage and contain dispersed dense particles lacking an obvious orderly arrangement. The posterior eyes mostly contain mature melanosomes. Membranous compartments in the matrix of the posterior eye lenses are rare. Apart from longer peripheral cristae, lenses of 22-day-old embryos are identical with those of oncomiracidia. The evolution of mitochondrial lenses in Plathelminthes is considered.