The “Complex” Ascidiosperm of Aplousobranchs

Summary

The ascidiosperm of non-appendicularian tunicates has a basic plane: the “head” contains a rod-like nucleus flanked by a large mitochondrion; the acrosome is reduced to small vesicles; the “tail” has a 9+2 axoneme. In species with external fertilisation, the sperm is simpler than in internally fertilising species. Our data show that, in Trididemnum cereum, the long nucleus coils apically in a flange of 40 spires and is flanked by mitochondrion and vesicles; the flagellum has 9 accessory bodies. Other aplousobranchs, the polycitorid Polycitor adriaticus and the polyclinid Aplidium conicum, also have a long nucleus coiled apically in 8 or 6 spires, respectively. Aplidium conicum has a “dense groove” joining the plasma membrane to the nuclear envelope. Although sperm complexity may depend on the mode of fertilisation, it is possible to distinguish plesiomorphies and apomorphies in sperm and use them for the phylogenetic reconstruction of tunicates.