Ultrastructure and phylogeny of the spermatozoid of Chara vulgaris (Charophyceae)

At maturity, spermatozoids of the green alga Chara vulgaris are biflagellated, anterior of the cell contains an ovoid headpiece anchoring two slightly staggered basal bodies that are positioned above and directly in front of approximately 30 linearly arranged mitochondria. An elongated stellate pattern occupies the transition zone between the BBs and axonemes. Flagella emerge from the cell just in front of the nucleus and encircle the full length of the spermatozoid. The spline comprises a maximum of 38 microtubules surrounding the anterior mitochondria and gradually decreases posteriorly to a minimum of 11. The dense nucleus is narrow, cylindrical, and occupies the central revolution of the cell. Six starch-laden plastids and associated mitochondria are linearly arranged at the cell posterior. Phylogenetic analyses of charalean taxa and archegoniates based on spermatogenesis strongly support the order Charales, with Nitella as the sister group to Chara. Diagnostic features of Chara spermatozoids include absence of a lamellar strip and axonemes embedded in the cell for almost the entire length of the anterior mitochondria. Potential relationships among Charales, Coleochaetales and archegoniates are evaluated in regards to the probable course of evolution of streamlined biflagellated gametes.