| Abstract: | Many naked gametes are produced in each fusiform, male gametangium of Atractomorpha echinata Hoffman and are liberated through irregularly shaped pores in the gametangial wall. They are typically biflagellate, pyriform or fusiform in shape, 6-11 μm long, and only a few micrometers wide. A mature male gamete is characterized by: (i) a nucleus with condensed chromatin and no nucleoli, (ii) a reduced, starch filled chloroplast occupying a posterior position, and (iii) a cup shaped eyespot consisting of a single layer of plastoglobuli. The flagellar apparatus includes two types of flagellar roots alternating in a cruciate pattern. One type consists of two microtubules, while the other consists of microtubules of varying number, usually eight or nine, but rarely as many as eleven. The paired basal bodies are connected anteriorly by a broad, striated distal fiber; there is no dense apical cap as reported in Sphaeroplea sperm. A unique structure, consisting of three layers of small subunits (6–8 nm diameter) arranged in a paracrystalline array, is positioned beneath each basal body. Based on the structure of its male gametes, Atractomorpha clearly demonstrates affinity with the chlorophycean rather than the ulvaphycean line of evolution. Moreover, if phylogenetic affinities for the Sphaeropleaceae are to be sought among other groups of green algae, the Chlorococcales appears the most promising candidate. |
