Acoel spermatozoa: ultrastructure and immunocytochemistry of tubulin

Acoel spermatozoa are filiform and contain two parallel axonemes, which do not show the trepaxonematan 9 + '1' pattern, but instead, another kind of 9 + '1' pattern, or a 9 + 0 or 9 + 2 pattern. Spermatozoa have either cortic al singlet microtubules or central microtubules. Identification of these gr oups of microtubules and recognition of homologies between species is diffi cult with electron microscopy. In addition to conventional electron microsc opy, indirect immunofluorescence of tubulin was performed on three species (Symsagittifera schultzei, Symsagittifera psammophila, and Actinoposthia be klemischevi). This technique facilitated understanding of the general morph ology of the filiform spermatozoon and of the arrangement of the microtubul ar organelles along its length. We have found that different monoclonal ant ibodies (anti-alpha-, anti-alpha-acetylated- and anti-bt ta-tubulin) can di stinguish distinct subcellular populations of microtubules. The axonemes we re labelled by the three antibodies in all species. The cortical microtubul es (in Actinoposthia beklemischevi) were labelled by the three antibodies. The central microtubules (in Symsagittifera schultzei and S. psammophila) w ere labelled with the anti-beta-tubulin antibody and not labelled by the an ti-alpha- and anti-alpha-acetylated-tubulin. Similar experiments were perfo rmed on other Platyhelminthes and indicated that immunocytochemistry of spe rmatozoa may provide new characters for phylogenetic studies.