An actin-like component in sperm tails of a crane fly (Nephrotoma suturalis Loew).

In negatively stained preparations made from glycerinated crane fly spermatids and sperm, actin-like filaments are seen which bind heavy meromyosin (HMM) to form arrowhead complexes, the reaction with HMM being blocked by ATP and pyrophosphate. In preparations from young spermatids the actin-like filaments are found singly, or in small groups, while in those from mature sperm the actin-like filaments are organized into a structure which we call ' rods' . Both rods and single filaments come from lysed sperm tails. The actin-like filaments in rods bind HMM only when frayed out on a grid. In sections of normal or glycerinated spermatids or sperm, no actin-like filaments are seen, either because they are not preserved through the fixation and embedding procedures, or because they are present in a form which we do not recognize. In sections of glycerinated spermatids incubated with HMM, decorated filaments are seen in non-nuclear regions of spermatids (tails), oriented parallel to the axoneme. These probably correspond to the single filaments identified in the negatively stained preparations and not to the rods, because the actin-like filaments in rods bind HMM only after fraying out on a grid. HMM causes polymerization of filaments: actin-like filaments are not seen in negatively stained preparations of glycerinated cells subsequently incubated in salts solution, yet are seen in such cells after a further incubation in HMM. Thus some of the decorated filaments seen in sections may have been polymerized by the HMM, raising questions about the procedure. After rupture of living cells no single actin-like filaments are seen in negatively stained preparations, raising the question of whether glycerol as well as HMM can cause polymerization of filaments. Rods are seen in such preparations, and thus rods seem to exist as such in living cells. Single actin-like filaments and rods are seen even when cytoplasmic microtubules are not seen (after incubation in colchicine or vinblastine). Thus the filaments do not seem to come directly from cytoplasmic microtubules. The possible presence of actin-like filaments in cilia and in other flagella, the location of actin-like filaments in sperm tails, and the possible role of the actin-like filaments are discussed.