On the Evolutionary Stability of the 9 + 2 Pattern

Since a particular arrangement of filaments, which has come to be known as the 9 + 2 pattern, is found with one exception (9) in all motile cilia and flagella that have heretofore been examined with the electron microscope, some interest (2, 3, 5) has arisen in the remarkable stability of this pattern and its universal occurrence. Bradfield (3) has summarized three possibilities. First, the constancy of the 9 + 2 pattern might arise "from the intrinsic properties of the proteins forming the 11 fibers, such that during fibrogenesis only the observed configuration can be produced by the centriole or basal corpuscle ." Second, the pattern might be "the result of chance integration of. .. bacterial flagella, which subsequently became a rigid character for some genetical reason, and not because it constituted the only solution from a mechanical point of view ." Or, third, it might represent the "only possible mechanical solution" to the problem of ciliary motion. Serra (8) has commented extensively upon a fourth hypothesis, that the 9 + 2 pattern is stable because of spatial properties "owing to which no other number of fibers would be normally possible ." According to Serra, the central pair of filaments is produced by the centriole's