Microtubular diversity in insect spermatozoa: Results obtained with a new fixative

Abstract

The architecture of the tail axoneme of spermatozoa from 49 insect species representing 20 insect orders has been examined. Whereas the microtubular walls of the nine doublet tubules and the two central singlet tubules were found to consist of a fixed number of protofilaments, the walls of the accessory tubules may consist of 13, 15, 16, 17, or 19 microtubules and have diameters from 24 to 38 nm. Cytoplasmic microtubules in the spermatid cytoplasm, outside the axoneme, may have 13, 14, or 16 protofilaments. The number of protofilaments seems constant for each class of accessory tubule and, except in Diptera, seems fixed in each insect order. Pterygote insects have 16 protofilaments and this was found in all orders except Ephemeroptera and Psocoptera with 13, Phasmida with 17, and Trichoptera with 19 protofilaments. Within Diptera, 16 protofilaments were found within the family Mycetophilidae, which for this reason can be regarded as the most primitive, extant dipteran group; the majority of dipteran flies have 13 protofilaments. Two nematoceran families, Bibionidae and Culicidae, have axonemes of a 9 + 9 + “1” type and accessory tubules with 15 protofilaments. We feel that axonemal ultrastructure is important in studies of insect phylogeny. We conclude that spermatids can effect specification of different protofilament numbers with numerical exactitude for the different types of microtubules.