The structure of the 9 + 1 axonemal core as revealed by treatment with trypsin

Abstract

Possible homology between the central singlets of the 9 + 2 axoneme, characteristic of flagella of spermatozoa of certain acoel turbellarians ( 2, 7,8 ), and two tubular helical elements described as possible components of the central core of the “9 + 1” axoneme of spermatozoa of other platyhelminths ( 11 ) is of interest in understanding evolutionary relationships among the turbellarians ( 8 ). Although examination of both sectioned and negatively stained preparations of 9 + 1 axonemes has failed to elucidate the structural basis for the appearance of the helical elements, trypsin has a differential effect on the components of the core, allowing examination of its substructure. After internal components have been digested, and only the peripheral sheath remains, the double helical pattern is retained, suggesting that the tubular helical elements in question do not exist; instead, the image results from the structure of the overlying sheath. It is therefore doubtful that homologies exist between the central regions of the 9 + 2 and 9 + 1 axonemes.