FIGURE 1. Electron micrographs of Nyctotherus ovalis showing hydrogenosomes.

a,N. ovalis from the hindgut of Blaberus⁵. The hydrogenosomes (H) are surrounded by endosymbiotic methane-producing Archaea (dark spots); N, macronucleus; n, micronucleus; V, vacuole. Visible by MnO₄fixation/Epon. b, c, e,N. ovalis from the hindgut of Periplaneta americana⁵. Immunogold labelling of glutaraldehyde-fixed and Unicryl-embedded sections; m, methanogenic Archaea (endosymbionts). b,DNA antiserum (Boehringer) labels the matrix of about 80% of the hydrogenosomes on randomly chosen sections with 3-10 grains. The difference in DNA concentration causes the label over the endosymbiotic methanogens to be heavier. c,Immunogold labelling obtained with a polyclonal antiserum against hydrogenosomal adenylate kinase (hdgAK2L2) from the anaerobic chytrid Neocallimastix sp. L2 (F. V. and B. B., unpublished). Matrix of hydrogenosomes and endosymbiotic methanogens is labelled. e,Labelling of the hydrogenosomes (and methanogens) with an antiserum against an [Fe]-hydrogenase of Trichomonas vaginalis⁶. There is about 40% amino-acid sequence identity with the [Fe]-hydrogenase described here. d,Electron micrograph of a hydrogenosome of N. ovalis from Blaberus. OsO₄fixation/Epon. Cristae are clearly visible. Arrows indicate ribosomes. Scale bars are 1 m, except for that in a,which represents 10 m.