Envelope formation and structure in the Euglenoid genus Trachelomonas

Ultrastructural details of envelope formation are described comparatively for several species of the genus Trachelomonas. The first-formed envelope component in T. oblonga var. punctata Prings. is an external “skin”. Next, a fibrillar layer is produced between the skin and the cell surface. Ferric hydroxide and manganese compounds are precipitated on this inner layer to form a thick envelope, species-specific in size, shape, ornamentation and collar characteristics. The original skin is then lost.

A similar sequence of events occurs during envelope formation in other species, but differences exist in the details of the process and in the texture and complexity of construction of the mature envelopes. T. volvocina Ehrenb. has a fibrillar inner layer and granular precipitation, but produces a complex three-layered envelope with a honeycombed central layer. This is compared with the superficially similar three-layered envelope of an unnamed species from a wild collection, and subtle differences in ultrastructural patterning are discussed. T. pertyi Prings. forms a two-layered spiny envelope, less highly organised than that of T. volvocina. The spiny and porous envelopes of T. hispida (Perty) Stein em. Defl., T. echinata Singh and T. lefevrei Defl. are quite different in texture, being composed of flaky material woven into a spongy matrix. The presence of large amounts of iron in the mature envelopes of all the species under study was confirmed by microanalysis in the electron microscope using X-ray spectroscopy.

Details of envelope ultrastructure are discussed in relation to taxonomy, cell metabolism, the problem of why Trachelomonas cells form envelopes when those of other euglenoid genera do not, and the problem of cell control of species-specificity of envelope characters. Electron microscopy has added considerably to our knowledge of the structural details of trachelomonad envelope formation, but it has been less successful in helping to elucidate some of the enigmas associated with this phenomenon.