Cell division and morphogenesis of the labiate process in the centric diatomDitylum brightwellii

Summary

Cells of the centric diatomDitylum brightwellii were filmed undergoing cell division and valve secretion, and were fixed for transmission electron microscopy. Attention was directed particularly at the origin of the Labiate Process Apparatus (LPA).

As reported previously (li andVolcani 1985 a), the nucleus, centrally situated during interphase, moves laterally to undergo mitosis against the girdle bands. We describe the spindle which splits up into numerous fibres of overlapped polar microtubules (MTs) by metaphase. The chromosomes are diffuse and the spindle elongates rapidly during anaphase. A complex of organelles is found at the poles and ill-defined, dense material extends to the nearby plasmalemma from prophase on. The two Silica Deposition Vesicles (SDVs) are initiated during anaphase close to the poles and by midcleavage, the dense LPA arises on each SDV close to dense polar material. After cleavage, the daughter protoplasts round up and the SDV, already containing a nascent valve, expands over the cleavage furrow. The labiate process, a long straight hollow tube of silica, is rapidly (ca. 25 minutes) secreted from directly under the LPA; a fibrous plug (polysaccharide?) always appears in the SDV immediately adjacent to the LPA during the initiation of this secretion. The ill-defined Microtubule-Organizing Center (MC) from the spindle pole remains close to the LPA and in it can be seen the tiny presumptive primordial spindle on the nuclear envelope.

The raphe and the labiate process (LP), both highly differentiated apertures in the valve, probably function in a specialized form of the mucilage secretion involved in generation of movement in raphid diatoms, and in a simple form of movement in some centrics. Morphogenesis of the LP is associated with the LPA while differentiation of the raphe is almost associated with the MC; both MC and LPA have an intimate ontological relationship with the spindle pole and the postmitotic cytoskeletal system of MTs. This association also is seen in the formation of the LP in an araphid pennate,Diatoma (work in progress). Therefore, from functional, morphogenetic and ontogenetic observations, we support the proposal that the raphe of pennate diatoms arose from the LP of centric diatoms.