Thallus development in Halopteris filicina (Phaeophyceae, Sphacelariales)

Abstract

The apical cell of H. filicina is intensely and permanently polarized, exhibiting a tip-like growth. It undergoes two types of highly asymmetrical divisions, resulting in the formation of primary segments, lateral branches and axillary hairs.

Critical asymmetrical divisions for thallus morphogenesis occur in the daughter cells of the apical cell derivatives, producing epidermal and medullary cells. Asymmetrical divisions of particular epidermal cells produce the rhizoid mother cells. These give rise to multicellular rhizoids, which form a sheath around the basal thallus portion.

Following experimental removal of the apical cell, the living cells at the wounded thallus face are differentiated into apical cells. This process is carried out by: (a) intense protoplasmic synthesis; (b) some organelle dedifferentiation; (c) cell polarization—formation of a tip growth centre.

The secondary apical cell segments, as well as their first derivatives, are large and highly vacuolated. The thin peripheral cytoplasmic zone, in which the plastids are usually distributed, is connected with the central perinuclear zone, where most of the other organelles are accumulated, by thin cytoplasmic strands. Microtubules clustered in bundles and ER membranes traverse these strands. The probable role of the microtubules and the differentiation of two cell types are also discussed.