Developmental aspects of branching in filamentous Cyanophyta/Cyanobacteria

Branching in filamentous cyanobacteria results in a wide range of phenotypic expressions that have traditionally been used for taxonomic delimitations. Branching patterns are often complex, but consistently specific for particular groups. Their development frequently involves several steps of cause-consequence relationships. These properties indicate a certain degree of genetic control of the branching process. The present paper evaluates causalities involved in the branching process and maps the spectrum of the resulting morphologies for a wide range of filamentous cyanobacteria. Definitions of various types of false and true branching are complemented or modified, based on developmental analysis at the population level. Distinction between false and true branching is based on identification of ‘branch-point cells' having at least three contacts with neighboring cells. Two basic types of false branching (by bundle partitioning and by lateral trichome protrusion) are evaluated in relation to sheath production and heterocyst differentiation. Four types of true branching ("X", "T», "V" and reverse-"Y") are evaluated in reference to the changes in division plane, orientation and position of the partitioning wall, number of cells engaged in branching and their origin, sequence of events during branch formation, as well as limitation of branch growth. Filament multiseriality is explained in terms of true branching.