JANUS CELLS UNVEILED: FRUSTULAR MORPHOMETRIC VARIABILITY IN GOMPHONEMA ANGUSTATUM

Abstract

A population of Gomphonema angustatum (Kützing) Rabenhorst with a high proportion of Janus (heteromorphic) cells was examined with respect to the variability of continuous morphometric valve characters at three nested levels: among sectors within valves, among valves and among frustules. The pattern of several characters within valves varied asymmetrically about the apical and transapical axes, reflecting the heteropolar shape of the valve and ontogenetic processes generating the valve structure. Continuous characters among valves showed a wide range of relative variability, and characters were generally strongly correlated with either length or width. The distributions of differences between valves in the same frustule in mean interstriae distance and canonical characters were continuous and normal. Cells showing a relatively large degree of intrafrustular difference are one tail of these continuous distributions and provide the basis for the observation of Janus cells in G. angustatum. Ontogenetic models of striae pattern generation, in which both linear and random components are valve length dependent and in which pattern variation is independent of environmental variation, are sufficient to explain the production of Janus cells in G. angustatum. Intrafrustular differences provide an estimate of the reaction norms for genotypes and facilitate assessment of the correspondence of genotypic and phenotypic variation in the population.