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Keywords:

  • Bacillariophyta;
  • evolution;
  • Gomphoneis;
  • Gomphoneis mesta;
  • Gomphonema;
  • initial frustules;
  • life history;
  • ontogeny perizonium;
  • ultrastructure

ABSTRACT

The morphology of initial frustules of Gomphoneis mesta Passy-Tolar & Lowe was studied with light and scanning electron microscopy The structure of the raphe, central nodule, stigma, striae, and apical pore field in post-auxospore generations was described. Discovery of internal chambers and an axial plate in very early stages of the life cycle of G. mesta reveals a possible evolutionary connection with the Herculeana group (sensu Kociolek and Stoermer 1989) and suggests that the loss of axial plates is a secondary event. Polarization of the characters “stigma number” and “outline of the internal stigmal opening” is proposed. A perizonium, composed of transverse and longitudinal bands, is reported for the first time in the genus Gomphoneis.