Sex cells and reproduction in the diatom Nitzschia longissima (Bacillariophyta): discovery of siliceous scales in gamete cell walls and novel elements of the perizonium

Abstract

I. Kaczmarskam, N.A. Davidovich and J.M. Ehrman. 2007. Sex cells and reproduction in the diatom Nitzschia longissima (Bacillariophyta): discovery of siliceous scales in gamete cell walls and novel elements of the perizonium. Phycologia 46: 726–737. DOI: 10.2216/07–04.1

We examined the fine structure of gametes and the auxospore wall in the heterothallic pennate raphid diatom Nitzschia longissima (sensu Karsten 1897). The presence of scales in the gamete wall of both sexes is demonstrated for the first time for a pennate diatom species. Silica in scales was documented by energy dispersive x-ray spectroscopy. This finding supports our earlier hypothesis that scales are present in pennate sex cells more often than previously considered and reflect their evolutionary relationship to scale-bearing ancestors. In addition, the auxospore was shown to contain both transverse and longitudinal perizonia as well as longitudinal bands with fimbriate margins. Longitudinal bands are also novel components of a pennate auxospore. All these structures are either difficult to resolve or irresolvable in light microscopy (LM), clearly demonstrating that LM is not sufficient to investigate diatom sex cell structure. The sexual process employed by N. longissima is compared to its congenerics. Unlike other species from this genus, our diatom is morphologically and behaviourally anisogamous. In addition, the copulation of gametangia is not aided by copulation tubes, papillae or copulation jellies. Therefore, sexual reproduction in N. longissima shows little similarity to that of the few other species in the genus Nitzschia thus far examined.

Key Words::

• Auxospores
• Diatoms
• Gametes
• Nitzschia
• Perizonia
• Scales

ACKNOWLEDGMENTS

We thank S.S. Bates and C. Léger for maintaining cultures. J. Harris and A. Steeves performed initial mating experiments. This work was supported in part by an NSERC Discovery Grant to I.K.