Diatoms as dandelions: convergent evolution in the reproductive biology of small Nitzschia species (Bacillariophyta) and its possible taxonomic consequences

ABSTRACT

Nitzschia soratensis and N. inconspicua are two small diatoms that are extremely similar in the light microscope though separable in subtle aspects of valve and girdle ultrastructure. They are not closely related in molecular phylogenies and differ in their ecological preferences, though they sometimes co-occur in the same communities. To test further their functional equivalence, we investigated the reproductive biology of N. soratensis, for comparison with a previously published account of N. inconspicua. Both species are automictic, lacking pairing between gametangia and gamete exchange. Nuclear staining shows the presence of two nuclei in some auxospores of N. soratensis, which are smaller than the nuclei of vegetative cells and appear to be haploid, indicating prior meiosis. These auxospores, surrounded by incunabula containing tangles of silica strips, give rise to uninucleate initial cells 18–21 µm long. Frequently, however, small N. soratensis cells produce two spherical ‘pseudogametes’, some of which abort while others expand and form initial valves shorter than those produced by binucleate auxospores (maximum length 14.5 µm). Similar uniparental auxosporulation, with silica strip formation, occurs in N. aff. hantzschiana, N. acidoclinata, N. fonticola and N. angustata, which are close relatives of N. soratensis according to morphological and/or molecular evidence. This group and the N. inconspicua complex may be the diatom equivalents of apomictic angiosperms, e.g. dandelions, and likely contain a multitude of microspecies.

KEYWORDS:

• Automixis
• Auxospores
• Diatoms
• Girdle structure
• Incunabula
• Life cycle
• Taxonomy

ACKNOWLEDGEMENTS

We thank Dr Martyn Kelly for collecting the sample from Houselop Beck, County Durham, UK, from which N. soratensis was isolated. Thanks are also due to Frieda Christie for her invaluable oversight of the SEM facility during the period when this study was made. The comments of two reviewers were constructive and we are grateful for them.

DISCLOSURE STATEMENT

No potential conflicts of interest are reported by the authors(s).

Supplementary Information

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00318884.2024.2309855

Additional information

Funding

We acknowledge support from the CERCA Programme/Generalitat de Catalunya. A SYNTHESYS award (funded by EU FP7 under the “Capacities” Programme) supported a visit of Rosa Trobajo to RBGE in 2011. The Royal Botanic Garden Edinburgh is supported by the Scottish Government’s Rural and Environment Science and Analytical Services Division. DGM thanks the Royal Society for a grant that enabled the purchase of the Polyvar photomicroscope.