Vegetative cell enlargement in selected centric diatom species – an alternative way to propagate an individual genotype

ABSTRACT Vegetative cell enlargement (VCE) and structures involved are documented for the first time using epifluorescence and electron microscopy. We examined three centric diatom species with polar and non-polar valve outlines. VCE progressed in two ways. The first involved liberation of the protoplast, nearly isodiametric expansion, and building of the large frustule in a more or less spherical cell. Protoplast retraction as observed in sexual auxospores before the formation of initial valves was never observed. Cell walls produced a variety of scales, including valve-like scales before the first enlarged epivalve was deposited. VCE wall structures were not similar to auxospore walls of the same species. The morphology of the first enlarged valve, the epivalve, differed from that of typical vegetative valves to a varying degree, depending on the species. The second way to vegetatively enlarge cells involved expansion of the girdle region of the existing small frustule. The circumference of the girdle region grew due to the deposition of irregular, supernumerary copulae. These copulae produced one or more protuberances, resulting in a misshapen cell. This misshapen cell then divided mitotically, and the first enlarged valve was a hypovalve. Hypovalve morphology strongly differed from that of typical vegetative valves. Several subsequent mitotic divisions were required to restore normal, species-specific frustules. Over time, irrespective of the manner of enlargement, the enlarged cells grew well and eventually became more frequent than small cells in our cultures. The scale factors of enlarged cells compared with pre-enlargement size in our experiments (1.1–5.2) were similar to those reported for other species (1.4–7.5). Additionally, we found that size increases from VCE were comparable to those resulting from sexual reproduction of the same species. Therefore, VCE can potentially expand the resident time of the population in the environment to the same degree as would production of sexual initial cells. HIGHLIGHTS Species vegetatively restore large cell size in different ways. Scale factors in sexually and vegetatively enlarged cells are similar. VCE may not be as rare as reported in nature.