Gametogenesis and Auxospore Development in Actinocyclus (Bacillariophyta)

Abstract

cGametogenesis and auxospore development have been studied in detail in surprisingly few centric diatoms. We studied the development of sperm, eggs and auxospores in Actinocyclus sp., a radially symmetrical freshwater diatom collected from Japan, using LM and electron microscopy of living cultures and thin sections. Actinocyclus represents a deep branch of the ‘radial centric’ diatoms and should therefore contribute useful insights into the evolution of sexual reproduction in diatoms. Spermatogenesis was examined by LM and SEM and involved the formation of two spermatogonia (sperm mother-cells) in each spermatogonangium through an equal mitotic division. The spermatogonia produced a reduced ‘lid’ valve, resembling a large flat scale with irregular radial thickenings. Sperm formation was merogenous, producing four sperm per spermatogonium, which were released by dehiscence of the ‘lid’ valve. The sperm were spindle-shaped with numerous surface globules and, as usual for diatoms, the single anterior flagellum bore mastigonemes. One egg cell was produced per oogonium. Immature eggs produced a thin layer of circular silica scales before fertilization, while the eggs were still contained within the oogonium. Sperm were attracted in large numbers to each egg and were apparently able to contact the egg surface via a gap formed between the long hypotheca and shorter epitheca of the oogonium and a small underlying hole in the scale-case. Auxospores expanded isodiametrically and many new scales were added to its envelope during expansion. Finally, new slightly-domed initial valves were produced at right angles to the oogonium axis, after a strong contraction of the cell away from the auxospore wall. At different stages, Golgi bodies were associated with chloroplasts or mitochondria, contrasting with the constancy of Golgi–ER–mitochondrion (G-ER-M) units in some other centric diatoms, which has been suggested to have phylogenetic significance. Electron-dense bodies in the vacuole of Actinocyclus are probably acidocalcisomes containing polyphosphate.

Spermatogenesis and auxospore structure in the multipolar centric diatom Hydrosera

“…We have recently observed auxosporulation in several centric diatoms, some with circular valves (e.g., Actinocyclus ; Idei et al. ), others with more complex shapes, including the large multipolar centric Hydrosera , which we found reproducing sexually in natural populations from southern Japan. In this paper we document the development of the Hydrosera auxospore and compare it with that of other “polar” centric diatoms, especially Chaetoceros , Bacteriastrum , Attheya , Odontella , Biddulphia , Lithodesmium and Triceratium , for which brief but invaluable data were published by Drebes (, ) and von Stosch ().…”

“…However, sexual reproduction is infrequent in natural populations (e.g., Mann 1988) and the factors controlling sexualization are poorly understood (Chepurnov et al 2004), which probably explains why auxospore development, and hence shape morphogenesis, has been so little studied. We have recently observed auxosporulation in several centric diatoms, some with circular valves (e.g., Actinocyclus ; Idei et al 2012), others with more complex shapes, including the large multipolar centric Hydrosera, which we found reproducing sexually in natural populations from southern Japan. In this paper we document the development of the Hydrosera auxospore and compare it with that of other "polar" centric diatoms, especially Chaetoceros, Bacteriastrum, Attheya, Odontella, Biddulphia, Lithodesmium and Triceratium, for which brief but invaluable data were published by Drebes (1972, 1977a) and von Stosch (1982.…”

“…In contrast, it is more difficult to argue that the perizonial bands are homologous with the scales present in the walls of many centric diatom auxospores (e.g., Crawford , Roemer and Rosowski , Schmid , Idei et al. , this paper), since scales are present in addition to properizonia and perizonia in a variety of centric and pennate diatoms (e.g., von Stosch , Sato et al. , Mann et al.…”

“…, Mizuno ) have indicated instead that the pattern of spermatogenesis is rather labile, sometimes even within the same genus (see also Idei et al. ).…”

“…Medlin and Kaczmarska (2004) considered spermatogenesis to be a conservative feature in diatoms and suggested that the radial centrics (their Coscinodiscophyceae) are generally merogenous, whereas the polar centrics (their Mediophyceae) are generally hologenous. However, further reviews of the few available data (Jensen et al 2003, Mizuno 2008 have indicated instead that the pattern of spermatogenesis is rather labile, sometimes even within the same genus (see also Idei et al 2012).…”