Spermatogenesis and auxospore structure in the multipolar centric diatom Hydrosera

Abstract

Spermatogenesis and auxospore development were studied in the freshwater centric diatom Hydrosera triquetra. Spermatogenesis was unusual, lacking depauperating cell divisions within the spermatogonangium. Instead, a series of mitoses occurred within an undivided cell to produce a multinucleate plasmodium with peripheral nuclei, which then underwent meiosis. 32 or 64 sperm budded off from the plasmodium leaving a large residual cell containing all the chloroplasts. Similar development apparently occurs in Pleurosira, Aulacodiscus, and Guinardia, these being so distantly related that independent evolution of plasmodial spermatogenesis seems likely. After presumed fertilization, the Hydrosera egg cell expanded distally to form a triangular end part. However, unlike in other triangular diatoms (Lithodesmium, Triceratium), the development of triradiate symmetry was not controlled by the "canonical" method of a perizonium that constrains expansion to small terminal areas of the auxospore wall. Instead, the auxospore wall lacked a perizonium and possessed only scales and a dense mat of thin, apparently entangled strips of imperforate silica. No such structures have been reported from any other centric diatoms, the closest analogs being instead the incunabular strips of some raphid diatoms (Nitzschia and Pinnularia). Whether these silica structures are formed by the normal method (intracellular deposition within a silica deposition vesicle) is unknown. As well as being more rounded than vegetative cells, the initial cell is aberrant in its structure, since it has a less polarized distribution of the "triptych" pores characteristic of the species.

Morphological and transcriptomic evidence for ammonium induction of sexual reproduction in Thalassiosira pseudonana and other centric diatoms

“…Oogonia, auxospores, and spermatogonia in the other species studied displayed similar morphologies to those observed in T . pseudonana and other centric diatoms (Fig 3L–3O, S2M–S2P Fig, Fig 4A) [11, 16, 28, 33, 35, 37, 38, 40, 41]. …”

Ontogenetic analysis of siliceous cell wall formation in Triparma laevis f. inornata (Parmales, Stramenopiles)

“…), and Hydrosera triquetra (Idei et al. ). It is entirely reasonable therefore to assume that the presence of the annulus and sternum as pattern centers is synapomorphic for the Parmales and diatoms: that is, the sternum cannot be regarded as an autapomorphy of pennate diatoms, since their last common ancestor already had both pattern centers, annulus and sternum.…”

Ardissonea crystallina has a type of sexual reproduction that is unusual for centric diatoms

“…It is worth noting that so far, the most closely-related species to A. crystallina whose auxosporulation has been studied in detail is Hydrosera triquetra Wallich, which undergoes a completely different kind of sexual reproduction than that seen in Ardissonea . In Hydrosera , as is typical of centrics, sexual reproduction is oogamous and involves flagellated sperm 27 . Auxospores have also been observed in an unidentified species of Lampriscus 28 , a genus appearing to be the closest to the Toxariids in each molecular phylogeny 9 , 11 , 13 , 14 , 29 .…”