Morphological and transcriptomic evidence for ammonium induction of sexual reproduction inThalassiosira pseudonanaand other centric diatoms

Abstract

The reproductive strategy of diatoms includes asexual and sexual phases, but in many species, including the model centric diatom Thalassiosira pseudonana, sexual reproduction has never been observed. Furthermore, the environmental factors that trigger sexual reproduction in diatoms are not understood. Although genome sequences of a few diatoms are available, little is known about the molecular basis for sexual reproduction. Here we show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica. RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium. Some of the induced genes are linked to meiosis or encode flagellar structures of heterokont and cryptophyte algae. The identification of ammonium as an environmental trigger suggests an unexpected link between diatom bloom dynamics and strategies for enhancing population genetic diversity.

Thoughts on the evolution and ecological niche of diatoms

“…Phenomenologically, such a strategy can help reconcile observations of strongly species-dependent sinking events under favorable growth conditions, such as observed during NABE08. It is noteworthy here that, in the experimentally-triggered study on Thalassiosira pseudonana (Moore et al 2017), sexual reproduction was accompanied by a high level of TEP production (K.H. Halsey, personal observation).…”

“…In other words, the evolution of physiological attributes that allow a given diatom species to sustain accelerations in division rate for a sufficient duration to achieve high concentrations may be based on improvements in sexual reproduction success that enhance genetic invention. If this is the case, then this advantage will be enhanced if sexual reproduction of a population is coordinated and especially if it is communicated or triggered once the population has achieved its critical density (Sterner and Elser 2002, Edlund and Stoermer 1997, Moore et al 2017. Recently, it was shown that sexual division in the diatom, Thalassiosira pseudonana, can be experimentally triggered by high concentrations of ammonium, but only after reaching maximal cell density (Moore et al 2017).…”

“…If this is the case, then this advantage will be enhanced if sexual reproduction of a population is coordinated and especially if it is communicated or triggered once the population has achieved its critical density (Sterner and Elser 2002, Edlund and Stoermer 1997, Moore et al 2017. Recently, it was shown that sexual division in the diatom, Thalassiosira pseudonana, can be experimentally triggered by high concentrations of ammonium, but only after reaching maximal cell density (Moore et al 2017). Chemicallycoordinated synchronization of sexual reproduction has also been indicated in other diatom species, along with the exudation of chemotaxic pheromones that may be involved in optimizing encounter rates between eggs and sperm (Koester et al 2007, Amin 2012.…”

Chlorella vulgaris genome assembly and annotation reveals the molecular basis for metabolic acclimation to high light conditions

“…In particular, 78.2% of the genes in CiliaCut that were present in the diatom Thalassiosira pseudonana were present also in C. vulgaris 211/11P (Table S10): 84.2% of the T. pseudonana genes in MotileCut (genes in CiliaCut involved in motile flagella functions) were present also in C. vulgaris 211/11P. This result suggested that C. vulgaris 211/11P might be able to form gametes with motile flagella as previously observed for T. pseudonana during gametogenesis (Moore et al ., ).…”