Abstract
Summary Microalgae play a major role as primary producers in aquatic ecosystems. Cell signalling regulates their interactions with the environment and other organisms, yet this process in phytoplankton is poorly defined. Using the marine planktonic diatom Pseudo‐nitzschia multistriata, we investigated the cell response to cues released during sexual reproduction, an event that demands strong regulatory mechanisms and impacts on population dynamics.We sequenced the genome of P. multistriata and performed phylogenomic and transcriptomic analyses, which allowed the definition of gene gains and losses, horizontal gene transfers, conservation and evolutionary rate of sex‐related genes. We also identified a small number of conserved noncoding elements.Sexual reproduction impacted on cell cycle progression and induced an asymmetric response of the opposite mating types. G protein‐coupled receptors and cyclic guanosine monophosphate (cGMP) are implicated in the response to sexual cues, which overall entails a modulation of cell cycle, meiosis‐related and nutrient transporter genes, suggesting a fine control of nutrient uptake even under nutrient‐replete conditions.The controllable life cycle and the genome sequence of P. multistriata allow the reconstruction of changes occurring in diatoms in a key phase of their life cycle, providing hints on the evolution and putative function of their genes and empowering studies on sexual reproduction.
Highlights
Phytoplankton feature prominently in aquatic ecosystems, showing striking morphological and functional diversity and accounting for one-half of the Earth’s primary productivity (Falkowski & Knoll, 2011)
The sequencing and assembly yielded a genome of 59 Mb composed of 1099 scaffolds with an N50 of 139 kb
Estimated heterozygosity was 0.18% and the distribution of allele frequencies peaked at c. 0.5, indicating a diploid clonal strain
Summary
Phytoplankton feature prominently in aquatic ecosystems, showing striking morphological and functional diversity and accounting for one-half of the Earth’s primary productivity (Falkowski & Knoll, 2011). The first assembled genomes of a centric (Thalassiosira pseudonana, Armbrust et al, 2004) and a pennate (Phaeodactylum tricornutum, Bowler et al, 2008) diatom were small in size (27– 32 Mb) with 10 000–14 000 genes. They contained only one-half of the genes with an annotated function, and c. 5% of P. tricornutum genes were predicted to be acquired by HGT from bacteria These genomes contributed towards an understanding of the genes and pathways involved in nutrient assimilation and metabolism of diatoms. To improve our understanding of the evolution and adaptation of this highly diverse group of organisms, additional diatom genomes were sequenced, such as those of the open-ocean centric diatom Thalassiosira oceanica (Lommer et al, 2012), the oleaginous Fistulifera solaris (Tanaka et al, 2015) and the polar diatom Fragilariopsis cylindrus (Mock et al, 2017), instrumental for the study of iron physiology, lipid metabolism and adaptation to cold, respectively
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