Abstract
BackgroundFlagella have been lost in the vegetative phase of the diatom life cycle, but they are still present in male gametes of centric species, thereby representing a hallmark of sexual reproduction. This process, besides maintaining and creating new genetic diversity, in diatoms is also fundamental to restore the maximum cell size following its reduction during vegetative division. Nevertheless, sexual reproduction has been demonstrated in a limited number of diatom species, while our understanding of its different phases and of their genetic control is scarce.ResultsIn the transcriptome of Leptocylindrus danicus, a centric diatom widespread in the world’s seas, we identified 22 transcripts related to the flagella development and confirmed synchronous overexpression of 6 flagellum-related genes during the male gamete formation process. These transcripts were mostly absent in the closely related species L. aporus, which does not have sexual reproduction. Among the 22 transcripts, L. danicus showed proteins that belong to the Intra Flagellar Transport (IFT) subcomplex B as well as IFT-A proteins, the latter previously thought to be absent in diatoms. The presence of flagellum-related proteins was also traced in the transcriptomes of several other centric species. Finally, phylogenetic reconstruction of the IFT172 and IFT88 proteins showed that their sequences are conserved across protist species and have evolved similarly to other phylogenetic marker genes.ConclusionOur analysis describes for the first time the diatom flagellar gene set, which appears to be more complete and functional than previously reported based on the genome sequence of the model centric diatom, Thalassiosira pseudonana. This first recognition of the whole set of diatom flagellar genes and of their activation pattern paves the way to a wider recognition of the relevance of sexual reproduction in individual species and in the natural environment.
Highlights
Flagella have been lost in the vegetative phase of the diatom life cycle, but they are still present in male gametes of centric species, thereby representing a hallmark of sexual reproduction
Leptocylindrus danicus showed a higher representation of genes associated with sexual reproduction such as the Gene ontology (GO) terms ‘cilium’, ‘cilium morphogenesis’, ‘female meiosis’, ‘female gonad development’, ‘female sex differentiation’, ‘development of primary female sexual characteristics’ and ‘sex differentiation’ (Additional file 7: Table S5B), no specific conditions that could stimulate sexualization had been applied to the culture used for RNA sequencing
In L. aporus often the genes associated with these GO terms were absent, whereas genes associated with the GO terms ‘androecium development’, ‘stamen development’, ‘stamen formation’ and ‘stamen morphogenesis’ were found, which were almost absent in L. danicus
Summary
Flagella have been lost in the vegetative phase of the diatom life cycle, but they are still present in male gametes of centric species, thereby representing a hallmark of sexual reproduction. This process, besides maintaining and creating new genetic diversity, in diatoms is fundamental to restore the maximum cell size following its reduction during vegetative division. Following the high number of mitotic divisions during a bloom (1–2 per Nanjappa et al BMC Genomics (2017) 18:813 day or more), the average cell size considerably decreases, until gametes are formed and conjugation occurs. The soft cell wall of the zygote allows cell enlargement, and a new silica shell is formed only when the maximum size is reached again
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