Exploring Molecular Signs of Sex in the Marine Diatom Skeletonema marinoi

Abstract

Sexual reproduction plays a fundamental role in diatom life cycles. It contributes to increasing genetic diversity through meiotic recombination and also represents the phase where large-sized cells are produced to counteract the cell size reduction process that characterizes these microalgae. With the aim to identify genes linked to the sexual phase of the centric planktonic diatom Skeletonemamarinoi, we carried out an RNA-seq experiment comparing the expression level of transcripts in sexualized cells with that of large cells not competent for sex. A set of genes involved in meiosis were found upregulated. Despite the fact that flagellate gametes were observed in the sample, we did not detect the expression of genes involved in the synthesis of flagella that were upregulated during sexual reproduction in another centric diatom. A comparison with the set of genes changing during the first phases of sexual reproduction of the pennate diatom Pseudo-nitzschiamultistriata revealed the existence of commonalities, including the strong upregulation of genes with an unknown function that we named Sex Induced Genes (SIG). Our results further broadened the panel of genes that can be used as a marker for sexual reproduction of diatoms, crucial for the interpretation of metatranscriptomic datasets.

Keywords: Skeletonema marinoi; diatom; flagella; meiosis; sex induced genes (SIG); sexual reproduction; transcriptomics.

Figure 1

Schematic drawing of the life cycle of a centric diatom: When a vegetative cell divides mitotically (1), each daughter cell inherits one of the two halves (thecae) of the rigid silica cell wall (orange and green lines) and builds a new smaller theca (purple lines). This leads to a progressive reduction of the cell size of the cell population (2). Above a species-specific sexualization size threshold (SST), (3) diatom cells are incapable of performing sexual reproduction. Below the SST, if a proper trigger is present (such as a salinity shock for Skeletonema marinoi), meiosis is induced and cells can either produce one egg or four sperms (4). Gametes conjugation (syngamy) leads to the formation of a zygote that becomes an auxospore, a soft stage which can expand (5). A new cell wall is built inside the auxospore, which eventually becomes an initial cell of the maximum cell size (6).

Figure 2

Experimental setup: (A–C), conditions tested in the experiment. The scanning electron micrographs show two S. marinoi cells in a chain. (D) Comparisons considered for the gene expression analysis. (E) Scheme of the sexual stages of S. marinoi detected at different time points after application of the salinity cue (0 h) (see Reference [8]) and time points chosen for the transcriptomic analysis (black arrows). At 20 h (gray arrow), only vegetative cells were observed. The yellow arrows, the pink arrow, and the blue arrow indicate the presence of gametangia, gametes, and auxospores, respectively. The sexual stages, indicated by thin black arrows, are shown in the images above with a color-matching frame. Scale bar, 5 µm.

Figure 3

Number of genes differentially expressed in the SEX and SALT comparisons: (A) Number of genes up- and downregulated at the two time points. (B) Venn diagram showing the overlap between different datasets of differentially expressed genes in the SEX and SALT conditions at the two time points.