Transcriptomic Analysis Reveals the Regulation Mechanism of Sporogenesis in Ulva prolifera

Ulva prolifera, the main causative species of green tide, has drawn much attention. Spore reproduction was one of the main reproduction strategies that could be induced by changing environmental factors, but the mechanism of spore formation remains obscure. Here, we cultured U. prolifera (segments) under the optimal sporulation condition, and four states in total from vegetative to reproductive were defined. Additionally, the chlorophyll fluorescence and transcriptome analysis were determined at these four states. The results showed that: (1) Compared with state I, the photosynthetic capacity (F-v/F-m, F-v '/F-m ', rETR(max)) and chlorophyll content (Chl a, Chl b, carotenoids) were enhanced in state II, whereas it decreased in state III and IV (the spore formation period); (2) a total of 41,058 unigenes were expressed during the spore formation process; (3) compared with state I, the genes related with photosynthesis, terpenoid backbone biosynthesis, and carotenoid biosynthesis were significantly upregulated in states II, III, and IV whereas glycolysis was downregulated in state I; (4) some genes of the transcription factors families, such as the C3H family, may be one of the key factors that regulate genes in the spore formation; (5) 574 of the differentially expressed genes (DEGs) associated with flagella biosynthesis were annotated according to Gene Ontology and Kyoto Encyclopedia of Genes and Genomes Orthology, including 20 unigenes encoding intraflagellar transport proteins (IFTs) which had not been reported in previous transcriptome analysis in U. prolifera. This study provides a new perspective of spore formation at the gene transcriptional level, although the detailed transcription regulatory spore formation network remains to be unveiled.