Transcriptome sequencing and comparative analysis of the gametophyte thalli of Pyropia tenera under normal and high temperature conditions

The marine red alga Pyropia tenera grows on intertidal rocks, where it undergoes dynamic environmental changes including temperature, desiccation, osmotic shock, and changes in light intensity. Therefore, Pyropia have developed a variety of strategies and mechanisms to overcome those environmental stressors. In an effort to identify the genes involved in the high-temperature tolerance of P. tenera, we generated 368,334 expression sequence tags (ESTs) using 454 sequencing technology and 3,331 ESTs using the Sanger method. Among the total ESTs, 222,024 reads were generated from gametophyte thalli under control condition and 149,641 reads were generated under high temperature condition. These ESTs were assembled into 17,870 contigs consisting of 336,016 reads, whereas 35,924 sequences remained as unassembled ESTs. Only 16.5 % of contigs shared significant similarity with an E value of a parts per thousand currency sign1E- 10 with UniProt sequence. The 95 different SSR motifs were discovered in 1,586 contigs. Trinucleotide repeat was absolutely predominant (90.2 %) SSR, and GGC was the most common motif. A comparison of the ESTs from gametophyte thalli under normal and heat stress conditions enabled us to identify the transcripts that were up or downregulated by high temperature. Most of transcripts produced under the high temperature condition belong to heat shock protein family and novel transcripts not matched to known genes in current public databases. These ESTs will provide valuable information to identify the DNA markers for the Pyropia species and the genes involved in the molecular mechanism of thermotolerance in red algae.