TRANSCRIPTOME ANALYSIS OF COMPATIBLE AND INCOMPATIBLE INTERACTIONS OF ECTOCARPUS WITH EURYCHASMA DICKSONII USING RNA-SEQUENCING

The widely distributed filamentous macroalga Ectocarpus siliculosus serves as a model organism for genetic and genomic approaches in brown algae. Eurychasma dicksonii, is one of the most common pathogenic oomycete of brown macroaglae, and infects more than 40% of the tested algal species. Importantly, some host species are resistant (R) to the infection, whereas some are susceptible (S). The understanding of the algal defense mechanisms is unclear. To identify transcriptional mechanisms underpinning the resistance, we analyzed RNAseq data of three diffferent species of Ectocarpus: i. Ectocarpus siliculosus (genome strain (S)) ii. Ectocarpus sp. (R) iii. Ectocarpus fasciculatus (R) treated with two strains of Eurychasma dicksonii. The sequence reads were aligned to de novo transcriptome assemblies, analyzed to measure gene expression levels in order to identify genes that are differentially expressed between the treatments. The orthologous genes in three different Ectocarpus species were identified through PicoPLAZA: 11,094, 1,504 and 696 of the Ectocarpus genes were shared across all three species, across E. fasciculatus and E. siliculosus and across E. sp and E. siliculosus respectively. We also compared the expression levels of these orthologous genes across the Ectocarpus transcriptomes. Using the available high quality genome annotation of E. siliculosus, we were able to functionally annotate more than 70% of the E. sp and E. fasciculatus transcriptomes. Using this approach, we identified classical defense response and detoxification genes i.e. vanadium bromoperoxidase (vBPO), manganese superoxide dismutase (MnSOD), glutathione-S-transferases (GSTs) were induced during resistant interactions. Most interestingly, we also found that the Ectocarpus viral-1 genes (EsV-1) were 1-to-2 fold induced/repressed during the compatible/incompatible interactions. Our RNA seq results will
be experimentally tested using the Fluorescent in situ hybridization (FISH) technique for our top 5 candidate gene/ gene families. The results will help to identify key resistant/ susceptible genes and contribute to a better understanding of the mechanisms of interaction between Ectocarpus and Eurychasma.