library_construction_protocol: Embryos were dissected on a 1.5% agarose-coated dish with a fine glass 'hair' smeared with cellular debris. A single isolated cell was transferred in 0.5μl of seawater into a 0.2ml PCR tube containing 9μl RNase-free water. After visual inspection of the presence of the cell under stereomicroscope, 1μl of 10x Reaction buffer (SMARTer Ultra Low Input RNA Kit, Takara Bio) was added and the tube was vortexed immediately and stored at -80°C before being processed for library preparation. Indexed Illumina libraries were prepared using Nextera XT kits (Illumina) from cDNAs generated with SMARTer Ultra low input RNA kit (Takara Bio) according to manufacturers' instructions. Sequencing was conducted in the UCA GenomiX genomics platform (Sophia Antipolis, France) using Illumina NextSeq500 system. For each library, single end 75bp reads were obtained with sequencing depth of around 8 million reads.