Transcriptome Regulation Of Stress Response In The Eastern Oyster(Crassostrea Virginica) And Oysters' Phylogenetic Analyses

Oysters first appeared in fossil records about 270 million years ago.Since then,they have experienced several mass extinctions.Oysters flourished during the cretaceous period,with many living species remaining today.Oyster diversity and their adaptation to challenging intertidal environments are not well understood.The sequencing of the oyster genome has provided opportunities to study the molecular mechanisms of oyster's adaptation.Studies have revealed key elements of Pacific oyster's adaptation.Its sister-species,eastern oyster Crassostrea virginica,that shows stronger resistance to environmental stress has received little attention.In this study,we sequenced transcriptomes of eastern oysters under different stresses and resequenced candidate genes to under eastern oyster's remarkable resilience and identify candidate stress resistance genes.The following are the main findings:1.Transcriptome response under environment stressOyster's response to environment stress conforms to three different stages:alert,resistance and exhaustion.Transcriptome response differs among different stages.Genes and pathways related to apoptosis,immunity,metabolism and endocrine system are important to oyster's stress response.Compared with air exposure and low salinity stress,high-temperature induced upregulation of the largest number of genes.The number of differentially expressed genes increased over time,but declined on the third day.Excessive energy consumption and cellular damage may be the leading causes of death.Endoplasmic reticulum stress response and apoptosis pathways play important roles in heat resistance.Eastern oyster has remarkable tolerance to low salinity.It response to the variable salinity stress by adjustting free amino acid pool conservatively.This experiment set up a salinity very low that it near the critical point of easten oyster's tolerance.We found genes belong to solute carrier protein family members(SLCs)are up-regulated,which may help maintain cells nutrients absorb.Extracellular matrix remodeling associated genes such as Zinc dependent metalloprotease and tissue inhibitor of metalloproteinase up-regulated too,which may help keep cell morphology under environment of low permeability.A large number of ribosomal proteins are up-regulated under air exposure.Amino acid and lipid metabolism are enhanced and may become the main source of energy.When eastern oyster dying under dry condition,the mitochondrial oxidation reaction enhanced.Under cold stress,oysters turn to suppress metabolic activities expecailly xenobiotics metabolism and genes related to transcription initiation.Dying oysters show unexpectedly high activities of gene transcription,with genes related to translation and apoptosis inhibition being most active in both heat and air exposure treatment groups.Based on the function analyse of relative down regulated genes,the down-regulated of genes involved to the formation of neuromuscular connection may be the key reson for oysters'exhaustion in dying.2.Resequencing of 1002 candidate stress-resistance genes before and after stress-caused mortalities using AmpliSeq identified a set of genes whose allele frequency showed significant changes.These genes and variations may be important to the eastern oyster's remarkable resistance to environmental stress.The oysters' species diversity and adaptability of the oysters are the foundation of the oyster's evolution.We identified two Ostreid oysters from Peru as Undulostrea megodon Hanley,1846 and Talonostrea talonata Li&Qi,1994,and renamed U.megodon as Ostrea megodon Hanley,1846 and T.talonata as Crassostrea talonata Li&Qi,1994.We identified three oyster species from Myanmar:Crassostrea belcheri Sowerby 1871,a common species found in Indo-Pacific;Saccostrea malabonensis Faustino 1932 and Crassostrea gryphoides tanintharyiensis.They are the first confirmed records in Myanmar.These findings highlight the need for molecular classification of oysters to understand oyster's global diversity,distribution,adaptation and human-facilitated transfers.