Transcriptome analysis of high mortality phenomena in polyploid fish embryos: An allotriploid embryo case study in hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

Abstract

Triploidization of diploid fish is an ideal means for achieving polyploid breeding. During the preparation of allotriploid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂), the allotriploid embryos (T) had a much higher morality from the high blastula to middle gastrula stage compared to the normal hybrid diploids (D). Therefore, the internal regulation of gene expression in allotriploid fertilized grouper eggs was explored by comparing the transcriptional profiles of T and D mapped at the high blastula (1), middle gastrula (2), and crystal (3) stages using the Illumina RNA-seq technology. There were 2357 differentially expressed genes (DEGs) found only in T1_vs_T2 and 1585 DEGs found only in T2_vs_T3. According to the GO functional enrichment analysis, when T developed from the high blastula to middle gastrula stage (i.e., pre-embryonic development; T1_vs_T2), the DEGs showed significant enrichment in terms associated with the cell cycle and cytoskeleton, indicating that cell proliferation was inhibited. KEGG pathway analysis of DEGs showed significant enrichment in mRNA processing and translation and in pathways associated with programmed cell death (PCD). Cell differentiation can trigger PCD and the combined effects of up-regulated ripk1, dnm1l, pgam5, Slc25a5, and vdac1 expression and down-regulated fadd expression during pre-embryonic development may have been the main cause of mass mortality in embryos up until the middle gastrula stage. A two-by-two comparison of D2_vs_T2 DEGs at the middle gastrula stage to the aforementioned DEGs expressed only in T revealed that the Parp4 and pole were down-regulated in the pre-development of T, which might lead to decreased DNA repair efficiency. These results could have reflected either (1) the altered physiological state of the cells due to cold-shock treatment or (2) the sensitivity of pre-embryonic development to an elevated number of chromosomes, which induced disrupted gene expression and abnormal cell division. In conclusion, the above study helps explain why allotriploid grouper exhibit high mortality at the middle gastrula stage, improves our understanding of the regulation of triploid embryo development, and provides basic information for reducing mortality at the middle gastrula stage.