Unisexual Development Potential Mechanism Of Diploid Eggs

Gynogenesis is an important fish breeding method for the only maternal genetic composition of the offspring.Usually,haploid eggs,produced by the diploid fish,are unable to develop into viable individuals directly via gynogenesis but needing double chromosomes of embryos by physical or chemical methods.The diploid eggs,produced by the allotetraploid fish which are obtained via distant hybridization in this laboratory,can directly develop into diploid progeny via gynogenesis.This diploid progeny can produce unreduced diploid gametes,which display a higher hatch rate by gynogenesis.The above developmental phenomena indicate that diploid eggs(especially gynogenesis-derived)show obvious developmental advantages during parthenogenesis.However,the research on the molecular mechanism of this developmental advantage is almost blank until now.Mid-blastula transition of the fish embryo occurs after 10 divisions,that is,1K-cell stage,transitioning the embryo development regulation from maternal to zygotic.Previous studies showed that the maternal materials in eggs were not only meet the demands of cell division during early embryonic development,but also could change the modification state of chromosomes to regulate the activation of the zygotic genome.Therefore,we compared the different reserves of haploid eggs anddiploid eggs from sexual reproduction and gynogenesis by transcriptomic analysis.Moreover,we analyzed the histone post-modification state and gene expression of diploid embryos(inbred-derived,gynogenesis-derived)and haploid embryos at 1K-cell stage to initially reveal the molecular mechanism of the developmental advantage of diploid eggs.The main results obtained from research are as follows:(1)The transcriptomic comparative analysis of diploid eggs and haploid eggs showed that there are 1212 up-regulated genes and 672down-regulated genes in diploid eggs,which are mainly concentrated in ribosome and oxidative phosphorylation signaling pathways.This result indicated that diploid eggs displayed more abundant maternal source material storage than haploid eggs,providing material preparation advantages for subsequent embryonic development.The transcriptomic comparison analysis of diploid and haploid embryos at the 1K-cell period showed that there are 72 down-regulated genes and 31 up-regulated genes in both two diploid embryos.The functions of up-regulated genes are mainly zinc finger proteins,transcription factors,etc.,which regulate the expression of downstream genes.(2)Comparing the histone post-modification state of haploid and diploid embryos at the 1K-cell period,it was found that the modification levels of H3K27 ac and H3K27me3 in haploid embryos were lower than that of diploid embryos.Changes of the above two modifications may berelated to abnormal gene expression in haploid embryos.(3)Transcriptome analysis of diploid eggs derived from two reproduction methods revealed that diploid eggs from gynogenesis had 58down-regulated genes and 44 up-regulated genes when compared with sexual reproductive derived.Comparing the transcriptome analysis of the two kinds of diploid embryos at the 1K-cell stage,925 genes were up-regulated and 681 genes were down-regulated in embryos developed by diploid eggs derived from gynogenesis.The gene expression analysis lasted to gastrula confirmed this significant difference between the two diploid embryos.Differential expressed genes are mainly enriched in pathways including cell cycle,FoxO signaling pathway,etc.,indicating that the differential expression of maternal genes plays a very important role in embryo development.(4)Comparing the 1K-cell stage embryos developed from diploid eggs produced by gynogenesis and sexual reproduction,it was found that the modification level of H3K27 ac in the former was higher than that in the latter,and the modification level of H3K27me3 was reversed.This indicated that the gene expression regulated by H3K27 ac may be related to the developmental advantage of diploid eggs derived from gynogenesis.In summary,this study revealed the important role of maternal factors and histone modifications on the regulation of embryonicdevelopment,and initially explained the molecular mechanism of the diploid egg's parthenogenetic developmental advantage,which is of great significance in the theoretical research of developmental biology,and also provides theoretical guidance for fish genetic breeding.