Camellia Meiocarpa And Camellia Oleifera Phylogenetic Relationship And Fruit Size Differentiation Transcriptome Analysis

Oilseed camellia is one of the most important woody oil crops in China.Camellia oil is known as "oriental olive oil" and is a healthy and high-quality edible vegetable oil.Strive to develop the oilseed camellia industry is one of the development strategies of China's oilseed crop industry,helping to meet the growing demands for healthy and high-quality vegetable oils.Camellia oleifera is the most widely distributed and productive oilseed camellia species in China,with large fruits and high seed number but thick pericarp and low seed to fruit ratio.Camellia meiocarpa is secondly to C.oleifera in terms of cultivation area and yield,with small fruits,low seed number,thin pericarp and high seed to fruit ratio.Integrating the advantages of both,breeding oilseed camellia with large fruits,thin pericarp and high seed to fruit ratio can significantly increase yield.However,the phylogenetic relationship between C.meiocarpa and C.oleifera is still controversial,and the molecular mechanism of fruit size trait differentiation is still unclear,which limits the exploitation and utilization of their genetic resources.In this study,we assembled the complete chloroplast genomes of 19 species of the Sect.oleifera(including C.oleifera and C.meiocarpa),Sect.Paracamellia,Sect.Camellia of the genus Camelia,by high-throughput sequencing and analyzing the chloroplast genomes.Sequence variation between species was analyzed by comparing chloroplast genomes,especially for the differences between C.meiocarpa and C.oleifera.Based on chloroplast genomic data,phylogenetic trees were constructed and the phylogenetic relationship between C.meiocarpa and C.oleifera was analyzed.In addition,the fruit growth and development processes of C.meiocarpa and C.oleifera were observed.RNA-seq was performed on pericarp and seed samples at key stages of fruit growth and development,and differences in gene expression in different tissues at different stages of growth and development were analyzed to screen for genes that may be associated with fruit size differentiation.The main results and conclusions of this study are as follows:1.High-quality complete chloroplast genomes of 19 species from the Sect.oleifera,Sect.Paracamellia,Sect.Camellia were assembled.The chloroplast genomes size of these species spanned from 156754 bp to 157005 bp,relatively conserved in structures and similar in characteristics.The nucleotide diversity of the chloroplast genomes was low,sequences were highly conserved,and polymorphic loci were located mainly in non-coding regions.At the atpH-atpI intergenic region,a 400 bp long fragment deletion was found in the chloroplast genome of C.meiocarpa being compared to that of C.oleifera.2.The phylogenetic tree based on the complete chloroplast genome is divided into three large clades;species of the Sect.Oleifera are dispersed into two large clades,while species of the Sect.Paracamellia are scattered in all three large clades.The results do not support the current partition of the Sect.Oleifera and the Sect.Paracamellia in the genus Camellia.Camellia oleifera and C.meiocarpa were located in different large clades,suggesting that they have different maternal ancestors,and C.meiocarpa and C.oleifera may be different species3.The fruit size changes of C.meiocarpa and C.oleifera show a typical"slow-fast-slow" "S"-type change trend,which can be divided into three stages,i.e.,flat growth period,rapid expansion period and platform period.In all periods C.oleifera fruits were significantly larger than those of C.meiocarpa.Besides,the rapid expansion period of the fruit may be the key period that determines the differentiation of fruit size between them.Fruit transcriptomes of C.meiocarpa and C.oleifera were consistent with trends in fruit size changes.Among them,the number of differentially expressed genes was the highest in the pericarp from the flat growth period to the rapid expansion period,and mitosis related gene expression was suppressed in the pericarp cells,while pericarp expansion related genes were up-regulated in this process.4.Twelve hub-genes associated with the fruit size trait were found in C.oleifera and eight hub-genes associated with the fruit size trait were found in C.meiocarpa.Some hub-genes are localized to regulatory pathways associated with mitotic processes in fruit cells;others are closely related to regulatory pathways for fruit cell expansion.This study can provide scientific and technological supports for the efficient exploitation and utilization of genetic resources in C.oleifera and C.meiocarpa.