The Sericulture Team published landmark achievements on Molecular Plant

　　 　　

On May 16th, the Institute of Sericulture and Silk, the College of Animal Science and Technology, the State Key Laboratory of Silkworm Genome Biology and the Institute of Silkworm Science and Systems Biology of Southwest University jointly published an online paper entitled "Chromosome-level reference genome and population genomic analysis into the evolution and improvement of domesticated mulberry (Morus alba L)" on the "Molecular Plant". Associate Professor Jiao Feng and doctoral students Luo Rongsong, Dai Xuelei and Liu Hui of Northwest A&F University are co-first authors, and Professor Qian Yonghua and Professor Jiang Yu of Northwest A&F University and Professor Zhao Aichun of Southwest University are co-correspondents.

After ten years' hard work, the sericulture team of the College has made long-term research on three key issues: mulberry species classification, genome ploidy identification and the origin of mulberry, and made a series of important discoveries. This is another major breakthrough in mulberry research since Southwest University obtained the genome framework map of wild mulberry: the reference genome of mulberry cultivated species was obtained; the ploidy of cultivated mulberry trees was defined; the classification of mulberry species was re-identified at the molecular level and the transfer process of mulberry industry center in China was analyzed from the origin of lake mulberry. This research result provides a theoretical basis for mulberry evolution, character improvement and functional gene research, changes the situation of sluggish progress in basic research of mulberry, and provides a theoretical basis for solving the bottleneck problem of production. It is another major achievement in mulberry research in China, indicating that the research in this field of our school has reached the world leading level.

In this paper, the species classification of cultivated mulberry was defined by genome data for the first time, and it was considered that Morus alba L., Morus multicaulis Perr. and Mizuho all belonged to one species, namely Morus alba L.

Mulberry cultivation has a history of more than 5000 years, which is recorded in ancient agricultural books. Although it is divided into various types, it lacks scientific and systematic species classification. In view of the differences in mulberry classification at present, 132 mulberry cultivars (except Guangdong mulberry) were re-sequenced, and the 14Mb SNP was used to cluster the other four mulberry species except Guangdong mulberry, but the clustering results were not similar to those of morphological classification. The genome data supported the conclusion that several cultivated mulberry species were merged in Flora of China. In this paper, the species classification of cultivated mulberry was defined by genome data for the first time, and it was considered that Morus alba L., Morus multicaulis Perr. and Mizuho all belonged to one species, namely Morus alba L.

The reference genome of cultivated mulberry "Lake Mulberry 32" was reported for the first time, and it was confirmed that cultivated mulberry with 28 chromosomes belongs to diploid.

Chromosome ploidy of cultivated mulberry, represented by Morus alba L species, is a widely controversial issue. With the help of genome third generation sequencing and Hi-C technology, the genome was assembled to chromosome level, and the collinearity analysis within and between species was used. It was found that Mulberry only had γ hexaploid events common to dicotyledonous plants, and no new genome-wide doubling events occurred. Through cytological observation, it was found that during meiosis, 28 chromosomes formed 14 pairs of regular bivalents, and pollen activity reached about 90%. These evidences show that Morus alba L with 28 chromosomes belongs to diploid. In addition, through evolutionary analysis, it is speculated that wild mulberry and cultivated mulberry have been diverging for 10 million years. This study provides an example for the basic research of ploidy of perennial woody plants in the genome era, and the method of identifying ploidy of mulberry trees by using genome data has important reference value for the ploidy research of perennial woody plants. Fig. 1. Chromosome and genome assembly of Morus alba L variety "Lake Mulberry 32" (14 chromosomes) Fig. 2 (a) The differentiation distance between Morus alba L and M. notabilis is about 10 million years. (B) Gene collinearity analysis showed that there was no doubling event between Morus alba L ，Vitis. vinifera，and Prunus persica. This paper confirmed that mulberry distributed in Jiangsu and Zhejiang provinces is a unique variety branch formed after long-term strong artificial selection.

Fig. 1 Chromosome and genome assembly of Morus alba L variety "Lake Mulberry 32" (14 chromosomes)

Fig. 2 (a) The differentiation distance between Morus alba and M. notabilis is about 10 million years. (B) Gene collinearity analysis shows that there is no doubling event between Morus alba，Vitis. vinifera，and Prunus persica genomes.

This paper proves that lake mulberry species distributed in Jiangsu and Zhejiang provinces is a unique variety branch formed after long-term strong artificial selection.

Lake mulberry is an excellent cultivated mulberry variety group distributed in Hangzhou-Jiaxing-Huzhou area, which is of great value to the prosperity of sericulture and regional economic and social development in this area since the Southern Song Dynasty. Ancient agricultural historiography has long been controversial about its origin and evolution, and has always lacked experimental evidence to support it. Resequencing analysis of 134 mulberry varieties from China and Japan showed that the genetic distance between Chinese mulberry varieties and Japanese mulberry varieties was far away, and the differentiation distance between lake mulberry variety population and mulberry varieties from northern and southwestern China was obvious. Compared with other varieties, the genetic diversity is significantly lower, and it has obvious artificial selection signal. Therefore, the lake mulberry varieties in Hangzhou-Jiaxing-Huzhou area have an ancient and unique origin and are a unique variety branch. This can be mutually confirmed with the palynological study of Songze site and the archaeological evidence of Qianshanyang in Wuxing (Jiang Youlong, 1987). It is also consistent with the history that since Song Dynasty, the key silk production areas have been transferred to Hangzhou-Jiaxing-Huzhou area, and high-yield and high-quality mulberry varieties have been bred.

Fig. 3 Genetic structure analysis of Morus alba population showed that phylogenetic relationship (B) and principal component analysis (C) showed that the differentiation distance between lake mulberry and non-lake mulberry in Chinese mulberry population was far away

The genome data of Morus alba has been uploaded to NCBI database with the entry numbers of genome PRJNA597121, resequencing PRJNA597170 and transcriptome PRJNA597172.

Paper link: http://www.cell.com/molecular-plant/fulltext/S1674-2052 (20) 30140-4