First revealed! Mulberry trees have two sets of chromosomal cardinality

◎ Science and Technology Daily reporter Yong Li

Recently, the Chinese scientific and technological journal Horticultural Research published the paper "Chromosome Reconstruction and Number Change of Sichuan Mulberry and White Mulberry in the Evolutionary Process" published by the State Key Laboratory of Silkworm Genome Biology (hereinafter referred to as the "Laboratory") of Southwest University, which found for the first time in the world that mulberry trees have 14 and 7 sets of two sets of chromosome bases, and proposed the theory of "mulberry chromosome rupture-fusion cycle". This research result has laid the foundation for drawing the "family tree" of mulberry kinship, and will also provide more accurate guidance for the development of mulberry disciplines and industries.

From discovery to confirmation, theory was proposed and proved, and the Mulberry Team at Southwest University took 10 years. In order to keep the experiments from interruption, working on holidays has also become a habit of the research team.

On February 10, a reporter from Science and Technology Daily came to the laboratory to explore the story behind this achievement.

It took 10 years to solve the mystery of the mulberry chromosome

Mulberry tree is a traditional indigenous tree species in mainland China, and it is also an important ecological and economic tree species. Up to now, there are more than 30 species and more than 7,000 mulberry germplasm resources in the world, and the mainland accounts for more than 50% of the world's mulberry germplasm resources.

"In addition to sericulture, mulberry trees are also good soil and water conservation, ecological restoration of tree species." Associate Professor Liang Jiubo, a team member, talked about mulberry trees, saying that the State Key Laboratory of Silkworm Genome Biology of Southwest University had released the world's first sericulture genome framework map, which was at the forefront of the research of silkworms, and as the leader of the laboratory, academicians of the Chinese Academy of Engineering proposed to zhonghuai that they should also study mulberry trees and let it play a greater role.

"Mulberry trees are extremely rich in chromosome ploidy, which gives it the ability to adapt quickly to different environments." Liang Jiubo explained that the number of chromosomes in mulberry trees totals 10 kinds of chromosomes, such as the number of chromosomes in Xinjiang medicine mulberry has reached 308, which is the largest number of known plants. Through genetic sequencing, the team found that mulberry trees actually belong to the order Rosaceae, and in the past 100 million years alone, the mulberry gene has evolved at about 3-4 times faster than other species of the order Rosacea.

In order to clarify the various germplasm resources of mulberry trees and the "kinship" relationship between them, and sort out the core advantages of mulberry germplasm that are conducive to silkworm farming, medicinal use, ecological restoration, etc. among thousands of mulberry germplasm, the laboratory team, under the leadership of Professor He Ningjia, will focus on the carrier of genes - chromosomes.

In conventional understanding, an organism has only one set of chromosome bases, such as human cells are composed of a set of 46 chromosomes. Previous research literature has set the mulberry chromosome base to 14 as a haploid for the sequencing of the mulberry genome, but in the study, He Ningjia's team found that many chromosome numbers are not multiples of 14, and in the breeding process, it was also found that breeding in the traditional way sometimes does not get high-quality varieties.

In 2012, the team found that some mulberry trees have a chromosome base of 7. However, what is the relationship between this and the 14 chromosomes, why this happened, the team spent 10 years to build a molecular cytogenetics platform, analyze the whole genome of mulberry trees, and demystify the evolution of mulberry chromosomes, repeatedly confirming that 14 and 7 are the chromosome cardinal bases that make up the smallest genetic unit of mulberry trees.

Xuan Yahui (left) and Liang Jiubo (right) examine cultivated mulberry saplings in the laboratory. Science and Technology Daily reporter Yong Li photographed

Probe reveals the "points" and "combinations" of mulberry chromosomes

In the laboratory, postdoc Xuan Yahui skillfully operated the robotic arm to move the distance of microns in front of a high-power microscope to conduct a mulberry chromosome microseparation experiment.

"A chromosome is only a few microns long, and we use a special glass needle to isolate the chromosome from the cell." Xuan Yahui said that each separation is no less than a high-intensity competition, under the microscope, it is necessary to concentrate, through the robot arm to move the micron level of distance to provoke the chromosomes, and finally successfully separate the chromosomes, it takes a few minutes to separate a chromosome alone.

This is a necessary preparation to demystify the mystery of the mulberry chromosome, so that some special DNA sequences can be screened out as probes for the study of the structure of this chromosome, helping to resolve the function of important genes on this chromosome and the evolutionary mechanism of this chromosome.

It is understood that because most of the chromosomes of mulberry trees are dotted and close in length, especially in polyploid mulberry trees, dozens to hundreds of chromosomes, it is very difficult to analyze their karyotyping, and it is necessary to use fluorescence in situ hybridization technology to locate, but it is also difficult to determine the unique gene sequence. They developed a unique gene sequence on each chromosome as a probe, which is as unique as a human fingerprint, so as to accurately identify each chromosome of the mulberry tree from a multitude of chromosomes.

Based on such a set of techniques, He Ningjia's team drew the karyotype of wild mulberry tree Sichuan mulberry (2n=2x=14), and the 14 chromosomes were accurately divided into seven pairs, laying a foundation for the study of mulberry trees and building a reliable technical platform.

Liang Jiubo introduced that after countless comparisons and screenings, the team identified the ploidy level of mulberry trees and drew a karyotype map of chromosomes; and found that the two sets of chromosomes 14 and 7 are not simple multiple relationships and cannot be replaced by each other, they are caused by the rupture of 1 chromosome to form multiple chromosomes or the fusion of multiple chromosomes end to end.

"Through this research, we will be able to firmly hold the mulberry germplasm resources in our own hands." Professor He Ningjia, deputy director of the laboratory, said that in the next step, they will continue to study to clarify the relationship between other polyploid mulberry species and better guide the basic research and breeding of mulberry trees.

Source: Science and Technology Daily

Editor: Wang Yu

Review: Julie

Final Judge: He Yi