It was discovered more than a century ago that the B chromosome exists in the genomes of many plants, animals and fungi. B chromosomes are not necessary for the life activities of individuals, but they still exist in the population through different mechanisms. For example, the B chromosome of maize does not pair with any A chromosome, and its transmission does not follow Mendelian law of inheritance. The B chromosome will undergo nondisjunction during the second mitosis of pollen. The sperm nucleus containing the B chromosome preferentially fertilizes the egg cell, enabling Pass and accumulate in the group. Previously, maize geneticists used the A-B chromosome translocation system to create abundant chromosomal variation materials, which were applied to maize genetics, artificial chromosomes, gene dosage effects, and centromere studies. For example, it is found that inactivated B centromeres in maize can be inherited stably (Han et al., PNAS, 2006); inactivated B centromeres still have the function of non-segregation and can restore activity (Han et al., PNAS, 2006). , Plant Cell, 2007; 2009); use B centromere and A chromosome translocation line to explore the BFB theory proposed by McClintock many years ago (Birchler and Han, Plant Cell, 2018); use B centromere to split materials Study the formation of new centromeres (Zhang et al., Plant Cell, 2013; Liu et al., PNAS 2015). The B chromosome has made an important contribution to the development of plant chromosome biology, but the key issues such as the origin, evolution and molecular mechanism of the maize B chromosome accumulation in the population are still unclear.
The research group of James A. Birchler, professor of the University of Missouri, the research group of the Institute of Experimental Plants of the Czech Academy of Sciences, and the research group of Han Fangpu, a researcher of the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences, jointly completed the genome map of the maize B chromosome and analyzed the maize The origin, evolution and non-segregation mechanism of the B chromosome. The research team used chromosome sorting, Illumina sequencing, Bionano optical atlas, Hi-C and other methods to assemble 328 B chromosome-specific scaffolds with a total length of 125.9 Mb. The study uses special materials such as B-A chromosome translocation, B centromeric splitting and B chromosome breakage to construct a molecular map at the chromosome level, and annotates 758 protein-coding genes, of which at least 88 are expressed. The homologues of B chromosome protein coding genes are widely scattered on 10 A chromosomes of maize, but the A chromosome does not detect the B chromosome colinearity gene region. It is speculated that the current B chromosome genes are continuously transferred from the A chromosome during the long-term evolution. As a result of partial degradation, these retained genes represent selected participation in the maintenance of B chromosome functions, such as the”centromeric region”,”spindle filament microtubules,””cytokinesis regulation”, and”histone serine phosphorylation”. Chromosome segregation” and so on. Further analysis of the transposable elements found that 60%of the maize B chromosome sequence is composed of transposable elements, and its type is basically the same as that of the A chromosome. The content of B chromosome genes and transposable elements and selection analysis of transposable protein coding genes indicate that the B chromosome has existed for millions of years in the evolutionary generation.
A 574-kb functional centromere sequence was identified on the B chromosome. In addition to sharing the CentC and CRM type repeats with the A centromere, it also contains the B centromere-specific tandem repeat sequence B-repeat. The centromere structure analysis showed that the structure and function of the B centromere CENH3 nucleosome is similar to that of the A centromere. High-quality B chromosome sequence maps and genetic analysis revealed that cis-factors related to B chromosome non-segregation are scattered on the centromere and surrounding B chromosome-specific repetitive sequences. Through B-3Sb deletion mapping, it was identified that 34 protein-coding genes at the end of B chromosome may be involved in the process of non-segregation as trans-acting elements. The B chromosome transposition repeat gene may evolve many new functions to maintain the existence of this unnecessary chromosome in the population. The maize B chromosome fine map provides reference information for further research on the characteristics of the B chromosome.
On June 5, relevant research results were published online on PNAS (DOI:10.1073/pnas.2104254118). James A. Birchler, Han Fangpu are the co-corresponding authors of the paper. The research work is supported by the key international cooperative research projects of the National Natural Science Foundation of China.
Maize B chromosome genome map and function research progressed