Recurrent breakdown and rebalance of segregation distortion in the genomes: battle for the transmission advantage
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aBIOTECH
REVIEW
Recurrent breakdown and rebalance of segregation distortion in the genomes: battle for the transmission advantage Fan Xia1 , Yidan Ouyang1& 1
National Key Laboratory of Crop Genetic Improvement and National Centre of Plant Gene Research (Wuhan), Huazhong Agricultural University, Wuhan 430070, China
Received: 23 February 2020 / Accepted: 28 May 2020
Abstract
Mendel’s laws state that each of the two alleles would segregate during gamete formation and show the same transmission ratio in the next generation. However, an unexpected biased allele transmission was first detected in Drosophila a century ago, and was subsequently observed in other animals, plants, and microorganisms. Such segregation distortion (SD) shows substantial effects in population structure and fitness of the progenies, which would ultimately lead to reproductive isolation and speciation. Here, we trace the early investigations on the violation of Mendelian genetic principle, which appears as a wideexistence phenomenon rather than a case of exception. The occurence of SD in the whole genome was observed in a number of plant species at the single- and multi-locus level. Biased transmission ratio might occur at meiosis stage due to asymmetric movement of the chromosome; transmission ratio advantage is also caused by interaction and battle between the alleles from respective genomes at the genetic and molecular level. The origin of a SD system is likely to be determined by coevolution of the killer and protector via recurrent breakdown or rebalance loop. These updated understandings also promote genetic improvement of hybrid crops.
Keywords Segregation distortion, Killer-protector system, Breakdown and rebalance model, Crop genetic improvement
INTRODUCTION Species tried to improve its adaptation to the environment during the evolution. The common strategy is to increase the frequencies of genes with higher fitness, because these genes might increase the ability of their hosts in survival or reproduction. However, a special group of genes, i.e., genes causing segregation distortion (SD), can spread in the population through transmission advantage despite being harmful to the fitness of the offspring. Generally speaking, based on law of segregation and law of independent assortment, alleles for each gene would segregate independently during meiosis, resulting in euqal transmission of each allele and 1:2:1
genotype ratio over generations. Different from such normal case, SD shows a selective advantage to one allele or a certain genotype for its own favor, but results in a cost of another allele or the other genotypes. The favored allele or genotype can spread in the populations over time regardless of its fitness, which would affect the composition of a population. Therefore, the evolution of SD changes the the allele frequency and genotype frequency from generation to generation, which would ultimately show substantial effects on population structure. In addition, biased transmission ratio is always accompanied with reduced adaptation
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