Tandem duplications, eupolyploidy and secondary diploidization – genetic mechanisms of plant speciation and progressive evolution

Abstract

The article considers the genetic mechanisms of plant speciation. The importance of interspecific hybridization and polyploidy (whole genome duplication, WGD) as the main mechanisms of plant speciation is shown. There are three main ways of transformations of the hybrid genome associated with speciation. In the first way, the ploidy of the offspring does not change in comparison with the parents’ ploidy; the genomes of hybrid lines are stabilized through backcrosses and introgression without polyploidization. In the second way, the interspecial hybridization followed by whole genome duplication. Then, the initially unstable neopolyploid genome gradually passes into a stable eupolyploid state with the preservation of the polyploid number of chromosomes but with the diploid type of chromosome conjugation in meiosis. This is a widespread and rapid mechanism of plant speciation and genus formation in higher plants, which ensured the morphological and genetic isolation and adaptability of at least 15 % of modern species of higher plants. However, this is a path that usually does not lead to aromorphoses, this is speciation at an already achieved level of complexity. The third way of speciation is realized through dysploidy and secondary diploidization of the genome. In this case, the neopolyploid undergoes significant genomic rearrangements and loses most of the duplicated gene copies, its number of chromosomes is radically reduced. In different individuals of a species that has embarked on the path of stochastic genome fractionation and dysploidy, the initial genetic redundancy of various genome components multiplied after WGD is transformed in an unpredictably unique way, which leads to a radical increase in intraspecific genomic and epigenetic polymorphism and provides rich material for natural selection. It was also shown that in eupolyploids and paleopolyploids, a significant role in heritable adaptations to environmental conditions and in anatomical and morphological innovations is played by segment and tandem duplications not associated with WGD. Some of the paleopolyploids, which turned out to be evolutionarily progressive morphotypes, possessing aromorphoses with diploidized genomes, give rise to new phylogenetic branches, new suprageneric taxa. The article proposes to assign both genera carrying a unique two-chromosomal genome Zingeria and Colpodium (x = 2; 2n = 4, 8, 12) into subtribe Zingeriinae Rodionov, subtrib. nov. – Type: Zingeria P. A. Smirn. In addition, the accomodation of the genera Helictochloa and Molineriella into one subtribe Helictochloinae Röser et Tkach seems to us unreasonable from a genomic point of view, since the fundamental difference between representatives of these two genera is that Molineriella species carry an unusual 4-chromosomal genome, while speciation in genus Helictochloa(2n = 14 – 154) goes through the combinations of different 7-chromosome subgenomes, denoted by the letters E, L, B, C, M, V, G, U. Therefore, we consider it necessary to assign Molineriella into a monogenic subtribe MolineriellinaeRodionov, subtrib. nov. – Type: Molineriella Rouy.