Does one subgenome become dominant in the formation and evolution of a polyploid?

Abstract

Background: Polyploids are common in flowering plants and they tend to have more expanded ranges of distributions than their diploid progenitors. Possible mechanisms underlying polyploid success have been intensively investigated. Previous studies showed that polyploidy generates novel changes and that subgenomes in allopolyploid species often differ in gene number, gene expression levels and levels of epigenetic alteration. It is widely believed that such differences are the results of conflicts among the subgenomes. These differences have been treated by some as subgenome dominance, and it is claimed that the magnitude of subgenome dominance increases in polyploid evolution.

Scope: In addition to changes which occurred during evolution, differences between subgenomes of a polyploid species may also be affected by differences between the diploid donors and changes which occurred during polyploidization. The variable genome components in many plant species are extensive, which would result in exaggerated differences between a subgenome and its progenitor when a single genotype or a small number of genotypes are used to represent a polyploid or its donors. When artificially resynthesized polyploids are used as surrogates for newly formed genotypes which have not been exposed to evolutionary selection, differences between diploid genotypes available today and those involved in the formation of the natural polyploid genotypes must also be considered.

Conclusions: Contrary to the now widely held views that subgenome biases in polyploids are the results of conflicts among the subgenomes and that one of the parental subgenomes generally retains more genes which are more highly expressed, available results show that subgenome biases mainly reflect legacy from the progenitors and that they can be detected before the completion of polyploidization events. Further, there is no convincing evidence that the magnitudes of subgenome biases have significantly changed during evolution for any of the allopolyploid species assessed.

Keywords: Polyploidy; polyploid evolution; subgenome dominance; variable genome; whole-genome duplications.

Fig. 1.

A diagram illustrating two possible scenarios of subgenome biases in polyploid formation and evolution as discussed herein, one being that the degree of increases of subgenome biases over time (A) and the other that subgenome biases mainly reflect differences between diploid donors (B). Each of the four oval shapes contained in each of the nuclei (circles) represents a haploid gamete produced by a diploid individual. Darker colours represent higher degrees of subgenome biases.

Fig. 2.

A diagram showing the major steps, thus factors that need to be considered, in studying subgenome biases in the formation and evolution of allopolyploids. Six individuals are used to represent each of the populations for both the diploid donors and the natural polyploids derived from them.