Gene-body methylation in plants: mechanisms, functions and important implications for understanding evolutionary processes

Gene body methylation (gbM) is an epigenetic mark where gene exons are methylated in the CG context only, as opposed to CHG and CHH contexts (where H stands for A, C, or T). CG methylation is transmitted transgenerationally in plants, opening the possibility that gbM may be shaped by adaptation. This presupposes, however, that gbM has a function that affects phenotype, which has been a topic of debate in the literature. Here, we review our current knowledge of gbM in plants. We start by presenting the well-elucidated mechanisms of plant gbM establishment and maintenance. We then review more controversial topics: the evolution of gbM and the potential selective pressures that act on it. Finally, we discuss the potential functions of gbM that may affect organismal phenotypes: gene expression stabilization and upregulation, inhibition of aberrant transcription (reverse and internal), prevention of aberrant intron retention, and protection against TE insertions. To bolster the review of these topics, we include novel analyses to assess the effect of gbM on transcripts. Overall, a growing body of literature finds that gbM correlates with levels and patterns of gene expression. It is not clear, however, if this is a causal relationship. Altogether, functional work suggests that the effects of gbM, if any, must be relatively small, but there is nonetheless evidence that it is shaped by natural selection. We conclude by discussing the potential adaptive character of gbM and its implications for an updated view of the mechanisms of adaptation in plants.

Natural diversity in flowering responses of Arabidopsis thaliana caused by variation in a tandem gene array

Tandemly arrayed genes that belong to gene families characterize genomes of many organisms. Gene duplication and subsequent relaxation of selection can lead to the establishment of paralogous cluster members that may evolve along different trajectories. Here, we report on the structural variation in MADS AFFECTING FLOWERING 2 (MAF2) gene, one member of the tandemly duplicated cluster of MADS-box-containing transcription factors in Arabidopsis thaliana. The altered gene structure at the MAF2 locus is present as a moderate-frequency polymorphism in Arabidopsis and leads to the extensive diversity in transcript patterns due to alternative splicing. Rearrangements at the MAF2 locus are associated with an early flowering phenotype in BC(5) lines. The lack of suppression of flowering time in a MAF2-insertion line expressing the MAF2-specific artificial miRNA suggests that these MAF2 variants are behaving as loss-of-function alleles. The variation in gene architecture is also associated with segregation distortion, which may have facilitated the spread and the establishment of the corresponding alleles throughout the Eurasian range of the A. thaliana population.

Unequal sister chromatid and homolog recombination at a tandem duplication of the A1 locus in maize

Tandemly arrayed duplicate genes are prevalent. The maize A1-b haplotype is a tandem duplication that consists of the components, alpha and beta. The rate of meiotic unequal recombination at A1-b is ninefold higher when a homolog is present than when it is absent (i.e., hemizygote). When a sequence heterologous homolog is available, 94% of recombinants (264/281) are generated via recombination with the homolog rather than with the sister chromatid. In addition, 83% (220/264) of homolog recombination events involved alpha rather than beta. These results indicate that: (1) the homolog is the preferred template for unequal recombination and (2) pairing of the duplicated segments with the homolog does not occur randomly but instead favors a particular configuration. The choice of recombination template (i.e., homolog vs. sister chromatid) affects the distribution of recombination breakpoints within a1. Rates of unequal recombination at A1-b are similar to the rate of recombination between nonduplicated a1 alleles. Unequal recombination is therefore common and is likely to be responsible for the generation of genetic variability, even within inbred lines.