RNA Directed DNA Methylation (RdDM) is a pathway that mediates de novo DNA methylation, an evolutionary conserved chemical modification of cytosine bases, which exists in living organisms and utilizes small interfering RNA. Plants utilize DNA methylation for transposable element (TE) repression, regulation of gene expression and developmental regulation. TE activity strongly influences genome size and evolution, therefore making DNA methylation a key component in understanding divergence in genome evolution among seed plants. Multiple proteins that have extensively been studied in model plant Arabidopsis thaliana catalyze RNA dependent DNA Methylation pathway along with small interfering RNA. Several developmental functions have also been attributed to DNA methylation. This review will highlight aspects of RdDM pathway dynamics, evolution and functions in seed plants with focus on recent findings on conserved and non-conserved attributes between angiosperms and gymnosperms to potentially explain how methylation has impacted variations in evolutionary and developmental complexity among them and advance current understanding of this crucial epigenetic pathway.

Myriophyllum, among the most species‐rich genera of aquatic angiosperms with ca. 68 species, is an extensively distributed hydrophyte lineage in the cosmopolitan family Haloragaceae. The chloroplast (cp) genome is useful in the study of genetic evolution, phylogenetic analysis, and molecular dating of controversial taxa. Here, we sequenced and assembled the whole chloroplast genome of Myriophyllum spicatum L. and compared it to other species in the order Saxifragales. The complete chloroplast genome sequence of M. spicatum is 158,858 bp long and displays a quadripartite structure with two inverted repeats (IR) separating the large single copy (LSC) region from the small single copy (SSC) region. Based on sequence identification and the phylogenetic analysis, a 4‐kb phylogenetically informative inversion between trnE‐trnC in Myriophyllum was determined, and we have placed this inversion on a lineage specific to Myriophyllum and its close relatives. The divergence time estimation suggested that the trnE‐trnC inversion possibly occurred between the upper Cretaceous (72.54 MYA) and middle Eocene (47.28 MYA) before the divergence of Myriophyllum from its most recent common ancestor. The unique 4‐kb inversion might be caused by an occurrence of nonrandom recombination associated with climate changes around the K‐Pg boundary, making it interesting for future evolutionary investigations.

Heat shock factor (HSF) genes are essential in some of the basic developmental pathways in plants. Despite extensive studies on the structure, functional diversification, and evolution of HSF genes, their divergence history and gene duplication pattern remain unknown. To further illustrate the probable divergence patterns in these subfamilies, we analyzed the evolutionary history of HSF genes using phylogenetic reconstruction and genomic syntenic analyses, taking advantage of the increased sampling of genomic data from pteridophytes, gymnosperms and basal angiosperms. We identified a novel clade that includes HSFA2, HSFA6, HSFA7, and HSFA9 with a complex relationship, which is very likely due to orthologous or paralogous genes retained after frequent gene duplication events. We hypothesized that HSFA9 derives from HSFA2 through gene duplication in eudicots at the ancestral state, and then expanded in a lineage-specific way. Our findings indicate that HSFB3 and HSFB5 emerged before the divergence of ancestral angiosperms, but were lost in the most recent common ancestors of monocots. We also presumed that HSFC2 derives from HSFC1 in ancestral monocots. This work proposes that during the radiation of flowering plants, an era during which there was a differentiation of angiosperms, the size of the HSF gene family was also being adjusted with considerable sub- or neo-functionalization. The independent evolution of HSFs in eudicots and monocots, including lineage-specific gene duplication, gave rise to a new gene in ancestral eudicots and monocots, and lineage-specific gene loss in ancestral monocots. Our analyses provide essential insights for studying the evolutionary history of this multigene family.