Evolutionary conservation of epigenetic control of plant sexual reproduction
During the course of evolution, land plants evolved a peculiar haplodiplontic life cycle in which both the haploid gametophyte and the diploid sporophyte are multicellular. Within the life cycle, the phase transition between these two stages is called the alternation of generations. However, within...
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|Summary:||During the course of evolution, land plants evolved a peculiar haplodiplontic life cycle in which both the haploid gametophyte and the diploid sporophyte are multicellular. Within the life cycle, the phase transition between these two stages is called the alternation of generations. However, within land plants the dominant generation varies. Following the alternation of generations on molecular level is difficult to analyse in flowering plants due to their diploid-dominant generation and their highly reduced gametophytic generation. Bryophytes, in contrast, are haploid-dominant plants, have more easily tractable (morphologically larger) generations and share with flowering plants the fundamental regulatory networks for switching between vegetative and reproductive growth. This switch is most often regulated by transcription associated proteins (TAPs). Combining an orthology detection tool (proteinortho), protein-family categorization (TAPscan), literature search and phylogenetic inference a set of candidate genes, representing single copy TAPs potentially involved in embryo development, was identified. In bryophytes the knowledge of sexual reproduction and evolution of plant sexual reproduction is limited as compared to seed plants. Even though, bryophyte plants defective in gametangiogenesis or embryo/sporophyte development often can be grown as gametophytic tissue and vegetatively propagated contrary to flowering plants. Two master regulators (swi3a/b, hag1) were identified. It could be shown that they are indeed crucial for sexual reproduction and thus the knowledge about the regulation of sexual reproduction in bryophytes could be amended. Deletion of the respective genes led to a loss of sporophyte development due to an impairment in the male germ line in the latest steps of spermatozoid ripening on the part of swi3a/b, and an impairment in the male and female germ lines (disturbed ripening) on the part of hag1. Hag1 is a highly conserved gene throughout all eukaryotic kingdoms, which was proven by phylogenetic analyses and underlined by mutants impaired in the male germ line in moss as well as flowering plants. In contrast, the SWI3 family diversified during land plant evolution (one SWI3 gene in algae, four SWI3 genes in Arabidopsis thaliana). That was underlined by the phenotypical analyses in Physcomitrium patens and A. thaliana, which showed a shift from a male exclusive role of swi3a/b in P. patens towards an involvement in male and female gametogenesis as well as embryogenesis in A. thaliana. These findings underline the hypothesis that on one hand TAPs can be highly conserved and can serve as single copy phylogenetic markers, whereas on the other hand TAP families expanded with plant complexity as drivers for morphogenic evolution. All in all, TAPs are important developmental regulators, that can be used for analysing the plants’ potential to adapt to new conditions.|
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