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Comparative genome biology has unveiled the polyploid origin of all angiosperms and the role of recurrent polyploidization in the amplification of gene families and the structuring of genomes. Which species share certain ancient polyploidy events, and which do not, is ill defined because of the limited number of sequenced genomes and transcriptomes and their uneven phylogenetic distribution. Previously, it has been suggested that most, but probably not all, of the eudicots have shared an ancient hexaploidy event, referred to as the gamma triplication. In this study, detailed phylogenies of subfamilies of MADS-box genes suggest that the gamma triplication has occurred before the divergence of Gunnerales but after the divergence of Buxales and Trochodendrales. Large-scale phylogenetic and KS-based approaches on the inflorescence transcriptomes of Gunnera manicata (Gunnerales) and Pachysandra terminalis (Buxales) provide further support for this placement, enabling us to position the gamma triplication in the stem lineage of the core eudicots. This triplication likely initiated the functional diversification of key regulators of reproductive development in the core eudicots, comprising 75% of flowering plants. Although it is possible that the gamma event triggered early core eudicot diversification, our dating estimates suggest that the event occurred early in the stem lineage, well before the rapid speciation of the earliest core eudicot lineages. The evolutionary significance of this paleopolyploidy event may thus rather lie in establishing a species lineage that was resilient to extinction, but with the genomic potential for later diversification. We consider that the traits generated from this potential characterize extant core eudicots both chemically and morphologically. [ABSTRACT FROM PUBLISHER]

Recent molecular phylogenetic studies have shown that the Gunnerales, including Gunnera and Myrothamnus, form a clade, sister to other core eudicots. This systematic position puts Gunnera in a pivotal position regarding the evolution of morphological characters in eudicots. In particular, the dimerous flower organization of Gunnera has recently been considered precursor to the pentamerous condition found in most core eudicots. In order to understand the role of Gunnera in the flower evolution of core eudicots, new detailed studies of the flowers of Gunnera are needed. In this study, the flower ontogeny and morphology of Gunnera monoica and G. dentata as well as of G. macrophylla were studied with SEM. The new observations are compared with previous studies of G. manicata and G. herteri. The first four species have dimerous perfect or staminate flowers with two small median sepals, followed by two large petals opposite the two stamens. In G. herteri, the staminate flowers are reduced to single stamens. In the pistillate flowers, there are two sepals, alternate to the two stylodia above the ovary. In G. monoica, two petal primordia initiate but abort early in floral development; in G. dentata, petal primordia arise after the sepals but remain smaller in the flower; in G. macrophylla petals are lacking. We focus on the floral reductions found in Gunnera and how these are connected to wind pollination common to the entire genus. Clear reductive trends in the perianth can be demonstrated in Gunnera, culminating in the completely perianthless G. herteri. A comparison is also made between the flowers of Gunnera and other simplified flowers such as those of Myrothamnus and other early-diverging eudicots. The floral reductions of dimerous Gunnerales cannot be related to the pentamerous core eudicots. Floral evolution of Gunneraceae, as well as that of other wind-pollinated, basally placed eudicots, is best seen as a unique specialization linked to this pollination mode. [ABSTRACT FROM AUTHOR]