Since the identity of floral organs is specified by conserved floral organ-identity genes, clarifying the phylogeny of these genes and their protein products may provide us with valuable insights into the evolution of flowers. Analysis of numerous MADS box genes from mosses and ferns suggested that orthologues of floral homeotic genes are absent in non-seed plants. However, orthologues of class B and class C floral homeotic genes were identified in different conifers and in the gnetophyte Gnetum gnemon, orthologues of class C floral homeotic genes were also reported for Cycas and Ginkgo. This suggests that B- and C-type genes were established by gene duplications and sequence divergence in the lineage that led to extant seed plants, after the lineage that led to extant ferns had already branched off, i.e. 300–400 million years ago, even though some molecular clock estimates suggested a considerably earlier origin of these genes.
[...] Mutant analysis says that the structural evolution predicted in these hypotheses is developmentally possible. In the conifer, Picea abies ‘acrocona' hermaphroditic cones with female reproductive units at the top and male organs underneath have occasionally been observed. Among the molecular changes, which might have caused the modifications in B gene expression are changes in genes that control B gene expression (encoding ‘trans-acting factors') or changes in the cis-regulatory elements of the B genes themselves. Accordingly, conserved differences in upstream regulators of B gene expression between angiosperms and gymnosperms, specifically those that prevent B gene expression in the central part of the floral meristem or promote B gene expression specifically in the second and third whorl could provide evidence for the ‘out-of-male' scenario. [...]
[...] The out-of-male and out-of-female hypotheses are not the only scenarios on flower origin that can be considered from molecular evidence. Two alternative hypotheses have been inspired by phylogeny reconstructions of the floral meristem identity gene FLORICAULA (FLO)/LEAFY (LFY) and its orthologues. Extant gymnosperms have two paralogous copies of the LFY gene, termed LEAF and NEEDLE genes. Phylogenetic reconstructions suggest that the most recent common ancestor of extant gymnosperms and angiosperms had both a LEAF and a NEEDLE gene, but that the NEEDLE paralogue was lost in the angiosperm lineage shortly after its separation from the gymnosperm lineage. [...]
[...] Further investigations on these ‘evo-devo genes' thus promise a significant leap forwards towards a better understanding of flower origin and diversification. SUMMARY Analysis of numerous MADS box genes from mosses and ferns suggested that orthologues of floral homeotic genes are absent in non-seed plants. However, orthologues of class B and class C floral homeotic genes were identified in different conifers and in the gnetophyte Gnetum gnemon, orthologues of class C floral homeotic genes were also reported for Cycas and Ginkgo. [...]
[...] Defining the exact roles of class B genes in tulip and lily flower development still requires mutant analysis. However, these findings already support the view that shifts in the boundaries of class B floral homeotic gene expression and hence floral homeotic changes contributed to the difference between typical eudicot and monocot flowers. However, in the flowers of garden asparagus (Asparagus officinalis), another member of the lily family, class B genes are only expressed in the third (stamen) and second (inner tepal) whorl but not in the first (outer tepal) whorl, although asparagus has also two whorls of almost identical petaloid tepals. [...]
[...] To determine if changes in the spatial or temporal expression of floral homeotic genes have contributed to the structural diversification of the flower during angiosperm development, comparative expression studies in flowers with dissimilar architectures are necessary. In the following we discuss a few molecular case studies that corroborate the view that homeotic transitions played a role during the evolution of flowers. The perigon of Liliaceae Tulip (e.g. Tulipa gesneriana), lily (e.g. Lilium regale) and most of their relatives in the lily family (Liliaceae) have flowers displaying organ identities quite similar to the ones of higher eudicots. [...]
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