Expression of floral MADS‐box genes in basal angiosperms: implications for the evolution of floral regulators

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<jats:title>Summary</jats:title><jats:p>The ABC model of floral organ identity is based on studies of Arabidopsis and <jats:italic>Antirrhinum</jats:italic>, both of which are highly derived eudicots. Most of the genes required for the ABC functions in Arabidopsis and <jats:italic>Antirrhinum</jats:italic> are members of the MADS‐box gene family, and their orthologs are present in all major angiosperm lineages. Although the eudicots comprise 75% of all angiosperms, most of the diversity in arrangement and number of floral parts is actually found among basal angiosperm lineages, for which little is known about the genes that control floral development. To investigate the conservation and divergence of expression patterns of floral MADS‐box genes in basal angiosperms relative to eudicot model systems, we isolated several floral MADS‐box genes and examined their expression patterns in representative species, including <jats:italic>Amborella</jats:italic> (Amborellaceae), <jats:italic>Nuphar</jats:italic> (Nymphaeaceae) and <jats:italic>Illicium</jats:italic> (Austrobaileyales), the successive sister groups to all other extant angiosperms, plus <jats:italic>Magnolia</jats:italic> and <jats:italic>Asimina</jats:italic>, members of the large magnoliid clade. Our results from multiple methods (relative‐quantitative RT‐PCR, real‐time PCR and RNA <jats:italic>in situ</jats:italic> hybridization) revealed that expression patterns of floral MADS‐box genes in basal angiosperms are broader than those of their counterparts in eudicots and monocots. In particular, (i) <jats:italic>AP1</jats:italic> homologs are generally expressed in all floral organs and leaves, (ii) <jats:italic>AP3</jats:italic>/<jats:italic>PI</jats:italic> homologs are generally expressed in all floral organs and (iii) <jats:italic>AG</jats:italic> homologs are expressed in stamens and carpels of most basal angiosperms, in agreement with the expectations of the ABC model; however, an <jats:italic>AG</jats:italic> homolog is also expressed in the tepals of <jats:italic>Illicium</jats:italic>. The broader range of strong expression of <jats:italic>AP3/PI</jats:italic> homologs is inferred to be the ancestral pattern for all angiosperms and is also consistent with the gradual morphological intergradations often observed between adjacent floral organs in basal angiosperms.</jats:p>

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