Abstract
Background: The ABC model of flower development describes the molecular basis for specification of floral organ identity in model eudicots such as Arabidopsis and Antirrhinum. According to this model, expression of C-class genes is linked to stamen and gynoecium organ identity. The Zingiberales is an order of tropical monocots in which the evolution of floral morphology is characterized by a marked increase in petaloidy in the androecium. Petaloidy is a derived characteristic of the ginger families and seems to have arisen in the common ancestor of the ginger clade. We hypothesize that duplication of the C-class AGAMOUS (AG) gene followed by divergence of the duplicated AG copies during the diversification of the ginger clade lineages explains the evolution of petaloidy in the androecium. In order to address this hypothesis, we carried out phylogenetic analyses of the AG gene family across the Zingiberales and investigated patterns of gene expression within the androecium. Results: Phylogenetic analysis supports a scenario in which Zingiberales-specific AG genes have undergone at least one round of duplication. Gene duplication was immediately followed by divergence of the retained copies. In particular, we detect positive selection in the third alpha-helix of the K domain of Zingiberales AGAMOUS copy 1 (ZinAG-1). A single fixed amino acid change is observed in ZinAG-1 within the ginger clade when compared to the banana grade. Expression analyses of AG and APETALA1/FRUITFULL (AP1/FUL) in Musa basjoo is similar to A- and C-class gene expressions in the Arabidopsis thaliana model, while Costus spicatus exhibits simultaneous expression of AG and AP1/FUL in most floral organs. We propose that this novel expression pattern could be correlated with the evolution of androecial petaloidy within the Zingiberales. Conclusions: Our results present an intricate story in which duplication of the AG lineage has lead to the retention of at least two diverged Zingiberales-specific copies, ZinAG-1 and Zingiberales AGAMOUS copy 2 (ZinAG-2). Positive selection on ZinAG-1 residues suggests a mechanism by which AG gene divergence may explain observed morphological changes in Zingiberales flowers. Expression data provides preliminary support for the proposed mechanism, although further studies are required to fully test this hypothesis.
Figures
![Figure 1 Floral organ specification gene regulatory network (FOS-GRN), modified from Álvarez-Buylla, E.R et al. [6]). The circles (nodes) represent genes or proteins that are experimentally shown to participate in floral organ specification during flower morphogenesis in Arabidopsis thaliana. Arrows indicate positive interactions between nodes, while the T symbols indicate negative interactions between nodes. Fifteen genes are depicted: EMF1, LFY, AP2, WUS, AG, LUG, CLF, TFL1, PI, SEP, AP3, UFO, FUL, FT, and AP1. Direct interactions between AGAMOUS (AG) and other genes in the FOS-GRN are highlighted in black. Other interactions within the FOS-GRN not directly involving AG are depicted in gray, as well as genes that are not direct interactors with AG.](https://s3-eu-west-1.amazonaws.com/com.mendeley.prod.article-extracted-content/images/8a730845-61a9-3b64-b242-6f3ac5c4d62b/thumbnail-9053e0a7-7534-449f-969b-d78e991b926a-0.png)
![Figure 2 Phylogeny of Zingiberales with key events in androecial evolution. (A) Morphological character states of the androecium are mapped onto the most recent Zingiberales phylogeny [28]. The eight Zingiberales families are divided into two groups: the first diverging banana lineages (Heliconiaceae, Strelitziaceae, Musaceae, and Lowiaceae), and the derived ginger clade (Zingiberaceae, Costaceae, Marantaceae, and Cannaceae). Main changes in androecial morphology are depicted with numbers. (1) Reduction in the number of fertile stamens, from 5 to 6 fertile stamens in the banana lineages, to 1 fertile stamen in Zingiberaceae and Costaceae or ½ of a fertile stamen in Marantaceae and Cannaceae; (2) fusion of petaloid staminodes leading to the formation of the labellum. Five infertile stamens fuse in Costaceae, while 2 or 4 staminodes form the labellum in the Zingiberaceae; (3) laminar extension of the filament of the fertile stamen; (4) abortion of a theca of fertile stamen. (B) Costus sp. flowers. (La) labellum; Asterisk indicates the abaxial side of laminar connective of fertile stamen. (C) Canna indica one half fertile stamen. (Th) single theca; (Pa) petaloid appendage resulting from the laminar expansion of the filament [29].](https://s3-eu-west-1.amazonaws.com/com.mendeley.prod.article-extracted-content/images/8a730845-61a9-3b64-b242-6f3ac5c4d62b/thumbnail-663b72b1-e6cd-435f-bc35-587ae32fe4d8-1.png)
![Figure 4 Evolution of the AGAMOUS gene lineage. (A) Potential AGAMOUS (AG) gene copy histories within the Zingiberales. Scenario 1 assumes one single duplication event at the base of the Zingiberales order, leading to two distinct orthologous AG lineages (ZinAG-1 and ZinAG-2). Scenarios 2 and 3 depict alternative histories of duplications and losses of the AG copies, particularly in the Zingiberaceae lineage. In both cases, orthologous relationships would be complicated by the existence of subsequent duplication events, unique to the Zingiberaceae lineage, leading to the evolution of yet another copy of AG, ZinAG-3. (B) Shimodaira-Hasewaga test (Shimodaira & Hasewaga [37]), SH test) was performed using PAUP* on a constrained topology, where the two first paraphyletic lineages were forced to form a monophyletic clade. The likelihood score for the constrained topology was compared to the likelihood score of the unconstrained gene tree, as obtained on Bayesian and maximum likelihood phylogenetic analysis (Figure 3). The constrained topology shows a better likelihood score than the one calculated for the gene tree topology presented here (although not significantly different), supporting the idea that the first two paraphyletic lineages are actually derived from a single duplication event. This result supports the evolutionary history depicted by Scenario 1, in which ZinAG-1 and ZinAG-2 are orthologous lineages.](https://s3-eu-west-1.amazonaws.com/com.mendeley.prod.article-extracted-content/images/8a730845-61a9-3b64-b242-6f3ac5c4d62b/thumbnail-46fc8338-3d57-4961-95a5-125ccdead84b-4.png)
![Figure 6 Amino acid changes within positive selected sites for the two copies of the AGAMOUS (AG) gene across Zingiberales species. The asterisk depicts the evolution of androecial petaloidy within the Zingiberales order. Note that it also corresponds to the base of the ginger clade (in blue). Marked in yellow are the paraphyletic lineages of the banana grade. For amino acid comparisons, Musa acuminata (Musaceae), Costus spicatus (Costaceae), and Canna indica (Cannaceae) AG sequences were used. Logos for the specific codons of the banana grade (bottom) and ginger clade (top) are shown. Single-letter amino acid traditional names were used (colored boxes). On the far right, images of Canna indica fertile stamen (top; Th-theca; Pa-petaloid appendage of the stamen); Costus sp. labellum (La, middle image); and Musa basjoo flower (bottom; Fp free petal) are shown. Also, note that Musa acuminata has four AG sequences due to a subsequent whole genome duplication event after the divergence of the Musaceae lineage [45].](https://s3-eu-west-1.amazonaws.com/com.mendeley.prod.article-extracted-content/images/8a730845-61a9-3b64-b242-6f3ac5c4d62b/thumbnail-c69aebd1-9246-4f38-a2db-d453b5b82e21-6.png)
Author supplied keywords
Register to see more suggestions
Mendeley helps you to discover research relevant for your work.
Cite
CITATION STYLE
Almeida, A. M., Yockteng, R., Otoni, W. C., & Specht, C. D. (2015). Positive selection on the K domain of the AGAMOUS protein in the Zingiberales suggests a mechanism for the evolution of androecial morphology. EvoDevo, 6(1). https://doi.org/10.1186/s13227-015-0002-x
Readers over time
Readers' Seniority
Readers' Discipline
