Tracking the development of the petaloid fertile stamen in Canna indica: insights into the origin of androecial petaloidy in the Zingiberales

Abstract

Flowers of the order Zingiberales demonstrate a remarkable trend of reduction in the number of fertile stamens; from five or six fertile, filamentous stamens bearing two thecae each in Musaceae and Strelitziaceae to just a single petaloid stamen bearing a single theca in Cannaceae and Marantaceae. As one progresses from ancestral to derived floral forms, 5-6 fertile stamens are replaced by 4-5 petaloid staminodes. In Cannaceae and Costaceae, all members of the androecial whorls exhibit petaloidy, including the fertile stamen. In Costaceae, a single fertile stamen develops two thecae embedded on a broad petaloid appendage, while in Cannaceae the single fertile stamen is further reduced to a single theca with a prominent, expanded petaloid appendage. Whether petaloidy of the fertile stamen is a synapomorphy of the entire ginger clade (including Cannaceae, Costaceae, Zingiberaceae and Marantaceae), or the result of independent convergent evolution in Cannaceae, Costaceae, and some Zingiberaceae, is unclear. We combine a developmental series of the formation of the petaloid fertile stamen in Canna indica with data on the expression of B- and C-class floral organ identity genes to elucidate the organogenetic identity of the petaloid stamen and staminodes. Our data indicate that the single fertile theca in C. indica and its petaloid appendage are derived from one-half of the primordium of a single stamen, with no contribution from the remaining part of the stamen (i.e. the second theca primordium) which aborts early in development. The petaloid appendage expands later, and develops from the position of the filament/connective of the developing theca. Floral identity gene expression shows that petal identity genes (i.e. B-class genes) are expressed in all floral organs studied while C-class gene AG-1 is expressed in an increasing gradient from sepals to gynoecium, and AG-2 is expressed in all floral organs except the petals. The canonical model for molecular specification of floral organ identity is not sufficient to explain petaloidy in the androecial whorl in Canna sp. Further studies understanding the regulation of gene networks are required.

Keywords: Canna; MADS-box genes; Zingiberales; evo-devo; floral development; petaloid stamens; petaloidy.

Figure 1.

Phylogenetic context for studying comparative organogenesis in Zingiberales. (A) Zingiberales phylogeny according to molecular and morphological characteristics (Kress 1990; Kress et al. 2001). The dashed square highlights the ginger clade, comprising a monophyletic group of four families (Costaceae, Zingiberaceae, Cannaceae and Marantaceae). Photographs: C. indica (top); Costus spicatus (bottom). On the right, floral diagrams representative of flowers of the Cannaceae (top) and Costaceae (bottom) families. Light grey, sepals; white, petals; hashed, petaloid staminodes; dark grey, fertile stamen; *, aborted stamen; centre grey, gynoecium. (B) Canna indica half fertile stamen with petaloid appendage.

Figure 2.

Canna indica floral development series. (A) Floral initiation showing the protrusion of the sepal primordial. The arrowhead points to a floral primordium amplified in (B); (B, C) development of the ‘floral cup’; (D) sepal primordia already separated from the remaining floral primordium, and evident petal primordia; (E) early stages of fertile stamen development, with two theca primordia; (F, G) fertile stamen development; (H) later stages of fertile stamen development. A single theca has developed with its petaloid appendage, while the other theca arrests development (arrowhead); (I) an almost mature stamen with its petaloid appendage (arrowhead); and the aborted theca to its right. se, sepal; pe, petal; pe/sta, petal/stamen common primordium; std, staminode; the, fertile theca; app, fertile stamen appendage; x, aborted theca primordium.

Figure 3.

Expressions of B- and C-class MADS-box genes in the floral organs of C. indica as detected by RT-PCR. Each C. indica floral organ was dissected and RNA was extracted independently. The fertile stamen was divided into petaloid appendage and theca. sep, sepal; pet, petal; std, staminode; stm, stamen; pap, petaloid appendage of stamen; the, theca; gyn, gynoecium. Actin was used as an endogenous control for the cDNA synthesis. B-class genes: DEF, DEFICIENS; GLO-1, GLOBOSA-1; GLO-2, GLOBOSA-2. C-class genes: AG-1, AGAMOUS-1; AG-2, AGAMOUS-2.

Figure 4.

Summary results for gene expression and the corresponding floral organ morphology in Arabidopsis and Canna. (A) Classical ABC model of floral development based on Arabidosis thaliana. Only B-class (DEFICIENS and GLOBOSA) and C-class (AGAMOUS) MADS-box genes are depicted, as the role of A-class MADS-box genes in floral development in monocots awaits further investigation. In the classical ABC model, petal identity is a result of A- and B-class MADS-box gene expression, while stamen identity results from concomitant expression of B- and C-class MADS-box genes. (B) Canna indica B- and C-class MADS-box gene expression pattern. Canna indica contains two GLOBOSA genes (GLO-1 and GLO-2) and two AGAMOUS genes (AG-1 and AG-2). B- and C-class MADS-box genes are expressed in most of the floral parts studied here, and when compared with the classical ABC model, show an expansion in their expression domains. x, position of the aborted theca primordium relative to the half fertile stamen.