Floral Morphology and Development inAragoa(Plantaginaceae) and Related Members of the Order Lamiales

Abstract

Inflorescence and floral morphology and development were investigated in Aragoa (Plantaginaceae) and related genera. Each inflorescence of Aragoa is a reduced, axillary raceme, on which the actinomorphic floral apices generally arise successively. The inflorescences of Aragoa and Plantago are polytelic and lateral. The five sepals emerge from the abaxial to the adaxial side of the floral apex, but at maturity, the calyx is actinomorphic. The four stamens arise simultaneously and before emergence of the petals. The four petals emerge unidirectionally united, but the corolla becomes actinomorphic. Aestivation is cochlear ascendent. The two united carpels initiate simultaneously. The abaxial-adaxial inception of the calyx and corolla during early floral development in genera such as Aragoa, Digitalis, Plantago, and Veronica may indicate that the zygomorphic condition is ancestral in those genera. The tetramerous corolla, which is actinomorphic during middle and late development, and the presence of four stamens are possible synapomophies of the clade (Aragoa þ Plantago). Pentamery of the calyx and corolla appears to be plesiomorphic in the broader AragoaAngelonia clade. Characters related to development and morphology of inflorescences and flowers of Aragoa are essentially similar to those found in Plantago, which is consistent with the molecular-based sister group relationship between these genera.

Evolution of flower shape in Plantago lanceolata

“…However, the role of CYC-like genes in determining flower shape is not always coupled to a role in determining floral organ number: loss of CYC-like gene activity in Linaria vulgaris, a species very closely related to Antirrhinum, also results in actinomorphy, but with no affect on floral organ number (Cubas et al 1999b). In addition, although pentamery is often correlated with zygomorphy, a reduction from pentamerous to tetramerous corollas has occurred at least twice within the Lamiales without loss of zygomorphy (Bello et al 2004). This indicates that the shift to a tetramerous corolla may also be independent of CYC-like gene activity.…”

“…This indicates that the shift to a tetramerous corolla may also be independent of CYC-like gene activity. Scanning electron micrographs of early flower development in Plantago show slightly slower rates of development of petal and stamen primordia in dorsal compared to ventral positions, indicating a degree of zygomorphy that is not seen in mature flowers (Endress 1997, Bello et al 2004. It is unclear what role, if any, PlCYC has in establishing this early zygomorphic pattern of organ development because in situ hybridization does not detect PlCYC expression during floral organ initiation in Plantago.…”

Gradual disintegration of the floral symmetry gene network is implicated in the evolution of a wind-pollination syndrome

“…In contrast to the ancestral flower form found in Digitalideae, several Veroniceae species have flowers that are four-parted, radially symmetrical (polysymmetrical and actinomorphic), and/or wind-pollinated ( Fig. 1) (8,15,(20)(21)(22). The small flowers of Veronica are primarily fly-and/or self-pollinated, and develop four sepals and four petals (23,24).…”

“…The enlarged dorsal petal is inferred to develop from two dorsal petals that fuse early in development (11,25) and, consequently, Veronica flowers have retained bilateral symmetry within the petal whorl (11,25). Whereas the radially symmetrical showy flowers of Aragoa have retained the ancestral number of five sepals but have four identical petals and stamens (11,12,21), the diminutive radially symmetrical flowers of wind-pollinated Plantago are four-merous in all three outer organ whorls (Fig. 1).…”

“…1). In early development, both Aragoa and Plantago flowers are bilaterally symmetrical due to the unidirectional emergence of organs in a ventral-to-dorsal sequence (21). However, flowers at mid to late stages of development in both genera are fully radially symmetrical (21,26).…”