Floral ontogeny in Astragalus compactus (Leguminosae: Papilionoideae: Galegeae): variable occurrence of bracteoles and variable patterns of sepal initiation

Abstract

Comparative studies of floral ontogeny represent a growing field that promise to provide new insights on floral evolution. Floral ontogenetic information has been used successfully in Leguminosae for re-examining phylogenetic relationships at different levels. Using epi-illumination light microscopy, we present original ontogenetic data in Astragalus compactus, which was chosen because of its unusual arrangement of inflorescence and variable occurrence of bracteoles on flowers. Based on our results, uncommon ontogeny of the inflorescence led to the arrangement of flowers in four different positions. Variation was observed not only in the presence of bracteoles, but also in the order of sepal initiation in flowers of the same inflorescence. Surprisingly, besides the widely stated unidirectional pattern, bidirectional, sequential and an atypical unreported order were observed. High degree of overlapping between whorls and formation of two types of common primordia also were found. The variable occurrence of bracteoles suggests that the species is in an intermediate state towards fully lacking of bracteoles. We propose that the variability of the sequence of sepal initiation is possibly a consequence of the function of mechanical forces generated by surrounding leaves. Relationships between mechanical force and auxin signalling are discussed.

Comparative floral development in the tribe Mentheae (Nepetoideae: Lamiaceae) and its bearing on the evolution of floral patterns in asterids

“…The other three species examined ( Nepeta crassifolia , Salvia nemorosa , and Ziziphora tenuior ) lacked bracteoles entirely from floral initiation. This discrepancy between our observations and earlier reports could indicate variation in bracteole occurrence with a genus, as we have shown in Astragalus (Movafeghi et al, , ; Naghiloo et al, ). However, it is perhaps more likely that the high density of inflorescence branching, especially in Nepeta and Ziziphora compared with Rosmarinus , results in misinterpretation of the bracts subtending higher order inflorescences or flowers as bracteoles, given that they are initiated in the same positions as bracteoles.…”

“…A similar pattern exists among the genera studied here, with adaxial to abaxial initiation in the three thyrsoid genera ( Nepeta , Salvia , Ziziphora ), and reverse unidirectional sequence in the racemose inflorescences of Rosmarinus . Such differences could be due to the different arrangement of flowers in an inflorescence and variation in the influence exerted by the primary shoot or subtending bract (Endress, , ; Naghiloo et al, ). Studies on Arabidopsis have also shown that mechanical forces that act on the meristem surface could help to shape the apex (Hamant et al, ).…”

Floral development in Scutellaria pinnatifida (Lamiaceae): the ontogenetic basis for sepal reduction

“…It has been shown that different arrangements of flowers in various types of inflorescences could influence the order of organ initiation (Endress ; Naghiloo et al . ).…”

Floral anatomy and vegetative development in Ceratophyllum demersum: A morphological picture of an “unsolved” plant

“…Mechanical forces have been regarded as an important factor in determining morphogenesis, and external mechanics can alter plant morphology (Robinson et al, 2013). Naghiloo et al (2012) suggested that compressive forces from the subtending leaf determine the pattern of sepal initiation in flowers of Astragalus compactus . We also suggest that mechanical pressure on the floral apex of Ceratophyllum may retard the initiation of bract and stamen primordia on the adaxial side, which distorts the original spiral initiation patterns of floral organs (described by Iwamoto et al, 2003).…”