Different ways to obtain similar results: the development of the corolla and epipetaly in Rubieae (Rubioideae, Rubiaceae)

Background and aims – Rubieae is a tribe in the subfamily Rubioideae characterised by herbaceous plants with verticillate leaves and flowers with a rudimentary or absent calyx and a short, cup-shaped corolla. This is in contrast to the flowers of most other Rubiaceae, in which the tubular corolla is longer than the corolla lobes. Also, the description by Payer, a French 19th century pioneer of floral ontogenetic research, of the floral development in Asperula, Galium, and Rubia deviates from recent insights about the development of tubular corollas, which are based on investigations of flowers of tropical Rubiaceae. Tubular corollas are currently considered as resulting from the development of underlying annular intercalary meristems, whereas Payer explained the tubular corollas in the three taxa by postgenital fusion. We therefore tested both hypotheses in six Rubieae genera, including the three taxa studied by Payer.Methods – Floral ontogeny of ten species in six Rubieae genera based on scanning electron (SEM) and light microscopy (LM). Conclusions – Our results suggest that, in all species studied, the mature phenotype of the corolla as well as the epipetaly of the stamens is caused by a combination of three developmental processes (the development of a stamen-corolla tube, the development of a corolla tube sensu stricto, and postgenital fusion), and the relative moment of activation of each of these processes during floral development (plastochron variation or heterochrony).

Floral ontogeny of Illicium Lanceolatum (Schisandraceae) and its implications on carpel homology

There are two competing hypotheses for the origin of carpels. The traditional hypothesis favors a phyllosporous origin and regards a conduplicate carpel as an ancestral form that is the result of longitudinal folding of a leaf bearing ovules along its margins. Alternatively, the carpel formation is interpreted as the result of a fusion between an ovule-bearing branch and its subtending leaf-like structure. Illicium is a member of the Austrobaileyales, which are one of the three ANA lines that diverged before all other extant angiosperms. This genus with apocarpous gynoecium has various ancestral morphological characteristics in terms of carpel, ovule, and floral apex. Although various aspects of Illicium morphology have been previously investigated, many evolutionary characteristics remain poorly understood. In this study, the development of carpel, ovule, and floral apex of I. lanceolatum was studied using LM and SEM. The results showed that the ovule primordium originates in the axillary position between the flower axis and carpel wall. So the carpel of Illicium is a leaf-like structure that encircles the ovule. This kind of carpel favors the carpel as a result of fusion between two parts, ovule-bearing branch and its subtending leaf-like structure.

Interaction between Floral Merism and Symmetry: Evidence from Fasciated Mutant of Lupinus angustifolius L. (Leguminosae)

A significant diversity of flower structure in angiosperms is defined by few parameters, among which one may list floral merism and symmetry as the most important. However, observation of normal variation in different taxa indicates that high floral merism (more than six organs) is very rarely associated with a monosymmetry. Precise mechanisms underlying this tendency, as well as the mode of interaction between regulatory pathways of floral symmetry and merism, remain unidentified. In this work, we observed the floral ontogeny in normal plants of Lupinus angustifolius L. (Leguminosae), described the floral structure and its variations in fasciated mutant. Our results suggest that monosymmetry is regulated independently from the floral merism, so three types of petals differentiate properly even in flowers with an anomalously high number of organs. However, the adaxial floral domain is the most stable floral part, which is the least susceptible to the fluctuations of merism. This means that some genes responsible for the development of the adaxial flower domain may additionally stabilize it and hence prevent fluctuations of merism in the adaxial domain. The monosymmetry in Leguminosae is a character connected with pollination by insects and requires a precise interaction between all floral parts. That is why fluctuations, which impair this interaction and symmetry, may decrease the pollination efficiency.

Staminal wing and a novel secretory structure of asclepiads

Asclepiads (Asclepiadoideae, Apocynaceae) have morphologically complex flowers with a wide range of secretory structures related to their defense and pollination. A striking feature of the flowers is the presence of a guide rail formed by staminal wings, which is related to the guidance of the pollinator to the pollinarium translator and the retention of the pollinium. Previous analyses of floral ontogeny in asclepiads detected the presence of a possible gland on the staminal wing during flower development. The goal of the present study was to verify the occurrence of a secretory structure on the staminal wing and its possible function through ontogenetic and histochemical analyses of the presumed gland. Staminal wings are formed early in developing flowers, and a gland is originated in the outer and inner margins of the guide rail in all species. This gland releases a secretion composed of mucilage and lipids, which likely assists in the introduction of the pollinator appendage into the narrow guide rail by lubricating the staminal wings. The gland degenerates before anthesis, increasing the space within the guide rail and enabling the introduction of the pollinarium. This new secretory structure may be present in floral buds of all asclepiads.