Floral development and vascularization help to explain merism evolution inPaepalanthus(Eriocaulaceae, Poales)

Abstract

BackgroundFlowers in Eriocaulaceae, a monocot family that is highly diversified in Brazil, are generally trimerous, but dimerous flowers occur in Paepalanthus and a few other genera. The floral merism in an evolutionary context, however, is unclear. Paepalanthus encompasses significant morphological variation leading to a still unresolved infrageneric classification. Ontogenetic comparative studies of infrageneric groups in Paepalanthus and in Eriocaulaceae are lacking, albeit necessary to establish evolution of characters such as floral merism and their role as putative synapomorphies.MethodsWe studied the floral development and vascularization of eight species of Paepalanthus that belong to distinct clades in which dimery occurs, using light and scanning electron microscopies.ResultsFloral ontogeny in dimerous Paepalanthus shows lateral sepals emerging simultaneously and late-developing petals. The outer whorl of stamens is absent in all flowers examined here. The inner whorl of stamens becomes functional in staminate flowers and is reduced to staminodes in the pistillate ones. In pistillate flowers, vascular bundles reach the staminodes. Ovary vascularization shows ventral bundles in a commissural position reaching the synascidiate portion of the carpels. Three gynoecial patterns are described for the studied species: (1) gynoecium with a short style, two nectariferous branches and two long stigmatic branches, in most species; (2) gynoecium with a long style, two nectariferous branches and two short stigmatic branches, in P. echinoides; and (3) gynoecium with long style, absent nectariferous branches and two short stigmatic branches, in P. scleranthus.DiscussionFloral development of the studied species corroborates the hypothesis that the sepals of dimerous flowers of Paepalanthus correspond to the lateral sepals of trimerous flowers. The position and vascularization of floral parts also show that, during dimery evolution in Paepalanthus, a flower sector comprising the adaxial median sepal, a lateral petal, a lateral stamen and the adaxial median carpel was lost. In the staminate flower, the outer whorl of staminodes, previously reported by different authors, is correctly described as the apical portion of the petals and the pistillodes are reinterpreted as carpellodes. The occurrence of fused stigmatic branches and protected nectariferous carpellodes substantiates a close relationship between P. sect. Conodiscus and P. subg. Thelxinoë. Free stigmatic branches and exposed carpellodes substantiate a close relationship between P. sect. Diphyomene, P. sect. Eriocaulopsis and P. ser. Dimeri. Furthermore, the loss of nectariferous branches may have occurred later than the fusion of stigmatic branches in the clade that groups P. subg. Thelxinoë and P. sect. Conodiscus.

Comparative Floral Morphology and Anatomy of Gagea s. str. and Lloydia

“…Investigations of the vascular anatomy of flowers was introduced by van Tieghem (1871), and is still successfully applied in the investigation of monocots (Dyka, 2018;Novikoff & Kazemirska, 2012;Remizowa et al, 2010;Zalko & Deroin, 2018). Because of its evolutionary conservation, the floral vascular system can serve not only for direct comparison of different taxa, but also for the elucidation of fused organs and analysis of floral evolution and morphogenesis (Joshi, 1940;Novikoff & Jabbour, 2014;Nuraliev et al, 2021;Silva et al, 2016;Sokoloff et al, 2018). Similarly, the principles of the gynoecium vertical zonality were developed by Leinfellner (1950) and Baum (1952), but are still useful in solving phylogenetic and taxonomic issues, in combination with morphological and molecular data (Heigl et al, 2020;Odintsova et al, 2013;Oliveira et al, 2020;Silva et al, 2020).…”

Stability Despite Reduction: Flower Structure, Patterns of Receptacle Elongation and Organ Fusion in Eriocaulon (Eriocaulaceae: Poales)

“…In Poales, common petal-stamen primordia are present in Xyris [11,20] and Eriocaulon ( [8,11,23] and this study). The occurrence of common petal-stamen primordia in other genera of Eriocaulaceae, e.g., Paepalanthus, is controversial [11,28]. Remarkably, in most other monocots possessing common primordia, these occur only for inner whorl perianth members and corresponding stamens, even when the two perianth whorls are similar to each other in anthetic flowers (e.g.,Veratrum, Liliales, [2]; Dioscorea, Dioscoreales, [64]) or rarely for all perianth members and stamens on their radii.…”

“…The unusual nature of orientation of trimerous pentacyclic flowers in Eriocaulaceae highlights a need of more detailed comparative and developmental studies with attention to variation of flower groundplan in the family. The most obvious aspects of floral diversity in Eriocaulaceae are the reduction of the outer whorl of the androecium (its rudiments can be often traced in male flowers) in Paepalanthoideae, occurrence of dimery rather than trimery in some taxa, patterns of corolla tube formation and petal to stamen fusion [ 9 , 11 , 13 , 25 , 26 , 27 , 28 ]. In the present study, we explore the reported phenomenon of the absence of the inner perianth whorl (corolla) in female flowers of some species of Eriocaulon , e.g., [ 9 , 29 ].…”

Flower and Spikelet Construction in Rapateaceae (Poales)

“…Such simultaneous initiation of numerous organs requires more space and meristematic material, leaving only a small area for gynoecium inception. Among Poales, common petal-stamen primordia have been reported for Xyridaceae (Remizowa et al, 2012;Nardi et al, 2021) and Eriocaulaceae (Stützel, 1984;Silva et al, 2016;Sokoloff et al, 2020). Simultaneous organ initiation but with separate primordia is also found in Rapateaceae.…”