Abstract
Acianthera section Pleurobotryae is one of ten sections of the genus Acianthera and include four species endemic to the Atlantic Forest. The objective of this study was to describe comparatively the anatomy of vegetative organs and floral micromorphology of all species of Acianthera section Pleurobotryae in order to identify diagnostic characters between them and synapomorphies for the section in relation of other sections of the genus. We analyzed roots, ramicauls, leaves and flowers of 15 species, covering eight of the nine sections of Acianthera, using light microscopy and scanning electron microscopy. Acianthera section Pleurobotryae is a monophyletic group and the cladistic analyses of anatomical and flower micromorphology data, combined with molecular data, support internal relationship hypotheses among the representatives of this section. The synapomorphies identified for A. sect. Pleurobotryae are based on leaf anatomy: unifacial leaves, round or elliptical in cross-section, round leaves with vascular bundles organized in concentric circles, and mesophyll with 28 to 30 cell layers. Within the section, the clade (A. crepiniana + A. mantiquyrana) presented more differences in vegetative organ morphology and higher support values in combined analyses when compared to the second clade, (A. atropurpurea + A. hatschbachii). For each of these clades an exclusive set of homoplasies of vegetative and floral organs were also identified. The results support the argument that vegetative organs are more evolutionarily stable in comparison to reproductive organs and thus helpful for inference of internal phylogenetic relationships in Acianthera, while flowers are highly variable, perhaps due to the diversity of pollinator attraction mechanisms. The analyses indicate that the elliptical leaves observed in A. crepiniana have originated from round leaves observed in the other species of this section, suggesting an adaptation to increase the area of exposure of the leaf and better the efficiency of capture of sunlight in shaded environments such as the Atlantic Forest. The presence of papillose regions in both vegetative and floral organs indicated that micromorphological characters are also useful for the delimitation of species and sections within the genus.
Highlights
The family Orchidaceae represents approximately 7% of all angiosperms, and half of the Monocotyledons [1], with approximately 25,000 species and approximately 800 genera [2]
The analysis of molecular data combined with anatomical and micromorphological data of Acianthera section Pleurobotryae allowed us to identify shared anatomical characters observed exclusively in these species, besides supporting the internal relationships suggested by the molecular data set [15]: unifacial leaves, round or elliptical in cross-section, round leaves with vascular bundles organized in concentric circles, and mesophyll with 28 to 30 cell layers
A. section Pleurobotryae is a strongly supported monophyletic group with the following synapomorphies: leaves unifacial, round or elliptical in cross-section, round leaves with vascular bundles organized in concentric circles, and mesophyll with 28 to 30 cell layers
Summary
The family Orchidaceae represents approximately 7% of all angiosperms, and half of the Monocotyledons [1], with approximately 25,000 species and approximately 800 genera [2]. But the greatest diversity, including most of the epiphytic species, occurs in the tropics, especially in tropical mountains [3]. These species often establish complex interactions with other organisms for their growth, development and reproductive success. They are highly susceptible to environmental change and play an important ecological role in ecosystems [4]. Acianthera currently includes 291 species of terrestrial, rupicolous and especially epiphytic plants with long-repent or repent growth and variable size found throughout South America [8, 9]. The largest number of species is found in Brazil [10], many of them endemic [11]
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