Pollination ecology of two species of Elleanthus (Orchidaceae): novel mechanisms and underlying adaptations to hummingbird pollination

Abstract

Relationships among floral biology, floral micromorphology and pollinator behaviour in bird-pollinated orchids are important issues to understand the evolution of the huge flower diversity within Orchidaceae. We aimed to investigate floral mechanisms underlying the interaction with pollinators in two hummingbird-pollinated orchids occurring in the Atlantic forest. We assessed floral biology, nectar traits, nectary and column micromorphologies, breeding systems and pollinators. In both species, nectar is secreted by lip calli through spaces between the medial lamellar surfaces of epidermal cells. Such a form of floral nectar secretion has not been previously described. Both species present functional protandry and are self-compatible yet pollinator-dependent. Fruit set in hand-pollination experiments was more than twice that under natural conditions, evidencing pollen limitation. The absence of fruit set in interspecific crosses suggests the existence of post-pollination barriers between these sympatric co-flowering species. In Elleanthus brasiliensis, fruits resulting from cross-pollination and natural conditions were heavier than those resulting from self-pollination, suggesting advantages to cross-pollination. Hummingbirds pollinated both species, which share at least one pollinator species. Species differences in floral morphologies led to distinct pollination mechanisms. In E. brasiliensis, attachment of pollinarium to the hummingbird bill occurs through a lever apparatus formed by an appendage in the column, another novelty to our knowledge of orchid pollination. In E. crinipes, pollinarium attachment occurs by simple contact with the bill during insertion into the flower tube, which fits tightly around it. The novelties described here illustrate the overlooked richness in ecology and morphophysiology in Orchidaceae.

Species boundary and extensive hybridization and introgression in Petunia

“…The foraging preferences of hummingbird species involve the morphological matching between the bill and floral traits and are also a consequence of resource abundance (Maglianesi et al 2015a;b). In fact, specialized interactions have been observed in many plant-hummingbird pairs and are implicated as important drivers of plant diversification in the Neotropics (Nunes et al 2016;Serrano-Serrano et al 2017). Petunia exserta and P. axillaris present several floral traits in common (Stehmann et al 2009), such as a long and salverform corolla tube with nectar chambers at the base, which also exist in hybrid individuals.…”

Deep in the Jelly: Histochemical and Functional Aspects of Mucilage-Secreting Floral Colleters in the Orchids Elleanthus brasiliensis and E. crinipes

“…Elleanthus brasiliensis and E. crinipes are epiphytic or rupicolous herbs that occur in environments with high air humidity (Nunes et al, 2016). E. brasiliensis has pendent stems (Figure 1A), while E. crinipes has erect stems (Figure 1C), at the apex of which reproductive buds give rise to racemose inflorescences (Figure 1B,D,E).…”

“…f. is found in the humid forests of eastern Brazil and the Guianas, while Elleanthus crinipes Rchb. f. is endemic to southeastern Brazil, where it is found in highland forests in different physiognomies of the Atlantic Forest (Nunes et al, 2016). E. brasiliensis stands out from other species due to the large amount of mucilaginous secretion that covers its inflorescences, refracting the reddish color of the bracts and giving the globular inflorescence a bright appearance (Figure 1A,B).…”

“…E. brasiliensis stands out from other species due to the large amount of mucilaginous secretion that covers its inflorescences, refracting the reddish color of the bracts and giving the globular inflorescence a bright appearance (Figure 1A,B). In E. crinipes (Figure 1C), this secretion is conspicuously scarce (Figure 1D,E), although these two species may occur in the same environments and share the same pollinators (Nunes et al, 2013, 2016).…”

Pollination biology from micro‐morphological adaptations to community ecology of plant–pollinator interactions

“…In this Special Issue, Nunes et al . () report on hitherto undescribed micro‐morphological floral adaptations through which two Elleanthus orchids from the Atlantic forest of Brazil have evolved mechanical isolation barriers, and secrete nectar to attract their specialised pollinators, hummingbirds. The authors provide evidence that nectar is produced by the lip calli, through spaces between the medial lamellar surfaces of epidermal cells, and that original changes in the floral column – the presence of a ‘lever apparatus’ – result in attachment of the orchid pollinia to the hummingbird's bill.…”