A pollinator shift explains floral divergence in an orchid species complex in South Africa

Abstract

Aims Floral diversification driven by shifts between pollinators has been one of the key explanations for the radiation of angiosperms. According to the Grant-Stebbins model of pollinator-driven speciation, these shifts result in morphologically distinct 'ecotypes' which may eventually become recognizable as species. The current circumscription of the food-deceptive southern African orchid Eulophia parviflora encompasses a highly variable monophyletic species complex. In this study, two forms were identified within this complex that differ in distribution, floral morphology, scent chemistry and phenology, and a test was made of whether these differences represent adaptations for different pollinators. † Methods and Results Multivariate analysis of floral and vegetative traits revealed that there are at least two discrete morphological forms in the species complex. Field observations revealed that each form is pollinated by a different insect species, and thus represent distinct ecotypes. The early-flowering coastal form which has long spurs and floral scent dominated by sesquiterpene compounds is pollinated exclusively by the long-tongued bee Amegilla fallax (Apidae, Anthophorinae), while the late-flowering inland form with short spurs and floral scent dominated by benzenoid compounds is pollinated exclusively by the beetle Cyrtothyrea marginalis (Cetoniinae; Scarabaeidae). Choice experiments in a Y-maze olfactometer showed that beetles are preferentially attracted to the scent of the short-spurred form. A spur-shortening experiment showed that long spurs are required for effective pollination of the bee-pollinated form. Although it was initially thought likely that divergence occurred across a geographical pollinator gradient, plants of the long-spurred form were effectively pollinated when transplanted to an inland locality outside the natural coastal range of this form. Thus, the underlying geographical basis for the evolution of ecotypes in the E. parviflora complex remains uncertain, although early flowering in the long-spurred form to exploit the emergence of naïve bees may restrict this form to coastal areas where there is no frost that would damage flower buds. Later flowering of the short-spurred form coincides closely with the emergence of the pollinating beetles following winter frosts. † Conclusions This study identifies a shift between bee and beetle pollination as the main driver of floral divergence in an orchid species complex. Floral scent and spur length appear to be key traits in mediating this evolutionary transition.

Nectar sugars and amino acids in day- and night-flowering Nicotiana species are more strongly shaped by pollinators’ preferences than organic acids and inorganic ions

“…They typically have a very long proboscis supporting the hypothesis that flower shapes have co-evolved with the morphology of the mouth parts of their pollinators [23,24]. Particularly within the group of sphingophilous tobacco plants there exists a very high correlation between the corolla tube lengths and the proportions of sucrose (r = 0.857).…”

Orchid diversity: Spatial and climatic patterns from herbarium records

“…Therefore, the most important drivers of orchid speciation are likely to be both spatially linked and biotic. A critical factor is likely to be insect diversity; switching to a new pollinator species is instrumental in orchid speciation (Breitkopf, Onstein, Cafasso, Schlüter, & Cozzolino, ; Peakall & Whitehead, ; Peter & Johnson, ; Sun, Schlüter, Gross, & Schiestl, ). Mycorrhizal fungi diversity is less important than pollinators in orchid speciation (Phillips et al, ), but mycorrhizae do nonetheless determine the subset of sites that an orchid can colonize within its possible climatic range (McCormick & Jacquemyn, ; Nurfadilah et al, ).…”

Should I stay or should I go? Pollinator shifts rather than cospeciation dominate the evolutionary history of South African Rediviva bees and their Diascia host plants

“…Range expansions and contractions have been suggested for many other plants in the Greater Cape Floristic Region, especially during the Miocene (Cowling, Procheş, & Partridge, ), and were probably followed by pollinator shifts (van der Niet, Pirie, et al, ). Hence, we conclude that pollinator shifts seem to play a major role in the diversification of Diascia , as for other plants in the Greater Cape Floristic Region (Johnson, ; Johnson & Steiner, ; van der Niet, Pirie, et al, ; Peter & Johnson, ; Valente, Manning, Goldblatt, & Vargas, ; Valente et al, ) and the Drakensberg Mountain Centre (Springer, van der Niet, & Cron, ).…”