Philodendron adamantinum(Araceae) lures its single cyclocephaline scarab pollinator with specific dominant floral scent volatiles

Abstract

Cyclocephline scarabs and their host plants are documented as highly specialized plant-pollinator associations, with various fine-tuned adaptations. We studied the association between Philodendron adamantinum, a species endemic to the Espinhaço Range in Minas Gerais, South-East Brazil, and its exclusive pollinators. We focused on the pollination mechanism and reproductive success of P. adamantinum, analysed its floral scent composition, and performed field bioassays to verify the scent-mediated attraction of pollinators. The reproductive success of P. adamantinum depends on the presence of Erioscelis emarginata (Scarabaeidae, Cyclocephalini), its sole pollinator. At dusk, the inflorescences heat up to 18°C above the surrounding ambient air temperature and give off a strong sweet odour, from which 32 volatile compounds were isolated. Dihydro-β-ionone, the major constituent in the floral scent bouquet, lures individuals of E. emarginata when applied to scented artificial decoys, either alone or blended with methyl jasmonate. We attribute the low fruit set of P. adamantinum at our study sites to pollinator limitation of small and isolated populations and geitonogamic pollen flow of vegetatively generated clonal plant groups. The interaction between P. adamantinum and E. emarginata shows common traits typical of the known plant-pollinator associations involving cyclocephaline scarabs: the asymmetrical dependence of plants on their pollinators, and the scent-mediated interaction between flowers and beetles. In addition to updating the current catalogue of active compounds of cantharophilous pollination systems, further experimental studies should elucidate the role of the specific chemical compounds that attract pollinators along different time and biogeographic scales.

Synopsis of the cyclocephaline scarab beetles (Coleoptera, Scarabaeidae, Dynastinae)

“…A mixture of dihydro-β-ionone and methyl jasmonate was synergistically attractive to E. emarginata , which pollinates Philodendron adamantium Mart. ex Schott (Araceae) (Pereira et al 2014). Among Nymphaea spp.…”

“…In the case of Philodendron adamantium Mart. ex Schott, a single dominant flower scent compound (Dihydro-β-ionone) extracted from this species was sufficient to attract E. emarginata to scent traps (Pereira et al 2014). …”

Floral resins of Philodendron adamantinum (Araceae): secretion, release and synchrony with pollinators

“…However, it makes part of the fine-tuned pollination mechanism. The resin secretion covers the smooth body of the hairless scarab beetles and acts as an adhesive for pollen grains (Gibernau et al 1999;Gibernau & Barabé 2002;Pereira et al 2014).…”

“…Many species of the three subgenera of Philodendron release resin on the adaxial side of the spathe and, more rarely, on the stamens (Mayo 1991;Grayum 1996;Croat 1997). In Philodendron adamantinum, the pollinating scarab beetles Erioscelis emarginata, attracted by specific volatile compounds produced in the osmophores at the apex of sterile and fertile staminate flowers (Pereira et al 2014;Gonçalves-Souza et al 2017), enter the pollination chamber in the evening of the first day of flowering when pistillate flowers are viable. They remain in the chamber during the night and day two of inflorescence flowering.…”

“…From the late morning on, the release of resin droplets on the adaxial side of the spathe has been observed to occur in the period immediately preceding dehiscence of the anthers. The chamber closes due to the gradually constricting spathe and the beetles are expulsed from dusk on, covered with resin and loaded with pollen-dust (Pereira et al 2014).…”