Do we truly understand pollination syndromes in Petunia as much as we suppose?

Abstract

Petunia is endemic to South America grasslands; member of this genus exhibit variation in flower colour and shape, attracting bees, hawkmoths or hummingbirds. This group of plants is thus an excellent model system for evolutionary studies of diversification associated with pollinator shifts. Our aims were to identify the legitimate pollinator of Petunia secreta, a rare and endemic species, and to assess the importance of floral traits in pollinator attraction in this Petunia species. To determine the legitimate pollinator, field observations were conducted, and all floral visitors were recorded and evaluated. We also measured the nectar volume and sugar concentration. To characterize morphological cues for pollinators, we assessed the ultraviolet (UV)-light response in detached flowers, and characterized the floral pigments and pollen volatile scents for four different Petunia species that present different pollination syndromes. Petunia secreta shares the most recent ancestor with a white hawkmoth-pollinated species, P. axillaris, but presents flavonols and anthocyanin pigments responsible for the pink corolla colour and UV-light responses that are common to bee-pollinated Petunia species. Our study showed that a solitary bee in the genus Pseudagapostemon was the most frequent pollinator of P. secreta, and these bees collect only pollen as a reward. Despite being mainly bee-pollinated, different functional groups of pollinators visit P. secreta. Nectar volume, sugar concentration per flower, morphology and components of pollen scent would appear to be attractive to several different pollinator groups. Notably, the corolla includes a narrow tube with nectar at its base that cannot be reached by Pseudagapostemon, and flowers of P. secreta appear to follow an evolutionary transition, with traits attractive to several functional groups of pollinators. Additionally, the present study shows that differences in the volatiles of pollen scent are relevant for plant mutualistic and antagonist interactions in Petunia species and that pollen scent profile plays a key role in characterizing pollination syndromes.

What could be the fate of secondary contact zones between closely related plant species?

“…Although P. axillaris is primarily pollinated by hawkmoths, some bees and hummingbirds have been observed visiting its flowers (Lorenz-Lemke et al, 2006;Gübitz et al, 2009). P. exserta flowers display a set of traits usually seen in bird-pollinated species but share with P. axillaris some scent compounds that are attractive to bees (Rodrigues et al, 2018).…”

Pseudogenization and Resurrection of a Speciation Gene

“…At the same time, it is important to note that, although color and bee visitation have been restored, P. secreta retains the typical long-tube floral morphology, which limits bee visitors to collecting pollen. Pollinator observations found that P. secreta attracts small halictid bees (Apoidea and Halictidae; as opposed to short-tube species' solitary bee pollinators of the Andrenidae and Colletidae), who indeed collect pollen rather than nectar [58].…”

Species boundary and extensive hybridization and introgression in Petunia

“…These two Petunia species present suites of floral traits strongly related to their respective floral syndromes, hawkmoth in P. axillaris and hummingbirds in P. exserta (Stehmann et al 2009). No systematic pollination studies have been conducted in the field to identify which pollinators the intermediary colored plants attract and, except their pink shades that usually attract bees in Petunia species (Rodrigues et al 2018), other clues on pollinators remain unidentified.…”

“…Moreover, molecular characterization of genes and processes involved in floral syndromes shifts among Petunia species in general and P. axillaris and P. exserta species in particular (Amrad et al 2016;Sheehan et al 2016;Esfeld et al 2018;Rodrigues et al 2018), demonstrating the importance of a prezygotic barrier to reproductive isolation to maintain the species limits of this group. On the other hand, strong evidence of interspecific hybridization between P. axillaris and P. exserta was also obtained through molecular and morphological characterization, mostly at two sites in Guaritas (Turchetto et al 2019; LM Caballero-Villalobos unpubl.…”