Floral volatile alleles can contribute to pollinator‐mediated reproductive isolation in monkeyflowers (Mimulus)

Abstract

Summary Pollinator-mediated reproductive isolation is a major factor in driving the diversification of flowering plants. Studies of floral traits involved in reproductive isolation have focused nearly exclusively on visual signals, such as flower color. The role of less obvious signals, such as floral scent, has been studied only recently. In particular, the genetics of floral volatiles involved in mediating differential pollinator visitation remains unknown. The bumblebee-pollinated Mimulus lewisii and hummingbird-pollinated M. cardinalis are a model system for studying reproductive isolation via pollinator preference. We have shown that these two species differ in three floral terpenoid volatiles - D-limonene, β-myrcene, and E-β-ocimene - that are attractive to bumblebee pollinators. By genetic mapping and in vitro enzyme activity analysis we demonstrate that these interspecific differences are consistent with allelic variation at two loci – LIMONENE-MYRCENE SYNTHASE (LMS) and OCIMENE SYNTHASE (OS). M. lewisii LMS (MlLMS) and OS (MlOS) are expressed most strongly in floral tissue in the last stages of floral development. M. cardinalis LMS (McLMS) is weakly expressed and has a nonsense mutation in exon 3. M. cardinalis OS (McOS) is expressed similarly to MlOS, but the encoded McOS enzyme produces no E-β-ocimene. Recapitulating the M. cardinalis phenotype by reducing the expression of MlLMS by RNAi in transgenic M. lewisii produces no behavioral difference in pollinating bumblebees; however, reducing MlOS expression produces a 6% decrease in visitation. Allelic variation at the OCIMENE SYNTHASE locus likely contributes to differential pollinator visitation, and thus promotes reproductive isolation between M. lewisii and M. cardinalis. OCIMENE SYNTHASE joins a growing list of “speciation genes” (“barrier genes”) in flowering plants.

A key floral scent component (β‐trans‐bergamotene) drives pollinator preferences independently of pollen rewards in seep monkeyflower

“…Furthermore, three of these compounds (β‐myrcene, limonene and E‐β‐ocimene) were found to be the most abundant compounds in the floral scent of the bumblebee‐pollinated Mimulus lewisii , but were nearly absent in its sister species, the hummingbird‐pollinated M. cardinalis (Byers, Bradshaw, & Riffell, 2014a). These compounds were further shown to mediate bumblebee preferences for M. lewisii over M. cardinalis (Byers, Vela, Peng, Riffell, & Bradshaw, 2014b; Byers et al., 2014a). As hummingbirds forage on flowers exclusively for nectar rewards (Thomson, Wilson, Valenzuela, & Malzone, ) and are thought to have little sense of smell (Dobson, ), differences in the emission of these compounds by M. lewisii and M. cardinalis might reflect selection favouring honest olfactory signals of pollen reward quality in the bumblebee‐pollinated species.…”

Comparative analyses of flower scent inSileneaereveal a contrasting phylogenetic signal between night and day emissions

“…Similarly, compounds responsible for the global (E‐ β ‐ocimene, sabinene, linalool, phenylacetaldehyde and 2‐phenylethanol) and local (benzyl benzoate, benzaldehyde, β ‐pinene, camphene and sabinene) structures of day scents (pPCA analysis) potentially attract diurnal pollinators such as butterflies and bees (Andersson ; Dobson ; Byers et al. ). How is it possible that compounds phylogenetically constrained (high loading on global pPCA axes) seem to be similarly important for pollinator attraction as compounds phylogenetically unconstrained (high loading on local pPCA axes)?…”

Genomic studies on the nature of species: adaptation and speciation in Mimulus

“…) to confirm the function of flower colour and floral scent genes (Byers et al . ). Protocols for stable transformation and virus‐induced gene silencing (VIGS) are also available for M. aurantiacus (Streisfeld et al .…”