Floral fraudulence: Do blue Thelymitra species (Orchidaceae) mimic Orthrosanthus laxus (Iridaceae)?

Abstract

In Western Australia, Thelymitra crinita Lindl. and T. macrophylla Lindl. are pollinated by female, polylectic bees but offer no edible rewards. Flowers of Orthrosanthus laxus (Endl.) Benth. (Iridaceae) offer granular pollen and previous authorities suggest it is a Batesian model of T. crinita. We analyzed the floral fragrances and measured the floral dimensions of the orchid species, their putative hybrid, and O. laxus. Although the ‘scentless’ T. crinita emitted low levels of monoterpenoids and sesquiterpenoids, the pleasantly discernible fragrance of T. macrophylla was dominated by 2-phenylethanol. Their putative hybrid produced slightly lower levels of 2-phenylethanol compared with T. macrophylla and failed to produce any sesquiterpenoids associated with T. crinita. However, the hybrid produced higher volumes of the monoterpene linalool than either parent species. The fragrance of O. laxus contained 2-phenylethanol but lacked the sesquiterpenoids. We also measured perianth area and symmetry as well as the length and width of contrastingly pigmented floral centres for each taxon. Significant differences in floral area and symmetry were detected between the putative hybrid, the two parent species, and O. laxus. In contrast, the floral reward centre area (tuft of stamens) in O. laxus was significantly larger than the pseudo-reward centres (mitras) of both Thelymitra species and their hybrid. At the peak of their respective, but overlapping flowering periods, an inflorescence of T. macrophylla produced more than twice the number of open flowers as T. crinita and more than four times the number of open flowers on cymes of O. laxus. Based on scent production and visual displays, T. macrophylla appears more likely to be a Batesian floral mimic of O. laxus. We suggest that large-flowered Thelymitra species appear to produce a novel, visual and olfactory attractant pattern of fraudulence we call the ‘New Again, More Again Effect’.