Long-spurred Angraecum orchids and long-tongued sphingid moths on Madagascar: a time frame for Darwin's predicted Xanthopan Angraecum coevolution

摘要

Trait matching between the Madagascan orchid Angraecum sesquipedale, with a nectar spur of 33 cm, and a hawkmoth with a tongue almost as long has fascinated biologists since Darwin, who saw only flowers but correctly predicted the moth pollinator. This moth, Xanthopan morgani praedicta, was described from museum specimens in 1903 and documented as the pollinator in photographs and film in 1992 and 2004. However, Madagascar harbours c. 30 species of long-spurred orchids and seven species of long-tongued hawkmoths, and mutualisms between moths and 'Angraecum and other deep tubular flowers' (Darwin, 1862: p. 202) probably involve a network of interacting species. We infer the evolutionary time over which Madagascan sphingids and Angraecum have interacted, based on an orchid phylogeny that includes 62 of 144 Angraecum species on Madagascar and a moth phylogeny with all nine Madagascan Sphinginae. Clock models using either rate- or fossil-based calibrations imply that the Madagascan subspecies X. morgani praedicta and the African morgani diverged 7.4 +/- 2.8 Mya, which overlaps the divergence of An. sesquipedale from its sister, Angraecum sororium, namely 7.5 +/- 5.2 Mya; since both have extremely long spurs, long spurs probably existed before that. The phylogenies moreover show that several long-tongued moths and long-spurred orchids probably coevolved on Madagascar since the mid-Miocene, although field data on the moths' levels of polyphagy and pollen transport are lacking.