For those interested in technical issues, Nature 445 p816-817 27 Feb 2007 has a two page review on orchid pollinators. Entitled "The flower of seduction", its author is Heidi Ledford.
The basic question which the article seeks to answer is why insects seek out flowers which do not reward their visitors with nectar or oils, a category of which orchids are generally members. She reviews the work of Schiestl and Francke on odorant attracts, showing that these are extremely complex mixtures. Andrena nigroaenea bees, for example, are attracted to Ophrys sphegoides by a cocktail of 14 different odorants. These mixtures evolve from compounds which are a natural part of the cuticle of many plants, with different tweaks and emphasis attracting different insects. This has a direct line on speciation amongst orchids. Many sympatric, closely-related species flower at the same time, yet maintain reproductive isolation. It turns out that they manage this segregation by attracting different pollinator species, due to their distinctive odorants. Work in Australia on Chiloglottis in conjunction with Rod Peakall at ANU has looked at the genus, and found that sympatric (etc) species maintained isolation through different pheromone-mimic odorants. (Chiloglottis has a unique odorant called chiloglottone.) The different mixtures over the common musk of chiloglottone attracted different species of wasp, which only visited flowers of the same species. This may be the first evidence for a clear mechanisms for sympatric speciation - why lakes fill with hundreds of species of Tilapia, for example, or sea water with monstrous numbers of plankton species. To date, this has only been observed on remote peaks and lakes, and to see it on a continental scale is of considerable interest to evolutionary biologists. A similar DNA-based program - to see the developing clades - is now underway in Europe. And why do wasps pollinate orchids which do not reward them? It seems that juvenile males have to learn, and initially tend to leap indiscriminately on anything that looks vaguely attractive. Plus ca change... ______________________________ Oliver Sparrow +44 (0)20 7736 9716 www.chforum.org _______________________________________________ the OrchidGuide Digest (OGD) [email protected] http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
