A Unique Case of Floral Mimicry

Pollination is one of the major advantages flowering plants have over the rest of the botanical tree. With a few exceptions, flowers have cornered this market. It no doubt has played a significant role in their rise to dominance on the landscape. The importance of flowers is highlighted by the fact that they are costly structures. Because they don't photosynthesize, all plants take a hit on energy reserves when it comes time to flower. Sepals, petals, pollen, nectar, all of these take a lot of energy to produce which is why some plants cheat the system a bit. 

Sexual mimicry is one form of ruse that has evolved repeatedly. The flowers of such tricksters mimic receptive female insects waiting for a mate. The evolution of such a strategy taps into something far deeper in the mind of animals than food. It taps into the need to reproduce and that is one need animals don't readily forego. As such, sexually deceptive flowers usually do away with the production of costly substances such as nectar. They simply don't need it to attract their pollinators. 

By and large, the world of sexual mimicry in plants is one played out mainly by orchids. However, there exists an interesting exception to this rule. A daisy that goes by the scientific name Gorteria diffusa has evolved a sexually deceptive floral strategy of its own. Native to South Africa, this daisy is at home in its Mediterranean climate. It produces stunning orange flowers that very much look like those of a daisy. On certain petals of the ray florets, one will notice peculiar black spots. From region to region there seems to be a lot of variation in the expression of these spots but all are textured thanks to a complex of different cell types. 

The spots may seem like random patterns until the flowers are visited by their pollinator - a tiny bee-fly known scientifically as Megapalpus nitidus. With flies present, one can sort of see a resemblance. This would not be a mistake on the observers part. Indeed, when researchers removed or altered these spots, bee-fly visitation significantly decreased. Although this didn't seem to influence seed production, it nonetheless suggests that those spots are there for the flies. 

When researchers painted spots on to non-textured petals, the bee-flies ignored those as well. It appears that the texture of the spots makes a big difference to visiting flies. What's more, although female flies visited the flowers, a majority of the visits were by males. It appears that the presence of these spots is keying in on the mate-seeking and aggregation behavior of their bee-fly pollinators. Further investigation has revealed that the spots even reflect the same kind of UV light as the flies themselves, making the ruse all the more accurate. This case of sexual mimicry is unique among this family. No other member of the family Asteraceae exhibits such reproductive traits (that we know of). Although it doesn't seem like seed production is pollinator limited, it certainly increases the chance of cross pollination with unrelated individuals.

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The Sexual Ruse of the Bee Orchids

There are flowers out there that offer rewards far more enticing than any amount of pollen or nectar - they offer sex. Sexual deception is a rather tricky way of achieving pollination. By duping sex-crazed male insects into thinking they have found a female rather than a flower, such plants have tapped into an irresistible force of nature. Nowhere is this more beautifully illustrated than the orchids belonging to the genus Ophrys

Collectively referred to as the bee orchids, Ophrys grow native throughout much of Europe, North Africa, the Canary Islands, and parts of the Middle East. Phylogenetically speaking, they are a bit of a mess. Estimates of the number of species range from as few as 20 to as many as 130. The range of variation in floral color is staggering and has everything to do with the evolution of this genus. 

The reason they are called bee orchids is because that is exactly what they have entered into an evolutionary syndrome with. And what an evolutionary relationship it is! The bee orchids have evolved to trick male bees into thinking their flowers are receptive females. 

The most obvious aspect of this ruse is their appearance. Though there is quite a lot of variation, the overall theme is that the labellum acts as a female dummy complete with hairy abdomens and, in some species, iridescent wing marks. The ruse does not end there. Far more convincing than their appearance is the odor released by each flower. 

Ophrys produce chemical compounds called "allomones." These allomones closely mimic the pheromones released by female bees. What's more, each species of bee orchid produces allomones specific to the species of bee they are trying to attract. For some this can be very specific, attracting males of only a single species. For others it would seem that a small handful of different species have fallen for the orchid's trick. 

Regardless, male bees find these flowers irresistible at first, often preferring flowers to actual females. However, the males soon learn to avoid flowers, which results in consistently low pollination rates. This doesn't seem to be much of an issue for these orchids as a single plant can produce tens of thousands of seeds. 

This pollination syndrome has obviously worked for this genus. Slight mutations on the allomones produced have led to the massive radiation of Ophrys species we see today. Even more amazing is that research suggests that most of this radiation has occurred since the Pleistocene. 

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Further Reading: [1] [2] [3] [4]

Shhhh... Let Him Finish

Sexual deception is rampant in the orchid family. Orchid genera all over the world produce flowers that trick sexually charged male insects into failed mating attempts. The orchids go to great lengths to resemble females both in appearance and smell. Indeed, many sexually deceptive orchid species emit odors that precisely mimic the pheromones of specific insect species. 

In many instances, the orchids ruse is so powerful that male insects will often preferentially visit the flower over an actual female. For many of the sexually deceptive orchids, all that is required is the male to pay a visit. No attempt at copulation is necessary, though that doesn't stop vigorous attempts. Because of this, it is easy to see how the minute cost incurred to the insects is not enough to drive evolution away from deception. However, there is a group of tongue orchids (genus Cryptostylis) from Australia that seem to throw a wrench into this finely tuned system.... or do they?

The tongue orchids rely on deceiving male wasps in the genus Lissopimpla into mating with their flowers. As mentioned above, the males simply cannot resist the attempt. However, unlike many other reported cases, the male wasps actually mate to completion, depositing their sperm onto the flower. This should be disastrous for the wasps since males not only prefer flowers to wasp females, but they also waste their precious few mating attempts. How could this have evolved?

Most sexually deceptive orchids rely on bees and wasps (family Hymenoptera) for their pollination. Thus, the answer to this evolutionary conundrum lies in the mating system of these insects. Queens are genetically haplodiploid. I will spare you the details on that but basically what it means for Hymenoptera is that female offspring are produced via fertilized eggs whereas male offspring are produced via unfertilized eggs. 

The orchids have (unknowingly of course) tapped into this system to their benefit. If by mating with the flower and not a female wasp meant that no offspring were produced, this system surely would not have evolved to the level that it has. Instead, female wasps that have not been mated with or received less sperm than usual end up producing a higher amount of male offspring.

The orchids are effectively skewing the sex ratio of their pollinators! "How is this a sustainable system?" you may be asking. Well, by causing female wasps to produce more males, the orchids are ensuring that there will be more naive males in the population the next time they are in bloom. Also, by skewing the sex ratio towards males, there are now fewer females to mate with so that males become less choosy and more readily mate with orchids. Finally, with more sexually charged males flying around, each female has a greater chance of being fertilized. Because of the unique mating system that has evolved in Hymenoptera, the orchids have thus been able to evolve this pollination strategy with little harm to the pollinators.

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