by Aditi Mishra – Imagine a situation where you have to depend on mailmen for transferring some crucial package, but most mailmen are likely to misplace it. What do you do then? The logical strategy would be to identify the few mailmen who can be trusted, and somehow ensure that only they get to handle your package. Use a passcode maybe? Plants have to deal with the same issue every day, and they employ a similar strategy. They desperately need to exchange pollen, to produce seeds. They can’t move, and the pollen exchanged must be conspecific i.e. of the same species.
Over the course of evolutionary history, plants have not only figured out the perfect way to make their precious pollen reserves super-secure; they have also made the passcode simple and fundamental enough that insect pollinators can use it. Do you realize how cool it is? Its human equivalent would be making a bird feeder that only birds with specific songs can access, plus teaching the birds that they have to sing the same song each time they want to munch on some seeds. Plants invest considerable resources into pollen and nectar production- the payments given to pollinators for the service provided. It would be very counterproductive if pollinators picked up pollen and deposited it on the stigma of some other species.
So what smart solutions do plants employ? “Buzz pollination”, also known as sonication, is one. About 20,000 species including tomato and brinjal (eggplant) have poricidal anthers i.e. they release pollen via pores or slits only. The pollinator then has to vibrate at the right resonant frequency to coax pollen out. Some pollinators, e.g. carpenter bees, beat their wings at a specific frequency; other bees decouple the wings from flight mechanism and beat their thoracic muscles instead. Beetles grab the anther by their mandibles while buzzing, which is aptly called “milking the anthers”. The resonant frequencies are pretty high (ranging from 14,000 to 29,000 vibrations per minute) and only a fraction of the pollen stock is released in each buzzing event. The first few buzzes can release up to 60% of the pollen while subsequent buzzes may yield only 10%.
The high energy cost of this behaviour excludes many other pollen-feeders from the precious pollen, leading to reduced competition for the pollinators (especially, from pollen robbers). Releasing a variable fraction of the pollen is a strategy to encourage the insect visitor to come again. In the pursuit of pollen the same insect would visit similar flowers again and again, sometimes even when all the pollen has been eaten up – making pollen transfer more species specific. Apart from that, poricidal anthers may ensure that pollen gets ejected onto safe areas of the pollinators’ body – areas where there is less chance of pollen being groomed and eaten up.
“Behaviour is the pacemaker of evolution.” -Ernst Mayr
Buzz pollination has evolved independently 65 times, making it a very good example of both convergent evolution and co-adaptation. The most eloquent example of it would be the case of Orpheum frutescens and the carpenter bee Xylocopa caffra. The anthers of Orpheum frutescens, or as it is commonly called “sea rose”, are twisted with only two pores from which it dispenses most of its pollen. The anther of the flower is fairly separated from its stigma. This arrangement avoids self-fertilization and damage to the stigma when the female carpenter bee is strumming the anthers in a middle C note. Only a single insect can pollinate this flower, demonstrating how true the words of Ernst Mayr are. Behaviour is truly the pacemaker of evolution