Sunday, March 14, 2010

Hay fever and the evolution of pollination

(This woman is enjoying a good sneeze thanks to anemophily.)

April and May are my favorite months in upstate New York, because overt biological activity returns to the landscape. It is also my most miserable time—I suffer from allergies to pollen. My eyes water and itch, my nose tickles and runs, my throat is scratchy, and I sneeze a lot. I can take medicine but it makes me sleepy, and if I have to drive, being drugged is a bad idea. I am not alone. Approximately 20-30 million Americans suffer from outdoor allergies, mostly plant pollen. Although I have never been tested for the specific pollen to which I am allergic, I am pretty sure that maple, oak, ash, and possibly pine cause my problems, based on which flowering trees are in abundance near my house every year. I am also very allergic to grass pollen, so I simply stay out of meadows and hayfields during mid summer.

The enemy of those of us who suffer from allergies to plant pollen is “anemophily”. We all learned that flowering plants produce pollen, which is equivalent to sperm in vertebrates. The pollen must reach the plant carpel, or female part, of the plant so that the DNA in the pollen grain can join the DNA from the female gamete found in the ovule to produce a seed. We know that some plants are self-fertile, but others require that the pollen from the male structure get to a female flower elsewhere on the same plant, or to another female flower on another conspecific individual in the landscape. In some species, like aspen, you have male trees that produce only pollen, and female trees that produce only female flowers. So pollen often has to find a female flower of the same species somewhere in the landscape many meters or even hundreds of meters away. Most plants rely on insects, bats, or birds to move pollen from point a to point b, but about 20% of plant species rely on wind for pollen transport; this form of pollen dispersal is called anemophily. And those plants are the problem for hay fever sufferers, because their pollen is in the air to enter our eyes and nose.

Plants with showy, colorful flowers are always used in those television commercials that advertise an allergy medicine. You know the ones. The woman wants to garden, they show her walking through a yard full of black-eyed susans, Echinacea, lupine, and penstemon, and the scene implies that all those flowers are causing her itchy eye problem. Wrong! Plants that have large, or colorful, or aromatic flowers evolved those structures to attract some animal that can see or smell those characteristics. Those tend to be the plants we put in our gardens, because humans simply enjoy the sight. Plants did not evolve those beautiful structures for our enjoyment. Natural selection has responded to the potential suite of pollinators that existed out there. In fact, the tremendous diversification of flowering plant species coincides with the diversification of insect species during the Jurassic about 190 million years ago, although there is controversy surrounding the cause and effect of plant-insect evolution.

The plants that cause our problems are wind-pollinated, and they have small, inconspicuous flowers. How many of you know what a grass flower looks like? You need a compound scope to see them. But when they are at their peak flowering, if you hit the spike that contains those flowers, a small dust cloud of pollen will billow into the air. If I walk through such a field for 10 minutes, I need to reach for the Benadryl.

I have never suffered from hay fever in the tropics, however. This is curious, because there are many more plant species near the Equator than in upstate New York. But maybe that incredible diversity is part of the explanation as to why I am symptom-free in Costa Rica. There are many species, but it seems that the number of individuals in each of those species in any given location is not so great. The chances of a wind-blown grain of pollen landing on the female flower of another individual of the same species would seem to be low, or even remote, and not very efficient. If plants were selected naturally to develop a flower that attracts a particular species of fly or beetle or hummingbird, which visit to collect nectar or even pollen itself, that mobile organism is much more likely to visit another flower of the same species, probably within minutes. Many, but not all, of these animal pollinators are real specialists, and tend to visit only one species of plant. This is a much more focused system than relying on wind, which works just fine in the forest around my house where I have dozens of maple and ash and pine trees per acre, for example. (The story gets a bit more complicated. Red maple, which I have always thought caused my allergies, has small, red flowers. They are wind-pollinated, but they are also visited by bees. The fact that they are red suggests that they are not strictly wind-pollinated).

So now you have something to think about. It is fun to look at a flower and attempt to hypothesize what pollinates it. But also, the next time someone complains about their hay fever symptoms and points an accusing finger at the large yellow flowers growing along the side of the road, you can give a little fake sneeze and smile knowingly to yourself.