Cracking the Pollen Case
Every beekeeper has a secret recipe for fall feeding and tonight I was preparing mine: Canola oil, sea salt, fresh pepper rubbed over a thick rib eye and fried in a pan to a rich, juicy crust. Oh, perhaps I should mention this was beekeeper feed. Honeybees aren’t carnivorous, or else open feeding would just involve tethering a goat in the apiary and making a run for it. I prefer to get my protein on a plate with a pat of butter; honeybees fortunately do not.
I have a lot more in common with wasps than honeybees. Wasps have the jaws of hunters and weapons to match. They are smooth, sleek, and deadly. When they need protein to raise their brood they hunt it down and kill it. Then there’s the honeybee. Their jaws are not designed to rip and slice; their stings are not laced with paralytic poisons. When it comes to selecting a protein source the honeybee prefers something less mobile, something more defenseless. Something that doesn’t run away when you land near it, fight back when you grab it, or have teeth, stingers or other dangerous weaponry: something like pollen.
Plants create pollen in order to make more plants, not to make more bees. Every bee book I’ve read talks about the miraculous relationship between the bees and the flowers. How the flowers produce pollen and the bees ferry it back and forth for the flowers and in return the flowers let them gather nectar. When a bearded man at a chicken fried steak buffet transfers some gravy from one plate to another we don’t call it a miracle. For honeybees, transferring pollen from flower to flower is more a side effect then an end goal. They might transfer some pollen for the flowers but most of it is packed away for the bees to eat. Using pollen as a primary source of protein is a great choice for a bee that’s not built to hunt but it does present its own problems.
The sperm cells contained in a pollen grain are extremely fragile. Left on their own they’d be destroyed long before even a rocket propelled honeybee could transfer it from one flower to another. In nature the approach taken is to build layers of protection around the genetic payload. Almonds are covered in a nut shell. Eggs have a hard layer around the embryo inside and the world outside. Even teenage drivers get wrapped in Concorde station wagons with wood panel sides. Pollen gets wrapped up too.
As a grain of pollen matures it is prepared to leave the safe harbor of the flower and take a voyage beyond the edge of the petals. The delicate genetic package is enclosed carefully in a cellulose layer called intine which acts like a baggie to keep the DNA goo together. This inner core of cytoplasm is the real source of nutrition in pollen but the cellulose by itself blocks easy digestion. On top of the intine is the exine which acts like a bullet proof vest for the pollen grain. The exine is composed of sporopollenin, which is the scientific term for “incredibly tough.” Sporopollenin is a compound resistant to most acids, bases, temperatures up to 250 degrees Celsius and immense pressure. On top of the exine is the pollenkitt, or “Sticky stuff on pollen.” Pollenkitt acts like glue to stick pollen where it lands, as well as providing an additional layer of bubble wrap around the exine. Together these layers form a miniature plant which will germinate under the right conditions, growing a tube that can reach up to ten inches to deliver the sperm cells right into the heart of the plant. You can think of a grain of pollen like a chocolate covered almond, if the almond were still in the shell when it was dipped. With that many layers between the honeybee and the tasty genetic filling, how does the honeybee manage to digest pollen at all?
In order to understand how a honeybee turns a pollen grain from lock box to lunch box, you have to go inside a bee. You don’t have to very far, just past the nectar crop, to the entrance to a bee’s digestive organ: the ventriculus. The ventriculus is the organ in which digestion of pollen is performed. How do we know this? We looked inside a bee. Well, not me specifically. Researchers did. Researchers with sharp scalpels and a lot of free time cut tiny windows into the abdomen of bees and then fed them pollen. Then they watched the bees digest the pollen.
On my sixth birthday I received two rectangular gifts. One came from my parents and I shredded the paper to find Batman, in plastic action figure glory. I looked at the other box and saw the shape, the outline, and I knew: It must contain the Joker. That package came from my grandparents in Slaton, Texas. It was wrapped in plastic foil, followed by brown shipping paper, layered in shipping tape. I began to claw at the package. After a few minutes my mother tried to help. She broke two nails and passed it off to my Dad. He stabbed at it with a pocket knife. For the next few minutes every family member did their best to pry open that package. Finally my Dad went to the garage and came back with his toolbox. Batman and the tin snips defeated my grandmother and the US postal service but it took a while. Inside I found another Batman. That day I learned an important lesson – regardless of what is inside, some packages are harder to get into than others. The same is true with pollen.
The first stage of digestion involves unwrapping the pollen grain. The lipids in the pollenkitt are stripped away like wrapping paper. Unlike wrapping paper (which is rarely chewed up and swallowed), the pollenkitt is nutritious, rich in lipids. Under the pollenkitt lies the exine. The exine structure varies from species to species. Some exines are solid smooth and rely on the host plant triggering germination. Other exines have gaping holes in them so wide you could back a (tiny) loading truck right up to the pores to offload the cytoplasm. Under the exine the intine stands as a final barrier between the bee and the feast. In the ventriculus the honeybee employs a number of methods to get past the hard outside to the creamy filling. The first of these is osmotic shock. Osmotic shock causes the pollen exine to rupture like a balloon bursting. Some types of pollen are like Ford Pintos, ready to explode. Other pollen grains are sturdy and require the honeybee to “pick the lock.” These pollen grains have germination pores, which function as the plant equivalent of the exit rows on an airplane. When the pollen touches down after a crash landing on the pistil the grain germinates, opening at the pores. It then grows a tube that acts as an emergency slide for the genetic material. Honeybees use this method to open some pollen grains. Inside the ventriculus, nice and warm, the germination pores begin to swell and open. What happens next is like a scene from a alien invasion movie as the digestive enzymes seep in and out, leaving nothing behind but the empty shell. It’s like eating an almond and digesting the almond from the inside of the shell. Some pollen grains are built like Volvos. These pass through the ventriculus and come out the other end not the least bit digested. While that’s a win for the pollen grain, it’s a loss for the bee in more ways than one. First there’s the fact that all that work didn’t yield protein. Secondly the bees have to burn carbohydrates on cleansing flights to get rid of the indigestible bits.
What about humans? The local health food store sells pollen. “Bee Pollen”, said the sign above the counter. “Nature’s Best Food,” said the writing underneath. “This is real bee pollen,” said the sales lady, “it’s expensive because it contains all the protein bees need to satisfy all their needs.” Road kill contains all the protein crows need to satisfy their needs, but people don’t pay $20 for a tiny bottle of crushed, dried possum. So how do we fare in the pollen digestion?
To answer that question let’s look at the six major methods known for digesting pollen. The first three involve specialized mouth parts for cracking, crushing or piercing it. Since most people don’t chew each grain of pollen individually or have needle like teeth we’ll proceed to methods four, five and six, which are osmotic shock, pseudo germination or just flat out dissolving the sporopollenin and intine. Six is a long shot - only a few insects are capable of dissolving the sporopollenin (and most of the compounds that would digest it would digest us). That leaves osmotic shock and pseudo germination. Pseudo germination is possible in honeybees because the pollen is sitting in the nectar crop and exposed to warm dilute sugar – perfect for convincing a pollen grain it’s time to grow. The human digestive system features hydrochloric acid. In the spring when my wife starts our garden we have rows seed trays. “Soak generously in warm hydrochloric acid” never appears in the germination instructions on the seed packets, and for good reason. Even if we could get the pores to open the intine is made of cellulose and we are notoriously bad at digesting it. Without the nectar crop of a bee and the ability to induce pseudo germination we average digestion of 48% of pollen material. That leaves osmotic shock, and at least here we have good news. A pollen grain whose external barriers explode is every bit as digestible as a steak, even if it isn’t as tasty.
So honeybees have a specialized digestive tract for extracting the nutrients from pollen. Humans can gain nutritional content from raw pollen, but the number of intact and partially digested pollen grains at the end of our gut suggests we do a poor job of it. Compared to the honeybee we’re downright abysmal. I say let the bees have the pollen. You can’t pan fry pollen with butter, garlic, salt and pepper. The bees have no idea what they are missing.