When life was in its early stages, trying every possible conformation, with every possible means available, nature soon figured out a way to perform essential processes in an efficient manner. Nature never aims for perfection. Changes happen, organisms evolve randomly and the best of the available options is selected,… that too on the basis of the selection pressure applied. Back then, organisms didn’t show much of an interaction.
Things gradually changed with the emergence of multicellularity ( which led to an efficient division of labour).
The motility problem
Natural selection soon chose sexual reproduction over asexual as it provides the essential mingling of alleles of same character, leading to a more “fit” individual ( which is purely a play of chance). This, sort of, left the non-motile organisms on the lagging side. This population consisted of all plants. In order for the gametes to fuse, the mating partners should, in some way, come in contact.
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Hiring an agent
Nature found an amazing way out of this problem. What if the temporarily inactive gametes were packaged as parcels to the site of action and then again turned active..? This marks the advent of pollination in the plant kingdom.
A general postal service requires a delivery agent. The agents for pollination are termed as pollinators. Pollination can be carried out by many agents, such as- wind, water, insects, birds, bats, snakes, elephants etc.
Insect pollinators include- bees, ants, beetles, flies, moths, butterflies, wasps, mosquitoes etc, and is the largest contributor.
Now let us have a look at some stats. Three-fourths of the world’s flowering plants and about 35 percent of the world’s food crops depend on animal pollinators to reproduce. So, in absence of it we may have some stuff removed from our kitchen.
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Male part ( androecium) of the flower forms male gametes (microgametophytes) which are stored as pollen inside bag-like sacs called pollen sacs. These pollen grains are to be transferred onto the receiving end, the stigma of the female part (gynoecium) of another flower, of the same species.
The pollen, when lands onto the stigma, forms a long tube piercing through the style or stalk of the gynoecium. Through this the male gametes (2 in number) make their way towards the ovum. Ovum has a peculiar geometrical arrangement of seven cells, whereby lie the different cells which form the future fruit (the fertilized ovary). It has an egg cell (megagametophyte) and six other subsidiary cells. Out of the seven cells, two get fused by the male gametes. Since two fusions happen at once, this event is also known as double fertilization.
Pollen adaptations (on the basis of pollinators)
All such pollination is carried out in species which have an in-built mechanism to produce flowers, which facilitate easy transfer of pollens by their respective pollinators. For example, anemophilous plants produce terminally located inconspicuous flowers with pollen sacs light enough to be ruptured by the stroke of wind and pollen light enough to be carried away. For hydrophilous plants, pollen grains are waterproof and, depending on whether it happens underwater or on its surface, it may have drifting or floating abilities respectively.
The plant doesn’t have to do much to persuade the carrier to do the job for it. However, that is not the case with plants having animal pollinators.
The pollen grains of entomophilous species are heavier, relatively larger, and are with various types of exine ornamentation. This pattern helps in the adherence of the grains to the appendages of the pollinating agents.
Some of the flavonoids that contribute to the viable colour of the pollen grains provide a fragrance or taste, which either encourages or discourages the pollinating insects.
The pollen transported by insects has rich electron dense homogenous pollenkitt spread all over the exine surface and this adhesive factor has an adaptive value. In other words, plant lures the insect towards its flower, provides a reward (nectar, fragrance etc) and loads its pollen onto it.
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Protecting the gamete
Pollen have a double wall. The two gametes are covered by a cellulose containing thin wall called intine, while the outermost wall is tough and resistant made primarily of sporopollenin ( one of the most chemically inert biological polymers ). The exine has spines and folds which are unique for a given species.
The pollen surface is covered with waxes and proteins. Exine, which prevents the pollen from shrinking and desiccation, is made up of two layers viz,. tectum and foot layer. The outer wall is constructed with a resistant bio-polymer called sporopollenin.
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With all this information, one should be thinking that… if all the pollen have a basic sketch then how does the female flower recognize the pollen from the male flower of its own species? A whole lot of chemical conversation starts as soon as the pollen lands onto the stigma. Female flower releases chemicals all over the pollen. If the pollen is of the correct type, the pollen tube formation begins,… if not, those chemicals degrade the pollen. This check-post validates female incompatibility with the pollen.
Let us suppose that there is a random mutation somewhere in a plant cell genome which changes a trivial but necessary character of its flower. Considering that the plant is entomophilous and the mutation causes some change due to which the pollinator is not able to recognize this particular flower.
Animal pollinators don’t just wander around lavishly and visit every flower they see. They have a basic pre-acquired information about the species they visit and have certain cues (either visible or chemical) so as to pick the correct one.
Now, getting back to our mutated plant. The small change will now lead to no pollinator visiting our plant. This will lead to no successive progeny of that plant. Now thinking on a much bigger and idealized scale. Consider there happened a catastrophe which led to wiping out of most plants of this species, except the ones which had the mutation we discussed about, which was also in some way responsible for their survival( the genes for the two cases are assumed to be linked). Since the plants which could have been pollinated are now wiped out, and the ones which can survive cannot be pollinated, the species will soon be extinct.
Animal pollinators are not too choosy, and so, have a string of preferences to collect nectar from. Let us now take a similar situation with a changed scenario, whereby the above conditions still prevail in addition that all other plants of that particular locality have also been wiped out. In such a case the pollinator would starve which may soon lead to extinction of its species. If it happens so, that there appears a variant in the species which visited our flower( remember, our mutated plant still produces nectar… it just cannot lure the pollinator in) and has somehow developed some new recognition for the plant, then these variants will get selected over others and the plant-pollinator relationship will now thrive again. This is what co-evolution speaks about.
Most of the plant-pollinator models show the affect of each of the candidates driving the evolution of the other.
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“Good things cost more”
So far so good. It seems a very well designed model of nature, which led to successful survival of the species in question. However, a thought comes to the mind saying that,… isn’t this “chance-based”?? Plants don’t have flowers all year round. They bloom only during their breeding season. What if the wind, for an anemophilous plant in a given region, didn’t blew enough for the pollen to reach another flower of its species.
Thus, in order to enhance its chances, plants produce a huge number of pollen grains. Thousands of pollen are kept ready for their pollinators to pick the cargo up for delivery. This may not seem much of a problem but producing all these pollen requires a lot of input of raw materials and energy,… something which is not easy to fetch. Thus, pollination turns out to be an expensive deal for the plant. Out of thousands of pollen produced, just a few may reach the destination.
Thus, there always is a competition going around among the flowers of the plants in our garden, whereby, the one that produces the more numbers may have a better chance of surviving and preserving its genome for upcoming generations.
Most of you might have an uneasy feeling about this. But, it is what it is.
So, a wasteful process with relatively low success rates??? For now,.. let’s leave it to nature. So far, this is how it happened and still goes on like that, even if the plant has to face some painstaking consequences.