Monday, February 25, 2019

Pollination talk Part I The dance of co-evolution

The goal of every living organism, including plants, is to create offspring. 


Plants can reproduce vegetatively by pupping or sexually through seeds. While pupping produces genetically identical clones (vegetative reproduction), seed production is based on the recombination of genetic material that is important for evolution and adaptation and increased fitness. 



The sexual production of plant offspring requires Pollination. Pollination is the act of transferring pollen grains from the male anther of a flower to the female stigma. During fertilization genetic material from pollen enters the ovules in the ovary to produce the embryo that will be the central part of the seed.
Optimal recombination of genetic material occurs if pollen and ovules are from different plants (of the same species of course) - cross pollination.
Pollen can be transferred by wind (in grasses for example) and water (Abiotic Pollination), but most commonly pollination involves the help of animals as vectors (Biotic Pollination)


Pollination by Paul Mirocha, modified
Animals from several different groups act as pollinators. In our Southwest Desert the most important mammals may be two bat species that pollinate Saguaro flowers at night, here the lesser Long-nosed Bat. I fast fligh-by and hovering moves, bats lick nectar from the flowers. They also feed on some of the pollen that dusts their fine fur - they occasionally brush it into their mouths with their paws.



Others, like this squirrel, also do their share - as they should, because they will later happily gorge on the cactus fruit.



Birds are also excellent pollinators, with their easy mobility and taste for sweet nectar. Woodpeckers, Doves, Verdins, Orioles, and Hummingbirds all contribute.



Here are White-winged Doves, Gila Woodpecker, Oriole and Costa's Hummer at work. The WW Dove is probably one of the most important day-time pollinators of saguaro flowers. Flowers for bird and mammal pollinators tend to be sturdy and strong.


However, most flowers evolved to be pollinated by insects. In the Cretaceous age paleontologists find a simultaneous sudden radiation of Angiosperm plants and insect species that have developed to be pollinators. 


From: When did Angiosperms first evolve by Bruce Cornet and Labandeira et al. 2001
Around 175 million years ago, many insects changed from feeding on all kinds of plant material to selectively seeking out, and specialize on, protein rich pollen and energy rich nectar of plants. 
Then 100 million years ago, we see the species numbers of both angiosperm (flowering) plants and of insects suddenly explode. What happened?

Radiation Hypothesis
 Besides several other factors like changes in climate, the evolutionary advantage of insect facilitated gen-recombination became a driving factor of angiosperm speciation. Therefore, speciation was primarily driven by the development of new floral characters. Why? Plants  developed advertisement in form of showy flowers that attracted pollinators. This involved: 

-concentrating nectaries in the same area as sexual organs
-flower petals in colors easily seen by insects, often   with marks in the UV spectrum range that we   cannot see.
-Clustering of flowers to make them more visible
-Fragrances that highly olfactory-oriented insects   can  follow
-producing extra pollen, even pseudo-pollen
-offering other chemicals that insects may need 

To further encourage cross pollination:

-complex flower architecture 
-reactivity to certain buzz-frequencies
-producing insect-species specific scent molecules
The last three points serve to favor some visitors over others, essentially limiting access



From the Insects Point of View
Let's be very clear: Insects,  even honey bees, are never driven by the idea to service plant pollination, human agriculture or bee keepers' honey pots. They are in it for exclusively their own goals: the production of offspring of their own kind. 



Insect visit flowers to find nectar to fuel their flight muscles, pollen as protein for egg production or nursery provisions, to find insect prey, to find mating partners, to lay their eggs for their larvae to feed on the developing seeds, to lay their eggs to give their larvae access to other flower visitors as hosts, or for protection and cover. Plants adapted and capitalized on these instincts to use them for their own advantage.

Target: Cross-pollination
So insects visit flowers. But plants would not benefit if insects were just indiscriminately moving from flower to flower, with no regard to the plant species, thus spreading valuable pollen to flowers of different species and in fact wasting it. 

Exclusivity may be the most generally used tool that plants employ to keep their pollen from being wasted indiscriminately.
Flower shapes, colors, opening hours, buzz-triggered pollen release, and chemicals limit access to only specific faithful visitors, who then concentrate on just one plant species and thus reliably cross pollinate its flowers. (Anything that I am describing here as if there were strategical adaptations was of course completely based on a long process of random mutations and natural selection)   


The sphinx moths has a proboscis long enough to reach the nectar in the long spurs of the delphinium flower
To narrow the field of visitors, many flowers developed complicated shapes. Often we find actual physical barriers like deep, narrow throats, to only admit certain visitors with especially long mouth parts or small bodies. In response, some insect species developed longer and longer mouth parts to reach the nectar source.  I will come back to this later as this adaptation might pose an additional challenge to the plant.
Anthophora Bee has learned to land on the Astragalus flower in the 'intended' way. She gets access to rewards, while making close contact with anthers and stigma of the flower
Architectural challenges have a direct effect on pollinator faithfulness.
Many flowers respond to insects of the right shape and weight by moving their anthers and stigma against the body of the landing visitor, but only if the visitor approaches correctly.

The pollen color on the bee's leg is that of the Aloe that she is visiting, prove that she stayed with the same species 
 Even our broad generalist, the honey bee, is caught in this net. In this case the adaptation involves the behavior of the bees. Any but the simplest flowers pose a learning problem for the worker bee. So after solving the puzzle of a new nectar source, all bees of a given hive usually stick with that same type of flower as long as they can. Because constantly learning the secrets of different flower types would waste time and energy. So if you see a lot of bees with pollen packages of just the characteristic color of any particular plant, the strategy obviously worked.


Nectar thieves: Lasioglossum bee bites a hole into a manzanita flower, Nomada bee gets nectar through it. Photos Bob Parks
Of course, many bee species prove that they are not that easily outmaneuvered. They learned to bite through the side of flower's throat and get at the nectar without ever pollinating the flower. As soon as those holes are opened up, a number of other thieves uses them as well. In this case it's a male of a parasitic species: definitely not a pollen collector, just someone who needs fuel for his wing muscles.  These observations illustrate my point: Insects do not care about the needs of the plant.


The promise to make them more attractive to females makes Eupatorium flowers irresistible to  Danaini males
Enabling
To come back to another one of my slightly 'racy' labels for the plant's manipulative adaptations: 
Many flowers offer more than just sweets and proteins. The nectar of some Asteraceae  contains chemicals that Queen and Monarch butterflies use in their courtship. Blue Mist (Eupatorium sp.) offers
Pyrrolizidine Alkaloids that are absorbed by the male. He then sends out alluring chemical signals by way of his scent patches. The female will prefer to mate with this male because he is going to transfer the alkaloids to her with his sperm package. She will include them with her eggs. The chemicals will protect eggs and young larvae from predation.  (Older larvae will feed on toxic milkweed and gain their own chemical defense.) 


 The neotropical Orchid Bees add another example of a similar strategy: Male euglossine bees collect fragrant oils from the surface of the orchid flowers, dragging pollen from flower to flower in the process. They store the oils in specialized hollow pockets in their hind legs. These oils are then used to attract mates. Orchids pollinated by male euglossine bees have powerful, often resinous fragrances. 

Deception
Let's stay with orchids. As one of the most species rich groups of angiosperms (28000, as much as mammals, birds and reptiles together), they offer great examples of adaptive pollinator manipulation. In fact, I could probably use orchids for about all the concepts that I want to introduce here. 
In Europe we have a group of orchids in the genus Ophris that have small intricately patterned flowers which even to the human eye resemble bees or bumble bees.



They are velvety like a real bee's body, seem to have eyes, legs and antennae, a reflective bright spot where the wings would be ... and male bees find them irresistible and try to mate with them. But for the deception to work, those males have to land and try the mating act at least twice, once to pick up the sticky pollen packages and again to deposit them. In fact, the males seem to prefer the orchid flowers to their own females. So what makes the flowers so super-sexy? Looks alone? 



No. Bee orchids also emit just the right, bee-species-specific pheromone. This could be one of the examples that intelligent-design-believers use to prove their point. Much too complex to have developed by accidental mutation, isn't it? Not really. Plants and insects alike produce a number of simple hydro-carbons to protect their surface areas from dehydration. Those compound molecules are species-specific and function in many insects as pheromones. Given the high number of possible target species (insects) and the orchids' tendency to mutate, matches are not unlikely to happen (over thousands of generations) Once the plant's output matches a pheromone, the selective advantage is great because the result directly affects the orchid's offspring production. So this mutation is definitely a good basis for a new successful species. There may be further mutations/selection that may make the orchid even more bee like or maybe perfect the pollen-glue mechanism, resulting in a new orchid species, optimally matched to one dedicated pollinator species.  Perfect for  cross-pollination. But this tight dependency makes the orchid very vulnerable to any declines of the bee species. It's a fragile situation that often results in extinction.  thanks to: Orchid Pollination, Exploring a fascinating world. by Ron Mchatton    



Not only male insects fall for deception by plants seeking pollinators. I our backyard, every autumn the huge flowers of a Starfish Cactus open. In truth, Stapelia gigantea is no cactus at all but a succulent South African member of the Apocynaceae. That it finds pollinators here in AZ at all means that its strategy is not as narrow as that of the Ophris orchids.
The pinkish, veined flowers look like pieces of decaying skin, even complete with a fringe of fur, and they also smell the part. The flowers emit H2N(CH2)4NH2, called putrescine, and H2N(CH2)5NH2, called cadaverine,  foul-smelling compounds usually found in decaying flesh. Since they are derivatives of the Purine metabolism, it is not surprising that plants can generate them (Purine is part of the metabolic pathway to build or degrade several amino acids).
Smell and looks attract green Bottle Flies and Flesh Flies that lay their eggs and larvae into the center of the flower as if they had indeed found decaying flesh. Of course their larvae are bound to starve. But the flies visit more than one flower, so their role as pollinators is fulfilled. Interestingly, braconid wasps that are parasites of flesh flies and ants that feed on the flies' eggs and larvae are also soon around.    



Deception and entrapment The Southwestern Pipevine Aristolochia watsonii should really be called Mouse-Ear plant. Not only does its fluted flower look somewhat like the ear of a (rather big) rodent but it also emits a musty smell that is supposedly similar to that of a mouse ear. This is the olfactory stimulus that attracts  flies of the family Ceratopogonidae (Biting Midges). They are small, blood sucking insects, some of the no-see-ums that pester humans and other animals on warm, humid summer days. They often seek access to the blood vessels in rodent ears that are especially highly vascularized in desert species (thermoregulation!).
 Drawn by odor and ear-like shape of the flower, and expecting a blood meal, the midges enter the flower. The shape of the flower and inward-directing hairs in its narrowing throat trap the flies temporarily, often over night when pollen release is at its peak. In their attempts to escape the flies dislodge pollen and transfer pollen they may have already brought with them to the stigma. In the morning the hairy barriers wilt and pollinated flower releases the captives. Because the flowers provide super-stimuli, the flies' instinctual reaction is to fall for the same deceit over and over.

Addiction:

I mentioned earlier that I'd come back to the long-throat-long proboscis coadaptation of flowers and insects. Here is our Sacred Datura and its main pollinator, a Manduca moth. If you pull out the moths proboscis, it can reach easily 7 inches. The throat of the manduca is not that deep. So the moth could just hover and sip through its straw. Not a lot of pollen transfer would occur. But instead, many moths take the risk to land, crawl into the flower, actually wallow in there. Then, dusted in pollen, they eagerly fly to the next Datura flower. In fact their eagerness is so great that at dusk, several moths can be found waiting in front of not quite unfurled flowers. Datura  is a chemical powerhouse that produces potent Alkaloids. It is assumed that chemicals in the nectar may be slightly addictive and thus keep the moths faithful -  assuring that precious pollen reaches its goal and does not get wasted. It also helps that Manduca caterpillars are immune to the negative effects of those alcaloids, which protect their preferred food from other herbivores and themselves from predators. So the pollinating moths can lay their eggs right on their favorite plant to produce a new generation for this symbiotic relationship.

Codependency:



Some flowers are pollinated by insects that also lay their eggs into the flower so their larvae will feed on the growing seeds. They usually do not destroy all of the seeds that they helped generate, so the relationship is mutually beneficial. In the case of the chicory moth, you see that the adults look just like 'their' flowers, so the relationship is probably a close one, at least from the side of the moth.  


More extreme forms of Codependency evolved in Yuccas and their Moths, and Figs and their wasps


The female of the Yucca moth, (genus Tegeticula) gathers pollen from one flower, rolls it into a ball, flies to another flower, inserts the pollen mass into the stigma. This is the most 'purposeful' act of pollination I ever heard of.  The female also lays 4 or 5 eggs in the ovary.  The caterpillars live in the developing fruit and eat about half the approximately 200 seeds produced by the plant. The yucca can be fertilized by no other insect, and the moth can utilize no other plant (obligate pollination mutualism ). There are 4 species of Yucca moths. One of them can supposedly also utilize agaves, one is a cheater that lays eggs but does not pollinate.

Another example of obligate pollination mutualism
Fig wasp - Fig 


Hundreds of fig flowers are arranged on the inside of a syconium.
After entering a young fig syconium, female wasps lay their eggs in individual flowers, in the process pollinating them. After a few weeks adult wasps emerge from each minute fruit inside of the syconium and mate. Males, in an unusual exception to the pattern seen in most sexually dimorphic insects, are blind and wingless, and their only function is to inseminate females and help them get out of the mature fig through holes chewed into the walls. Then the males stay behind and die while the females go out into the world to find more young figs to start the process again.  Figs and their wasps are so closely dependent on each other that only certain wasp species service certain fig species.  We mostly eat  'Mission Figs' introduced by the Spaniards  from the Mediterranean. Most of them produce unfertilized fruit due to the lack of 'their' wasps here in the US. So you are not necessarily eating little wasps when you feast on figs. But at least a couple of European fig wasps can be found in California by now.

I introduced the examples of these special pollinator-plant relationships because I think that they explain best the concept of co-adaptation between insects and plants.
But of course there are many plants that are simple and open to all kinds of generalist pollinators. Or at least seem to be. 

Insects of most orders regularly visit flowers. 
But not all flower-visiting insects are good pollinators. 
First condition: They need to be mobile enough. 



An assassin bug of the genus Phymata  may never leave its chosen ambush location where he is hidden by camouflage and immobility. Not a pollinator.



 Honeypot Ants certainly seek out flowers (and extra-floral nectaries) but their action radius may also be too small and their bodies too smooth.



If flower visitors are very mobile, but keep themselves clean of pollen by standing high on stilted legs and use la long proboscis to get to the nectar, they may also not be ideal pollinators. Many butterflies fall into this group, as much as butterfly lovers try to call them pollinators.



Some flowers may have adapted to the stand-offishness of butterflies - note how anthers and pistil of this Mexican Bird of paradise are reaching out.



Many beetles seem to be excellent pollinators, even if they are more the mess-and-fess kind. They fly well, are often quite hairy, and often show a tight relation to 'adult host plants'. 



While most beetles have biting-chewing mouth-parts and may feed on pollen or other flower parts, a few, like this blister beetle in the genus Nemognatha have specially adapted elongate mouth-parts to get to the nectar of tiny asteraceae florets. 



Beetles, in the most divers order of all, come in all sizes and can service many different types of flowers. Hundreds of  Soft-winged Flower Beetles (Melyridae), Carpet beetles, and Bean and Pea Weevils (Bruchinae) can be found on mesquite catkins.  (Bruchins will lay their eggs so the larvae will develop in the mesquite beans.)



Many larger beetles with unspecialized mandibles prefer the simple architecture of generalist flowers. These often draw attention on mountain meadows because they are arranged in big clusters - easy to find and easy to land on. They also offer access to a lot more nectar than a stand-alone flower and may keep pollinators 'faithfull' that way. Heracleum on Mount Lemmon and Elder Berry are local examples. 



Wasps, butterflies and flies are also drawn to the big white umbels of Heracleum in the mountains. Nothing else really competes with this showy display, so even these generalist pollinators have little reason to stray and little pollen is wasted 


Many flies are excellent pollinators, highly mobile, with specialized mouthparts to suck nectar from complex flowers and often covered in setae that catch and carry pollen from flower to flower.



Bee Flies and many Syrphids resemble (honey) bees in shape and behavior so they are often mistakenly identified as such. This may partly be mimicry, as bees are armed and flies are not, but I think that similar nectaring behaviors also contributed to the expression of similar adaptive features.


Pseudomasaris vespoides Pollen Wasp
Generally wasps eat meat (other bugs) and bees eat pollen. Pseudomasaris vespoides is one exception. Unlike most other wasps, this pollen wasp species collects pollen from penstemon flowers to feed it’s young. Many other wasps are nectar feeders for their own use (not for their larvae) So they visit a lot of flowers and pollinate in the process.

continue with Pollination talk, Part II Bees, Master Pollinators and Pollen Pigs


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