Trees In Spring

Spring is a wonderful time to observe trees. After a long, dreary winter they burst into action. For many species, spring is the time for reproduction.

Species in this episode:

-Serviceberry (Amelanchier sp.)

-Norway maple (Acer platanoides)

-Eastern redcedar (Juniperus virginiana)

-Sugar maple (Acer saccharum)

-Saucer magnolia (Magnolia x soulangeana)

Producer, Writer, Creator, Host: Matt Candeias (http://www.indefenseofplants.com)

Producer, Editor, Camera: Grant Czadzeck (http://www.grantczadzeck.com)

Pollen Competition

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The animal kingdom is rife with sexual conflict. We are all aware of what is going on when two stag deer lock antlers or when a group of male sage grouse flaunt themselves on leks as females look on. But what about plants? Is there sexual conflict among plant species? Whether pollen ends up on a stigma via wind or animal, is there any way for a plant to "choose" who gets to fertilize the ovule?

It turns out, yes, there is. Sexual competition is part of the pollination process. In fact, some of the most familiar floral morphologies may have evolved as a way of weeding out weak paternal lines. To understand this process better, though, we must first quickly review exactly what goes on during pollination.

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Pollen is a male gamete. Each grain is haploid and contains only a single copy of a plant’s chromosomes. When a pollen grain lands on a stigma, the grain germinates like a tiny seed, sending down a root-like growth called a pollen tube. This tube grows down into the ovary until it finds an unfertilized ovule. At this point, sperm travels down the pollen tube where it can unite with the ovule, thus forming a seed.

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It’s the formation of this pollen tube that introduces the idea of competition among pollen grains. Again, whether by wind or animal, the pollen arriving to a new plant generally doesn't come from a single individual. Pollen from many potential paternal lines can arrive all at once. As such, the race to fertilize the ovules can be quite intense, and this is where competition begins.

Remember, pollen only contains a single set of chromosomes from the parent plant, thus all alleles, both functioning and deleterious, are represented. During the growth of the pollen tube, upwards of 60% of the pollen genome is actively transcribed. Any pollen containing lots of deleterious alleles is going to have a much harder time competing with pollen grains that have fewer deleterious alleles. Their tubes have a harder time making it to the ovules in time to fertilize them.

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It is thought that the length of the style (the stem connecting the stigma to the ovaries) may also provide a sort of "proving ground" for pollen too. For instance, picture the flowers of a lily or a mallow. Those long, slender styles may actually be acting like a race track. Only the pollen with the best selection of genetic material will be able to grow their pollen tubes fast enough to reach the ovules, leaving the weaker competition in the dust. In this way, plants may actually be sorting out stronger paternal lines, which makes sense for sessile organisms that can't see.

As with everything in nature, there is far more nuance to this than what I have outlined above. Much work is being done to test some of the earlier assumptions and data surrounding this concept of pollen competition. It certainly happens but the degree to which any given species utilizes such methods is up for debate. Still, it paints a much more interesting picture of mate selection in plants. Static, plants are not!

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4] [5] [6] [7]

 

How Plants Influence Honeybee Caste System

Is has long been known that food fed to larval honeybees influences their development and therefore their place in the hive. Larvae fed a mixture of pollen and honey, often referred to as "bee bread," develop into sterile workers whereas larvae fed special secretions termed "royal jelly" from nurses within the colony will develop into queens. Despite this knowledge, the mechanisms underpinning such drastic developmental differences have remained a mystery... until now.

A team of researchers from Nanjing University in China have uncovered the secret to honeybee caste systems and it all comes down to the plants themselves. It all has to do with tiny molecules within plants called microRNA. In eukaryotic organsisms, microRNA plays a fundamental role in the regulation of gene expression. In plants, they have considerable effects on flower size and color. In doing so, they can make floral displays more attractive to busy honeybees.

Photo Credit: [1]

Photo Credit: [1]

As bees collect pollen and nectar, they pick up large quantities of these microRNA molecules. Back in the hive, these products are not distributed equally, which influences the amount of microRNA molecules that are fed to developing larvae. The team found that microRNA molecules are much more concentrated in bee bread than they are in royal jelly. Its this difference in concentrations that appears to be at the root of the caste system.

Larvae that were fed bee bread full of microRNA molecules developed smaller bodies and reduced, sterile ovaries. In other words, they developed into the worker class. Alternatively, larvae fed royal jelly, which has much lower concentrations of microRNA, developed along a more "normal" pathway, complete with functioning ovaries and a fuller body size; they developed into queens.

All of this hints at a deep co-evolutionary relationship. The fact that these microRNA molecules not only make plants more attractive to pollinators but also influence the caste system of these insects is quite remarkable. Additionally, this opens up new doors into understanding co-evolutionary dynamics. If horizontal transfer of regulatory molecules between two vastly different kingdoms of life can manifest in such important ecological relationships, there is no telling what more is awaiting discovery. 

Further Reading: [1]

 

Why We Shouldn't Rag on Ragweed

Common ragweed (Ambrosia artemisiifolia), the bane of hay fever sufferers. This could quite possibly be one of the most despised plants whether people realize it or not. It is ragweed, not goldenrod, that is responsible for causing hay fever. All this is thanks to the copious amounts of pollen it wafts into the breeze. With all that being said, I could not call this In Defense of Plants if I did not come to the defense of ragweed.

Despite all the suffering it causes, ragweeds are enormously important plants ecologically. We already know they produce a lot of pollen, but that pollen is doing more than just making you stuffy and fertilizing other ragweeds. It is also feeding bees. Because it flowers so late into the season, ragweed offers up a prodigious source of protein-rich pollen for bees gearing up for fall and winter. Even before they flower, ragweed is a valuable food source for the caterpillars of many butterflies and moths including species like the wavy-lined emerald and various bird dropping moths. It's not just insects either. The seeds of ragweed are rich in fatty oils. Birds and small mammals readily consume ragweed seeds to help fatten up for the lean months to come.

Ragweed also offers us some cultural significance too. Before European settlement, ragweed is believed to have had a much narrower distribution. Palynologists use pollen taken from lake and bog sediment cores to track ancient climates and plant communities. Because ragweed produces so much pollen, it is a useful species to look for when studying core sediments. As pollen falls out of the air and settles on lakes or bogs, it eventually sinks to the bottom where it can remain buried in a rather pristine state for millennia. Palynologists have actually been able to use ragweed pollen as a way of tracking the settlement history of North America. As colonies advanced further and further, they opened up huge chunks of land, inadvertently creating ample opportunities for ragweed to expand its range. As such, ragweed pollen taken from lake cores has proven to be a pretty precise clue for studying our own history.

For as much as we despise it, ragweed thrives on the kind of disturbance that we humans are so good at creating. We are the ones to blame for our own suffering when it comes to hay fever, not the plants.

Photo Credit: Frank Mayfield

Further Reading:

http://bit.ly/2c2HpOG

http://bit.ly/2c7hx6X

http://bit.ly/2c6mtsh

http://bit.ly/2bRPf2T

http://bit.ly/2c7hrwi

Throwing it to the Wind

Though many of you may be cursing this fact, in the temperate regions of the north, wind pollinated trees are bursting into bloom. Their flowers aren't very showy. They don't have to be. Instead of relying on other organisms for pollination, these trees throw it to the wind, literally.

It is an interesting observation to note that the instances of wind pollinated tree species increases with latitude and elevation. This makes a lot of sense. It is most effective in open areas where wind is at its strongest. That is why many wind-pollinated trees get down to business before they leaf out.

 

 

 

The fewer obstructions the better. Also, pollinators can be hard to come by both at high elevation and high latitudes. Therefore, why not let the wind do all the work? This is also why wind-pollination is most common in early succession and large canopy species. Similarly, this is also why you rarely encounter wind-pollinated trees in the tropics. Leaves are out year round and pollinators are in abundance.

Without pollinators, wind-pollinated trees don't need to invest in showy flowers. That is why they often go unnoticed by folks. Instead, they pour their energy into pollen production. Your irritated sinuses are a vivid reminder of that fact. Wind pollination is risky. It relies mostly on chance. Therefore, the more pollen a tree pumps out, the more likely it will bump into a female. However, some trees like red maples (Acer rubrum) combine tactics, relying on both wind and hardy spring pollinators for their reproduction.

Whether you love this time of year or dread it, it is nonetheless interesting to see how static organisms like trees cope with the difficulties of sexual reproduction. I enjoy sitting in my yard and watching pines billow pollen like smoke from a fire. If anything, it is a stark reminder of how important sexual reproduction is to the myriad organisms on this planet.

Further Reading:
http://bit.ly/1qnRUm2

The Crazy World of Cycad Sex

When we think about plants, swimming ability generally doesn't come to mind. As kids we learn that one of the major differences between plants and animals is that plants generally can't move on their own volition. Certainly there are exceptions to this rule - sensitive plants and Venus flytraps to name a few. However, there are plants out there in which swimming is a crucial component of their life cycle. Though it isn't the plant itself that does the swimming, some of the ancestral plant lineages alive today have motile sperm!

Swimming sperm is a throwback to the early days of plant evolution. Because they arose from aquatic algae, a sperm's ability to swim to an ovule helped increase the chances of reproduction. Today we see this adaptation in plants like liverworts, mosses, and ferns, which still require water to complete their life cycle. However, swimming sperm are not restricted to the cryptograms. This adaptation also can be found in cycads (as well as ginkgoes). Their sperm are super strange too. They look like little seeds covered in concentric rings of beating flagella. Unlike cryptograms, however, their swimming ability doesn't come into play until pollen comes into contact with the ovule.

Cycads are either male or female. Each produces cone-like structures called strobili. This is where the magic happens. When pollen from a male plant finds its way onto the ovule of a female, it does something quite strange. It fuses with the ovule and begins to grow. In essence it acts almost like a parasite, sucking up nutrients from the ovule tissue and destroying it in the process. This is okay because once this happens, these tissues soon become obsolete. What matters is the female gametophyte, which is embedded inside the ovule.

The pollen begins to grow a tube down into the ovule. Once it has gained enough energy, the pollen will then burst and release its sperm. This is where the flagella come in. Each sperm is like a tiny submarine, capable of swimming around inside the ovule until it locates the female gametophyte. Then and only then is fertilization accomplished. Pretty wild for an otherwise sessile organism, wouldn't you say?

Photo Credit: http://www.slideshare.net/ericavanet…/lab-5-origin-of-plants

Further Reading:

http://bit.ly/20Wt7CZ

http://bit.ly/20Wt9L3

Mighty Mighty Squash Bees

It's decorative gourd season, ladies and gentlemen. If you are anything like me then you should be reveling in the tastes, smells, and overall pleasing aesthetics of the fruit of the family Cucurbitaceae. If so, then you must pay your respects to a hard working bee that is responsible for the sexual efforts of these vining plants. I'm not talking about the honeybee, no no. I am talking about the squash bees. 

If we're being technical, the squash bees are comprised of two genera, Peponapis and Xenoglossa. They are not the hive forming bees we generally think of. Instead, these bees are solitary in nature. After mating (which usually occurs inside squash flowers) the females will dig a tunnel into the ground. Inside that tunnel she places balls of squash pollen upon which she will lay an egg. The larvae consume the protein-rich pollen as they develop. 

The story of squash bees and Cucurbitaceae is a North American story. Long before squash was domesticated, these bees were busy pollinating their wild relatives. As a result, this bee/plant relationship is quite strong. Female squash bees absolutely rely on squash flowers for the pollen and nectar needs of their offspring. In fact, they often dig their brood tunnels directly beneath the plants. 

Because of this long standing evolutionary relationship, squash bees are the best pollinators of this plant family. The flowers open in the morning just as the squash bees are at their most active. Also, because they are so specific to squash, the squash bees ensure that pollen from one squash flower will make it to another squash flower instead of an unrelated plant species. Honeybees can't hold a candle to these native bees. What's more, crowds of eager honeybees may even chase off the solitary squash bees. For these reasons, it is often recommended that squash farmers forgo purchasing honeybee hives for their crops. If left up to nature, the squash bees will do what they are evolutionarily made to do. 

Photo Credit: MJI Photos (https://www.flickr.com/photos/capturingwonder/4962652272/)

Further Reading:
http://www.researchgate.net/profile/Victor_Parra-Tabla2/publication/226134213_Importance_of_Conserving_Alternative_Pollinators_Assessing_the_Pollination_Efficiency_of_the_Squash_Bee_Peponapis_limitaris_in_Cucurbita_moschata_(Cucurbitaceae)/links/549471010cf20f487d2a95b8.pdf

http://www.jstor.org/stable/25084168?seq=1#page_scan_tab_contents

http://extension.psu.edu/plants/sustainable/news/2011/jan-2011/1-squash-bees