Gymnosperms and Fleshy "Fruits"

Fleshy red aril surrounding the seeds of  Taxus baccata.

Fleshy red aril surrounding the seeds of Taxus baccata.

Many of us were taught in school that one of the key distinguishing features between gymnosperms and angiosperms is the production of fruit. Fruit, by definition, is a structure formed from the ovary of a flowering plant. Gymnosperms, on the other hand, do not enclose their ovules in ovaries. Instead, their unfertilized ovules are exposed (to one degree or another) to the environment. The word “gymnosperm” reflects this as it is Greek for “naked seed.” However, as is the case with all things biological, there are exceptions to nearly every rule. There are gymnosperms on this planet that produce structures that function quite similar to fruits.

Cross section of a  Ginkgo  ovule with red arrow showing the integument.  Photo copyright Bruce Kirchoff, Licensed under CC-BY

Cross section of a Ginkgo ovule with red arrow showing the integument.

Photo copyright Bruce Kirchoff, Licensed under CC-BY

The key to understanding this evolutionary convergence lies in understanding the benefits of fruits in the first place. Fruits are all about packing seeds into structures that appeal to the palates of various types of animals who then eat said fruits. Once consumed, the animals digest the fruity bits and will often deposit the seeds elsewhere in their feces. Propagule dispersal is key to the success of plants as it allows them to not only to complete their reproductive cycle but also conquer new territory in the process. With a basic introduction out of the way, let’s get back to gymnosperms.

“Fruits” of  Cephalotaxus fortunei  (Cephalotaxaceae)

“Fruits” of Cephalotaxus fortunei (Cephalotaxaceae)

There are 4 major gymnosperm lineages on this planet - the Ginkgo, cycads, gnetophytes, and conifers. Each one of these groups contains members that produce fleshy structures around their seeds. However, their “fruits” do not all develop in the same way. The most remarkable thing to me is that, from a developmental standpoint, each lineage has evolved its own pathway for “fruit” production.

Ginkgo  “fruits” are full of butyric acid and smell like rotting butter or vomit.

Ginkgo “fruits” are full of butyric acid and smell like rotting butter or vomit.

For instance, consider ginkgos and cycads. Both of these groups can trace their evolutionary history back to the early Permian, some 270 - 280 million years ago, long before flowering plants came onto the scene. Both surround their developing seed with a layer of protective tissue called the integument. As the seed develops, the integument swells and becomes quite fleshy. In the case of Ginkgo, the integument is rich in a compound called butyric acid, which give them their characteristic rotten butter smell. No one can say for sure who this nasty odor originally evolved to attract but it likely has something to do with seed dispersal. Modern day carnivores seem to be especially fond of Ginkgo “fruits,” which would suggest that some bygone carnivore may have been the main seed disperser for these trees.

“Fruits” contained within the female cone of a cycad ( Lepidozamia peroffskyana ).

“Fruits” contained within the female cone of a cycad (Lepidozamia peroffskyana).

The Gnetophytes are represented by three extant lineages (Gnetaceae, Welwitschiaceae, and Ephedraceae), but only two of them - Gnetaceae and Ephedraceae - produce fruit-like structures. As if the overall appearance of the various Gnetum species didn’t make you question your assumptions of what a gymnosperm should look like, its seeds certainly will. They are downright berry-like!

Berry-like seeds of  Gnetum gnemon .

Berry-like seeds of Gnetum gnemon.

The formation of the fruit-like structure surrounding each seed can be traced back to tiny bracts at the base of the ovule. After fertilization, these bracts grow up and around the seed and swell to become red and fleshy. As you can imagine, Gnetum “fruits” are a real hit with animals. In the case of some Ephedra, the “fruit” is also derived from much larger bracts that surround the ovule. These bracts are more leaf-like at the start than those of their Gnetum cousins but their development and function is much the same.

Red, fleshy bracts of  Ephedra distachya .

Red, fleshy bracts of Ephedra distachya.

Whereas we usually think of woody cones when we think of conifers, there are many species within this lineage that also have converged on fleshy structures surrounding their seeds. Probably the most famous and widely recognized example of this can be seen in the yews (Taxus spp.). Ovules are presented singly and each is subtended by a small stalk called a peduncle. Once fertilized, a group of cells on the peduncle begin to grow and differentiate. They gradually swell and engulf the seed, forming a bright red, fleshy structure called an “aril.” Arils are magnificent seed dispersal devices as birds absolutely relish them. The seed within is quite toxic so it usually escapes the process unharmed and with any luck is deposited far away from the parent plant.

The berry-like cones of  Juniperus communis .

The berry-like cones of Juniperus communis.

Another great example of fleshy conifer “fruits” can be seen in the junipers (Juniperus spp.). Unlike the other gymnosperms mentioned here, the junipers do produce cones. However, unlike pine cones, the scales of juniper cones do not open to release the seeds inside. Instead, they swell shut and each scale becomes quite fleshy. Juniper cones aren’t red like we have seen in other lineages but they certainly garnish the attention of many a small animal looking for food.

I have only begun to scratch the surface of the fruit-like structures in gymnosperms. There is plenty of literary fodder out there for those of you who love to read about developmental biology and evolution. It is a fascinating world to uncover. More importantly, I think the fleshy “fruits” of the various gymnosperm lineages stand as a testament to the power of natural selection as a driving force for evolution on our planet. It is amazing that such distantly related plants have converged on similar seed dispersal mechanisms by so many different means.

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8]

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

Aloe or Agave?

Aloe vs agave.jpg

Convergent evolution is the process by which unrelated organisms evolve similar traits in response to similar environmental constraints. One amazing example of convergent evolution has occurred among the Aloe and Agave. These two distinct lineages are separated both in space and time and yet they often look so similar that it can be hard for the average person to tell them apart. With that in mind, lets consider the similarities and differences between these two lineages.

To start, Aloe and Agave hail from two completely different spots on the botanical family tree. Each also has its own unique geographic origin. Agave is a New World genus with species ranging in their distribution from tropical South America north into arid portions of North America. Genetic analysis places the genus Agave in the family Asparagaceae.

Agave americana  in bloom

Agave americana in bloom

Aloe, on the other hand, enjoys an Old World distribution, from Africa and Madagascar to the Arabian Peninsula as well as many islands scattered throughout the Indian Ocean. Taxonomically speaking, Aloe has undergone more than a few revisions through time, however, recent genetic work suggests that the Aloe belong to the family Asphodelaceae.

Experts believe that the lineages that gave rise to these two distinct genera branched off from a common ancestor some 93 million years ago. Despite all of that intervening time and space, the rigors of their arid habitats have managed to shape these plants in strikingly similar ways. Morphologically speaking, there is a lot of superficial similarity between Aloe and Agave.

Aloe hereroensis in situ

Aloe hereroensis in situ

Both groups exhibit water-storing, succulent leaves arranged in rosettes. These leaves are often adorned with spines or other protrusions aimed at deterring herbivores. Both groups also utilize CAM photosynthesis for their energy needs. When it comes time to flower, both groups frequently produce brightly colored, tubular flowers arranged at the tip of long stalks.

It is worth noting that the harsh environments that have shaped these two plant lineages also seems to have induced a backup plan for reproduction. Both Aloe and Agave produce tiny offshoots called "pups." These pups gain nourishment from the parent plant until they are large enough to fend for themselves. All pups are clones but if the parent plant had what it takes to survive in that spot, there is a good chance that its cloned offspring will as well. That way, even if sexual reproduction fails, these cloned progeny will get another shot.

Despite all of this convergence, these two lineages nonetheless exhibit vastly different developmental pathways and thus there are plenty of differences separating the two. For starters, slice into the leaves of each type and you will quickly find one major morphological difference. As many already know, Aloe leaves are largely filled with a gooey pulp and not much else. Aloe leaves function as water storage organs. Agave also store plenty of liquid in their leaves, however, they also produce numerous long strands of fiber that provide much more structural integrity.

Cross section of an Aloe leaf showing gelatinous pulp.

Cross section of an Aloe leaf showing gelatinous pulp.

Agave leaf showing fibrous interior.

Agave leaf showing fibrous interior.

Aloe and Agave each have evolved their own reproductive strategies as well. Aloe are perennial bloomers. Under the right conditions, many Aloe species will produce a profusion of flower stalks year after year. The stalks emerge from between the leaves and are largely pollinated by birds and insects in their native habitats. Agave, on the other hand, are monocarpic meaning they invest all of their energy into one single bloom. The Agave flowering stalk emerges from the center of the rosette and are pollinated by myriad insects, birds, and even bats. After flowering is complete, the main Agave plant dies.

Aloe flowers

Aloe flowers

Agave flowers

Agave flowers

Convergent evolution will never cease to amaze me. Despite millions of years and hundreds of miles separating these two lineages, Aloe and Agave have nonetheless been shaped in similar ways by similar environmental conditions.

Photo Credits: Wikimedia Commons

Further Reading: [1]

Convergent Carnivores

A carnivorous lifestyle has evolved independently in numerous plant lineages. Despite the similarities between genera like Nepenthes, Sarracenia, and Cepholotus they are not closely related. Researchers have wondered how the highly modified leaves of various carnivorous plant species evolved into the insect trapping and digesting organs that we see today. Thanks to a recent article published in Nature, it has been revealed that the mechanisms responsible for carnivory in plants are a case of convergent evolution.

This research all started with the Australian pitcher plant Cepholotus follicularis. More closely related to wood sorrels (Oxalis spp.) than either of the other two pitcher plant families, this species offers a unique window into the genetic controls on pitcher development. Cepholotus produces two different kinds of leaves - normal, photosynthetic leaves and the deadly pitcher leaves that have made it famous the world over.

By observing which genes are activated during the development of these different types of leaves, the research team was able to identify which alleles have been modified. In doing so, they were able to identify genes involved in producing the nectar that attracts their insect prey as well as the genes involved in producing the slippery waxy coating that keeps trapped insects from escaping. But they also found something even more interesting.

Next, the team took a closer look at the digestive fluids produced by Cepholotus as well as many other unrelated carnivorous plant species from around the world. In doing so, the team made a startling discovery. They found that the genes involved in synthesizing the deadly digestive cocktails among these disparate lineages have a similar evolutionary origin.

Although they are unrelated, the ability to digest insects seems to have its origins in defending plants against fungi. You have probably heard someone say that fungi are more similar to animals than they are plants. Well, the polymer that makes up the cell walls of fungi is the same polymer that makes up the exoskeleton of insects - chitin. By comparing the carnivorous plant genes to those of the model plant Arabidopsis, the team found that similar genes became active when plants were exposed to fungal pathogens.

It appears that carnivorous plants around the world have all converged on a system in which genes used to defend themselves against fungal infection have been co-opted to digest insect bodies. Taken together, these results show that the path to carnivory in plants is surprisingly narrow. Evolution doesn't always require the appearance of new alleles but rather a retooling of genes that are already in place. 

Photo Credits: [1] [2]

Further Reading: [1]