Hydatellaceae: The Other Basal Angiosperms

Photo by Kevin Thiele licensed under CC BY 2.0

Though rather obscure to most of the world, the genus Trithuria has enjoyed somewhat of a celebrity status in recent years. A paper published in 2007 lifted this tiny group of minuscule aquatic plants out of their spot in Poales and granted them a place among the basal angiosperm lineage Nymphaeales. This was a huge move for such little plants. 

The genus Trithuria contains 12 species, the majority of which reside in Australia, however, two species, T. inconspicua and T. konkanensis, are native to New Zealand and India. They are all aquatic herbs and their diminutive size and inconspicuous appearance make them easy to miss. For quite some time these odd plants were considered to be a group of highly reduced monocots. Their original placement was in the family Centrolepidaceae. All of that changed in 2007.

Close inspection of Trithuria DNA told a much different story. These were not highly reduced monocots after all. Instead, multiple analyses revealed that Trithuria were actually members of the basal angiosperm lineage Nymphaeales. Together with the water lilies (Nymphaeaceae) and the fanworts (Cabombaceae), these plants are living representatives of some of the early days in flowering plant evolution. 

Of course, DNA analysis cannot stand on its own. The results of the new phylogeny had to be corroborated with anatomical evidence. Indeed, closer inspection of the anatomy of Trithuria revealed that these plants are truly distinct from members of Poales based on a series of features including furrowed pollen grains, inverted ovules, and abundant starchy seed storage tissues. Taken together, all of these lines of evidence warranted the construction of a new family - Hydatellaceae.

The 12 species of Trithuria are rather similar in their habits. Many live a largely submerged aquatic lifestyle in shallow estuarine habitats. As you may have guessed, individual plants look like tiny grass-like rosettes. Their small flower size has lent to some of their taxonomic confusion over the years. What was once thought of as individual flowers were revealed to be clusters or heads of highly reduced individual flowers. 

Reproduction for these plants seems like a tricky affair. Some have speculated that water plays a role but close inspections of at least one species revealed that very little pollen transfer takes place in this way. Wind is probably the most common way in which pollen from one plant finds its way to another, however, the reduced size of these flowers and their annual nature means there isn't much time and pollen to go around. It is likely that most of the 12 species of Trithuria are self-pollinated. This is probably quite useful considering the unpredictable nature of their aquatic habitats. It doesn't take much for these tiny aquatic herbs to establish new populations. In total, Trithuria stands as living proof that big things often come in small packages. 

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Further Reading: [1] [2] [3]

Meeting Amborella trichopoda

When I found out I would be seeing a living Amborella, a lump formed in my throat. There I was standing in one of the tropical houses at the Atlanta Botanical Garden trying to keep my cool. No amount of patience was ample enough to quell my excitement. How was I going to react? How big were these plants? Would I see flowers? Could I touch them? What were they growing in? My curiosity was through the roof.

Naturally this sort of excitement is reserved for those of us familiar with Amborella trichopoda. This strange shrub is not something that would readily stand out against a backdrop of tropical flora. However, if life history and ecology were to be translated into outward appearances, Amborella would likely be one of the most gaudy plants on this planet. What I was about the lay eyes on is the only member of the sole genus belonging to the family Amborellaceae, which is the sole member of the order Amborellales.

Even more exciting is its position on the angiosperm family tree. As flowering plants go, Amborella is thought to be the oldest alive today. Okay, so maybe this shrub isn't the oldest flowering plant in the world. It is likely that at one time, many millions of years ago, there were more representatives of Amborellaceae growing on this planet. Until we turn up more fossil evidence it is nearly impossible to say. Still, Amborella's place in the story of flowering plant evolution is consistently located at the base.

That is not to say that this shrub is by any means primitive. I think the first thing that shocked me about these plants is just how "normal" they appear. Sans flowers, I didn't see much out of the ordinary about them. They certainly look like they belong on our timeline. Without proper training in plant anatomy and physiology, there is little one could deduce about their evolutionary position. Regardless of my ignorance on plant morphology, there is plenty to look at on Amborella.

For starters, Amborella has tracheids but no vessel elements, making its vascular system more like that of a gymnosperm than an angiosperm. Its small flowers are borne in the axils of the evergreen leaves. It has no petals, only bracts arranged into a spiral of tepals. The female flowers consist of 4 to 8 free carpels and do not produce a style. Male flowers look like nothing more than a spiral cluster of stamens borne on short filaments.

If plant anatomy isn't enough to convince you, then the genetic analyses tell a much more compelling story. DNA sequencing consistently places Amborella at the base of the flowering plant family tree. Again, this is not to say that this shrub is by any means "primitive" but rather its lineage diverged long before what we would readily recognize as a flowering plant evolved. As such, Amborella offers us a window into the early days of flowering plants. By comparing traits present in more derived angiosperms to those of Amborella, researchers are able to better understand how the most dominant group of plants found their place in this world.

Another interesting thing happened when researchers looked at the DNA of Amborella. What they found was more than just Amborella genes. Inside the mitochondrial DNA are an unprecedented amount of foreign DNA from algae, lichens and mosses. In fact, an entire chunk of DNA corresponded to an entire mitochondrial genome of a moss! Researchers now believe that this is a case of extreme horizontal gene transfer between Amborella and its neighbors both growing on and around it. Both in the wild and in cultivation, Amborella is covered in a sort of "biofilm." Whether or not such gene transfer has assisted in the conservatism of this lineage over time remains to be seen.

At this point you may be asking how this lineage has persisted for over 130 million years. For the most part, it is probably due to chance. However, there is one aspect of its ecology that really stands out in this debate and that is its geographic distribution. Amborella is endemic to Grande Terre, the main island of New Caledonia. This is a very special place for biodiversity.

New Caledonia is a small fragment of the once great super-continent Gondwana. New Caledonia, which was part of Australia at that time, broke away from Gondwana when the super-continent began to break up some 200-180 million years ago. New Caledonia then broke away from Australia some 66 million years ago and has not been connected to another land mass since. A warm, stable climate has allowed some of the most unique flora and fauna to persist for all that time. Amborella is but one of the myriad endemic plants that call New Caledonia home. For instance, 43 species of tropical conifers that grow on these small islands are found nowhere else in the world. The whole region is a refugia of a long lost world.

Being a biodiversity hot spot has not spared New Caledonia from the threats of modern man. Mining, agriculture, urbanization, and climate change are all threatening to undo much of what makes this place so unique. The loss of a species like Amborella would be a serious blow to biodiversity, conservation, and the world as whole. We cannot allow this species to exist only in cultivation. New Caledonia is one place we must desperately try to conserve. Meeting this species has left a mark on me. Being able to observe living Amborella up close and personal is something I will never forget as my chances of seeing this species in the wild are quite slim. I am so happy to know that places like the Atlanta Botanical Garden are committed to understanding and conserving this species both in the wild and in cultivation. For now Amborella is here to stay. Long may it be that way.

Further Reading:

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Anise: An Angiosperm Success Story

Illicium floridanum Photo by Scott Zona licensed under CC BY-NC 2.0

I must admit there are few flavors I loath more than anise (and consequently licorice and fennel). Regardless of the flavor, I nonetheless find myself enamored by their whorled seed capsules of star anise. In an attempt to reconcile my feelings towards anise in a culinary sense, I decided to get to know the plants that are responsible for it and I am so glad that I did. As it turns out, this group of small trees and shrubs offer us a glimpse at some of the earliest branchings on the angiosperm family tree.

We get star anise from the genus Illicium. Native to humid tropical understories, there are roughly 40 species scattered around southeast Asia, southeastern North America, the Caribbean, and parts of Mexico. Molecular as well as fossil evidence suggests this group diverged during the mid to late Cretaceous, not long after flowering plants came onto the scene. Indeed, along with Amborella and Nymphaeales, Illicium represent the three lineages that are sister to all other flowering plants alive today.

Illicium henryi Photo by Scott Zona licensed under CC BY-NC 2.0

To call them primitive, however, would be a serious misnomer. Because they diverged so early on, these lineages represent serious success stories in flowering plant evolution. Instead, think of them as fruitful early experiments in angiosperm evolution. Illicium has characteristics that set it out as being sister to all other flowering plants. For instance, the vascular tissues more closely resemble those of gymnosperms than they do angiosperms. Also, like the other sister angiosperms, Illicium blur the line between the long standing categories of monocot and eudicot. As such, they are sometimes referred to as "paleoherbs." Another key diagnostic feature lies in their floral morphology.

They don't have what could be considered true petals or sepals. Instead, they have whorls of tepals, which start off sepal-like and gradually become more petal-like as you near the center of the flower. The stamens, which are laminar or leaf-like, are also arranged in a dense whorl surrounding a yet another whorl of fused carpels. Once fertilized, each carpel gives rise to a hard, glossy seed. As the carpels mature and begin to dry, the individual capsules get tighter and tighter until at some point the seed is pinched so hard that it is ejected from a slit in the fruit in projectile fashion.

Illicium verum. Photo by Tim Waters licensed under CC BY-NC-ND 2.0

Although this research will never rectify the taste of this spice, it nonetheless has given me a new found respect and sense of awe for this genus. To look upon the fruit of Illicium is to look at a biological structure that has stood the test of time. These plants are evolutionary successes that should be admired for their unique place in the story of flowering plant evolution.

Photo Credits: Scott Zona and Tim Waters

Further Reading: [1]