The succulent and climbing members of the milkweed family (Apocynaceae) have been gaining a lot of popularity among houseplant growers and for good reason. These wonderful plants produce some of the most elaborate flowers most of us will ever encounter and many of them smell quite strongly. Whereas houseplant enthusiasts recognize multiple genera of these spectacular plants, recent taxonomic work suggests lumping them all into one single genus - Ceropegia.
Such a massive taxonomic move has caused its fair share of drama. Folks seem to get quite ornery when it comes to shifts in nomenclature, especially when it involves this many species. However, when you dive into this group of plants, you really start to see how shaky the ground was that supported the previous classification systems. Evolution, after all, is not a neat and tidy process and we can learn a lot from the succulent and climbing asclepiads regarding the importance of combining morphological and genetic data into our taxonomic decisions.
Botanists have been obsessing over this group for decades. Historically speaking, four major groups have been recognized: those with caudiciform stems (genus Brachystelma), the stem succulent stapeliads (which include genera such as Stapelia, Huernia, Orbea, Caralluma, and others), the climbers (genus Ceropegia), and the so-called early divergent group (which include genera such as Anisotoma, Conomitra, Dittoceras, and others). Together these groups total something like 762 species and represent the tribe Ceropegieae.
The taxonomic status of the various members of Ceropegieae have always been up for debate. Early work was based on surprisingly few species and relied heavily on morphological characters such and corolla shape, stem anatomy, and pubescence. Since the 1950’s, many more species have been discovered and that is where a lot of the trouble began. Much of the early characters that were used to draw lines between various groups were suddenly blurred. Genera were created and absorbed by various authors in an attempt to get a handle on how this tribe evolved.
Things got even more complicated as various stapeliads and Ceropegia attracted the attention of horticulturists. As new species became available, many varieties were haphazardly named and genera such as Stapelia were further split to accommodate some of the peculiar nuances in floral shapes, colors, and sizes. It wasn’t until some genetic work was done that the need for a major overhaul of the Ceropegieae tribe became apparent.
Unfortunately, this early molecular work suffered from low resolution. Very few genera were used and among those, only a handful of gene regions were analyzed. Still, the picture that was developing was that the historical understanding of Ceropegieae was surprisingly misleading. For instance, the genera that made up the stapeliad group appeared to be nested quite firmly within the genus Ceropegia. Though equally as limited in scope, consecutive work in the early 2000’s added further evidence to the idea that the four groups that made up Ceropegieae were so genetically similar that most should be nested somewhere within Ceropegia.
Though not without controversy, this early molecular work convinced enough taxonomists to take a closer look at each of the four groups. With more resolution and a finer grasp on the diversity in form of these plants, taxonomists started to question the validity of some taxa. Indeed, the closer anyone looked, the more the lines between genera started to blur.
For example, Ceropegia and Brachystelma have long been separated on the basis of floral structure. Ceropegia were considered to adhere to a single corolla structure involving long, tubular flowers whereas Brachystelma were thought to be more variable in form. The discovery of new species clearly demonstrates that there are far too many exceptions to this system for it to be valid.
Such is also the case for other anatomical features such as whether plants climb or not. Again, there are plants in both genera that deviate from these patterns, thus making it impossible to nail down any set of characters that maintain the split between these two genera. Also, it would seem that some authors were trying to pull a fast one on readers. Back in 2007, Meve and Liede-Schumann claimed there were “a wide array of morphological features” that separate these two genera but failed to reveal any but those mentioned here. There are multiple species of Ceropegia and Brachystelma that simply do not conform to this historical classification.
Similarly, Ceropegia and the various stapeliads have been separated on the basis of stem and floral anatomy. Historically speaking, the stapeliads were thought to consist of fleshy, succulent stems with tubercules and reduced or absent leaves, whereas Ceropegia were considered to be slender climbers. Again, with more species having been discovered, these distinctions grew more and more blurry.
It turns out that there are many Ceropegia with fleshy, succulent stems and the only major difference between the two genera is the lack of angles in the stems of some Ceropegia. The structure and presentation of their flowers also stands on shaky ground. There is so much similarity between the flowers of some of the succulent Ceropegia and the early diverging stapeliads that one would be hard pressed to identify any character that clearly separates them.
Between all of the molecular work and the anatomical scrutiny, it was clear that something needed to be done to clean up the taxonomic status of Ceropegieae. Keeping things separate may make sense to some but considering the group as a whole instead of from a collector’s standpoint, trying to find enough distinct characters to preserve the historical treatment would make things way too messy. In 2017 it was suggested that because there are no clear differences between the four groups within this tribe, all members were to be lumped back in to the genus Ceropegia.
Although this most recent treatment still recognizes some morphological differences between these plants (thus multiple subsections are recognized), the lack of genetic differentiation between groups long thought to be distinct really does support this decision. Because of historical precedents, Ceropegia won out as the main generic classification.
Personally I find this work to be extremely exciting. It involved a lot of wonderful detective work and a whole lot of attention to detail. I think the end result paints a far better picture for our understanding of how these plants evolved. I am especially floored that some of the earlier morphological notes turned out to be quite useful in this modern understanding. Even more exciting is the fact that now we know that many of what we thought were “unique” characters amoung the various species actually evolved multiple times throughout the history of this group. This is why I will never get upset by taxonomic changes. They may be working documents but each step we take helps us understand evolution that much more.
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