#3 Random ramblings about Asclepias
I recently started an instagram page (@bay_state_botanist) to document my learning about the flora of New England, with reference to my South African perspective where there is any connection or relevance.
After seeing my first individuals of the striking Asclepias tuberosa on Martha's Vineyard last weekend, I was reminded of the few Asclepiad species I've seen in Cape Town, specifically A. crispa and Gomphocarpus physocarpus. Here is what I wrote:
"The striking orange flowers of the butterfly weed, Asclepias tuberosa, one of the more common milkweed (Apocynaceae) species of eastern and south western North America. These ones I saw between Edgartown and Oak Bluffs on Martha's Vineyard were being pollinated by a variety of beetles, flies and bees.
Milkweeds are usually known for their stems that contain milky saps which drip out when the stem is broken. This milkweed is an exception. Nevertheless it is still toxic to consume in large amounts, and like other American milkweeds, the leaves are an important food source for monarch butterfly caterpillars.
Native Americans have traditionally used milkweed roots as treatment for pleurisy (inflammation of tissues around the lungs and chest cavity) and other pulmonary issues. As far as I understand this is why the genus was named "Asclepias", after Asclepius who in Greek mythology was a mortal who eventually became the god of medicine. His rod, a staff with snakes wrapped around it, is still used in many medical symbols today. [I have more to say about this; the rod is sometimes that of Hermes/Mercury rather than Asclepius, but I won't get too sidetracked...]
There's an interesting connection between my two worlds of the US and SA regarding this genus. Asclepias are found in the Americas and in southern Africa. The 5th photo shows an Asclepias crispa I saw growing near Rhodes Memorial on Devil's Peak in Cape Town. Disjunct distributions of closely related species like these provide support for the continental drift theory. The idea is that at one time many millions of years ago these lands were connected, and the common ancestor of these two Asclepias species lived on that land. Since then they have speciated and adapted to their different environmental conditions.
The last two photos show the invasive Gomphocarpus physocarpus (balloon plant) growing in Kirstenhof wetlands in Cape Town. It's a species of milkweed that has spread across much of the world. You can see how similar its flowers are to the Asclepias. The main difference between the two genera is that Gomphocarpus has fibrous roots and woody branched stems while Asclepias has tuberous roots and annual, unbranched stems.
The milkweeds of North America are famous for being the food source of the monarch butterfly Danaus plexippus which migrates from Mexico to the US each year. Interestingly the southern African milkweeds also have their own African monarchs, Danaus chrysippus. Both butterfly species can ingest the milkweed toxins and store them in their bodies, making them distasteful to predators. The close genetic relationship and behavioral similarities of these two monarch species on different continents also suggests a once uniform land mass that since broke apart. In biogeography they would be called vicarious species.
Molecular work to better understand the genetic differences between Asclepias and Gomphocarpus is ongoing. My filed guide to the flora of Table Mountain from the early 2000s actually refers to Asclepias crispa as Gomphocarpus crispa, which confused me when I first started botanizing and led me to research this family a bit deeper in this post."
I've done a little more browsing of the scientific literature on Asclepiad biogeography and it seems my assumption about African-North American vicariance is not necessarily supported by the research.
Rapini et al. (2007) write that the Asclepiadoideae subfamily "colonized the New World at four different times, suggesting independent dispersals from the Old World rather than any kind of vicariant event. The clade with Metastelmatinae, Oxypetalinae, and Gonolobinae (MOG, see introduction) was the first to arrive, at 32 Ma; three other invasions were more recent, with Cynanchum at around 24 Ma, Asclepias 20 Ma, and Marsdenia 16 Ma. Between 32 and 16 Ma, South America was a continental island (Raven & Axelrod, 1974a, b). In contrast, North America was connected to eastern Asia through the Bering Strait, a route probably permeable for temperate taxa until the end of the Oligocene, but possibly later."
"The assumed Asian sister group of Asclepias, however, is hypothesized as extinct."
I guess this is still vicariance in a way, just not in the direction I had assumed.
Though I'm not a taxonomist or biogeographer, I find piecing together the evolutionary trajectory of taxa and trying to decipher the complex puzzle of patterns we see today, very interesting. If you are curious about the dispersal and diversification of the other Asclepiadoideae clades in the New World, I'd recommend reading Rapini et al. (2007). It's available through Lib Gen if you don't have an account with the publisher (DOI: http://dx.doi.org/10.3417/0026-6493(2007)94[407:DOAAIT]2.0.CO;2).
Fishbein et al. (2011): "Phylogenetic Relationships of Asclepias (Apocynaceae) Inferred from Non-coding Chloroplast DNA Sequences" is also a good read.
Other sources used for my IG post:
https://www.greeka.com/greece-myths/asclepius/#:~:text=Asclepius%20was%20originally%20a%20mortal,science%20and%20the%20healing%20arts.
https://www.wildflower.org/plants/result.php?id_plant=astu
https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=276924
https://www.phillipskop.co.za/flora/gomphocarpus-cancellatus
Thanks for reading if you made it this far.
