Rubus (subgenus Rubus) species
Seeking botanical good times in the Seabranch State Park swamp this week and last, near Hobe Sound Florida, John and George just can’t stay out of the swamp, though the mosquitoes may shift that obsession. John shot a gigapan panoramic image of the mire. (See if you can find me mooning the reader therein.) Also underfoot were germinated fern spores, more properly known as fern gametophytes, a pretty picture for today although not a topic to explore right now.
Some of the more eye-grabbing and pants-grabbing specimens are blackberries, in full bloom in January. Let’s give them their due. I’ve seen blackberries called a “taxonomist’s nightmare,” but that would be a taxonomist who feels that variation must conform dutifully to a human concept of distinct species. I see blackberries a little differently—as a taxonomist’s dream come true, in the sense of a complex dynamic pattern of organization that couldn’t give a hoot about human preconceptions.
Nobody can say how many species of blackberries exist, because they do not sort into traditional species. (Allow me now for convenience to expand the conversation to embrace blackberries, raspberries, and other close relatives making us the entire genus Rubus.) Worldwide there are perhaps 700-1000 “species” of sorts, but more interestingly there are also thousands (repeat, thousands) of widespread genetically identical clonal variants, hybrids, possible ancient cultivars, and sundry evolutionary offshoots, including strains with abnormal chromosome numbers. (In short, pseudo-species separated by small genetic differences arising in a moment by cloning, as opposed to true species evolving gradually by accumulated genetic processes.)
At least four population characteristics make blackberries so devilishly interesting:
1. Everything eats them. As the most delicious food on earth, blackberries feed everything from rodents to raccoons to bears to birds. I once had a golden retriever who enjoyed berry picking. The creatures move them all over the place aided by little piles of natural fertilizer. This might help explain why so many “types” of blackberries are so geographically widespread. As an example, cloudberry (Rubus chamaemorus) circles the globe at northern latitudes, wobbling as far south as Long Island.
2. Everybody eats blackberries. As long as there have been hunter-gather humans they have certainly hunted and gathered blackberries. Blackberry seeds dot coprolites. (The Coprolites were not an ancient mesopotamian kingdom.) Blackberries long long ago were probably ancient camp followers thriving on waste heaps near human settlements, where humans could go select, perpetuate, and spread their favorite strains, probably creating ancient cultivars. You can be sure that our ancestors helped stir the blackberry genetic pot bringing different variants together, inadvertently producing hybrids, and moving them around.
3. Many blackberries reproduce asexually. They clone. Many form non-sexual seeds genetically identical to the mother plant. This skill allows minor genetic variants, hybrids, and clones favored by bears, birds, Neanderthals, or the climate to expand their populations and spread.
4. Species of Rubus can be careless about their chromosomes. “Normal” plant species (with very many exceptions) have chromosomes in pairs. But blackberries and their relatives sometimes sport multiple chromosome sets and other chromosomal aberrations. You can get away with that when you reproduce asexually, as the main problem with screwy chromosomes is a thwarted sexual cycle. And blackberries are happy to hybridize.
So let’s sum up the messy situation. Here you have a group of plants moved around by every living thing and monkeyed with by every prehistoric human. Mobility brings divergent evolutionary lines together, providing chances to hybridize, which blackberries are so willing to do. Hybrids on average have a rough time facing the real world, unless they are able to clone asexually; oh yeah, did I mention blackberries do that…and then move around again by crows or Cro-Magnons just to stir things up more.
One way to tackle such a complex situation is to grab one thread and yank on it. Let’s do that for our local blackberries. Even that’s not so easy to do, as you may understand from reading this, because a glance at different references reveals the expected disagreement as to what species of Rubus live in our local counties. Let’s go arbitrarily with one modern reference and pull forth three species names: Rubus cuneifolius, R. pensilvanicus, and R. trivialis. Are any of these locals fuzzy to define or otherwise involved in genetic mischief?
Rubus cuneifolius is a nice “diploid” (with paired chromosomes) species, or is it? Strains with chromosomes in sets of three and four are reported. One sign of taxonomic confusion within a species is synonymy, that is, the existence of additional names interpretably pertaining to that species. I got bored and quit counting after finding 18 synonyms, including the “Rubus dixiensis.” Makes me want to whistle.
Another interesting measure of messiness is finding documented hybrids involving a purported species. I quickly found five and quit counting. One of them is especially intriguing. Our Rubus cuneifolius is in South Africa an invasive exotic, and seems to hybridize with multiple African species, most saliently with Rubus longepedicellatus. These two species have spawned what’s known as a hybrid swarm. The swarm is a geographically widespread series of novel strains not belonging to either parental species. “Shake-n -bake” instant species!
Some books say we have Rubus pensilvanicus, others that we have R. argutus. I sure don’t want to quibble on this question, because the whole point of this post is to underscore the murkiness. So easy to be expert when simplistic! Botanical life gets more complex than “either-or.” Rubus pensilvanicus is no clean-living species. On the U.S. West Coast, it has generated a hybrid mess with at least one western species, a pattern reminiscent of our sordid South African story.
OK then, what about Rubus trivialis…do we have one true blue species here? Naw—guess what one of its hybridization partners is, our own Rubus cuneifolius, the same species that mixes it up in South Africa fools around here in Florida with R. trivialis.
So when John and I snag in blackberry bramble and say “oh rats,” is it Rubus pensilvanicus, R. argutus, R. trivialis, R. cuneifolius, or none of the above…or a mix of the above? Or a mix of the above and more? (I did not label the photos.)