Discussion of what defines plant "carnivory"

[Apoplast]

Hi Dennis - Started a new thread so I didn't hijack your Beginner's Guide. Sorry I did as much as I did.

Jan 4, 2017 22:51:55 GMT -5 Dennis Z said:

I probably shouldn't be one trying to argue with a well educated fellow in botany and evolution like yourself

Dennis, please don't say that. Everyone should feel free to contribute to discussions like this. I'd had this discussion with many people, including people who study carnivorous plants, and you have brought up many of the same ideas they ave expressed. You are lesser to no one.

I also believe conversations like this are important to have, even if they seem pedantic. The discussion of what constitutes carnivory in plants is similar to discussions about what constitutes succulence in plants (a definition that is much better worked out, though not perfectly), or what plants have structures that qualify a bulbs (incidentally I believe some Mexi-Pings qualify - a discussion I've had a bit with DVG here, and I believe he disagrees with me entirely). On a smaller scale, this happened a number of years back with tuberous sundews, and whether they are actually tubers (BTW, they turned out to definitively be stem tubers). Hopefully these conversations spur research, as well as deepening our understanding of the amazing adaptations plants have evolved.

Jan 4, 2017 22:51:55 GMT -5 Dennis Z said:

the presence of structures and chemicals are evidence that they are used for "eating insects" unless you are suggesting that they are vestigial

So this is not an uncommon line of thinking about evolution. It's called adaptationism. It's a pitfall that many have suffered from, including many scientists (the evolutionary biology literature from the 1980's is littered with this issue). Simply because a character exists, does not mean it have evolved for that function. I the case of Stylidium, it's entirely possible that the glandular trichomes have evolved as floral defense, but an enzyme production mutation occurred, and the individuals or species with this mutation can scavenge extra nutrients. Such a trait certainly wouldn't be selected against, but would still not likely constitute carnivory as it is not under selection to maximize trapping of animals to gain significant nutrients. I'm not saying this is happening, just that there has been a rush to add Stylidium to the list of carnivorous plants without solid evidence.

Roridula however has evolved gaps in it's cuticle, and costly adaptation where it not for the need to gain nutrients from the prey they capture. That is actually a good way to assess adaptations. Typically there are trade-offs in any adaptation. Whether it is the tubular leaves of pitcher plants that are terrible for photosynthesis, or the self shading caused by the glandular trichomes on sundews, most adaptations come at a cost for other functions. The maintenance of these traits in the face of their costs is often evidence of their positive selection for the other function. This is what is missing from so many CP studies.

[lloyd]

There's always a continuum. On one side something is "obviously" carnivorous, on the other side, not. In the middle, not so simple.

[Apoplast]

Hi Lloyd - Completely agree! Actually, that is where the genus Stylidium would benefit from a comparative and evolutionary approach.

I think Stylidium would make an exemplary system in the study of the evolution of carnivory in plants. There are a few hundred species, which makes for great comparative work. Few of these have been studied in depth, and they may represent stages of the evolution of plant carnivory. This is something we currently lack in our understanding of how plant carnivory evolves. A diverse group where there are multiple stages would be wonderful!

Other diverse groups like Drosera, or Utricularia are pretty well carnivorous, with possibly some reversion tendencies in a few species. The nearest thing we have is the genus Brocchinia, which has probably two carnivores (B. reducta & B. hechtioides), and one myrmecophyte (B. acuminata - a very cool one in fact), with few intermediate forms and the complications of tank material accumulation which roots can grow into making them uptake in no serious way much different than most plants. Stylidium may solve this issue, if we are willing to pull back and investigate it in a broader context.

Most of the species in Stylidium have weakly glandular inflorescences. Some of these can produce protease enzymes. Perhaps not all though. Some species have glaucous inflorescences, which are unlikely to trap or digest anything. A few species seem to be densely glandular. Imagine what a thorough study or set of studies could tell us about how plants evolve carnivory (and all the grey area in between) if there were comparisons made of the gland structures as they relate to capture, and how capture rates in the field (yes, it must be in the field) relate to enzyme production, and what contribution to the N or P any insect captures make to the plant. The list goes on. It potentially represents the only viable extant system to study what factors in the "grey area" push plants toward carnivory (however you want to define it), and which attributes are important steps or which ones seem to be hurdles. Of course the danger for those who like to lump the genus in as "carnivorous", it seems at least in part, because they have rapid movement in their flowers so attract the same folks who like VFT and thus many CP hobbyists, is that we might find out the glands have nothing to do with a carnivorous nature.

But that would make them ripe for defensive floral gland comparisons, which would be awesome for entirely differently reasons!

[Apoplast]

With few posts on the Forum since I last posted on this thread, I feel a bit as if I have killed this forum. I certainly hope not!

And then too, I hope this is not piling on, but this is a great article from last month that (though it take a more genomics approach) is essentially why I feel as though we have rushed to judgement on calling Stylidium species (and Philcoxia for that matter)mas carnivorous, and why we need to take a broader evolutionary approach, which includes recognizing drift and randomness.

If you are interested:
www.ncbi.nlm.nih.gov/pmc/articles/PMC5180405/

[lloyd]

Ok, I read the article and now my brain hurts 🤕. Still it makes sense that we have to move from magical thinking (Sir James George Frazer, the Golden Bough) to Galileo's science.

[amriko]

Is it possible that pineapple enzymes (the ones used for tenderizing meat) are related to the carnivorous bromeliad and it's enzymes as they are both bromeliads?

[nimbulan]

I never saw this thread before. I tend to subscribe to a definition with 3 requirements:

1) The plant must attract prey
2) The plant must trap prey
3) The plant must be able to absorb nutrients from that prey in some way other than through the roots (sorry, sheep-catching brambles) and benefit from this process. This can occur through digestive enzymes or through a mutualistic organism such as bacteria or predatory insects.

This definition covers all known carnivorous plants, including Roridula and Ibicella. I was not aware of Roridula species' adaptation for cuticle gaps though, that certainly adds further evidence in their favor.

I'd also like to add that there was an article covering two Sylidium species (S. debile and S. fimbriatum if I remember right) in the latest ICPS newsletter showing that they do in fact absorb nutrients from insects they catch.