Orchid Ant Farms

I am beginning to think that there is no strategy for survival that is off-limits to the orchid family. Yes, as you may have figured out by now, I am a bit obsessed with these plants. Can you really blame me though? Take for instance Schomburgkia tibicinis (though you may also see it listed under the genera Laelia or more accurately, Myrmecophila). These North, Central, and South American orchids are more commonly referred to as cow-horn orchids because they possess hollow pseudobulbs that have been said to been used by children as toy horns. What is the point of these hollow pseudobulbs?

A paper published back in 1989 in the American Journal of Botany found the answer to that question. As it turns out, ants are quite closely associated with orchids in this genus. They crawl all over the flowers, feeding on nectar. The relationship goes much deeper though. If you were to cut open one of these hollow pseudobulbs, you would find ant colonies living within them. The ants nest inside and often pile up great stores of food and eventually waste within these chambers. The walls of the chambers are lined with a dark tissue that was suspect to researchers.

Using radioactively labeled ants, the researchers found that the orchids were actually taking up nutrients from the ant middens! What's more, nutrients weren't found solely in adjacent tissues but also far away, in the actively growing parts of the roots. These orchids are not only absorbing nutrients from the ants but also translocating it to growing tissues.

While orchids without a resident ant colony seem to do okay, it is believed that orchids with a resident ant colony do ever so slightly better. This makes sense. These orchids grow as epiphytes on trees, a niche that is not high in nutrients. Any additional sources of nutrients these plants can get will undoubtedly aid in their long-term survival. Also, because the ants use the orchids as a food source and a nest site, they are likely defending them from herbivores.

Photo Credit: Scott.Zona (http://bit.ly/1hvWiGX)

Further Reading:
http://www.jstor.org/stable/2444355

When a Mutualism Becomes Obligate

Mutualism. The word invokes this warm and fuzzy "you scratch my back and I'll scratch yours" feeling. It is easy to grasp how a mutualism would develop and be maintained. But, in any system, there are bound to be cheaters. Cheaters reduce the fitness of one of the partners so to avoid such things, some species up the ante by resorting to some interestingly "sinister" methods.

Acacias and ants have quite the relationship. Acacias protect themselves by offering ants hollow spines and branches where their colonies can live. They even sweeten the deal via extrafloral nectaries. These are glands on the stems that secrete nectar that the ants eat. In some ant species, this is their only source of food. Needless to say, the ants become highly protective of their acacia trees. They readily attack herbivores and even go as far as to prune away vegetation that may interfere with their host. This seems like a pretty straight forward mutualistic relationship, right?

Ah, but it goes deeper. To make sure that the ants will solely rely on the acacia and are thus completely tied up in the well being of their host, the acacia alters the ants phenotype at birth. Normally these ants have no issues digesting sucrose. Researchers found that the nectar in the extrafloral nectaries contains a protein called "chitinase." Chitinase inhibits the ability of the ants to digest sucrose. When ant eggs hatch into larvae, their first meal is nectar from the extra floral nectaries. Once the larvae ingest this protein they are no longer able to feed on anything other than their hosts nectar. Thus their very survival is completely tied to the Acacia.

I am positive that more examples of such obligate mutualisms abound in nature. We only have to ask the right questions to discover them. It is also interesting considering what we are finding out about our own behavior and how it relates to the microbiome living on and within us. What about human behavior could be described in the context of a relationship similar to ants and acacias?

Photo Credit: Tony Rodd

Further Reading:
http://www.ncbi.nlm.nih.gov/pubmed/24188323