Seed dispersal has always proven to be advantageous for plants to reduce competition and inbreeding. This mechanism results in much better chances of success and genetically fit offspring. Seeds may be dispersed by different agents like wind, water, and even animals and for instance, seeds may stick to the bodies of animals and get displaced, or may even disperse from an animal’s faeces. However, of all creatures great and small, did you ever imagine ants to play a role in dispersing seeds? This type of ant-mediated seed dispersal is termed myrmecochory.
Seeds dispersed by ants usually have an oily, lipid-rich attachment called the elaiosome. The entire game of dispersal of seeds by ants is played by these elaiosomes, which accounts for almost 5-10% of the entire volume of seed, which, for a tiny seed, is a huge investment! But why are these elaiosomes so important? Research suggests that the elaiosomes harbour chemicals, which effectively help in luring ants to carry the seeds to their nests. The ants then separate the elaiosomes from seeds, feed it to the larvae and dump the ‘waste’ seeds at a disposal area around their nest. Now that the seeds have been transported to a different area, they may happen to germinate there, away from the parent plant, thus widening its range!
However, what are these ‘chemicals the elaiosomes are made up of? To understand elaiosomes and their importance for the plants, Rohit Sasidharan and Dr Radhika Venkatasan from NCBS, Bangalore, studied the elaiosomes of different varieties of castor oil plants (Ricinus communis) found across the country. When researchers laid out castor seeds at a field site, they found that out of 7 different species of ants recorded at the study site, only 2 species of ants, namely, Aphaenogaster beccarii and Pheidole grayi were seen to disperse seeds by carrying them to their nest. This intrigued the researchers further- what exactly was going on!? They studied the chemical composition of these elaiosomes to reveal something really cool!
So let us start from the basics.
When a myrmecochorous seed falls on the ground, it can have three possible outcomes. First, in rare situations, ants do not come across these fallen seeds; second, ants approach the seeds and remove the elaiosome on-site, without actually displacing the seed; and third, the ants may do exactly what the plant wants it to do -- seeds are carried to ant nests, and elaiosomes are removed and consumed by these ants. These seeds are then dumped near the nests and may germinate into a new plant, with more favourable nutrient conditions at the nest site than in the surrounding areas.
The study shows that the elaiosome has a germination inhibitor which delays the sprouting of the seed. If ants do not happen to remove the elaiosome, the seed still doesn’t go to ‘waste’. Well, it is too precious for a plant to allow it to go waste! Instead, fungal breakdown of the nutrient-rich elaiosome eventually leads to its germination, but this takes longer than when the ants themselves remove the elaiosome.
If ants like Solenopsis spp. (fire ants) attempt to cheat plants and simply use seeds for their own benefit, plants make sure to outsmart them. They have developed a mechanism, wherein the size, volume, and chemicals like 2-decenal present in the elaiosome, help the plant to associate with true myrmecochorous species by specifically attracting them and avoid, or at the very least, minimise cheaters like fire ants.
If ants decide to carry the seeds along with them to their nests, chemicals in the elaiosomes could also help them in navigation. Several tropical ants forage at night, and their compound eyes cannot provide sufficient night vision for them. The oily components of elaiosomes have scents, which the ants use as trails to forage and navigate in dark. However, the job is not done once the seeds are dumped near the nest! Researchers noted that the disposal sites showed higher levels of carbon and nitrogen as compared to other randomly sampled points on the field. As a result, these sites are well suitable for the growth of the seedling. The elaiosome and in turn its chemistry is therefore crucial for the propagation of these plants with the help of ants.
Well, it is quite evident that plants like castor depend primarily on myrmecochory for their dispersal, and elaiosomes play a crucial role in achieving this complex goal! But isn’t it interesting how such a tiny, overlooked component of seed can play such a crucial role in ensuring that their progeny have maximum chances of survival?
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Original research paper: Sasidharan R and Venkatesan R (2019). "Seed Elaiosome Mediates Dispersal by Ants and Impacts Germination in Ricinus communis". Front. Ecol. Evol. 7:246. DOI: 10.3389/fevo.2019.00246
Link to the original paper: https://www.frontiersin.org/articles/10.3389/fevo.2019.00246/full
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This article has been contributed by Anuja Vartak, edited by Devica Ranade