29 March 2020

Deceiving dispersal: Lying for a living

A tale of dung-like seed dispersal and seed-like insect eggs.

Three seeds with orange hair like tufts

Seeds travel by land, sea and air to conquer new territories. 

Many have developed flight capabilities: wings, parachutes, gliders and exotic hydraulic explosive systems to propel them meters.

With hooks to hold on to fur or feathers, or found within a sweet tasting fruit, seeds can also use animals to hitch a ride far away from their parents. 

Some seeds, however, have extreme disguises.   
 

Two very similar looking dark green speckly balls
Left: Bontebok dung, Right: Ceratocaryum argenteum seed. They're not easy to tell apart if you’re a beetle [From: Midgley, J. et al. (2015)].

Dung-rolling away

The work of a dung beetle is simple: Find dung, roll dung, bury dung and repeat. 

But not in South Africa, where one plant (Ceratocaryum argenteum) has found a way to exploit the system. 

Its seeds are spherical, very similar to the local antelope’s droppings. 

As if this visual trick was not enough, these seeds emit certain volatile compounds like those found in antelope dung. 

Oblivious to the complex disguise, dung beetles transport the seeds far away and bury them when they see fit. 

Only is it then when they realize that what they thought were soft tasty dung meals, are just hard thick seed coats with no other rewards than disappointment.

Moving gif of beetle rolling dung like seed
Oblivious to the fraud in place, dung beetles transport Ceratocaryum argenteum seeds...
Moving gif of beetle rolling a dung like seed into hole
...and are tricked into burying them [source https://www.youtube.com/watch?v=ugxRx7alXb4].

While disguising as dung (faecal mimicry) is a unique strategy, flesh and faeces smells are not new in plants (known as sapromiophily). 

Many flowers pollinated by flies and beetles attract their pollinators this way, such as the corpse lily (Rafflesia arnoldii) and the 'corpse flower' (Titan arum; Amorphophallus titanum).

A bit of an extreme solution to solve their dispersal problems, but these plants are not the only beings lying for a living… 

A bug’s life

Ants have been helping to move seeds around (an activity called myrmecochory) for a while, but not for free. 

Some seeds have a nutritious lipid‐rich appendage (elaiosome) which serves as a payment for the movement of the seed to an ant nest where it’s buried. 

This quid pro quo is found in thousands of plants species, but this method has been exploited by stick and leaf insects.

The exceptional disguises of stick and leaf insects limit their motility and so they have trouble expanding their populations. 

But evolution has found a solution. 

Chemically similar to the elaiosomes of ant-attracting seeds, some stick and leaf insect eggs have a tasty treat for ants to take away (known as capitula). 

Insects just drop or flick their eggs and wait for the ants to hide them away from predators and other threats. 
 

A selection of eggs and similar looking seeds
Can you tell the difference? Left: Stick and leaf insect eggs [From Robertson, J. A., Bradler, S., & Whiting, M. F. (2018)]. Right: Ant-dispersed seeds © P.Gomez, W.Stuppy, H.Morales, G.Hoyle, E.Vaes/RBG Kew.

Do the evolution!

Millions of years of evolution lie behind these dung-like seeds and seed shaped insect eggs. 

Mimicry (or the art of looking something else) and cases of different organisms developing similar traits (convergent evolution) are widely present on Earth.

Among them are remarkable examples of plant and animal species developing a common solution to similar problems. 

What is certain is that evolution does know how to “kill two birds with one stone”.

References

Denoeud, F., Carretero-Paulet, L., Dereeper, A., et al. (2014). The coffee genome provides insight into the convergent evolution of caffeine biosynthesis. Science, 345: 1181-1184.

Giladi, I. (2006). Choosing benefits or partners: a review of the evidence for the evolution of myrmecochory. Oikos, 112: 481-492.

Midgley, J. J., White, J. D., Johnson, S. D., & Bronner, G. N. (2015). Faecal mimicry by seeds ensures dispersal by dung beetles. Nature Plants, 1: 1-3.

Robertson, J. A., Bradler, S., & Whiting, M. F. (2018). Evolution of oviposition techniques in stick and leaf insects (Phasmatodea). Frontiers in Ecology and Evolution, 6: 216.
 

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