by Matthew Cobb
One of the most amazing things about social insects – bees, wasps, ants and termites (hornets are just big wasps) – is the way they have a division of labour. At its simplest, this division of labour is morphological and involves having a reproductive individual or group of individuals who is the queen. She is genetically identical to all the other females in the colony/hive, but she is specialised for producing loads of eggs. We know there’s nothing genetic about being a queen – there’s no special ‘queen gene’. It’s simply a matter of how much food she is given when she’s a larva. The same seems to apply to those ant species that have specialised workers – ‘soldiers’ – that carry out defensive or aggressive tasks. There’s no gene involved in being a soldier, it is environmentally determined.
This raises a fascinating question: how does the genome of a single species code such a wide variety of forms? In other words, how are the different developmental pathways turned on and off?
Things get even odder when you realise that although many ant or termite species have ‘soldiers’, there are no known cases of bees or wasps having morphological worker casts. There are no ‘soldier bees’ or ‘soldier wasps’. Why not?
Two recent papers – one in Science on ants, the other on bees in Proceedings of the National Academy of Sciences (USA) (PNAS as it is generally known) – have addressed these issues, giving some insight into how the ability to have soldiers may have evolved, and revealing that in one species of bee at least, there ARE soldiers!
The PNAS article, from researchers in Sussex (UK) and Sao Paulo (Brazil), studied the neotropical stingless bee Tetragonisca angustula. This species has two kinds of guards that protect the nest: standing bees near the entrance, and ‘hovering bees’ that do pretty much that. Together, only about 1% of T. angustula workers in a nest are guards.
The guards are much bigger (both in size and weight) than their sisters who go out foraging, or those who take the trash out, suggesting that they are actually determined to perform this function (no, bees don’t have free will, either). Furthermore, they are not simply scaled up versions of their smaller sisters, bits of them are proportionately bigger, suggesting that they are specialised in certain functions.
Having shown that this species of bee does have ‘soldiers’ – a morphological worker subcaste – the next question was ‘why’? These stingless bees can’t defend themselves by stinging (the clue is in the name), and yet they are regularly attacked by much larger robber bees (the clue’s in the name there, too).
In a final step the scientists carried out a classic experiment: they set up a bug fight. T. angustula guards were paired off with their deadly foes, Lestrimelitta limao robber bees. The larger the guard, the longer the fight lasted.
The increased size, and perhaps the altered shape, of the guards, may be involved in their defensive ability. As the authors say in a nice closing sentence: their discovery ‘serves as a reminder that stingless does not mean defenceless’.
In the best scientific tradition, this finding merely raises more questions: if T. angustula has morphological soldiers, why don’t other bee species, or wasp species, for that matter? Is there something different about the ecology of ants and termites (you’ll have noticed they are both terrestrial) compared to that of bees and wasps that means that the evolution of soldiers is more likely?
In some closely related ant species can vary for the presence of soldiers. For example, in the genus Pheidole, although all species have two worker subcastes (minor workers and soldiers) as well as the queen, eight species also have ‘supersoldiers’ – much larger individuals, with significantly wider heads and with tiny vestigial wings. It is thought that juvenile hormone (JH) levels – largely influenced by the amount of food the insect has received as a larva – are responsible for these switches. The presence of apparently useless vestigial wings in supersoldiers is probably due to some developmental link between extra-high JH levels and wing development, which is required for queen development. Don’t forget, all these kinds of ants have the same genes, they are just being activated in different ways at different times in different tissues, due to the amount of JH, which in turn is affected by food.
A study in Science by an international group of researchers looked at the evolution of supersoldiers, and how they might develop. The starting point was a chance observation on a field trip, where they found some odd soldiers in a nest of Pheidole morrisi, which does not have supersoldiers.
These odd ants were larger than their sisters (see below), and looked a bit like supersoldiers – right down to the vestigial wings – suggesting that this species might also possess the developmental pathway to produce these extra-large soldiers, even though they are not normally found in nature.
By looking at a number of Pheidole species, the authors concluded that naturally-occurring supersoldiers had probably evolved separately, even though similar developmental mechanisms seem to be involved.
To investigate what those mechanisms might be, the researchers put methoprene (an analog of insect juvenile hormone – JH) onto larvae of P. morrisi and produced extra-large ants a bit like the ones they had seen in the wild. They then tried a similar experiment in other species that do not have supersoldiers and found similar results. It seems that all Pheidole ants they studied share the ability to produce supersoldiers, if the right environmental conditions apply.
To confirm that JH levels are involved in the production of supersoldiers, they studied a species that has this caste, and put JH on larvae that had yet to ‘decide’ whether they were going to be ordinary soldiers or supersoldiers. The treated larvae produced a higher proportion of supersoldiers, suggesting that JH is indeed involved in determining whether a supersoldier is produced.
This neat study doesn’t resolve the issue of why only some Pheidole species have this extra caste, but it seems very likely to be related to their ecology – some of the species with supersoldiers have to fight off attacks of army ants.
Whatever the case, JH levels, driven by food supply, appear to be responsible for morphological variation in these ants – and, I would bet, in the stingless bee soldiers too. The question of why such variability occurs remains unclear, and probably does not even have a single answer.
References (links to abstracts; pay wall for full articles)
Rajendhran Rajakumar, Diego San Mauro, Michiel B. Dijkstra, Ming H. Huang, Diana E. Wheeler, Francois Hiou-Tim, Abderrahman Khila, Michael Cournoyea, and Ehab Abouheif (2012) ‘Ancestral developmental potential facilitates parallel evolution in ants’ Science 335: 79-82.
Christoph Grütera, Cristiano Menezesb, Vera L. Imperatriz-Fonsecab and Francis L. W. Ratnieks (2012) ‘A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee’ PNAS Early Edition.