Mechanisms behind eusocial societies in hymenoptera

What were the mechanisms that drove solitary hymenoptera into taking that first step into social societies and drove further advances into Eusociality? The main mechanisms that have been hypothesised are Kin Selection and Parental Manipulation (Brian, 1983; Gauld & Bolton, 1988). This blog entry will focus on the first mechanism - Kin Selection.

Kin selection relies heavily on genetic asymmetry in haplo-diploid systems – genetic asymmetry means a female has three quarters of her genes in common with her sister but only half in common with her mother and one quarter in common with her brothers (Brian, 1983). A female that assists in the rearing of her sisters will be genetically better off than having her own offspring which would only have half their genes in common (Brian, 1983). This is known as inclusive fitness and is stronger than direct fitness in haplo-diploid animals (Brian, 1983).

Oecophylla smaragdina (green weaver ants) adults caring for young in brood nest. Cairns, Queensland, Australia. Image courtesy of Alex Wild. 

Genetic fitness may also explain why most social hymenoptera produce very few males in comparison with females – males are much larger than workers and require more energy from the colony to produce but they provide very little and are essentially just breeding stock (Brian, 1983). Females however provide protection, brood care and food provisions making them a much wiser investment for the colony as a whole (Brian, 1983).

Apis mellifera (European honey bee) queen being tended to by her colony. Photo courtesy of Kathy Keatley Garvey.

Kin Selection alone cannot entirely answer the question of why socialism developed (Gauld & Bolton, 1988).  If kin selection alone was the driving mechanism then why don't other insects with haplo-diploid sex determination that display groups of adults and juveniles have more complex social structures like that of the Hymenoptera? The only insect species outside hymenoptera that are eusocial are Termites that are in the Blattodea order containing Cockroaches (Gauld & Bolton). Perhaps the answer is in the combination of both kin selection and parental manipulation. 

In the next blog entry I will discuss Parental Manipulation as a driver behind eusocialism in the hymenoptera and reproductive altruism. Here is a video that goes into more details about haplodiploidy in social insects.

References:

Gauld, I & Bolton, B 1988, The hymenoptera, Oxford University Press, Oxford.

Brian, MV 1983, Social Insects: ecology and behavioural biology, Chapman and Hall, London.

Image 1, Oecophylla smaragdina: Alex Wild: The diversity of insects, viewed 29 March 2015, <http://www.alexanderwild.com/Ants/Taxonomic-List-of-Ant-Genera/Oecophylla/i-kxngrZp/2/XL/smaragdina32-XL.jpg >

Image 2, Apis mellifera: Bug Squad: happenings in the insect world, viewed 29 March 2015, <http://ucanr.edu/blogs/bugsquad//blogfiles/14212_original.jpg>

Video: ‘Haplodyploidy in honeybees’, youtube, viewed 29 March 2015,  <https://www.youtube.com/watch?v=pkzTHwWwhQk>

Social Lives of Ants, Bees and Wasps...

Social life history in insects has evolved multiple times over diverse species and so understandably has a few different evolutionary paths and some very broad adaptations (Zablotny, 2009). Many solitary species of bees and wasps have communal nesting, where many females make their burrows within close quarters to each other but do not provide any provisions to each other (Gauld & Bolton, 1988). It is not certain whether communal nesting started to address a need for protection from predators, or due to adults returning to their birthplaces generation after generation or simply members of the same species just being mutually attracted to each other (Gauld & Bolton, 1988). Whatever the reason, this was the first step of socialism in the hymenoptera. 

Image 1: communal nest site of Cactus Bees (Diadasia australis). 

The different stages of socialism in insects can be difficult to grasp, so in 1971 the renowned American biologist E. O. Wilson established a standard vocabulary for describing the degrees of insect sociality from solitary to eusocial (Zablotny, 2009):

•  Solitary species do not have cooperative brood care, reproductive castes or overlapping generations.
• Quasi-social colonies share a nest, cooperate in brood care and also assist each other in providing food, nest building and protection but do not have overlapping generations or reproductive castes.
• Semi-social colonies are those that have some caste structure (a worker caste caring for a reproductive caste) as well as having cooperative brood care.

Image 2: semi-social nest of the stick-nest brown paper wasps (Ropalidia revolutionalis)
 in South East QLD 

• True social colonies are referred to as Eusocial. The generally accepted idea of a eusocial colony in insects is a colony which has cooperative brood care, an overlap of two or more generations & reproductive division of labour or ‘castes’ – in bees the castes are the reproductive queen, the non reproductive female workers and the reproductive male drones. 

Image 3: difference in honey bee (Apis mellifera) castes, from L to R: smaller non reproductive female worker, reproductive male drone and reproductive female queen. Notice the large abdomen on the queen for storage of eggs. The pale green dot on the queens back is common to help ID and spot queens in a hive. 

The haploid-diploid sex determination system assisted the evolution of social hymenoptera - Sex of offspring can be determined by the queen and she readjusts the sex ratio to suit the needs of the colony (Gauld & Bolton, 1988). Large numbers of female workers can be produced until the need for males arises. 

These however are not the only forms of socialism in the hymenoptera. There is social parasitism where a queen will take over an already functioning nest (either by killing or living alongside the original queen of another closely related species, parabiosis where 2 ant species may utilise the same nests and trails but do not share, lestobiosis where a smaller and larger species of ant may co-inhabit a nest and others (Gauld & Bolton, 1988).

I will leave you with a video of Dulosis or ‘slave-making’ which is common form of socialism in ant species - one species kidnaps pupae of another species to function as workers in their own nest.

References:

Gauld, I & Bolton, B 1988, The hymenoptera, Oxford University Press, Oxford.

Zablotny, J 2009, ‘Sociality’, Encyclopedia of insects. Oxford, United Kingdom: Elsevier Science & Technology, viewed 23 March 2015, Retrieved from <http://search.credoreference.com.elibrary.jcu.edu.au/content/entry/estinsects/sociality/0>

Image 1 – Cactus Bee Nests on The Happy Scientist, viewed 22 March 2015, <http://thehappyscientist.com/files/DailySciencePhoto/870c.jpg>

Image 2 – Stick-nest Brown Paper Wasp Ropalidia revolutionalis on Brisbane Insects, viewed 22 March 2015, <http://www.brisbaneinsects.com/brisbane_vespoidwasps/images/DSC_4461.jpg>

Image 3 – Bee Castes, viewed 22 March 2015,<http://www.brisbaneinsects.com/brisbane_vespoidwasps/images/wpe52.jpg>

Video – ‘Slave raid of P.americanus a obligatory social parasitic ant’, Youtube, viewed 22 March 2015, <https://www.youtube.com/watch?v=YdzEpd657RU>

Introducing the Hymenoptera: much more than aggressive stingers and jam jar raiders.

The Hymenoptera are one of the dominant life forms on Earth in both the number of species and in the diversity of life styles; however it is impossible to guess how many individual species there are with any accuracy (Austin & Dowton 2000). The order is so diverse there is no common name for the order like that given to the other insect orders, for example Coleoptera are commonly known as beetles, and Lepidoptera are referred to as butterflies and moths.

Four features are the main factors that separate the Hymenoptera from the rest of the insects and are crucial to the evolution and diversity of the order (Austin & Dowton 2000, Gould & Bolton 1988). These four factors are:




Figure 1: Aphidius ervi - biocontrol wasp attacking pea aphids. A parasitic wasp lays its egg inside a host,
the young feed on the host.
Image courtesy of www.alexanderwild.com.

1. Ovipositor is used for egg laying and venom delivery

 

2. Food is provided to the larvae by adults (bees/wasps create a cell for young and feed until pupation) or egg is laid in food source (such as parasitic wasps).

3. Larvae eat a range of foods – waste is stored in a gut cavity that does not form an intestine until the final stages of development.

 

4. Haplo-diploid sex determination: males are developed from non fertilised eggs therefore only receive mother’s genetics and females develop from fertilised eggs so genes are provided by mother and father.

Figure 2: honey bee feeding larvae inside cell.
Image courtesy of Maryann Frazier/Penn State.

 



These factors, along with environmental pressures have contributed to the amazing diversity of life seen in the order – there are parasites, parasites of parasites, herbivores, seed eaters, gall formers, specialised predators, pollinators and both social and solitary species (Austin & Dowton 2000).

 

 

 

Social Hymenoptera are said to represent the absolute peak of the evolutionary summit of invertebrates – social structures, agriculture and slave-keeping were present in Hymenopteran society well before humans even picked up their first tool (Gould & Bolton 1988). The video below shows how parasitic wasps can turn other insects into their 'slaves' or 'zombies'.

 

It has been suggested that the diversity of Hymenoptera is an example of how environmental pressures and ancestry can influence the evolution of a species and are useful subjects for the studies of how these processes can both play a part in understanding insect evolution (Strand 2000). It could be argued that the Hymenoptera are possibly the most important insects to evolutionary research.

Stay tuned for more from the exciting world of Hymenoptera.  

References:

Austin, AD & Dowton, M 2000, 'The hymenoptera: an introduction’, in AD Austin & M Dowton (ed.), Hymenoptera: evolution, biodiversity and biological control, CSIRO Publishing, Melbourne, pp. 3-7.

Gould, I & Bolton, B 1988, The hymenoptera, Oxford University Press, Oxford.

Strand, MR 2000, 'The effects of life history on development of the hymenoptera’, in AD Austin & M Dowton (ed.), Hymenoptera: evolution, biodiversity and biological control, CSIRO Publishing, Melbourne, pp. 11-16.

Figure 1: Alex Wild: The diversity of insects, viewed 10 March 2015. http://www.alexanderwild.com/Insects/Insect-Orders/Bees-Wasps-and-Sawflies/i-W8t97Kt/2/XL/Aphidius12-XL.jpg
 
Figure 2: Penn State News 2010, Penn State University, Pennsylvania, viewed 10 March 2015. http://news.psu.edu/story/301619/2014/01/27/research/common-crop-pesticides-kill-honeybee-larvae-hive

youtube video "Zombie caterpillar controlled by voodoo wasps": New Scientist 2008, Reed Business Information Ltd, viewed 10 March 2015. http://www.newscientist.com/article/dn14053