The Evolution of Eusociality in Bees

Nissa Osheim 4/4/2007 The Evolution of Eusociality in Bees Evolution has been called the survival of the fittest. It is a competition between individuals to see which genes will get to replicate. However, cooperation and even altruism has managed to evolve and in some cases prosper. The insect world has many forms of social insects. Bees are among the most social of all insects. They are willing to perform acts of extreme altruism for the good of their colony. In this review we will look at this behavior and how it might have evolved. There are two major theories for the evolution of altruism. At their core, they both need the idea that if individuals corporate, everyone gains more on average than they would of if they had all acted selfishly. In one theory, individuals remember who cooperated and who acted selfishly, and the cooperators can then band together and cooperate with individuals that have cooperated with them in the past and there by gain an advantage over those individuals that act selfishly[1]. The other theory is called sibling altruism. It is the idea that individuals will help close relatives even to the point of hurting themselves because the relatives genetic code is similar to theirs and if the relative reproduces it is almost as good as the individual reproducing, as far as evolution is concerned[2]. It seems very likely that something like sibling altruism is responsible for bee socialization. First bees have very small brains. It is unlikely that they have the memory 1

or recognition capabilities necessary to make the first theory work. Second, the bees in a colony are usually almost all sisters, often genetically identical. In addition, the worker bees are sterile. So, the only way to get their genetic code passed on, is to help the queen reproduce. This form of social living is called eusociality. Most bees have currently evolved a very complex social behavior. Most are eusocial. Eusociality is defined by overlapping generations, parental brood care, and reproductive division of labor. The sterile workers care for and protect the queen who alone has the ability to breed and pass on their genetic code. The decision of whether a bee will become a worker or a queen seemed to be made early on by the feeding of a substance called Royal Jelly[3][4]. This mixture of amino acids, lipids, vitamins, acetylcholine, and other nutrients is produced by the worker bees and stimulates the young larva to develop into queens. The proteins that have been identified as responding to this Royal Jelly share an evolutionary origin with the Yellow protein. This Yellow protein has been identified in many other insects. The role of queen and worker is a very important component in the bee s eusociality. Therefore, looking at the origin of the protein that determines this, my help track down how bees evolved their eusociality. Another clue to how bees evolved their social behavior can be found in the intermediate steps between solitary and eusociality. Unfortunately, for this puzzle, most bees developed their eusociality long ago and the intermediary steps have disappeared. However there are still some types of bees that are solitary or that have very rudimentary social behavior. The halictines and allodpines groups have species which are solitary, eusocial, and in-between[5]. Looking at these groups will help in the understanding of how eusociality evolved. 2

To follow the evolution of bees it makes sense to start at the genetic code. Unfortunately gene sequences are very hard to compare and bees evolved to be eusocial so long ago that we can find no evidence of the creatures they were before. We can how ever look at proteins that seem to play a role in the bee society and see if we can find traces of that protein in other insects. If so, we can get a little bit of an idea of how they may have evolved. Major Royal Jelly Protein (MRJP) plays a big role in bee societies. In part, it is used by the workers to produce a substance called Royal Jelly. This is very nutritional and is fed to all bee larvas for the first few days of development. If it continues to be fed to the larva past the first couple of days, that larva will developed into a queen bee, hence the name Royal Jelly[3][4]. Using data from A.melifera genome project, they found a cluster that seems to code for this protein. This protein is found in many other insects including fruit flies and social ants. There is evidence that this protein may have evolved at the same time bees became eusocial. The gene group that has been linked to MRJP has also been linked to having a common origin with the yellow protein of Drosophila melanogaster. This yellow protein is common in many other insects. So it appears that as socialization developed this yellow protein changed into the MRJP. It is unknown whether this helped drive socialization or weather socialization drove the changes to the protein. Most highly eusocial insects use pheromones as a form of communication. Communication and coordination are essential for very large communities. Pheromones are useful for communication because a few individuals can send a message that can be 3

picked up by the rest of the colony. Also, they are short lived so the message can be easily changed. This pheromone communication has developed the ability to communicate what roles need to be filled in the community and seem to influence the behavior and sometimes the development of individuals. Again, it is unknown weather they already had this pheromone and it helped them become social or weather as they became social these pheromones developed. Since physiology tends to follow behavior, it is more likely that as they were becoming social these pheromone communicators developed. It is also likely that they already had the pheromone. It was just being used for a different purpose, most likely mate attraction, and the role it was being used for just changed over time[10]. Another way to look at the evolutionary process is to look at what may have driven this process. There needed to be an evolutionary benefit to socialization for it to have been able to evolve. Bees are willing to sacrifice themselves for the hive. This behavior must give them an evolutionary benefit. Also, there needs to be benefit for the behavior of all the evolutionary steps. The first step is likely to be forming small communities of simi-related individuals. benefit The Australian Dunatothrips forms such communities, building domiciles out of the leaves of their host plant, Acacia. The domiciles take a reasonable expenditure of energy to create, and the per individual breeding rate decreases as the colony size becomes large. Available room on the host plant doesn t seem to change the size of the colonies. So, they are not just forming the colonies to combat space constraints. They have found that the chance of surviving to adulthood increases for those born in 4

domiciles. Also, the chance of fighting off the kleptoparasite Xaniothrips mulga increase as the community size increases. So, it appears that the benefits of forming these domiciles outweigh the costs[2]. Models of social evolution suggest that the dominate individuals give some breeding rights to the subordinates in order to elicited cooperation. The amount of incentive a subordinate needs in order to be willing to stay and help and not challenge for dominance vs. the incentive the dominate individuals are willing to give to the subordinates for there support determines if peaceful socialization is possible. This chance increases when there fewer constraints on the number of offspring the colony can support. In this case allowing the subordinate to breed more doesn t hurt the chances of the dominate individuals offspring as much. Harsh environmental conditions which make it harder to survive without a colony will also increase the chance for socialization. The subordinates are willing to stay for less reward because it is safer that way. Finally, if the subordinate is related to the dominate individual they will again be willing to stay for less reward. Since they are related, helping the dominate individual s offspring survive will help a similar genetic code survive. Not as good as your own offspring, but better than nothing[12]. The Macrogalea species from Malawi is another example of this type of reward process. The females form brooding colonies of sizes up to 13 adult females. They raise their young together. All the females will help to raise all the young. These colonies are better protected the larger they are, but there are limited resources. Colonies sizes over 3 adult females decrease the per capita reproduction rate. So, there is competition on which females get to breed. Among the older individuals, this manifest in only the lager 5

females being allowed to reproduce. However, the younger ones get to reproduce regardless of size. It is thought that this is because the younger ones still have potential in helping to raise the offspring. So, the bigger ones allow them to breed as incentive to get them to stick around and help raise their young. The older ones are going to die soon. So, there is no reason to offer them resources to get them to stick around[7]. Eusociality includes multiple generations. So, the next step towards it, or maybe a parallel step is the idea of siblings helping to raise the next generation of siblings. In the Australian species E. setosa from the subgenus Exoneurella, I female will raise her first set of offspring alone, but she times it so that her next set of offspring is hatching about the time the first set is ready to leave the nest. This gives the older siblings the choice of staying and helping to raise their younger siblings or leaving and searching out breeding opportunities of their own. Having their siblings survive will help their gene code continue, but not as much as breeding. However, if they don t manage to survive long enough to breed then they will not have helped their genetic code replicate at all. So, many stay and help raise their siblings forming the beginnings of Eusociality. However there is another species, sensu stricto, in the subgenus Exoneurella. This species has stopped overlapping their generations. Instead they from groups and raise their offspring together. These groups are from the same generation and are usually sisters. So, there is still some pressure to help the other s offspring because they are nieces and nephews, but this isn t as much pressure as there is to help a sibling. Since part of eusociality is overlapping generations, this species has evolved away from it. The northern social spider Anelosimus studiosus, is another example of siblings staying to rear younger siblings. This spider lives at various altitudes. The spiders living 6

at higher altitudes have a more sever environment, and therefore their casualty rate is higher. These spiders will tend to stay in the web and help raise their younger siblings because the chances of them surviving long enough to raise their own young are smaller. The ones at lower altitudes where casualty rates are lower will tend to leave the web earlier. Their chances of surviving to breed on their own are greater, and therefore it is worth the risk to gain the benefit of having their own offspring[15]. Eusociality also requires reproductive division of labor. How it came about that evolution caused some bees to give out their ability to reproduce is much harder to see. Reproduction is what drives evolution. So, it is hard to see what could have caused it to make some bees not reproduce. Both the queen and the workers have the same genetic code. The queen was given royal jelly during its development and this caused it to develop into a queen. Not all worker bees have the same number of ovaries. Most have 1-4 ovaries. There are also many with 5-7. Very few worker bees have over 7 ovaries. Bees with between 1-4 ovaries start foraging later than the other bees. They spend more time taking care of the young. If the queen dies the bees with more ovaries are more likely to become the new queen. This seems to suggest that the workers would want the queen to die so they would get a chance at being queen, but the colony would destroy itself if the workers behaved in such a selfish manner. Bees have been called super organisms because they are like a single organism that can break itself into many pieces. A mammal could be thought of as a colony of millions of single celled organisms all clones. Each of these organisms work for the benefit of colony completely sacrificing themselves if necessary. Most of these 7

organisms are sterile, only a few have the ability to reproduce, and the entire colony works to protect these individuals and help them reproduce. The worker bees are willing to give their lives for the queen and her offspring because the queen the same, or very similar genetic code. So, the queen reproducing is like the worker had reproduced. Therefore the workers are being selfish when they protect the queen and the hive. A greenbeard gene is a gene that produces some feature that can be recognized by others with. This feature could be a green beard, some smell, a color pattern, or any other recognizable feature. It will also cause those with the gene to act a certain way towards others with the gene. This has been proposed as a possibility for the evolution of some cooperative traits. Those with the gene will be able to recognize and thereby cooperate with others with the gene. They can be reasonably assured that the others with the gene will cooperate back. There is always the danger of another gene mimicking the physical feature that makes them recognizable, but doesn t also give the behavior of cooperation. A type of fire ant, Solenopsis invicta, has an identified greenbeard gene. Workers will kill a queen without this gene. The workers which do the killing are primarily those with the gene itself[9]. It has been proposed that bees have a greenbeard gene which they use to identify nest mates, and which causes them to cooperate with one another. It is possible that this played a role in the evolution of their eusociality. Siblings could recognize other siblings with this gene and know that it would benefit them, or rather their DNA, to help their sibling. This could give them greater incentive to stay and raise their siblings instead of going off to try to breed themselves. This could continue until they loose that ability to breed along with the desire. 8

Statistic models have been created to try to determine if there is a single origin or duel origin for highly eusocial behavior in corbiculate bees. Because it is a model, there is a great deal of simplification done and various parameters must be guessed at and set. For these reasons the results were inconclusive. One model concluded a single origin and another concluded a likely duel-origin. However, the model that favored duel-origin had some very precise and not very likely settings. So, it seems more likely that the model that favored single origin is closer to the truth. Interestingly enough, the model that favored duel-origin sets highly eusocial behavior as an evolutionary ending point. Insects do not evolve out of being highly eusocial. This seems to suggest that if there is a single origin, some may have evolved out of being highly eusocial. However, models are imprecise because there is no way to know and include all the factors present in nature which may have an impact[8]. In conclusion, bees evolved so long ago that almost all record of that evolution has been wiped out. A few clues can be found in the proteins and pheromones they use, but not many. There are bees that are less social. They may be evolutionary steps, or they may be offshoots. Either way, they and other social creatures offer insight into what may have happened. There are social insects together is small groups. This can help in efficient food gathering and protection. The stronger individuals may allow the weaker individual to breed just to get their help. Next we look at sibling altruism. Siblings are very genetically similar. So, helping ones sibling survive is almost as good as surviving and breeding. Bees have been seen forgoing possible breeding opportunities to help raise their siblings. This behavior my have become so prevalent that some bees gave up all their breeding opportunities. Then since they were not breeding anyway, evolution could 9

have changed it to make them unable to breed. This way they were not wasting energy developing organs they were not using anyway. It kept in the ability to change them back just incase they were needed to breed. From their evolution could have refined their communication and role allocation until we are left with the modern day bee. 10

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