Question 1: How are social insect colonies founded and how does this affect the queen’s ability to mate?
Answer 1: Most of the insects are not social barring a few that exists as social groups. There are a number of advantages in leading a social life such as collecting food, better defense strategies from predators which is better if there is a group, increased production, better care for young ones, availability of nest for retiring and better productivity of vital substances for sustenance. Most insect societies have some distinct characteristics. Thus in an insect colony there is slavery, territorial aggression/protection, farming and maintaining ambient nest conditions. In slavery basically insects from one species capture the young ones of another species, raise them and make them as workers, the pattern is so conspicuous that in some cases the dominant insects are unable to survive without the slave species. Insects in colonies are also very aggressive and territorial which leads to colony expansion. Forage and farming are also important characteristics. Insect colonies of one species subjugate and rear insects from another species, protect and save them from predators and in turn use the slave’s secretion for nourishment. In some cases insects grow fungi in the nest thus creating a readily available food source. Finally insect colonies also maintain necessary conditions like temperature and humidity within acceptable limits because if temperature is too high it will melt temperature sensitive substances and if it is too cold, it will result in death of individuals. Likewise if humidity is very low it will result in dryness which is very harmful for insects that have a soft exterior. Most insect colonies have a basic organization such as the reproducing group consisting of queen and drones, workers and soldiers. The main function of the queen is to procreate and reproduce. She also determines the gender in some cases, i.e. if the eggs are fertilized they develop into females else males. Most of the individuals in a colony are offspring form the queen and the queen secretions also helps to identify individuals from different castes. Drones are males that serve only reproductive functions while workers that do most of all the work such as colony protection, farming and forage. In addition workers also get to determine whether an eggs need fertilization or not, in essential they determine how many males and females are required. Even among the females produced, workers determine which larva develops for the purpose of reproduction and which develops into a worker. This is achieved by feeding royal jelly for 3 days for larvae destined to become a worker and almost always for larvae destined to develop only for the purpose of reproduction. The organization of the colony determines the queen’s ability to mate. The workers are the ones that determine the number of drones needed for maintenance of the progeny. They perform this task by preventing fertilization of eggs that then develop into males. In some situations it is seen that queens actually mate with several males. This is believed to reduce the kin similarity between the female workers who then initiate the generation of more males. Such a strategy increases colony fitness. Sometime multiple queens arise. The dominant queen then executes the other queens even before they have a chance to contest with her thus ensuring the success of her progeny in one case. In the other case workers may also execute the dominant queen (one that is not determined by them in case of an accident). When there is more than one queen, then the basis tenet of worker production is delayed, instead males are produced first. The males then fight among themselves and the victorious one then gets to mate with all the queens. These males live exceptionally long. Ultimately the ability of the queen to mate is decided by workers in single queen colonies and by queens in multiple queen colonies.
Question 2: How do we account for the pattern of eusociality in hymenoptera?
Answer 2: Eusociality refers to a greatest degree of social behaviour and cooperation in insects characterized by reproductive fitness, rearing and nursing of young ones and defense/protection of colony. Basically three hypotheses have been postulated for the pattern of eusociality such as kin selection, parental manipulation and mutualism. In kin selection, workers gain fitness; in parental manipulation, the queen facilitated the development of individuals in spite of the low fitness level and in mutualism, there is mutual increase in fitness levels. Eusociality also depends on the life cycle of an insect. The three important factors favoring the pattern of eusociality seen in hymenopterans are the development pattern, mating characteristics and male haploidy. Eusociality is favored more in bivoltine cycles than univoltine cycles. Likewise, eusociality gets reduced by multiple mating especially due to the involvement of the workers, although the benefits and advantages for the mother do not change. Also production of haploid males also favors eusociality because this confers the ability to workers who can alter the ratio between males and females. This is also assisted by the fact that virgin females can produce males adding another reason to up regulate the production of more females. Also the presence of diploid females and haploid males is thought to contribute to the development of unique and advanced eusocial traits compared to situations in where both males and females are diploid. Studies have shown that the ability to alter a harmful or deleterious allele is several orders of magnitude higher in haplo-diploid systems rather than diploid systems. In addition haplo-diploid systems favor the workers who are the main regulators of a colony thus ultimately leading to eusociality. Although these factors are important for the pattern of eusociality seen, they are not exhaustive and there are far more factors that contribute to the phenomenon.
In hymenopterans, there is an absence of male workers. This is thought to occur mainly due to the low quality of parental care that the developing larvae receive. Most of the insect colonies in hymenopterans contain haploid males but occasionally diploid males also arise. It is very difficult to explain such phenomena. It is believed that this phenomenon can be best explained by in breeding that is thought to occur for several generations resulting in the development of the diploid males. But inbreeding is not enough to explain the pattern of eusociality seen in hymenopterans as there is not an involvement of a positive genetic selection and other associated ecological and biological factors must be considered for the observed pattern. Based on this once eusociality begins, it results in the production of workers that are sterile and live long with sterility or have a very short duration in the cycle. Several models have been proposed to explain this hypothesis. It is generally thought that there is a specific reason for making the workers sterile. Sterile workers move away from the nest in search of food. Once they find the food they bring it to the nest for the consumption of nest members. This behavior prevents the consumption of vital nest resources and ultimately results in the development of new traits in the workers over the course of evolution. In hymenopterans, there is a low level of eusociality compared to other insect groups. Most of them share similarity in life cycle pattern with other highly eusocial insect groups, yet lack eusociality. The hymenopterans have been in the evolutionary ladder for a very long time and still lack eusociality. Even the existing trends seen in the colony do not hold the promise of a future development of eusociality. This said, it may be safe to assume that in hymenopterans, there may be some negative selection or deleterious factors that may interfere with likelihood of a progression towards eusociality.