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= Caste determination in social insects = =Definitions= Caste: A morphologically and/or behaviourally distinct subset of individuals within an insect colony.

Epigenetics: Phenotypic change resulting from gene expression but not changes to the genome.

Eusocial: the support of reproductive castes by sterile ‘worker’ castes to improve colony fitness in ants, bees, termites, wasps and other social insects.

Phenotype: The physical appearance of an individual brought about by both the environmental and genotype

Phenotypic plasticity: The ability of a single genotype to produce different phenotypes due to environmental conditions.

Polyethism: The functional specialisation of the individual members of a insect colony, which facilitates the division of labour

Polymorphism: The existence of two or more distinct body forms amongst individuals of the same age in a single colony OR The existence of two or more distinct body forms amongst genetically identical individuals

Totipotency: The potential of an individual to express the full range of behaviours of the population even if never actually expressed.

Trophogenic polymorphism: polymorphism brought about by dietary influences rather than genetic differences.

= Introduction =

Caste determination in social insects is the process by which physiological, environmental, and/or genetic factors give rise to morphologically and behaviorally dimorphic individual within the same colony. Insect caste determination facilitates the allocation of colony labor between specific forms of phenotypically plastic, totipotent, or genetically distinct individuals. Distinct castes may or may not display visible differences in appearance, but always display different behavioral and functional roles within the colony. Caste determination is crucial to the allocation of labor between morphologically distinct individuals for colony formation, maintenance and survival.

Caste determination is most apparent in Hymenoptera; ants, bees, wasps, and Isoptera; termites. Hymenoptera and Isoptera larvae have branching developmental pathways leading to multiple castes within a colony. Hymenoptera colonies usually consist of at least two castes defined by their reproductive ability. Theses castes are generally referred to as 'reproductives' that produce offspring and 'workers' who are either sterile or who’s reproductive ability is suppressed. Subsets of workers perform tasks such as: foraging, protection and maintenance of the colony. Some Hymenoptera, such as ants also have an additional 'soldier' caste. Isoptera colonies have three or more castes including: reproductives, workers, and soldiers.

=Evolution of Insect caste determination= Caste determination appears to have evolved independently on a number of occasions; this is reflected by the absence of common underlying mechanisms dictating caste determination between and even across species. Caste determination has evolved independently a total of 10-11 times in the Hymenoptera, and at least once in the Isopteran order. While the evolutionary history of caste determination remains widely unknown, there is evidence of its existence in 'primitively eusocial' wasp species, suggesting an ancient origin. For the genus Melipona, phenotypic and trophogenic as apposed to genetic caste determination appears to be phylogenetically more ancient, with genetic caste determination resulting from mutation.

=Function of caste determination= Caste determination allows the specialization of function of the individual for the benefit of the colony. Caste determination allows workers to become better workers, and reproductives to become better reproductives increasing the efficiency with which tasks are carried out, increasing the fitness of the colony as a whole.

= Factors controlling caste determination = Caste determination is a multistage process whereby larvae progress along distinct developmental trajectories to a final adult form. Depending on the species, and the caste, caste determination can occur at fertilization, during larval development, or throughout the individual’s life as a result of age-based polymorphism. For social insects determination between female-male specific castes occurs at fertilization, where as queen-worker and worker-worker caste determination occurs either during larval development, or throughout the life of the individual. For some species, there is only a small window of time during which specific castes can be induced, while in others the individual retains a degree of totipotency throughout its life .For example, in the honeybee, male-female determination occurs at fertilization. All male offspring belong to the reproductive caste and play no direct role in colony maintenance. Female queen-worker caste determination occurs during larval development. In the paper wasp, queen-worker caste determination occurs later in development. In certain ant and termite species caste is directly related to the age of the individual.

Despite similarities in colony organization between social insects, there is little evidence of a single set of factors influencing caste determination across multiple insect taxa. Transcriptome-wide analysis of three hymenopteran social lineages has shown that despite overlap in pathways and biological functions of caste determination, there is very littler overlap of caste differentially expressed transcripts. Some insect species appear to have a highly regulated system of caste determination with a genetic basis, while others appear to have a high degree of phenotypic plasticity, where a range of external factors such as nutrition maternal care and environment influence specific castes.

Sex determination in social insects
For many social insects, caste determination is sex specific. In the Hymenoptera, soldier and worker castes are always female whereas in Isoptera, soldier and worker castes contain both males and females. For hymenoptera control over sex of offspring is crucial to the production of sufficient quantities of workers.

In social Hymenoptera queens exhibit exquisite control over the sex of offspring. Sex is determined by whether or not an egg is fertilized. Unfertilized eggs develop into haploid male offspring while fertilized eggs develop into diploid female offspring. As the worker castes in eusocial Hymenoptera are all females, this gives the queen control over the proportion of colony workers. This process of asexual production of males, and sexual production of females is referred to as haploidiplody There appears to exist a relationship between caste determination and haploidiploidy sex determination in social insects, with the majority of haploidiploidy species exhibiting caste determination behaviour.

In Isoptera, worker castes are both male and female. Isoptera queens produce both male and female castes sexually through fertilization. Both male and female offspring are diploid and contribute to colony function.

Environmental Caste Determination
The central dogma of worker-queen caste determination is that specific castes develop based on environmental factors acting on the totipotent genome. However the mechanisms by which environmental factor such as nutrition, age of queen, temperature, pheromones and maternal care influence caste determination appear both species specific and incredibly complex.

Environmental conditions during early development, and throughout life interact with the individuals’ genome by triggering specific developmental switches, altering gene expression, inducing morphologically and behaviorally distinct individuals. This interaction of the environment and the genome to produce specific phenotypes is referred to as epigenetic caste determination. Some of the most striking evidence for epigenetic influence on caste determination is present in the Hymenoptera and Isoptera where gene expression is governed by factors such as diet induced DNA methylation, hormone levels, alternative splicing, and maternally governed signaling pathways, vitillogens and hormonal regulation.

Maternal influence on caste determination
Environmental influence on caste determination includes experiences encountered by the mother prior to fertilization and egg laying. Maternal environmental experience prior to laying has significant impact on caste determination of offspring due to regulation of gene expression through insulin/insulin like signaling (IIS) pathways, junior hormone levels (JH) and vitillogens (Vg). This interaction of epigenetic factors is observed across multiple taxa where certain environmental conditions are required for maternal production of specific castes. In Pognomyrmex seed harvester ants and Iridomyrmex humilis, prolonged exposure to cold temperatures is required for the production of next generation queens. For the ant species Pognomyremex rugosus production of queens and workers results from temperature induced up-regulation of genes expressed in the IIS pathway and genes coding for JH, which stimulates up-regulation of expression of Vg genes (Vg1, Vg2). Changes in Vg production in the mother are translated to eggs with Vg levels differing between queen and worker destines eggs. An extension of maternal influence of caste determination is the affect of queen age on offspring caste, with older Pognomyrmex queens required for the production of gynes. Maternal environmental caste determination is present not only in species where caste determination occurs through phenotypic plasticity but also on genetically pre-determined castes as in Ponomyrmex where queens abort queen-destined eggs based on environmental conditions.

Trophogenic influence on caste determination
For social insects, diet plays a significant role in determination of queen-worker and worker-worker castes across multiple insect taxa. Dietary influence over caste determination has been observed in Hymenopteran bees, ants, and wasps including primitively eusocial wasp species such as the Polistes wasp Dietary influence on caste determination has also been observed in the termite. In the termite, nutrition, and nutrition-related genes are associated with differences between queen-worker castes.

In the honeybee Apis melifera, diet, or more specifically the consumption of royal jelly, a food type selectively fed to larvae, can induce queen development through influencing DNA methylation and gene expression. While the mechanistic relationship between royal jelly and DNA methylation has not been established, royalactin a peptide in royal jelly has been seen to induce queen caste development by increasing body size, promoting ovarian development and inducing queen characteristics. Dietary control over caste determination involves the complex interaction of DNA methylation, stimulation of growth factors, hormonal changes and control over the expression of genes within signaling pathways which play a role in downstream caste-determining processes preceding JH levels.

Dietary induced DNA methylation occurs through the reversible addition of a methyl group to the carbon 5 position of the cytosine ring by DNA methyltransferases. DNA methylation has the ability to dictate and sustain phenotypic plasticity and thus influences caste determination in social insects. At the cellular level, despite genetic identically, DNA methylation, in combination with alternative splicing, histones, transcription factors and non-coding RNA's enables the production of a variety of distinct cells types, due to interaction with environmental and internal stimuli. Recent evidence suggests that DNA methylation can affect phenotype not only on a cellular level, but also on an organismal level. DNA methylation has been linked to queen caste determination in the honeybee Apis melifera, where interference with DNA methylation has been seen to have the same affect on honeybee larvae as a diet of royal jelly, inducing queen castes. Additionally genes in the brain of honeybee worker, queen, and drone castes appear to have different levels of methylation suggesting DNA methylation may play a role in caste behaviour. In the termite Zootermopsis nevadensis DNA methylation appears to control phenotypic plasticity similar to Hymenoptera whereby alternative splicing and DNA methylation interact with mRNA splicing to generate multiple phenotypes amongst genetically identical individuals.

Pheromones and caste reproductive control
In eusocial Hymenoptera such as bees, wasps, and ants, pheromones produced by the queen can induce caste-specific behaviour preventing worker castes from reproducing. For example, in some Ponerinae ants, the loss of reproductives, or pheromones produced by reproductives from a colony will result in battles between workers in which previously sterile individuals will become reproductively active. In the honeybee, the absence of a queen producing pheromones will result in workers becoming fertile, suggesting pheromone influence over worker reproduction. In wasp and ant species queen egg surface hydrocarbons control worker reproduction. When queen laid eggs are present, worker fertility is suppressed. In the species Camponotus floridanus, when both worker and queen laid eggs are present, worker laid eggs are destroyed irrespective of their viability, signaling queen control of reproduction by workers.

Genetic caste determination
For some species of insect there is a genetic basis to caste determination, where genotype may bias or even predict specific castes at the level of the individual. The underlying genetic influence of castes is most well studied in Hymenopteran ant species.

The leaf-cutting ant Acromyrmex echinatior has a highly genetic basis of caste determination with the caste fate of offspring determined by the paternal parent. Acromyrmex colonies generally divide colony labor between two physically dimorphic sub-castes namely ‘major’ and ‘minor’ workers. While partially plastic due to environmental influence, offspring with different fathers but within the same colony have different propensities to develop into major and minor workers. The exact mechanism underlying genetic caste determination in Acromyrmex, and how environment may play a role remains unknown.

For some social ant species, such as Pogonomyrmex barbatus and Solenopsis xyloni, colony formation is reliant on the interbreeding of co-occurring (sympatric) but genetically distinct lineages, this process is referred to as social hybridogenesis. Social hybridogenesis provided a novel study of genetic control of castes as queens are produced by asexual reproduction or ‘thelytokous parthenogenesis’, while workers are produced only by the interbreeding of two genetically distinct lineages. Sexual production of workers indicates that the alleles required for worker castes are carried on the male chromosome. Some species, such as Cataglyphis hispanica, have lost the ability to produce workers without social hybridogenesis, showing a clear reliance on genetic contribution for caste production. Less severe examples of genetic bias of caste are also found in the honeybee and the termite where tasks carried out by the individual are influenced by the paternal parent.