Ontogeny:

The hierarchical differentiation in eusocial insects can vary from the surprisingly simple to the royally complex. In all species, there is a differentiation at least between reproductive females and non-reproductive workers. (Note: there do exist some species of ants that are "queenless" and we will explore the ontogeny of their differentiation as well).

Different Morphs Found in Ant Species:

When considering ontogeny and its role on ant hierarchy, it is important to consider the different hormonal and genetic effects in conjunction with the overall morphology of the individual in order to more fully comprehend the different functions the over and under-expression of these certain chemical can cause within individual ants. In general, there tend to exist three types of morphs in ant species: Reproductively viable queens, sterile female workers, and haploid male workers.

The Queen:

The queen or queens of an ant colony spend their lives pumping out eggs for the colony. These individuals have phenotypes that are determined by the level of hormone expression during the larval stage and 1st and 2nd pupil instars, as well as by the collective choice (as determined by genetics) of the workers as to which queen reigns supreme. They tend to be bigger than the workers and have wings, which enable them to disperse to find new colonies if they so choose. (Schrempf 2006).

The Sterile Worker:

The sterile worker is a female phenotype resulting from differential hormone levels during development that robbed some females of the ability to reproduce and relegated them to the working class. These workers generally exhibit reduced ovaries and lack a spermatheca. (Lecoutey 2010). They also usually begin the development of wings in the larval stage, but these wings experience apoptosis before they leave the pupal stage of development. These individuals are diploid, having arisen from the fertilization of a queen?s egg, and do not exist in some so called ?queenless? species. (Miura 2005).

The Haploid Male:

Male ants are produced from the unfertilized haploid eggs of virgin queens. They contain exactly half of the genome of their mother queen, and are thus, more related to her than their worker sisters, whose genome comes half from their mother and half from their father. Males specialize in foraging and protection in most species. (Ometto 2010).

The Winged Male: The addition of juvenile hormone to males during the first and second (of three) pupil instars prompted the differentiation of individual males into a winged phenotype, which resemble typical males in other ant species and are peaceful observers. The upregulation of juvenile hormone is also responsible for the formation of wings and larger fat deposits in queen ants, showing that the mechanisms that determine male and female morph are probably quite similar. (Shrempf 2006).

The Wingless Male: Without the addition of juvenile hormone, the males develop into the more aggressive wingless morph, who engage more readily in fighting and try to guard sexual females for themselves. In most ant species, there is only one male morph, which typically is ergatoid, or without wings. (Shrempf 2006).

Social Structure As Related To Ontogeny:

Monogynes :Some species of ants are what we consider monogynous, in that they only have 1 queen ant per nest, and the rest of the virgin queens disperse once they reach sexual maturity, lest they be killed off by the workers if they aren?t lucky enough to be crowned supreme ruler, so to speak, by their subjects.

Polygynes: Other species (and even some subsets of the same species) are polygynous, and have multiple queens within each hive carrying out the task of reproduction, so most of their virgin queens remain in their original nest and carry out their egg laying responsibilities there.

Gp-9 and chemosignalling: Which social structure the ant nest exhibits, at least as far as the species Solenopsis invicta goes, depends on a single nuclear protein encoding gene named general protein-9 or Gp-9. Those colonies who?s workers are homozygous for this gene (Gp-9 ??)are monogynous, killing off all but one queen who is also homozygous for the gene and those that are heterozygous for the gene (Gp-9 ?b), are polygynous, killing only those potential queens who are homozygous for Gp-9, and letting the virgin queens who are heterozygous for the trait continue to develop and reproduce. This system suggests that somehow, the differences in the homozygous and heterozygous versions of this gene can be sensed by the worker ants, and triggers within them a drive to kill potential queens based on the aforementioned pattern. Research suggests that this could depend on the differences in pheromone production caused by allelic variation between individuals, because Gp-9 has been previously identified as an oderant-binding protein in other eusocial insect species. In general, individuals homozygous for Gp-9 were fatter and more likely to have wings, whereas individuals heterozygous for Gp-9 tended to lose their wings and weigh less (Deitrich 2007).

Queenless Colonies: There is another case to discuss, which is the case of so-called "queenless" ant colonies, whose workers and reproductives do not differ phenotypically. Thus, all females are theoretically viable to become reproductive queens. This, however does not happen. Once the females reach sexual maturity, the workers take it upon themselves to clip off the wing disks of many of the fertile females. This wing disk has evolved in these individuals to become a gland in the exocrine system and secretes certain reproductive pheromones. Upon the death of a queen, these reproductively viable workers begin gatherin fat deposits and releasing fertility signals to the rest of the hive. The workers subsequently clip off the wing disk gland from all of these workers but one (or in some cases, a few) determining their next queen. This is interesting, because it is a case in which phenotypic expression of reproductive viability is the same for all females, and subsequently becomes altered by members of the nest presumably based on some sort of physiological cue the workers pick up on. (Miura 2005) Furthermore, in another queenless colony, Diacamma sp, researchers found that reproductive differentiation between females is determined not at early oogenesis as previously thought, but during late oogenesis via the upregulation insulin signaling pathways in nurse cells during late oogenesis (Okada 2010).

The Second Pupal Instar and Hierarchical Predetermination:

Juvenile Hormone: Most of the differentiation between the different phenotypes does not actually become apparent until the second of three pupil stages. It turns out, phenotype is largely determined by the presence or absence of certain hormones during key stages of growth and sexual development. For example, the sexual viability of female ants depends on the presence of higher levels of juvenile hormone during key parts of the larval stage. The absence of this higher level of juvenile hormone similarly results in sterile and wingless female workers. In species that include a winged-male phenotype, the presence of upregulated juvenile hormone similarly induces wing development on the individual (Wurm 2010).

Differentiation of virgin queens after the orphaning of a hive also depends on increases in JH. Virgin queens have about the same levels of JH expression as do normal queens, but the expression of genes that degrade JH are also upregulated when compared to sterile workers. When a nest is orphaned, JH degrading genes become down-regulated, and JH levels within that individual rise. The virgin queen begins gathering fat deposits and what happens next depends on the species. (Ometto 2010).

 

gene expression

Fig. 1 Genes differentially expressed between S. invicta and S. richteri in pupae and adults. The light sections of each bar indicate genes that are differentially expressed between species exclusively in one caste. The dark sections indicate genes that are differentially expressed between species in two or all three castes.