Gamergate

Lateral view of a female Bothroponera strigulosa worker, the ant for which the term "gamergate" was originally coined^{[note 1]}

A gamergate /ˈɡæmərˌɡeɪt/ is a reproductively viable female worker ant that is able to reproduce with mature males when the colony is lacking a queen. Most commonly occurring within colonies of the primitive species of the poneromorph subfamilies, gamergate females differ from their fellow workers by a combination of elevated fecundity and aggression-related mutilation of competitors' secondary sexual characteristics. Subsequent to their first mating event, however, aggression is no longer needed as females secrete chemical signals that lead the workers to accept their role as reproducers for the colony.

Gamergates exist in colonies with winged and ergatoid queens as well as singly in monogynous colonies and alongside other reproductively viable gamergates in polygynous colonies. Most gamergate species are solitary generalist foragers living in arid environments.

Etymology

The term "gamergate" derives from the Greek words γάμος (gámos) and ἐργάτης (ergátēs) and means "married worker." It was coined in 1983 by geneticist William L. Brown^[1] and was first used in scientific literature by entomologists Christian Peeters and Robin Crewe in a 1984 paper published in Naturwissenschaften.^[2] The definition typically found in entomological dictionaries is "mated, egg-laying worker,"^[3][4] and is drawn from the glossary of Bert Hölldobler and E. O. Wilson's 1990 book, The Ants.^[3]

Overview

There exist between one hundred and two hundred different species of queenless ants (roughly 1% of all ants), most of which fall within the poneromorph subfamilies. Whereas female workers in colonies with a queen are morphologically incapable of mating, in gamergate societies a single reproductive worker and sometimes a cadre of dominant female workers have active ovaries.^[5] Gamergate lifespan is short compared to queens in queenright colonies, but gamergates can be replaced by other dominant workers in the colony without risking colony survival. Reproductive investment in gamergate females is thus optimized because non-differentiated gamergates (i.e. reproductively inactive workers) function as laborers.^[6]

Caste structure

Dorsal view of a nonreproductive female Diacamma australe worker. Note the lack of anterior thoracic gemmae (buds).

Within gamergate colonies all females are born reproductively viable and thus represent potential gamergates. Prior to differentiation as a gamergate, a dominant female worker must physically inhibit its sisters. For example in the case of Diacamma australe, the first females to reach maturity will clip off the thoracic gemmae of their sisters. This mutilation greatly reduces the attractiveness of the female as a mate. Thereafter persistent domination of worker females by gamergates via physical aggression all but ensure that they will not produce male offspring.^[7] In Diacamma nilgiri gamergates use dominance interactions to monopolize reproduction without mutilation of sister workers.^[8] The same is true for Streblognathus peetersi which engage in non-injurious aggression "games" to determine dominance.^[2] For all gamergate species, the act of mating eliminates the need to physically dominate female workers. Instead newly produced pheromones or signaling chemicals ensure that workers remain nonreproductive. Although it is unknown to what degree these chemicals act as pheromones or as signals, support for the signaling hypothesis can be found in the loss of gamergate reproductive inhibition of workers as the gamergate grows older and her fecundity diminishes.^[7]

When a reproductive gamergate dies, it is replaced by a former dominant worker who becomes a new gamergate. New gamergates often emerge from the female cohort closest to maturity at the time of death of the previous gamergate, but in some cases this event triggers adult female workers to differentiate directly to gamergates. For example, when a queen dies in a Harpegnathos saltator colony, workers of the colony will begin to fight for dominance to become the next queen and produce the next eggs.^[9] Because reproductively inactive workers are able to become reproductive again after the death of the gamergate, some gamergate species can be considered cooperative breeders rather than truly eusocial insects.^[10]

In colonies with queens, gamergates, and workers, gamergates occupy an intermediate caste.^[5] Research on Amblyoponini species displaying gamergate social structures has found that there is a fecundity-based hierarchy within the gamergate caste as well. In a study on Amblyopone sp., it was found that higher-ranked gamergates had more fully developed oocytes than low-ranked gamergates. The near absence of sterile gamergates suggests to researchers that particularly low-ranked gamergates may be expelled from their colonies.^[11] A three-tiered gamergate hierarchy also occurs within Streblognathus peetersi with only alpha-gamergates reproducing while beta- and gamma-gamergates await a chance to reproduce when an alpha-gamergate has lost her fecundity or died.^[2] Challenges to top-ranked gamergates from workers of the lower-hierarchies are risky for the challenger because gamergates of species like Dinoponera quadriceps may mark the challenger by stinging her with special chemicals only produced by the dominant gamergate. These chemicals signal other workers to immobilize the challenger by biting her appendages and holding her for up to a few days until the threat has passed.^[12]

The role that the dominated worker plays in selection of reproductive females is seen in the expulsion of infertile gamergates in Amblyopone colonies,^[11] and it is even more apparent in Dinoponera quadriceps where workers may choose to ignore the chemical marker applied by the dominant gamergate. If the dominant gamergate is less reproductively fit than the challenger, workers may instead bite the appendages of the gamergate and hold her immobilized allowing a more fecund challenger to establish herself as the new alpha-gamergate. By playing a part in the selection of gamergates, dominated workers are able to increase their indirect reproduction.^[5]

Social structure variation and ecology

There is variation within the social structure of ant colonies with a gamergate caste. Some species such as Harpegnathos saltator,^[13] Pachycondyla spp., Gnamptogynes menadensis, and Rhytidoponera confusa have a winged alate queen caste, a gamergate caste, and a non-reproductive worker caste.^[14] Some species that normally have a queen caste have been shown to persist for long periods of time by relying on the gamergate caste. For example, a colony of Myrmecia pyriformis in 1998 was collected and lasted three years reproducing without a queen ant.^[15]

Queenless species with only gamergates and workers may have a monogynous structure with a single gamergate female reproducing for the entire colony like a queen or they may have a polygynous structure with multiple reproductive gamergates laying diploid eggs. Examples of monogynous queenless species include Pachycondyla krugeri, P. sublaevis, Diacamma australe, D. rugosum, Platythyrea lamellosa, and Streblognathus aethiopicus among others.^[16] For monogynous gamergates, social regulation is based on morphometry (sexual attractiveness) and fecundity (ovarian oogenesis). Examples of polygynous queenless species include Ophthalmopone berthoudi, O. hottentota, Dinoponera quadriceps, and all known queenless species of Rhytidoponera spp.^[16] In the queenless Pachycondyla berthoudi (junior synonym of Bothroponera strigulosa), foreign males visit underground nests to mate with the polygynous gamergate workers.^[17]

Yet other systems exist for example in Pachycondyla spp. where gamergates and ergatogynes share a colony with workers.^[18]

Ecologically, gamergate species from different tribes and genera often tend to share certain characteristics. Most gamergate species are solitary generalist foragers living in arid environments.^[6] Like ergatoid systems, the evolution of gamergate social structure is hypothesized to be a response to frequent colonial fission events such as periodic flooding or changes in microclimate such as might occur in harsh arid climates. Myrmecologists Christian Peeters and Fuminori Ito have also suggested that "the evolution of gamergate reproduction appears strongly associated with the adaptive benefits of secondary polygyny (e.g. increased colony lifespan and resource inheritance), and it is the preferred option in species having workers able to reproduce sexually."^[14]

Classification dispute

The utility of "gamergate" as a morphological designation is not without critics. Within the field of myrmecology it is a matter of dispute whether caste should be defined primarily by reproductive role or by physical morphology. Notably, Alfred Buschinger has argued that the term "worker" should be applied only to those ants who make up the non-reproductive caste and "queen" should be applied only to reproductively viable female ants regardless of their physical appearance. Hölldobler and Wilson suggest that the two positions can be semantically resolved and that the most fruitful approach would be to keep classification "somewhat loose, incorporating either anatomy or roles in a manner that maximizes convenience, precision, and clarity of expression."^[19]

Genera with gamergates

This list may be incomplete and may require expansion:

• Poneromorph subfamilies
  • Amblyoponinae
    • Amblyopone^[11]
  • Ectatomminae
    • Rhytidoponera^[20]
  • Ponerinae
    • Anochetus^[21]
    • Bothroponera^[21]
    • Diacamma^[21][22]
    • Dinoponera^[21][23]
    • Hagensia^[21]
    • Harpegnathos^[21][24]
    • Leptogenys^[21]
    • Ophthalmopone^[21][25]
    • Platythyrea^[21][26]
    • Pseudoneoponera^[21]
    • Streblognathus^[21][27]
    • Thaumatomyrmex^[21]
• Myrmeciinae

• Myrmecia^[15]

• Myrmicinae

• Pristomyrmex^{[19][note 2]}

See also

• Laying worker bee
• Worker policing

Notes

References