Agamergate (/ˈɡæmərˌɡeɪt/GAMM-ər-gayt) is a matedworker ant that can reproduce sexually, i.e., lay fertilized eggs that will develop as females. In the vast majority of ant species, workers are sterile and gamergates are restricted to taxa where the workers have a functional sperm reservoir ('spermatheca'). In some species, gamergates reproduce in addition to winged queens (usually upon the death of the original foundress), while in other species the queen caste has been completely replaced by gamergates. In gamergate species, all workers in a colony have similar reproductive potentials, but as a result of physical interactions, a dominance hierarchy is formed and only one or a few top-ranking workers can mate (usually with foreign males) and produce eggs. Subsequently, however, aggression is no longer needed as gamergates secrete chemical signals that inform the other workers of their reproductive status in the colony.
Depending on the species, there can be one gamergate per colony (monogyny) or several gamergates (polygyny). Most gamergate species have colonies with a few hundred or fewer workers.
Gamergate derives from the Greek wordsγάμος (gámos) andἐργάτης (ergátēs) and means 'married worker'. It was coined in 1983 by geneticistWilliam Louis Brown[1] and was first used in scientific literature by entomologistsChristian Peeters andRobin Crewe in a 1984 paper published inNaturwissenschaften.[2] The definition typically found in entomological dictionaries is 'mated, egg-laying worker',[3][4] and is drawn from the glossary ofBert Hölldobler andE. O. Wilson's 1990 book,The Ants.[3]
There are 100–200 different species in which gamergates reproduce (roughly 1% of all ants), most of which fall within theponeromorph subfamilies. Whereas workers (which are all females) in most ant species aremorphologically incapable of storing sperm, in gamergate species one or several workers mate and have activeovaries.[5] Gamergate lifespan is short compared to queens inqueenright 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]
Within gamergate colonies, all workers are born reproductively viable and are thus potential gamergates. Prior to differentiation as a gamergate, a dominant worker must physically inhibit its sisters. For example, in the case ofDiacamma australe, the first female to become reproductively active will clip off thethoracic gemmae of her sisters, thus greatly reducing their sexual attractiveness. In other genera, persistent domination of worker females by gamergates via physical aggression all but ensure that they will not produce male offspring.[7] InDiacamma nilgiri, gamergates use dominance interactions to monopolize reproduction without mutilation of sister workers.[8] The same is true forStreblognathus peetersi, which engage in non-injurious aggression to determine dominance.[2] For most gamergate species, the start of ovarian activity eliminates the need to physically dominate nestmate workers. Instead newly producedpheromones 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 reproductive inhibition of workers as the gamergate grows older and herfecundity diminishes.[7]
Mechanisms of gamergate replacement vary among monogynous and polygynous species. When a gamergate dies, it is usually replaced by a formerly submissive worker who proceeds to mate and begins ovarian activity. A new gamergate often originates from a youngercohort. For example, when the original founding queen dies in aHarpegnathos saltator colony, younger workers begin to fight for dominance and some become the next reproductives.[9] Because reproductively inactive workers are able to activate their ovaries after the death of the gamergate, some gamergate species can be consideredcooperative breeders rather than trulyeusocial insects.[10]
In colonies with both queens and gamergates, the latter function as secondary reproductives.[5] Research onAmblyoponinae species has shown that there is a fecundity-based hierarchy among gamergates. InStigmatomma reclinatum, it was found that higher-ranked gamergates had more fully developedoocytes than low-ranked gamergates.[11] InStreblognathus peetersi, only the alpha worker mates and becomes the gamergate; younger workers await a chance to reproduce when the current gamergate exhibits decreased fecundity or dies.[2] Challenges to gamergates from subordinate workers are risky because the gamergate in species likeDinoponera quadriceps may mark the challenger by rubbing special chemicals produced only by the gamergate. These chemicals signal to other workers to immobilize the challenger by biting her appendages and immobilizing her for a few days until herhormonal levels return to normal.[12] Subordinate workers play an important policing role in theselection of future gamergates and are thus able to increase their indirect fitness.[5]
There is much variation in the social structure of ant colonies with gamergates. Some species such asHarpegnathos saltator,[13]Pseudoneoponera tridentata,Gnamptogenys menadensis, andRhytidoponera confusa have a wingedalate queen caste as well as gamergates.[14] Queenless species with only gamergates and workers may have amonogynous structure with a single gamergate or they may have apolygynous structure with multiple gamergates. Examples of monogynous queenless species includePachycondyla krugeri,P. sublaevis,Diacamma australe,D. rugosum,Dinoponera quadriceps,Platythyrea lamellosa, andStreblognathus aethiopicus.[15] Examples of polygynous queenless species includeOphthalmopone berthoudi,O. hottentota,[16] and all known queenless species ofRhytidoponera.[15] In the queenlessOphthalmopone berthoudi, foreign males visit underground nests to mate with young workers.[17]
Ecologically, gamergate species from differenttribes andgenera often tend to share certain characteristics. Many gamergate species are solitary generalist foragers living inarid environments.[6] Similar to species withergatoid queens, the evolution of gamergate reproduction is hypothesized to be associated with a shift tocolonial fission. Myrmecologists Christian Peeters andFuminori 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 store sperm."[14]
The utility ofgamergate as a morphological designation is not without critics. Within the field ofmyrmecology it is a matter of dispute whethercaste should be defined primarily by reproductive role or by physical morphology. Notably,Alfred Buschinger has argued that the termworker should be applied only to those ants who make up the non-reproductive caste andqueen 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."[18]
The existence of gamergates issymplesiomorphic, or ancestral, in ants.[19] They are known to be present in the following genera:
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