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Labial Gland Disease
"Pseudogynes" (Greek meaning "false queens") have long been known in various European Formica species, especially the wood ants. They were initially related to queens based on their similarity to true queens and because their origin was unclear. It was once thought that the guest beetle Lomechusa strumosa was responsible for their development, but this is not the case. It is now known that these individuals have enormously swollen labial glands (salivary glands), which are located in the mesosoma and exit to the mouth. This swelling is thought to be caused by a virus, however details are unknown. The method of spread and infection are also unknown. Infection causes a disease known as labial gland disease.
Labial gland disease causes swelling of the labial glands in the pupal stage of numerous Formica species. The resulting adults have enlarged thoraces and are called secretergates (Wasmann's pseudogynes). These adults infect younger larvae, probably during feeding. It soon becomes non-infectious outside the labial glands. Diseased queens were not seen to feed larvae, yet their offspring included secretergates. This discrepancy remains unexplained. (Elton, 1991)
It is unknown whether only deformed individuals are infected, or whether "normal" workers can also harbour the pathogen but without displaying morphological modifications. As infection normally (as far as known) takes place in the larval stage the conspicuously hunched thorax can develop during the pupal stage. If adult workers were to be infected, the shape of the thorax would already be fixed and would unable to change. Thus morphology alone may not be a reliable indicator of infection.
It has been proposed to differentiate between "Secretergate" (infected workers), "Secretogyne" (queens) and "Secretaner" (males). "Secretogyne", queens who are even more hunchbacked than normal thanks to the disease, can mate and lay fertile eggs; however, some infected queens do not lay eggs despite mating.
Labial gland disease in Formica polyctena. Healthy queen (top left; colour bleached by storage in ethanol), healthy worker (top right), and two “pseudogyne” or “secretergate” specimens with labial gland disease (bottom, note humped mesosoma) (from Laciny (2021), Fig. 7, originally from https://ameisenwiki.de/index.php/Pseudogynen_/_Secretergate_/_Viren. Accessed 15 Aug 2020).
Known Ant Hosts
Secretoforms have been found in six of the eight European species of the Formica rufa group, and may also occur in Formica uralensis and Formica pratensis (as Formica nigricans). They do occur in other species of the genus viz. in Formica sanguinea (Wasmann, 1915), Formica fusca (Wasmann, 1915), Formica lemani (Collingwood, 1956), Formica rufibarbis (Wasmann, 1915), and in some North American Formica species. They are a major mortality factor in F. sanguinea (Wasmann, 1915: 272-281) and could be so in other species. (Elton, 1991)
| Ant | Pathogen Species | Locality | Source | Notes |
|---|---|---|---|---|
| Formica fusca | unknown agent | Spain | Espadaler & Riasol, 1981; Elton, 1991 | |
| Formica lugubris | unknown agent | Spain | Espadaler & Riasol, 1981; Elton, 1991 | |
| Formica polyctena | unknown agent | Elton, 1991 | ||
| Formica pratensis | unknown agent | Japan | Elton, 1991 | as ''Formica nigricans'', needs confirmation |
| Formica rufa | unknown agent | Elton, 1975; Elton, 1991 | ||
| Formica rufibarbis | unknown agent | Elton, 1991 | ||
| Formica sanguinea | unknown agent | Elton, 1991 | ||
| Formica uralensis | unknown agent | Japan | Elton, 1991 | needs confirmation |
References
- Collingwood, C.A., 1956. Aberrations in British ants of the genus Formica. J. Soc. Br. Entomol. 5:193-196.
- Elton, E.T.G. 1975. Females of Formica rufa L. (Hym., Formicidae) with enlarged labial glands. Insect Sociaux 22: 405-414 (doi:10.1007/BF02224115).
- Elton, E.T.G. 1989. On transmission of the labial gland disease in Formica rufa and Formica polyctena (Hymenoptera, Formicidae). Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen: Series C: Biological and Medical Sciences 92: 415-459.
- Elton, E.T.G. 1991. Labial gland disease in the genus Formica (Formicidae, Hymenoptera). Insectes Sociaux 38, 91-93 (doi:10.1007/BF01242717).
- Espadaler, G.X., Riasol, B.J.M. 1981. Secretergates de Formica sp.: una morfologia de origen patologico en hormigas. Revista Ibérica de Parasitología 41: 539-549.
- Laciny, A. 2021. Among the shapeshifters: parasite-induced morphologies in ants (Hymenoptera, Formicidae) and their relevance within the EcoEvoDevo framework. EvoDevo 12, 2 (doi:10.1186/s13227-021-00173-2).
- Sonobe, R. 1974. On the occurrence of pseudogyne of Formica japonica Motschulsky (Hymenoptera, Formicidae) in Japan. Kontyû 42: 401-403.
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