Convergent development of ecological, genetic, and morphological traits in native supercolonies of the red ant Myrmica rubra

Abstract

Ant supercolonies (large networks of interconnected nests) represent the most extreme form of multi-queen breeding (polygyny) and have been found across ant lineages, usually in specific long-term stable populations. Many studies on the genetic population structure and demography of ant supercolonies have been done in recent decades, but they have lacked multicolonial control patches with separated colonies headed by a single or few queens so the origin of the supercolonial trait syndrome has remained enigmatic. Here, we set out to compare sympatric supercolonial and multicolonial patches in two natural Danish populations of the common red ant Myrmica rubra. We used DNA microsatellites to reconstruct genetic colony/population structure and obtained morphological and density measurements to estimate life history and ecology covariates. We found that supercolonies in both populations completely dominated their patches whereas colonies in multicolonial patches coexisted with other ant species. Supercolony patches had very low genetic differentiation between nests, negligible relatedness within nests, and lower inbreeding than multicolonial patches, but there were no significant morphological differences. One population also had nests that approached true outbred monogyny with larger workers and males but smaller queens than in the two other social nest types. Our results suggest that once smaller colonies start to adopt additional queens, they also gain the potential to ultimately become supercolonial when the habitat allows rapid expansion through nest budding. This is relevant for understanding obligate polygyny in ants and for appreciating how and why introduced North American populations of M. rubra have recently become invasive.

An Alpine ant’s behavioural polymorphism: monogyny with and without internest aggression inTetramorium alpestre

“…The aggression indices AI and MMAI yielded different values (Figs 2A–B and 3). AI integrates behaviour over time and is more balanced, in that brief aggressive interactions caused, for example, by disturbance (Huszar et al 2014) do not strongly affect AI. MMAI uses the mean of the most aggressive behaviour observed and helps detecting if any aggression occurs.…”

“…4), as workers generally attack non-nestmates to protect the nest against intruders (d’Ettorre & Lenoir 2009). Brief aggression observed in the first seconds of an encounter, however, might be a result of disturbance (Huszar et al 2014) indicating that workers eventually recognised each other and stopped aggressive behaviour. Also the average of pairwise intranest relatedness had no influence on the average aggressive behaviour (Spearman rank correlation) implying that aggression, if occurring at all, seems not to depend on intranest relatedness, at least for this pilot study.…”

“…They are extensive cooperative units with many queens and very many workers integrated harmoniously over several square metres to many square kilometres (Crozier & Pamilo 1996; Giraud et al 2002; Steiner et al 2009). Supercoloniality is often considered key to the success of invasive ant species (Holway et al 2002), and supercolonies have been thoroughly studied over the last decades (Tsutsui et al 2000; Giraud et al 2002; Pedersen et al 2006; Leniaud et al 2011; Huszar et al 2014; Kennedy et al 2014). However, the factors triggering their emergence remain largely unknown (Suarez & Suhr 2012).…”

Acceptance of alien queens by the ruby ant Myrmica rubra (Hymenoptera: Formicidae): Gene flow by queen flow

“…However, the high nest mate relatedness could be a result of mother-daughter queen pools in a founder population nest. The pattern in the relatedness for that population could also indicate multicolonial polygyny with few queens per nest (Huszár et al, 2014). The nests in that population were populous, in a line along the edge of a forest with distance between neighbouring nests less than 1 m. Myrmica rubra is very aggressive against alien conspecifi c workers, and thus, such a multicolonial population would be unstable and would eventually turn into a unicolonial population (Huszár et al, 2014) or a wider dispersion of the independent nests.…”

“…The tendency of M. rubra to accept alien queens may be one of the factors behind its invasion success, because it increases the survival of colonies and increases the gene pool (e.g., Huszár et al, 2014). In addition, accidentally translocated parts of colonies without queens may readily accept alien conspecifi c queens, which makes colonisation of new areas more successful and rapid.…”