Abstract
Wood ant colonies that appear to consist of individuals representing different species are described in several previous papers. The present study is the first to elucidate the genetic basis of the spectacular morphological variability observed within such colonies. Two seemingly colonies (FM-1 and FM-2) from southern Finland were investigated. On the basis of the morphology of their workers these colonies were comprised of individuals with phenotypes typical of Formica rufa L., F. polyctena Forst., and F. aquilonia Yarr. The sequence of an mtDNA fragment (5' end of the cytochrome b gene) was used to examine the phylogenetic relationships among haplotypes of workers from homogeneous colonies of different wood ant species and the two supposedly colonies, and to sort the individuals within the colonies into matrilines. Six microsatellite loci were used to analyse the genetic differentiation between colonies and among workers within colonies, and to detect putative hybrids. The results show that, independently of their phenotype, workers from the mixed colonies were genetically more similar to other individuals in their colony than to those in the homogeneous F. rufa, F. polyctena or F. aquilonia colonies. However, while colony FM-1 consisted of offspring of the same queen or more likely several maternally related queens, colony FM-2 consisted of the offspring of at least four unrelated queens. The data suggest hybridisation between F. polyctena × F. aquilonia and F. polyctena × F. rufa (and possibly subsequent mating between these hybrids) as the most probable mechanism leading to the existence of these two colonies, which implies that the hybrids are fertile. This study shows that colonies of wood ant hybrids can arise spontaneously and persist under natural conditions. The results also revealed that even some morphologically homogeneous colonies are genetically heterogeneous. In the case of closely related, morphologically similar species that interbreed, morphology can be a bad predictor of genetic differences between individuals.
Highlights
Wood ants, i.e. representatives of the subgenus Formica s. str. in general, and members of the F. rufa-group, well studied, still pose taxonomic problems and often puzzle field researchers
There is some evidence supporting the possibility of interspecific hybridisation within the F. rufa-group (e.g. Seifert, 1991, 1999; Czechowski, 1996; Goropashnaya et al, 2004; Gyllenstrand et al, 2004; Seifert & Goropashnaya, 2004; Sorvari, 2006), while on the other hand, there are data that reveal the occurrence of temporary social parasitism within the subgenus Formica s. str., i.e. colonies that are taken over by heterospecific queens
The first clade (2-1) contained the haplotypes that were found in 17 individuals of F. rufa, 8 of F. polyctena, 2 of F. lugubris, and all 17 workers from “mixed” colony FM-2
Summary
I.e. representatives of the subgenus Formica s. str. in general, and members of the F. rufa-group, well studied (see e.g. Cotti, 1963; Beretta Boera, 1979; Gösswald, 1989; Czechowski, 1996), still pose taxonomic problems and often puzzle field researchers. A quarter of a century ago, Vepsäläinen & Pisarski (1981) called the F. rufa-group “a taxonomic enfant terrible” among wood ants and described the state of its taxonomy as “chaos before order”. These vivid expressions have lost none of their relevance. Str., i.e. colonies that are taken over by heterospecific queens (see e.g. Pisarski & Czechowski, 1994; Czechowski, 1996). The latter phenomenon would result in temporarily mixed colonies
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