Mixed colonies and hybridisation of Messor harvester ant species (Hymenoptera: Formicidae)

Abstract

The Mediterranean harvester ant species Messor minor, M. cf. wasmanni, and M. capitatus can co-occur in the same habitat. In Italian populations, we encountered colonies that contained workers from more than one species as identified via standard morphology, as well as colonies with workers that appeared to be morphologically intermediate between species. This unusual finding required further analysis. We analysed such colonies using microsatellites, mitochondrial DNA and refined morphometrics, and a simple inference key for the colony-level interpretation of data from the three sources combined. We infer that Messor minor and M. cf. wasmanni engage in bidirectional interspecific gene flow. Hybrids between these two species are inferred to produce fertile offspring, which would indicate that barriers to hybridisation do not exist or can be completely overcome. This is unexpected, given that they are non-sister species and broadly sympatric in nature. Our findings also indicate the possible occurrence of hybrid-hybrid crosses, a phenomenon rarely observed in ants. We cautiously interpret the data at hand as in support of the interspecific gene flow considerably shaping the genetic makeup of populations, raising the question about a potential adaptive value of this hybridisation. Messor capitatus mixes with hybrids of the other two species, but we found no indication of hybridisation involving this species. We discuss various hypotheses on the causations of colony mixing and hybridisation in the three Messor species at the proximate and ultimate level.

Ants exhibit asymmetric hybridization in a mosaic hybrid zone

“…More research is needed to determine the prevalence of colony‐level mechanisms in speciation and reproductive isolation (Steiner et al . ).…”

“…; Steiner et al . ; Kulmuni & Pamilo ). Because of the haplodiploid genetic system, hybridization can lead to unusual genetic patterns in Hymenoptera.…”

Temnothorax crasecundus sp. n. – a cryptic Eurocaucasian ant species (Hymenoptera, Formicidae) discovered by Nest Centroid Clustering

“…In five data sets with a mean percentage of 18.9% hybrids, four of these did not involve cryptic species, the average agreement of NC-Ward and NC-K-means clustering with the LDA vector indication was reduced to 87.6%. They also stated that the best phenotypical identification of hybrids is given by their position along an interspecific vector (examples in Kulmuni et al 2010; Seifert et al 2010; Steiner et al 2011; Bagherian et al 2012). LDA vectors in particular are rather robust against the fluctuating nature of characters in hybrids.…”

“…Exemplary groups in animals are ducks (Phillips 1915, 1921), redstarts (Ertan 2002) or butterflies (Mavárez et al 2006, Kronforst 2008). Ants are no exception: as much as 18% of the Central European ant species are credibly shown to hybridize (Pearson 1983, Seifert 1984, Douwes and Stille 1991, Seifert 1999, 2006, Seifert et al 2010, Kulmuni et al 2010, Steiner et al 2011, Van der Have et al 2011, Bagherian et al 2012, Seifert 2013). A workable species concept has to recognize this.…”

Cryptic lineages hybridize for worker production in the harvester ant Messor barbarus

“…These lineages have been shown to harbour chimaeric genomes of two parental species, but the causal link between this hybrid origin and the actual system remains controversial [17]. Lineages of M. barbarus may also be of interspecific hybrid origin as unusual colony mixing and hybridization has been previously reported in the genus [18]. …”