Abstract
BackgroundHybridization can have complex effects on evolutionary dynamics in ants because of the combination of haplodiploid sex-determination and eusociality. While hybrid non-reproductive workers have been found in a range of species, examples of gene-flow via hybrid queens and males are rare. We studied hybridization in East African army ants (Dorylus subgenus Anomma) using morphology, mitochondrial DNA sequences, and nuclear microsatellites.ResultsWhile the mitochondrial phylogeny had a strong geographic signal, different species were not recovered as monophyletic. At our main study site at Kakamega Forest, a mitochondrial haplotype was shared between a "Dorylus molestus-like" and a "Dorylus wilverthi-like" form. This pattern is best explained by introgression following hybridization between D. molestus and D. wilverthi. Microsatellite data from workers showed that the two morphological forms correspond to two distinct genetic clusters, with a significant proportion of individuals being classified as hybrids.ConclusionsWe conclude that hybridization and gene-flow between the two army ant species D. molestus and D. wilverthi has occurred, and that mating between the two forms continues to regularly produce hybrid workers. Hybridization is particularly surprising in army ants because workers have control over which males are allowed to mate with a young virgin queen inside the colony.
Highlights
Hybridization can have complex effects on evolutionary dynamics in ants because of the combination of haplodiploid sex-determination and eusociality
Molestus-like population from Kakamega, and one D. molestuslike worker from Kakamega was assigned to the D. wilverthi-like population
Another noteworthy case of apparent mitochondrial introgression was found in the population from Tana River, where haplotypes from both sides of the river tend to cluster in clearly distinct clades (Figures 3 and 4) according to microsatellite data, all samples belong to a single population
Summary
(a) Morphological Analysis The D. wilverthi-like workers from Kakamega (see Figure 1 for geographic positions of study sites) had the posterior angles of the head prolonged into a raised point, but not recurved outwards as in specimens from more western populations (Figure 2a). More than half of the examined D. molestus-like workers lacked the characteristic petiolar tubercles Workers of both forms at Kakamega had relatively shorter antennal scapes than the workers from the respective “pure” allopatric populations (Figure 2b). The mitochondrial haplotype diversity was much lower in the D. wilverthi-like population (Na = 2; Rs = 1.55; N = 34) than in the D. molestus populations (Kakamega: Na = 7; Rs = 6.24; N = 42; Mt. Kenya: Na = 8; Rs = 6.94; N = 29; Tana River: Na = 5; Rs = 5.00; N = 11). According to this estimate, 18.2% of the genotyped workers from Kakamega Forest were hybrids.
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