Abstract
The Acacia drepanolobium (also known as Vachellia drepanolobium) ant‐plant symbiosis is considered a classic case of species coexistence, in which four species of tree‐defending ants compete for nesting space in a single host tree species. Coexistence in this system has been explained by trade‐offs in the ability of the ant associates to compete with each other for occupied trees versus the ability to colonize unoccupied trees. We seek to understand the proximal reasons for how and why the ant species vary in competitive or colonizing abilities, which are largely unknown. In this study, we use RADseq‐derived SNPs to identify relatedness of workers in colonies to test the hypothesis that competitively dominant ants reach large colony sizes due to polygyny, that is, the presence of multiple egg‐laying queens in a single colony. We find that variation in polygyny is not associated with competitive ability; in fact, the most dominant species, unexpectedly, showed little evidence of polygyny. We also use these markers to investigate variation in mating behavior among the ant species and find that different species vary in the number of males fathering the offspring of each colony. Finally, we show that the nature of polygyny varies between the two commonly polygynous species, Crematogaster mimosae and Tetraponera penzigi: in C. mimosae, queens in the same colony are often related, while this is not the case for T. penzigi. These results shed light on factors influencing the evolution of species coexistence in an ant‐plant mutualism, as well as demonstrating the effectiveness of RADseq‐derived SNPs for parentage analysis.
Highlights
Species coexistence, the question of how different species can coexist while competing for limiting resources, is a central question in ecology (Huston, 1979; Hutchinson, 1961)
We found no evidence that the genetic structure of colonies—in terms of polygyny, polyandry, or within-tree relatedness—underlies competitive ability, either within or between species, in the ant associates of A. drepanolobium
There was no association between polygyny and competitiveness: Our results do not support the hypothesis that the competitive C. sjostedti and C. mimosae are more polygynous than the less competitive C. nigriceps and T. penzigi (Palmer, 2004; Stanton et al, 2002)
Summary
The question of how different species can coexist while competing for limiting resources, is a central question in ecology (Huston, 1979; Hutchinson, 1961). The best-supported hypothesis to explain this coexistence is that a colonization-competition trade-off exists among the ant species: ants specialize either in colonizing new resources (i.e., unoccupied trees) or in competing for occupied resources. A primary hypothesis proposed is that more competitive species are polygynous, that is, they have multiple queens per colony (Palmer, 2004; Rubin et al, 2013). These queens are able to lay more eggs, thereby producing larger worker populations that can outcompete their neighbors. Using hundreds of single-n ucleotide polymorphisms for each of the four ant species, we reconstructed intra-colony relationships and were able to examine the degree of polyandry and polygyny of the ant species inhabiting A. drepanolobium
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