The role of microgynes in the reproductive strategy of the neotropical ant Ectatomma ruidum

Genome and cuticular hydrocarbon-based species delimitation shed light on potential drivers of speciation in a Neotropical ant species complex

Geographic separation that leads to the evolution of reproductive isolation between populations generally is considered the most common form of speciation. However, speciation may also occur in the absence of geographic barriers due to phenotypic and genotypic factors such as chemical cue divergence, mating signal divergence, and mitonuclear conflict. Here, we performed an integrative study based on two genome-wide techniques (3RAD and ultraconserved elements) coupled with cuticular hydrocarbon (CHC) and mitochondrial (mt) DNA sequence data, to assess the species limits within the Ectatomma ruidum species complex, a widespread and conspicuous group of Neotropical ants for which heteroplasmy (i.e., presence of multiple mtDNA variants in an individual) has been recently discovered in some populations from southeast Mexico. Our analyses indicate the existence of at least five distinct species in this complex: two widely distributed across the Neotropics, and three that are restricted to southeast Mexico and that apparently have high levels of heteroplasmy. We found that species boundaries in the complex did not coincide with geographic barriers. We therefore consider possible roles of alternative drivers that may have promoted the observed patterns of speciation, including mitonuclear incompatibility, CHC differentiation, and colony structure. Our study highlights the importance of simultaneously assessing different sources of evidence to disentangle the species limits of taxa with complicated evolutionary histories.

Nest architecture and colony composition in two populations of Ectatomma ruidum sp. 2 (E. ruidum species complex) in southwestern Colombia

Nest architecture plays a fundamental role in the adaptation of ants to their habitat, favoring the action of economically important species. Ectatomma ruidum sp. 2 (ruidum species complex) is a biological control agent in Neotropical agroecosystems, exhibiting high bioturbation impact due to high nest densities. The architecture and composition of 152 nests were studied in two Andean populations of southwestern Colombia, 24 of them being cast using the paraffin wax technique. Nest entrance was a single, circular, 4 mm hole at ground level, without any special external structure, connected to a single vertical tunnel communicating with successive half ellipsoidal chambers. Nests were extremely shallow (depth range: 28.7–35.4 cm), with an average of six chambers and an overall volume of 92.2 cm³ per nest. The deeper the chamber, the smaller its volume. Nest building was independent of plants or roots, and no surface or underground physical connections were found between neighboring nests. Few nests possessed a queen, and neither ergatoids nor microgynes were recorded. Despite significant interactions between localities and the number of both males and workers, queen presence had an overall highly positive effect on the number of workers and larvae and a negative one on the number of gynes. Overall, the studied Colombian populations of E. ruidum sp. 2 retained the simple nest structure described for other species of this species complex and for colonies of the same species from other geographical areas, though they constrasted in their extreme shallowness. Our data suggest that E. ruidum sp. 2, at the local level, does not follow the usual monodomic pattern of this species with facultative polygyny but, rather, has a polydomic pattern with monogyny, perhaps related to the extreme shallowness of the nests due to soil structure, which could significantly enhance the queen’s reproductive inhibition previously reported for this species.

Context dependent life-history shift in Macrodinychus sellnicki mites attacking a native ant host in Colombia

Ant parasitoidism has been reported in seven of the 26 recognized species of the mite genus Macrodinychus (Machrodynichidae). Macrodynichus sellnicki, previously reported as a parasitoid of the invasive ant Nylanderia fulva in Colombia, is now reported, in the same region, as attacking a native host, Ectatomma sp. 2 (E. ruidum complex). The mite develops within the protective silk cocoon of an Ectatomma pupa and waits for the emergence of the young ant before leaving the cocoon, unmolested. Overall nest prevalence was relatively high (34.6% of the 52 nests containing cocoons) but pupae prevalence was low (4.0%, n = 1401 cocoons). Mite life-history (parasite or parasitoid) was context dependent, shifting according to the intensity of the attack on a same host. Contrary to the strictly parasitoidic association of M. sellnicki with N. fulva, single mite attacks against E. ruidum did not result in host killing and solitary M. sellnicki (78.6% of the cases) behaved as parasites. However, in 21.4% of the attacks (0.9% of all available host pupae) more than one mite was involved and behaved as parasitoids, draining the host of its internal fluids and killing it. This is the first association of a macrodinychid mite with a species of the subfamily Ectatomminae, and the first ant associated mite for which such a context dependent life-style shift is described.