Macroevolutionary assembly of ant/plant symbioses:Pseudomyrmexants and their ant-housing plants in the Neotropics

Abstract

Symbioses include some of the clearest cases of coevolution, but their origin, loss or reassembly with different partners can rarely be inferred. Here we use ant/plant symbioses involving three plant clades to investigate the evolution of symbioses. We generated phylogenies for the big-eyed arboreal ants (Pseudomyrmecinae), including 72% of their 286 species, as well as for five of their plant host groups, in each case sampling more than 61% of the species. We show that the ant-housing Vachellia (Mimosoideae) clade and its ants co-diversified for the past 5 Ma, with some species additionally colonized by younger plant-nesting ant species, some parasitic. An apparent co-radiation of ants and Tachigali (Caesalpinioideae) was followed by waves of colonization by the same ant clade, and subsequent occupation by a younger ant group. Wide crown and stem age differences between the ant-housing genus Triplaris (Polygonaceae) and its obligate ant inhabitants, and stochastic trait mapping, indicate that its domatium evolved earlier than the ants now occupying it, suggesting previous symbioses that dissolved. Parasitic ant species evolved from generalists, not from mutualists, and are younger than the mutualistic systems they parasitize. Our study illuminates the macroevolutionary assembly of ant/plant symbioses, which has been highly dynamic, even in very specialized systems.

Partner abundance controls mutualism stability and the pace of morphological change over geologic time

“…Voucher information is reported in SI Appendix, Table S2. DNA extraction, PCR, sequencing, alignments, and phylogenetic analyses were performed as previously described (25) and are detailed in the SI Appendix, SI Materials and Methods.…”

“…Here we use a species-rich and diverse ant/plant interaction system to study the ecological context under which breakdowns of symbiotic mutualisms have occurred on a geologic timescale. Ant/plant mutualisms are ubiquitous in tropical ecosystems and encompass a wide range of strategies (22)(23)(24)(25). In Australasia, the majority of ant-plants are epiphytes and appear to be primarily involved in trophic mutualisms rather than defense mutualisms (22,24).…”

A review of the Pseudomyrmex ferrugineus and Pseudomyrmex goeldii species groups: acacia-ants and relatives (Hymenoptera: Formicidae)

“…These ants were revised taxonomically more than twenty years ago, with the consequent recognition of ten species (Ward 1993). Since that time no additional species of obligate acacia-ants have been discovered, but recent studies have revealed that two undescribed species of generalist twig-nesting Pseudomyrmex are nested phylogenetically within the P. ferrugineus group Chomicki et al 2015;Ward & Branstetter, 2017). These two species are described herein and the definition of the P. ferrugineus group is updated to incorporate greater phenotypic variation in workers and queens.…”

“…1); such taxa are highlighted in bold font in the key. Species groups have been modified from those recognized in , to reflect more recent knowledge of phylogenetic relationships (Ward & Downie 2005;Chomicki et al 2015;Ward 2017). The key also includes taxonomically isolated species that are unplaced to a group.…”

“…Most of the 200+ species are generalists, nesting in dead twigs or branches of a wide variety of plants, but about 40 species inhabit live domatia of specialized ant-plants (myrmecophytes) (Ward 1991;Ward & Downie 2005;Chomicki et al 2015). The species living in domatia usually have a mutualistic relationship with their host, defending the plant from herbivores and plant competitors, and receiving shelter and sustenance in return (Janzen 1966;Benson 1985;Davidson et al 1988;Ward 1991).…”