The following is based on the Sanchez, 2015.
Triplaris Loefl. (Eriogonoideae, Polygonaceae) is a genus that includes 18 dioecious species of trees with a Neotropical distribution from southern Mexico to southern Brazil. All species occur in lowland habitats from sea level to 2000 m in altitude and are considered important components in all stages of secondary succession. A conspicuous feature of all species in Triplaris is the hollow stems that harbor ants (Fig 1). Although the plants produce no food bodies or extra-floral nectaries,
rewards to the ants are provided by a third symbiont—scale insects (Coccidea, Hemiptera) in the form of honeydew. Triplaris is mainly colonized by Pseudomyrmex. Six species of Pseudomyrmex in the triplarinus subgroup have been recognized as obligate and specific mutualists to Triplaris (specialized ants that have not been found nesting outside their plants): Pseudomyrmex dendroicus, Pseudomyrmex mordax, Pseudomyrmex triplaridis, Pseudomyrmex triplarinus, Pseudomyrmex ultrix, Pseudomyrmex vitabilis. These species are distributed from southern Panama to southern Brazil. Some ecological experiments have studied the symbiosis between Triplaris and Pseudomyrmex, and it is thought that in exchange for the nesting sites, the ant partners protect their host plants against herbivore damage [23, 27, 32] and maintain the plants free of pathogens. Pseudomyrmex is also known to prune the vegetation around their host . Some other ant genera can opportunistically colonize Triplaris and it is not uncommon to find non-specialist ant genera such as Azteca, Camponotus, Cephalotes, Crematogaster, Dolichoderus, and Pheidole, as well as less specialized species of Pseudomyrmex (e.g., Pseudomyrmex elongatus, Pseudomyrmex gebellii, Pseudomyrmex longior, Pseudomyrmex rubiginosus, and Pseudomyrmex viduus.
Overall the relationships between Triplaris and Pseudomyrmex triplarinus subgroup are promiscuous; for most species there is no consistent pattern of specialization since one species of plant can associate with multiple species of ants and vice versa. Plants with a wide geographic distribution, such as Triplaris americana are inhabited by four obligate species of Pseudomyrmex as well as other ants, while species such as Triplaris longifolia were only collected in association with P. triplarinus. Species such as Triplaris cumingiana and Triplaris melaenodendron (from Colombia and Costa Rica) were mostly found with non-specific ants. In the case of Triplaris poeppigiana, no association with Pseudomyrmex was found, but instead with ants of the genus Azteca. For other plant species such as Triplaris punctata and Triplaris setosa, there is only one ant colony record at this time, making it difficult to understand the degree of specificity of the interaction. With the exception of T. cumingiana and T. melaenodendron (60% not colonized), the percentage of ant occupancy is high. Triplaris americana, T. longifolia, Triplaris peruviana and Triplaris purdiei were always found in association with ants, and Triplaris weigeltiana only had a 2.3% of individuals with no ant colony.
Ants that have a more extensive distribution associate with more than five species of Triplaris: P. triplaridis and P. triplarinus are associated with multiple host plants growing in sympatry. However, based on 37 collections of P. dendroicus from five different countries, there was faithfulness to a single host. These ants have only been collected in plants of T. americana. Species occur in sympatry (i.e. Triplaris dugandii, T. longifolia, T. peruviana, T. weigeltiana). Triplaris americana has a more ample range of distribution than P. dendroicus, but both organisms overlap in several regions of their range in Ecuador, Colombia, Peru, northern Bolivia, and western Venezuela and Brazil.
Pseudomyrmex triplarinus subgroup is also monophyletic and with the exception of Pseudomyrmex viduus, the clade is an exclusive associate to Triplaris. The Pseudomyrmex triplarinus subgroup is confined to South America and Panama with the exception of P. viduus, which has a widespread distribution, ranging from Mexico to Bolivia and Brazil. Three species have restricted ranges of distribution (Pseudomyrmex mordax, Pseudomyrmex vitabilis, and Pseudomyrmex ultrix), while the other three species (Pseudomyrmex dendroicus, Pseudomyrmex triplaridis, and Pseudomyrmex triplarinus) occur in sympatry and overlap extensively in the Amazon basin. Within the subgroup, there are two clades, one comprising P. triplaridis and P. viduus and another comprising P. mordax, P. dendroicus, and P. triplarinus. These clades are in accordance withWard . Several other species of Pseudomyrmex, outside the P. triplarinus subgroup, can also colonize the hollow stems of Triplaris (i.e., Pseudomyrmex elongatus, Pseudomyrmex gebellii, and Pseudomyrmex longior. These associations have evolved independently from the P. triplarinus subgroup and all of these species are considered generalists since they inhabit several different plant genera and in some cases, even dead twigs (i.e., P. elongatus).
Triplaris and Pseudomyrmex triplarinus subgroup overlap in most of their range of distribution. However, Triplaris has a larger range of distribution and occurs in areas where its obligate symbionts do not. Ants are, in general, sensitive to habitat and climatic changes, which may restrict their distribution to avoid drier (western South America, northeastern Brazil) and colder areas (the Andes Cordillera). When the ranges of both organisms are overlapped, there is a hotspot for the association in the western part of the Amazon Basin. In this area, six of the seven species of Pseudomyrmex triplarinus subgroup and 12 of the 18 described species of Triplaris occur.
From the ant perspective, most species are highly promiscuous. Species of ants such as Pseudomyrmex mordax and Pseudomyrmex triplarinus do not seem to discriminate between host species, since P. mordax colonizes the three species of Triplaris that overlap its distribution and P. triplarinus can colonize at least five host species growing in sympatry. Pseudomyrmex triplaridis may show some specificity towards T. weigeltiana but it still colonizes four other hosts. However, even when most ant species are promiscuous, one species seems faithful to its host. The 37 collection records for Pseudomyrmex dendroicus show the same pattern: they were all collected in a T. americana host. Despite the fact that more plant species are sympatric with T. americana, P. dendroicus has not been collected in any other host. Although this pattern could change as more collections of hosts and their resident ants are available, there is still a high degree of specificity by P. dendroicus. These two organisms co-occur in most of their range of distribution, which could have led to ecological sorting. Two less well-known ant species from the P. triplarinus subgroup, Pseudomyrmex ultrix and Pseudomyrmex vitabilis, were not collected in this study. Pseudomyrmex vitabilis has not yet been collected from a plant host (the description is based on a queen) and P. ultrix is only known from a single locality in Ecuador, on T. dugandii. More collections are therefore necessary to clarify the degree of specificity of these two ant species to their hosts.
- Sanchez, A. 2015. Fidelity and Promiscuity in an Ant-Plant Mutualism: A Case Study of Triplaris and Pseudomyrmex. PLoS ONE. 10. doi:10.1371/journal.pone.0143535