Mechanisms of speciation in Southeast Asian ant-plants of the genus Macaranga (Euphorbiaceae)

Abstract

The palaeotropic genus Macaranga (Euphorbiaceae) is an excellent model system to analyze co-evolutionary processes associated with myrmecophytism, a mutualistic interaction between plants and ants. Ant-plants like Macaranga provide nesting space and feed their partners, whereas the ants protect the plants from herbivores and competitors such as lianas. We used genome-based evidence to investigate speciation mechanisms in Macaranga ant-plants, and their co-evolution with ants from the genus Crematogaster. Our previous work had shown that myrmecophytic Macaranga species show little genetic differentiation, suggesting an adaptive radiation. We hypothesized that the obligatory symbiosis with ants may reduce gene flow among plant populations, eventually enhancing allopatric speciation. To test this hypothesis, we verified the monophyly of the investigated plant lineages by phylogenetic analyses, reconstructed parsimony networks based on chloroplast DNA (cpDNA) variation, and assessed population genetic parameters using nuclear microsatellites and cpDNA haplotypes. Our data provided evidence for vicariant events as well as for hybridization and cpDNA introgression among closely related Macaranga species. The extent of population differentiation within myrmecophytic versus non-myrmecophytic species proved to be in a similar range, indicating that our working hypothesis of enhanced allopatric speciation in myrmecophytes cannot be sustained by the present evidence. Nevertheless, the mutualistic interactions of Macaranga and associated ants may be a key innovation that opened an adaptive zone putatively exploited by the divergence of Macaranga.