Molecular Phylogeny of Rubiaceous Epiphytic Myrmecophytes Based on the atpB-rbcL Intergene Region of cpDNA.

Abstract

Various symbioses from facultative relationships to obligate mutualisms exist between the epiphytic myrmecophytes in 5 genera of Hydnophytinae (Rubiaceae) and the associated ants. However, the phylogenetic relationships and evolutionary processes have not been studied well. In the present study, a molecular phylogenetic relationship based on the DNA sequence of the atpB-rbcL intergene region was analyzed in 6 species of 4 genera in Hydnophytinae. The inferred phylogenetic relationship was shown as (Myrmecodia spp. + (Myrmephytum + (Anthorrhiza + Hydnophyfum spp.))). It was suggested that the ancestral taxon of Hydnophytinae was Myrmecodia or Myrmephytum because these genera were settled in the basal position in Hydnophytinae used in the present study. In addition, we discuss the evolution of diverse symbioses of Hydnophytinae based on the inferred phylogenetic relationships, together with ecological and geographical information. It is possible that the ancestor evolved as an epiphyte which lived in the nutrient-poor habitat of Southeast Asia and had a specialization for efficient and relatively obligate mutualism with ants by increasing their complex cavities as ant nesting spaces. After Myrmephytum and Anthorrhiza were respectively evolved in the Philippines and Papua New Guinea, the ancestor of Hydnophyfum evolved as a xeromorphic tuberous epiphyte in the dry environments of Southeast Asia and acquired facultative symbioses with arboreal ants.