Speciation in obligately plant-associated Crematogaster ants: Host distribution rather than adaption towards specific hosts drives the process

Ecological interactions among organisms may be an essential factor facilitating speciation processes. In one of the most species-rich ant-plant symbiotic systems worldwide pioneer trees of the euphorb genus Macaranga are inhabited by specific partner ants, mostly of the genus Crematogaster subgenus Decacrema. Both groups underwent radiation, with 30 species of Macaranga being colonized by eight species of Crematogaster. In this obligate association, the ants rely solely on their host for nutrition and nesting space. Hosts are distributed patchily in disturbed sites or gaps in primary forest. Association patterns are non-random in spite of the often sympatric occurrence of several host-plant species. Generally, each ant species colonizes two to seven different host species over its whole distributional range. Speciation processes in the ants may thus be driven either by adaptation towards alternative host species or by spatial patterns of host distribution, or by both factors. Limited dispersal of queens and nest site limitation due to the obligate association with a host were found to lead to significant isolation by distance on a small spatial scale in primary forest. Extremely high intraspecific genetic variation of mitochondrial markers was in contrast to the low genetic variability of nuclear markers, also pointing towards small population sizes of the ants and the importance of genetic drift in the diversification processes. Adaptation towards alternative hosts may occur as a by-product when different Macaranga hosts are colonized in different regions.