Coevolution between Two Epicephala and Two Breynia Figure 5. Morphological characters of Epicephala female genitalia. (A) E. lativalvaris with spine-shaped apex. (B) E. mirivalvata with blunt apex. (o) ovipositor. (a) antrum. (db) ductus bursae. (cb) corpus bursae. doi:10.1371/journal.pone.0041657.g005 through calyx lobes and ovary wall to lay eggs into the ovary tissue (Figure 2: D–F). So the external influence imposed on egg hatching could be greatly reduced, and the newly hatched larvae could instantly feed on seeds to ensure a higher survival rate. On the contrary, Epicephala mirivalvata has a clavate ovipositor with blunt apex (Figure 5: B), which could deposit eggs only between calyx lobes and ovary (Figure 2: G–I). So both eggs and newly hatched larvae were exposed to the influence of environmental factors. Moreover, the newly hatched larvae had to bore into the ovary, which could increase the mortal hazard and eventually decrease the population size. The phylogenetic analysis of Epicephala trees reveals that E. mirivalvata diverged firstly within all known pollinators of Breynia plants, that E. mirivalvata firstly established relationship with B. fruticosa and B. rostrata. However, its ovipositior structure and oviposition mode have resulted its inferiority in competition with E. lativalvaris. So it can be deduced that E. mirivalvata is likely to disappear or shift host to maintain population size, and E. lativalvaris may become the sole pollinator of B. fruticosa and B. rostrata in the future evolutionary course. Conclusion In Breynia–Epicephala diffuse coevolution, two morphologically and behaviorally distinct Epicephala species jointly pollinated two Breynia species, forming a new obligate pollination mutualism of two–to–two species speciality. We firstly describe this model of mutualism system in this study. Different phenology of B. fruticosa resulted in different life history of Epicephala species in Hainan and Fujian, and two hosts sharing pollination of the two Epicephala species has led to hybridization of Breynia plants. In this mutualism, two competitive Epicephala species overlap in the ecological niche. Epicephala mirivalvata is much smaller than E. lativalvaris in population size due to different oviposition mode, and therefore faces more selection pressure. PLoS ONE | www.plosone.org 6 July 2012 | Volume 7 | Issue 7 | e41657