Coexistence mechanisms of Colocasiomyia species (Diptera: Drosophilidae) sharing inflorescences of Alocasia odora (Araceae) as a host plant: Comparison between two- and three-species systems

There are two pollination-mutualistic systems between Colocasiomyia flies and Alocasia odora. The two systems are different in the number of fly species involved. One is a two-to-one system, where C. xenalocasiae and C. alocasiae share inflorescences/infructescences of A. odora in the Ryukyu Islands (Japan), Taiwan, Guangdong, and Guangxi (China). The other system, which additionally involves the third species, C. grandis, is seen from southern Yunnan (China) to northern Vietnam. To reveal coexistence mechanisms in these systems, we compared breeding habits of the component species between the two- and three-species systems in natural conditions, and undertook a field experiment to test a hypothesis whether oviposition sites of the component species are affected by interference competition between them. The observations under natural conditions confirmed the breeding niche separation of component species in the two-species system: C. xenalocasiae uses mostly the pistillate region of spadix, whereas C. alocasiae uses mostly the staminate region, with partial overlap of their oviposition sites in the lower intermediate region. In the three-species system, however, these two species separated their oviposition sites nearly completely from each other, suggesting that they are excluded from the lower intermediate region by the third species, C. grandis, which monopolizes there. The result of field experiments did not support this hypothesis: neither C. xenalocasiae nor C. alocasiae changed oviposition behavior regardless of the absence or the presence of C. grandis. Therefore, we propose an alternative hypothesis that the oviposition site segregation among the three species has evolved as a consequence of the past and/or ongoing competition of larvae.