A Theory of Ratio Selection—Lattice Model for Obligate Mutualism

Mutualisms are cooperative interactions between members of different species. We focus on obligate mutualism, where each species cannot survive without the other. From a theoretical aspect, obligate mutualism is similar to the relationship between male and female. Empirical data indicate a sex-ratio selection: male and female have a specific ratio in their population sizes. In the present paper, we apply lattice model to obligate mutualism between two species, and present a theory of “ratio selection” which is a generalization of sex-ratio selection. Computer simulations are carried out by two methods: local and global interactions. In the former, interactions occur between neighbouring cells, while in the latter they occur between any pair of cells. Simulations in both interactions show the so-called Allee effect: both species can survive, when both densities are large in some extent. However, we find a large difference between local and global simulations. In the case of local interaction, restriction for survival is found to be extremely severe compared to global interaction. Both species require a proper ratio for their sustainability. This result leads to the theory of ratio selection: when interaction occurs locally, the ratio of both species is uniquely determined. We discuss that the ratio selection explains not only the evolution of endosymbionts from free-living ancestors but also the evolution from endosymbionts to organelles.

Diversity and species-specificity of brood pollination of leafflower trees(Phyllanthaceae:Glochidion)by leafflower moths(Lepidoptera:Epicephala)in tropical Southeast Asia(Cambodia)

Glochidion(Phyllanthaceae;leafflower trees)is a genus of trees which is widely reported to be pollinated by leafflower moths(Gracillariidae:Epicephala)in temperate and subtropical Asia,Australia,and the Pacific islands.However,the pollination ecology of Glochidion is not well described from tropical Asia,the region where it is most species-rich at both local(<9 spp.)and regional(~200 spp.)scales.Here we report investigations of pollination biology and species-specificity of five Glochidion species in tropical Southeast Asia(Cambodia).Through nocturnal observations and fruit dissections,we find that at least three and likely five Glochidion species in Cambodia are pollinated by seed-parasitic leafflower moths.We find no evidence that any of these leafflower moths are non-mutualistic parasites,despite known examples of such parasites of this mutualism elsewhere in Asia.While the presence of a single larva in a fruit results in only a fraction of seeds being consumed,the presence of more than one larva per fruit-a frequent occurrence in some species—can result in almost all seeds within the fruit being infested.Multilocus phylogenetic analysis indicates that there are five different minimally monophyletic leafflower moth clades,each of which pollinates a unique Glochidion host species.Our results indicate that in its center of diversity in tropical Asia this system is an obligate pollination mutualism as previously described at the global margins of its distribution.These findings provide insights into the processes that generate and maintain biodiversity and maintain mutualism stability in plant-insect interactions in this biodiversity hotspot.