Abstract
Obligate out-breeding plants are considered relatively disadvantageous comparing with self-breeding plants when colonizing oceanic islets following long-distance dispersal owing to mate and pollinator limitation. The rarity of heterostyly, a typical out-breeding system, on oceanic islands seems a good proof. However, a heterostylous plant, Guettarda speciosa, is widely distributed on most tropical oceanic islets. Our research demonstrates that its heteromorphic self-incompatibility, plus herkogamy and long flower tube make it rely on pollinator for sexual reproduction, which is generally considered “disadvantageous” for island colonization. We hypothesize that available pollination niche will be a key factor for its colonization on islands. Our studies on remote coral islands show that G. speciosa has built equilibrium population with a 1:1 morph ratio. It could obtain pollination niche from the hawkmoth Agrius convolvuli. A pioneer island plant Ipomoea pes-caprae sustain the pollination niche by providing trophic resource for the larvae of the pollinator. Geographic pattern drawn by Ecological Niche Modelling further indicates the interaction between G. speciosa, A. convolvuli and I. pes-caprae can be bounded on those remote oceanic islands, explaining the colonization of G. speciosa distylous population. These findings demonstrated obligate out-breeding system could be maintained to acclimatize long distance dispersal, if the pollination niche is available.
Highlights
Islands, especially oceanic islands, are considered as “natural evolutionary laboratories” by biologists due to their uniqueness such as isolated biotas, distinctive environments, and fluctuant communities[1,2]
Despite that it’s long tubular flower lead to a dependence on pollinator for its sexual reproduction, it distributes on almost all tropical oceanic islands, and even becomes a dominant species, suggesting its successful adaptation modes for long distance dispersal and island habitat
The G. speciosa populations in Xisha Islands involved two discrete floral morphs differing in style length: long-styled morph (L-morph) flowers have stigma positioned above the anthers and slightly exserted out of corolla, while styled morph (S-morph) flowers have stigma positioned below the anthers
Summary
Especially oceanic islands, are considered as “natural evolutionary laboratories” by biologists due to their uniqueness such as isolated biotas, distinctive environments, and fluctuant communities[1,2]. It’s more difficult for plants with self-incompatibility systems (i.e. obligate out-breeding system) to adapt such condition which require more than one individual for population establishment and sufficient pollination service for sexual reproduction[1,2,8,9,10] For those obligate out-breeding plants colonized oceanic islands successfully following long-distance dispersal, why they can adapt this process remain underexplored[6]. For plants that are self-incompatible and outbreeding, pollination niche availability is a necessary condition to establish in new habitats through sexual reproduction It will be especially important for obligate out-breeding plants to establish on oceanic islands after long-distance dispersal. The heterosylous system of G. speciosa provides an ideal model to test our pollination niche hypothesis
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