Is heterostyly rare on oceanic islands?

Kenta Watanabe,Takashi Sugawara

doi:10.1093/aobpla/plv087

Kenta Watanabe, Takashi Sugawara

Open Access

https://doi.org/10.1093/aobpla/plv087

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Abstract

Heterostyly has been considered rare or absent on oceanic islands. However, there has been no comprehensive review on this issue. Is heterostyly truly rare on oceanic islands? What makes heterostyly rare on such islands? To answer these questions, we review the reproductive studies on heterostyly on oceanic islands, with special emphasis on the heterostylous genus Psychotria in the Pacific Ocean as a model system. Overall, not many reproductive studies have been performed on heterostylous species on oceanic islands. In Hawaiian Psychotria, all 11 species are thought to have evolved dioecy from distyly. In the West Pacific, three species on the oceanic Bonin and Lanyu Islands are distylous (Psychotria homalosperma, P. boninensis and P. cephalophora), whereas three species on the continental Ryukyu Islands show various breeding systems, such as distyly (P. serpens), dioecy (P. rubra) and monoecy (P. manillensis). On some other Pacific oceanic islands, possibilities of monomorphy have been reported. For many Psychotria species, breeding systems are unknown, although recent studies indicate that heterostylous species may occur on some oceanic islands. A shift from heterostyly to other sexual systems may occur on some oceanic islands. This tendency may also contribute to the rarity of heterostyly, in addition to the difficulty in colonization/autochthonous evolution of heterostylous species on oceanic islands. Further investigation of reproductive systems of Psychotria on oceanic islands using robust phylogenetic frameworks would provide new insights into plant reproduction on oceanic islands.

Highlights

Few heterostylous species and few studies of their reproductive biology are known from oceanic islands, and heterostyly is probably rare in such areas, as suggested by Pailler et al (1998b)
In the Hawaiian Islands, all 11 Psychotria species form a monophyletic group (Nepokroeff et al 2003), and it has been believed that they are probably derived from a distylous colonist (Beach and Bawa 1980; Sakai et al 1995b), since distyly is basal for the genus (Sohmer 1978)
This modification has contributed to a high proportion of dioecism and low proportion of distyly on oceanic islands

Summary

Introduction

Plant reproductive systems on islands have attracted the attention of many evolutionary biologists (Carlquist 1974; Ehrendorfer 1979; Bawa 1982; Baker and Cox 1984; Barrett et al 1996; Sakai and Weller 1999; Crawford et al 2011). Distylous species usually possess an incompatibility system that prevents self-fertilization and intramorph fertilization (heteromorphic incompatibility) This herkogamous floral dimorphism is generally thought to be controlled by ‘a heterostylous gene’ (Barrett and Shore 2008). It was often reported that heterostylous species were absent from oceanic island floras, e.g. Hawaii (Carlquist 1974) and Galapagos (McMullen 1987). A littoral woody plant of Lythraceae distributed widely on pantropical islands, is distylous at least on several Indian Ocean Islands (Lewis 1975). Few heterostylous species and few studies of their reproductive biology are known from oceanic islands, and heterostyly is probably rare in such areas, as suggested by Pailler et al (1998b). 1. Kondo et al (2007), 2. Sugawara et al (2014), 3. Watanabe et al (2014b), 4. McMullen (2012), 5. Schofield (1989), 6. Bramow et al (2013), 7. Pailler and Thompson (1997), 8. Pailler et al (1998b), 9. Thompson et al (1996), 10. Meeus et al (2011), 11. Olesen et al (2003), 12. Lewis (1975)

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