Ecological Barriers to Gene Flow Between Riparian and Forest Species of Ainsliaea (Asteraceae)

Abstract

Understanding the role of habitat-associated adaptation in reducing gene flow resulting in population differentiation and speciation is a major issue in evolutionary biology. We demonstrate a significant role for habitat divergence in species isolation between two naturally hybridizing riparian and nonriparian plants, Ainsliaea faurieana and A. apiculata (Asteraceae), on Yakushima Island, Japan. By analyzing the fine-scale population structure at six sympatric sites, we found that variations in leaf shape, geography,

Recent origin and adaptive diversification of Ainsliaea (Asteraceae) in the Ryukyu Islands: molecular phylogenetic inference using nuclear microsatellite markers

“…Therefore, interbreeding and gene exchange may occur between sympatric species. However, gene flow between A. linearis and A. apiculata is strongly restricted by divergent natural selection even when they occur in parapatry across riverbank-forest transitions and hybridize with each other (Mitsui et al 2011). Our population genetic analysis showed that the individuals of each species largely form a single group and are clustered by independent lineage, supporting the idea that genetic introgression among species could be restricted.…”

“…The process by which divergent selection leads to reproductive isolation is known as ''ecological speciation'' (Schluter Rundle and Nosil 2005). The rheophytic (riparian) and non-rheophytic (inland) environments are highly different (e.g., riparian habitats are exposed to frequent submergence during flooding periods and are subjected to strong solar irradiance, whereas inland habitats experience no flooding and are subjected to dim light conditions; Nomura et al 2006;Mitsui et al 2011). Natural selection arising from contrasting riparian and inland habitats should have facilitated adaptive divergence of rheophytic and inland species (Mitsui et al 2011).…”

“…Although A. apiculata is widely distributed in the Japanese Archipelago and the Korean Peninsula, the monophyly of this species is supported by the preliminary population genetic analysis in which the entire distribution range of A. apiculata was surveyed (Mitsui et al 2011). Thus, we used the populations of the northern Ryukyus (Yakushima and Tanegashima Islands) and the surrounding regions (Kyushu Island) for A. apiculata.…”

“…In reciprocal crossing experiments under greenhouse conditions, they were crossed successfully with each other and produced viable and fertile hybrids (Watanabe et al 1992;Mitsui et al 2011). Therefore, interbreeding and gene exchange may occur between sympatric species.…”

“…Moreover, no sequence variation occurs in more than 10 coding and noncoding cpDNA regions (about 6,000 bp) (Mitsui, unpublished data). Given the considerable molecular homogeneity and the fact that these species can be reciprocally crossed with each other (Watanabe et al 1992;Mitsui et al 2011), the phenotypic divergence and speciation in the Ryukyu Ainsliaea likely occurred relatively recently. In this study, we reconstructed the phylogenetic relationships within the recently diverged group of Ainsliaea in the Ryukyu Islands to estimate the origin and the process of adaptive radiation, with particular emphasis on the evolution of rheophytic species.…”