Local adaptation in parapatric and sympatric mosaic coastal habitats through trait divergence of Setaria viridis

Abstract

Abstract Maritime plants contain dwarf ecotypes that are adapted to exposure to strong winds transporting salt spray. When variants differentiated from local populations are well‐adapted and reproductively isolated, they become forerunners of ecotypes. This study aimed to determine how coastal variants arise from local populations under natural selection by studying phenotypic variations and survival of self‐fertilising Setaria viridis populations inhabiting pebble and sandy seashores in Japan. To identify adaptive traits characterising variants, common garden experiments were performed for determining the variation in morphology, salt spray tolerance and flowering time among populations in four adjacent habitats at two localities. To determine selective agents and their adaptation, field transplant experiments were performed, flowering phenology was monitored, and artificial crossing was conducted for F2 segregation. S. viridis populations comprised five coastal variants showing significantly higher salt spray tolerance than the inland variant. The most common of these was a short and tolerant variant adapted to cliff and pebble shore under intensive salt spray from the Pacific ocean. Three others were a tall and mid‐tolerant variant that endured mild breezes off the Seto inland sea, a tall and tolerant variant that survived sea splash from sporadic storms, and a prostrate and tolerant variant specialised for rock cracks exposed to strong sea winds. These variants imply that maritime plants first acquired salt spray tolerance for survival, after which compact plants evolved in habitats where strong winds caused damage from salt spray. Flowering in a fifth variant was delayed to avoid the fatal risk of reproducing during the mid‐summer drought. Although salt spray tolerance and late‐flowering were partially recessive, self‐fertilisation of S. viridis may have increased the homozygosity of recessive genes to generate novel variants. Synthesis. Diverse intensities of salt spray and wind as well as summer drought generated various coastal variants in parapatry and sympatry. This was partially attributable to self‐fertilisation. Among these, a compact variant adapted to common local seashores may become the forerunner of an ecotype. This study on the local adaptation in mosaic habitats will assist in determining adaptation modes of plants and the beginning of ecotype formation.