How does genetic differentiation begin? Shota Sakaguchi and colleagues have been studying the genetics of dwarf goldenrods. The team compared the dwarf goldenrods of Yakushima Island with their lowland relatives. Plant height and fecundity tend to correlate with plant height, and so dwarf populations should experience more genetic drift between populations than their taller counterparts.

To find out if this is the case in goldenrods, the team of botanists collected samples of Solidago minutissima from locations around 1700 m above sea level and up. S. minutissima is a small plant, just a few centimetres high. They then sampled populations of S. virgaurea, the species that S. minutissima evolved from. The parent species is a comparative giant reaching well over ten times the height.

A) Location of Yakushima Island in the southernmost area of the main islands of the Japanese Archipelago and the border of the Ryukyu Archipelago. (B) Geographical map of Yakushima Island. The altitudinal zonation and topography of the island are shown. The localities of lowland populations (L1–L5) and alpine dwarf populations (R1–R7, W1–W4) are shown as black circles. Two other known localities for lowland populations are indicated by open circles. (C) Stature of a flowering plant from the lowland population (KYO 00071707). (D) Scanned images of living plants from four alpine populations. (E) Alpine dwarf plants in divergent habitats: (E1) W3 population in wetland habitat; (E2) R1 population in rock habitat. Source Sakaguchi et al. 2020.

Sakaguchi and colleagues analysed the chloroplasts, nuclear microsatellites and double digest restriction-site-associated DNA. “The genetic analyses based on three types of molecular markers consistently revealed a clear divergence between the alpine dwarf and lowland Solidago populations. No chloroplast haplotypes were shared between the population groups, and the individuals were clustered separately for each of the alpine and lowland populations based on nuclear microsatellite and SNP analyses, except for one population, L5, showing an ‘admixed’ origin based on nuclear microsatellites,” write the authors in their article.

“This finding is in contrast to the previously reported population dynamics of alpine S. virgaurea populations in other mountain ranges, which repeatedly revealed little genetic differentiation from lowland populations.”

The authors note that in landscape genetics theory, variety in the landscape affects gene flow, causing differences in genetic distribution. The alpine landscapes of Yakushima Island create a number of microhabitats. “The apparent association between genetic variation and habitat type in the dwarf Solidago populations may be explained by a scenario in which gene flow predominantly links the populations in similar environments via selection against maladapted individuals that have migrated from alternative habitats... In fact, however, we found strong population isolation even between neighbouring wetland habitats, as evidenced by complete fixation to different chloroplast haplotypes and a predominance of different genetic clusters in similar habitat types.”

Sakaguchi and colleagues propose using transplants and common garden experiments to pull apart the effects neutral processes such as gene flow and natural selection in differentiating populations.

This paper is listed on ResearchGate.