Maintenance of soil ecotypes ofSolidago virgaureain close parapatry via divergent flowering time and selection against immigrants

Abstract

The often patchy distribution of serpentine geology can lead to abrupt changes in soil and microclimates. Thus, serpentine areas provide an ideal natural setting to understand how divergent selection drives the process of local adaptation in edaphically specialized plants. When the serpentine ecotype is surrounded by a related nonserpentine ecotype, a balance of natural selection and potential gene flow should maintain the different ecotypes over very short distances. We aimed to reveal the mechanisms allowing soil ecotypes of a goldenrod species to co‐occur sympatrically in Japan. We performed field surveys to characterize microenvironments and flowering phenology of each ecotype, common garden and reciprocal transplant experiments and artificial crossing, and population genetic analysis to investigate the levels of genetic differentiation between ecotypes. Growth chamber experiments show that serpentine plants showed lower specific leaf area (SLA) and greater resource allocation to their root systems than did their nonserpentine counterparts, a potential adaptation to drier and less fertile soil condition in serpentine habitats. Reciprocal transplant studies demonstrated a clear pattern of local adaptation in the plant growth rate. Importantly, serpentine populations completed flowering by midsummer versus late summer in nonserpentine plants. This pattern is consistent with the hypothesis that early flowering ensures reproductive success, before the microclimatic conditions becomes severe in open habitats. Although prezygotic isolation was a strong barrier to gene flow, genetic differentiation was very low, indicating a recent origin for the serpentine ecotypes and/or gene flow at low frequencies. Synthesis. The findings indicate that the early flowering times of serpentine ecotypes, which would have been selected for by microclimates in serpentine areas, can play roles in local adaptation, but also population isolation via a by‐product of diverged reproductive timings. This study contributes to general understanding of the initial stages of plant ecological speciation under potential gene flow in very small geographic scales.

Desert‐like badlands and surrounding (semi‐)dry grasslands of Central Germany promote small‐scale phenotypic and genetic differentiation inThymus praecox

“…Consequently, results strongly indicate locally differentiated genotypes. The observed site-differentiation patterns are supported by literature investigating genetic differentiation due to (micro)habitat differences (e.g., Ali et al, 2012;Linhardt & Grant, 1996;Odat et al, 2004;Sakaguchi et al, 2018). In general, we observed a remarkable phenotypic and genetic differentiation between local sites suggesting associated small-scale differentiation patterns.…”

“…Furthermore, in a sympatric situation, divergence in flowering time can isolate populations (Baack, Melo, Rieseberg, & Ortiz‐Barrientos, ). Slightly earlier flowering of badlands individuals is in accordance with the theory stating that before microclimatic conditions become worse in open habitats, earlier flowering can ensure the reproductive success of individuals (Sakaguchi et al, ). Furthermore, self‐compatible flowers, even when selfing occurs in low frequencies, guarantee the reproduction of badlands individuals that are less‐visited by pollinators promoting genetic differentiation among sites, besides limited gene flow as well.…”

“…Even within geographically small scales, environmental heterogeneity promotes plant phenotypic and genetic differentiation. Genetic variation with corresponding phenotypic (ecotypic) differentiation caused by habitat differences was found, for example, between ~100 km distant Solidago virgaurea serpentine and nonserpentine populations in Japan (Sakaguchi et al, ), between one to 17 km distant Ranunculus acris semi‐natural and agriculturally improved grassland populations in Central Germany (Odat, Jetschke, & Hellwig, ), or even between populations separated by less than few meters and/or centimeters that were under high selective pressure (Linhardt & Grant, ). Additionally, heterogeneous conditions at small scale also influence genetic differentiation by changing gene flow.…”

“…;Violle et al, 2012). Therefore, functional traits are appropriate to study population responses to environmental variability (see also Sakaguchi et al, 2018).…”

A snapshot of progenitor-derivative speciation in action inIberodes(Boraginaceae)

“…In the earliest stages of parapatric speciation mediated by ecological differentiation, reproductive barriers are expected to evolve in response to selection towards locally adapted, divergent ecotypes (Sakaguchi et al , 2019). The niche identity tests points to a role of ecologically-based selection in the speciation of the coastal lineage from I. linifolia.…”