Abstract
Plant species distributed along wide elevational or latitudinal gradients show phenotypic variation due to their heterogeneous habitats. This study investigated whether phenotypic variation in populations of the Solidago virgaurea complex along an elevational gradient is caused by genetic differentiation. A common garden experiment was based on seeds collected from nine populations of the S. virgaurea complex growing at elevations from 1,597 m to 2,779 m a.s.l. on Mt. Norikura in central Japan. Population genetic analyses with microsatellite markers were used to infer the genetic structure and levels of gene flow between populations. Leaf mass per area was lower, while leaf nitrogen and chlorophyll concentrations were greater for higher elevations at which seeds were originally collected. For reproductive traits, plants derived from higher elevations had larger flower heads on shorter stems and flowering started earlier. These elevational changes in morphology were consistent with the clines in the field, indicating that phenotypic variation along the elevational gradient would have been caused by genetic differentiation. However, population genetic analysis using 16 microsatellite loci suggested an extremely low level of genetic differentiation of neutral genes among the nine populations. Analysis of molecular variance also indicated that most genetic variation was partitioned into individuals within a population, and the genetic differentiation among the populations was not significant. This study suggests that genome regions responsible for adaptive traits may differ among the populations despite the existence of gene flow and that phenotypic variation of the S. virgaurea complex along the elevational gradient is maintained by strong selection pressure.
Highlights
Plant species distributed along wide environmental gradients often show large phenotypic variation reflecting their habitat heterogeneity (Leiblein-Wild & Tackenberg, 2014; Turesson, 1922; Vitasse, Delzon, Bresson, Michalet, & Kremer, 2009)
S. virgaurea complex populations along an elevational gradient were shown to be linked by substantial gene flow between neighboring populations by molecular analysis using neutral microsatellite markers
Many morphological and physiological traits and flowering phenology showed genetic differentiation along elevations of provenance sites in the common garden experiment. These results suggest that only the genome regions of adaptive traits may display differentiation due to strong selection pressures despite the existence of gene flow
Summary
Plant species distributed along wide environmental gradients often show large phenotypic variation reflecting their habitat heterogeneity (Leiblein-Wild & Tackenberg, 2014; Turesson, 1922; Vitasse, Delzon, Bresson, Michalet, & Kremer, 2009). Mountain ecosystems are ideal to study adaptive differentiation of plant species, because phenotypic variation is often found along elevational gradients, accompanied by drastic changes of environmental conditions at short geographic distances. Evaluation of gene flow and genetic structure behind geographic variation of phenotypes is important to improve our understanding of mechanisms of adaptation and evolution of plants. Many previous studies focused on morphological variation of reproductive organs (e.g., shape of involucral scales) to identify the two subspecies in Japan, they did not show whether morphological variation of the S. virgaurea complex along elevational gradients was caused by genetic differentiation and adaptation or phenotypic plasticity (Hayashi, 1976, 1978a, 1978b; Kitamura, 1957; Takasu, 1975). Are the nine populations along the elevational gradient genetically connected to each other by active gene flow?
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