Broad adaptive genetic differentiation in forage quality traits but
similar phenotypic plasticity in Tunisian populations of Brachypodium
hybridum in response to phosphorus deficiency
Abstract
Facing to the harsh environmental constraints, natural populations of
widespread plant species may express plastic response, evolve local
adaptation or combine both mechanisms in order to maintain their genetic
variation and pervasiveness across a wide ecological niche. Here, 79
lines representing height Tunisian populations of Brachypodium hybridum
were cultivated in two contrasting phosphorus (P) availability levels
and their genetic variation was investigated using forage quality traits
to identify the main mechanisms governing their genetic variation. Our
results revealed large variation among populations for most traits. PCA
analysis showed the separation between coastal and inland populations in
both P levels. Importantly, all traits revealed high heritability in
both P treatments and the response of most of them to P deficiency was
highly influenced by the population effect, which accounted 52.38% of
the total variation, indicating strong population differentiation.
However, although significant for most traits, only 18.13% of variation
was attributed to P treatment, suggesting little plastic response to P
deficiency. Importantly, as revealed by the lack of population*treatment
effect for most traits, the plastic response was probably common to all
populations. Moreover, we found that 56.99% of variation was attributed
to environmental factors with distance to coastline, precipitation
seasonality, and annual precipitation being the most important factors,
suggesting that the strong population differentiation likely reflects
local adaptation to specific environmental conditions. Contrary to the
coastal populations, which were characterized by high levels of
fiber-related traits levels, the inland ones displayed the high values
of most energy-related traits, suggesting that these latter were
characterized by relatively higher forage quality. Overall, our results
suggested that the effect of local adaptation overrides the effect of
phenotypic plasticity in explaining the variation of forage quality
traits, which indicate a broad adaptive genetic differentiation among
the Tunisian populations of B. hybridum in response to P deficiency.