Population Structure of a Widespread Species under Balancing Selection: The Case of Arbutus unedo L.

Abstract

Arbutus unedo L. is an evergreen shrub with a circum-Mediterranean distribution that also reaches the Eurosiberian region in northern Iberia, Atlantic France, and a disjunct population in southern Ireland. Due to the variety of climatic conditions across its distribution range, the populations of A. unedo were expected to display local adaptation. Conversely, common garden experiments revealed that diverse genotypes from a range of provenances produce similar phenotypes through adaptive plasticity, suggesting the action of stabilizing selection across its climatically heterogeneous range. Nonetheless, since a uniform response might also result from extensive gene flow, we have inferred the population structure of A. unedo and assessed whether its extended and largely one-dimensional range influences gene flow with the help of AFLP genotypes for 491 individuals from 19 populations covering the whole range of the species. As we had anticipated, gene flow is restricted in A. unedo, providing further support to the hypothesis that stabilizing selection is the most likely explanation for the homogeneous phenotypes along the range. The Euro-Siberian populations were not particularly isolated from the Mediterranean. Instead, there was a distinct genetic divide between the populations around the Mediterranean Sea and those sampled along Atlantic coasts from northern Africa up to Ireland. This genetic structure suggests the action of historic rather than biogeographic factors as it seems consistent with a scenario of independent glacial refugia in the Atlantic and Mediterranean portions of the range of A. unedo. Genetic exchange was likewise restricted within each set of populations. Nevertheless, isolation-by-distance (IBD) was stronger, and FST increased faster with distance, along the Atlantic, suggesting that gene flow might be larger among Mediterranean populations. Genetic diversity was significantly lower in NW Iberia and Ireland than in other populations whereas Ireland was more closely related to populations in NW Iberia than to a population in Atlantic France, suggesting a postglacial stepping-stone colonization of the Atlantic coast. Altogether, our results show that stabilizing selection is able to homogenize the phenotypic response even when population structure is strong, gene flow is constrained, and the phylogeographic past is complex.

Genetic diversity and divergence at the Arbutus unedo L. (Ericaceae) westernmost distribution limit

“…In the application of the “refugia within refugia” model to the Iberian Peninsula, the paradigm of Iberia as a single refugium during Pleistocene glacial maxima was challenged, supporting instead the presence of several Iberian refugia for a range of flora and fauna [4]. Thus, combining already available knowledge [12] with our results, which are based on a more intensive genetic sampling of A . unedo in this area, the hypothesis of the presence of different refugia cannot be excluded.…”

“…The species distribution is mostly typical of Mediterranean sclerophyllous and laurel-like vegetation, mainly within coastal and inland areas, where frost or summer dryness are not too intense [7]. Arbutus unedo genetic structure has been investigated at both local and biogeographical scale using different types of markers [5,8,9,10,11,12,13]. In Portugal, strawberry tree has a widespread distribution strongly related to soil characteristics and climate, as well as to landscape fragmentation, wildfires, and domestication [14].…”

The Contribution of Neutral and Environmentally Dependent Processes in Driving Population and Lineage Divergence in Taiwania (Taiwania cryptomerioides)

“…Selection driven by ecological factors acting on genetic variation of DNA sequences is of major importance in evolutionary biology (Schluter, 2001 , 2009 ). DNA sequence variation revealed by amplified fragment length polymorphism (AFLP) has been found to be closely associated with environmental conditions in shaping population adaptive divergence of many plant species (e.g., Fang et al, 2013 ; Huang et al, 2015a , b ; Santiso et al, 2016 ; Yang et al, 2016 ; Chen et al, 2017 ). There is an increasing interest in investigating environmentally dependent epigenetic variation in natural populations, which is also important for understanding the potential for adaptation of populations and species that are enduring rapid global environmental changes (Bossdorf et al, 2008 ; Johnson and Tricker, 2010 ; Alonso et al, 2015 ; Huang et al, 2015b ; Villota-Salazar et al, 2016 ).…”

Phylogeography and conservation genetics of the endangered Tugarinovia mongolica (Asteraceae) from Inner Mongolia, Northwest China

“…Finally, the best grouping was determined when the number of groups retained was maximized, F CT . However, this configuration was excluded when a single population appeared in the geographic group [40, 41].…”