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The genetics of enduring isolation: diversity, gene flow and offspring performance in glacial relict tree populations

Final Report Summary - RELICTOAK (The genetics of enduring isolation: diversity, gene flow and offspring performance in glacial relict tree populations)

The range dynamics of species in response to modern climate change are likely to be determined largely by population responses at range margins. In contrast to the expanding edge, the low-latitude limit of species ranges remains understudied, and the critical importance of its populations as long-term stores of species' genetic diversity and evolutionary history during the Quaternary has been little acknowledged. This research project investigated a set of 19 low-latitude marginal populations of Quercus robur (Pedunculate oak), one of the most important European forest trees, in order to elucidate how these have been able to achieve their long-term viability and genetic diversity, and which are their perspectives under modern climate change.

For this purpose, we investigate how genetic diversity is distributed and transmitted across populations. Because gene dispersal plays a central role in this process, we examine patterns of historical and contemporary gene flow involving three levels: within populations, amongst populations, and between species (i.e. through hybridisation with the co-occurring, more abundant Q. pyrenaica). Finally, measurements of acorn size, germination and plant growth under controlled conditions are used to assess whether low levels of genetic variation actually result in reduced fitness and resistance to abiotic stress. The ultimate objective of this study is to better understand how climate relict populations persist under adverse conditions and to provide valuable background information for the development of appropriate strategies for their conservation and management in a changing, and presumably drier, climate.

We geographical information system (GIS)-mapped and genotyped all adult Q. robur trees in the 15 populations whose size did not exceed 50 individuals, and 50 adult trees, respectively, in the four larger populations. Moreover, we genotyped up to five Q. pyrenaica trees in all populations where this species was present. Data from 12 nuclear SSR markers (microsatellites) revealed that the analysed Q. robur populations contain relatively high levels of genetic diversity while their mutual genetic differentiation is unexpectedly low and unrelated with the geographical situation of individual stands. A comparison of Q. robur genotypes with those of the sampled Q. pyrenaica individuals suggests that hybridisation between the two species is not uncommon; this phenomenon could explain the observed high diversity and weak geographical structure of Q. robur populations.

Pollen-mediated gene flow appears to be extensive in the investigated Q. robur populations. As a consequence, even the smallest stand (n is equal to 3 adult trees, nearest conspecific population > 10 km away) produces abundant crops in some years and shows ongoing regeneration. Preliminary results of a genetic study of pollen immigration into this stand indicate that long-distance gene flow is a common phenomenon. A second, larger study currently under way is investigating landscape-scale patterns of pollen gene flow and population connectivity across a set of 10 populations that grow scattered over > 20 km along a valley. All adult Q. robur trees in this area have been genotyped, which enables to assess which fractions of the > 1300 sampled acorns are sired by conspecifics from the same population, from other populations within the valley, from populations outside the valley, or by Q. pyrenaica. Morphometric measurements of the analysed acorns indicate that acorn size - a parameter known to influence demographic processes, such as seed dispersal, germination date and success, seedling growth - is strongly determined by the identity of the mother tree, virtually unaffected by the population of origin and to some extent affected by the affinity of the mother tree to Q. pyrenaica genotypes. Germination and growth experiments are still under way.

Overall, the present study underpins that efficient pollen exchange between the small and isolated Q. robur populations, aided by more or less regular hybridisation with the much more abundant sister species, has enabled the investigated stands to sustain a notably high genetic diversity and reproductive potential. The diversity and viability of marginal Q. robur populations, together with their potential to acquire genes from the more drought-resistant Q. pyrenaica would represent a powerful mechanism for adapting to a warmer and drier future climate. To investigate this mechanism in much greater detail is the goal of a recently initiated follow-up research project funded by the Spanish National Research Programme and led by the fellow. Results should provide valuable background information for developing appropriate strategies for the management and conservation of oak forests in a changing climate.