Christian Lexer
Group leader

We study the evolutionary genomics of species barriers and species differences in two wide-spread European species of the ‘model tree’ genus Populus  (poplars, aspens, cottonwoods). We focus on two species with strongly divergent ecological preferences, Populus alba  (White poplar) and P. tremula  (European aspen). The former is a foundation species in flood-plain forests and the latter is an upland pioneer. Numerous ecological differences separate the two species, including divergent abiotic tolerances (flooding, drought), biotic tolerances (defence against herbivores), and flowering phenology, and candidate traits and genes relevant to these ecological differences are being identified. Recent research on Populus  in our group has made use of natural hybrid zones between P. alba  and P. tremula  for identifying genomic regions and candidate genes that cross the species barrier more or less frequently than expected under neutrality, and for assessing the role of asexual reproduction in hybrid zone persistence. Ongoing British NERC and Swiss SNF funded work uses multiple ‘replicate’ hybrid zones throughout Europe to study variation for genomic isolation and the potential for adaptive introgression in different parts of the species’ ranges, using high-throughput microsatellite genotyping and next generation sequencing approaches. An important applied aspect of this work is the development of methods for mapping fitness-related regions of the genome using the concepts of `admixture mapping`. Future work will focus (among other topics) on the genetic basis of traits involved in response to abiotic stresses, herbivory, pathogen attack, and other biotic interactions in these species and hybrids. This research will reveal the role of ecological traits in speciation and species evolution in Populus in the face of interspecific gene flow. We also hope to gain insights into the role of genetic variation in dominant carrier or ‘foundation’ species in structuring the gene pools of associated organisms and their communities, which are topics of great current interest in conservation biology.

The molecular genetics of local adaptation

We study the genetics of neutral and non-neutral population differentiation in European Populus  species (poplars and aspens). We started with Populus tremula  because local populations of this species are close to random mating, which allows us to make efficient use of the tools of ‘population genomics’ to study the signature of local adaptation. Ongoing work is focused on the European portion of the species’ range and uses so-called genome scans for genes or markers that are more divergent between local populations than expected under neutrality, and genome scans for markers that exhibit greatly reduced genetic diversity in particular populations, indicative of recent ‘selective sweeps’. This work is greatly facilitated by emerging knowledge of variation in recombination rates across the Populus  genome; detecting departures from neutrality is easier in chromosomal regions with reduced recombination in these highly outcrossing species, because ‘genetic hitchhiking’ will extend over larger chromosomal distances there. Understanding the genetic basis of local adaptation is crucial for assessing the conditions under which long-lived organisms such as trees will be likely to adapt successfully in situ  to the expected rate of climate change. From a pragmatic point of view, it is important to indentify genome regions that depart from neutrality, because many applications in conservation biology require "neutral" genetic markers.

We are engaged in evolutionary and conservation genetics work in several biodiversity "hot spots", including Southeastern Brazil, Southern Africa, and Southeast Asia. In Brazil, our current focus is on the mechanisms of speciation and species cohesion in bromeliads (Bromeliaceae; pineapple family) adapted to isolated "inselberg" rock outcrops in the Atlantic Rain Forest biome. In Southern Africa and Southeast Asia, we study the drivers of genomic and phenotypic diversity in the radiations of restiads (Restionaceae), proteas (Proteaceae) and tropical stone oaks (Lithocarpus; Fagaceae). Common themes of these projects are (1) the goal to understand the evolution and maintenance of genomic diversity in highly structured and species-rich environments, (2) the transfer of knowledge and techniques from model systems such as Populus to highly speciose organismal groups. This research is carried out in collaboration and is supported by the Brazilian CNPq and CAPES, the Swiss SNF, the Portuguese FCT and the Chinese Academy of Sciences.