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 work uses multiple ‘replicate’ hybrid zones throughout Europe to study variation for genomic isolation in different parts of the species’ ranges. This work is accompanied by microarray-based functional genomics experiments to identify and map genes that are differentially regulated between the two species. An important applied aspect of this work is the development of methods for mapping ecologically important phenotypes to 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 maintaining species barriers 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.

Microevolution and speciation in terrestrial islands

Studies of organisms in ‘terrestrial islands’ can improve our understanding of two unresolved issues in evolutionary genetics: the likely long-term effects of habitat fragmentation, which is a topic of great current conservation concern world-wide, and the genetic underpinnings of continental species radiations in island-like terrestrial habitats. We have started to address these issues in plant species of the neotropical adaptive radiation Bromeliaceae (the pineapple family; bromeliads) adapted to isolated `inselberg` rock outcrops in the Atlantic Rainforest biodiversity hotspot of Brazil. I collaborate on these topics with two postdoctoral researchers associated with my lab, Thelma Barbará and Clarisse Palma-Silva. Our ongoing research on these topics benefits greatly from collaboration with several research groups in Brazil. Recent research in our group has shown that bat-pollinated bromeliad species of the genus Alcantarea  adapted to inselbergs in the Atlantic Rainforest resemble species distributed on oceanic islands with respect to their patterns of variability and gene flow. Thus, inselbergs not only facilitate research on the genetic consequences of long-term fragmentation in terrestrial island habitats, but also on the evolutionary forces that drive speciation in such island-like environments. Our neutral genetic marker studies also indicate that populations and species in this group vary greatly in mating systems and reproductive strategies (inbreeding vs. outcrossing; asexual vs. sexual reproduction; pollination by bats vs. insects), and flexible mating systems appear to have contributed to the ability of these species to colonise isolated inselberg rock outcrops. Future work will make use of these "replicated experiments of nature" seen on different inselbergs. Collaborative projects will focus (among other topics) on the ecology, genetics, and evolution of reproductive isolation between sympatric colour morphs of the bat-pollinated inselberg species Alcantarea imperialis, and on the role of variation in mating systems (selfing vs. outcrossing) in maintaining reproductive isolation among inselberg-adapted species of the bromeliad genus Pitcairnia.