Papers for labmeeting on November 24: genomics and sex ratio evolution

This Wednesday, we will discuss two short papers, one in Nature Genetics and the other one in Nature. They are both very short, but should make interesting reads. The first paper was suggested by Bengt Hansson, and is an example of how the sequencing of entire genomes can shed light over local adaptation, in this case of the plant Arabdopsis lyrata to serpentine soils. You can download that paper here. The second paper is an example of how climatic factors can drive population divergence in sex determination systems, in this case over an altitudinal gradient of an Australian lizard species. You can download that paper here. Abstracts of both papers are provided below.

I hope you enjoy these papers and that we will have a good discussion. Time and place as usual: "Darwin" at 10.15 (Wednesday November 24). Fika volunteers are encouraged to step forward.

Population resequencing reveals local adaptation of Arabidopsis lyrata to serpentine soils  

Thomas L Turner, Elizabeth C Bourne, Eric J Von Wettberg, Tina T Hu & Sergey V Nuzhdin

Nature Genetics 42: 260–263 (2010)

DOI: doi:10.1038/ng.515

A powerful way to map functional genomic variation and reveal the genetic basis of local adaptation is to associate allele frequency across the genome with environmental conditions. Serpentine soils, characterized by high heavy-metal content and low calcium-to-magnesium ratios, are a classic context for studying adaptation of plants to local soil conditions. To investigate whether Arabidopsis lyrata is locally adapted to serpentine soil, and to map the polymorphisms responsible for such adaptation, we pooled DNA from individuals from serpentine and nonserpentine soils and sequenced each 'gene pool' with the Illumina Genome Analyzer. The polymorphisms that are most strongly associated with soil type are enriched at heavy-metal detoxification and calcium and magnesium transport loci, providing numerous candidate mutations for serpentine adaptation. Sequencing of three candidate loci in the European subspecies of A. lyrata indicates parallel differentiation of the same polymorphism at one locus, confirming ecological adaptation, and different polymorphisms at two other loci, which may indicate convergent evolution.

Climate-driven population divergence in sex-determining systems

Ido Pen, Tobias Uller, Barbara Feldmeyer, Anna Harts, Geoffrey M. While & Erik Wapstra

Nature 468: 436–438 (18 November 2010)

DOI: doi:10.1038/nature09512

Sex determination is a fundamental biological process, yet its mechanisms are remarkably diverse. In vertebrates, sex can be determined by inherited genetic factors or by the temperature experienced during embryonic development. However, the evolutionary causes of this diversity remain unknown. Here we show that live-bearing lizards at different climatic extremes of the species’ distribution differ in their sex-determining mechanisms, with temperature-dependent sex determination in lowlands and genotypic sex determination in highlands. A theoretical model parameterized with field data accurately predicts this divergence in sex-determining systems and the consequence thereof for variation in cohort sex ratios among years. Furthermore, we show that divergent natural selection on sex determination across altitudes is caused by climatic effects on lizard life history and variation in the magnitude of between-year temperature fluctuations. Our results establish an adaptive explanation for intra-specific divergence in sex-determining systems driven by phenotypic plasticity and ecological selection, thereby providing a unifying framework for integrating the developmental, ecological and evolutionary basis for variation in vertebrate sex determination.

Goodbye to 20 % of all lizards by 2080?

An interesting, but depressing study was published in Science this Friday. A research team lead by Barry Sinervo, and also including my colleagues Donald Miles (University of Ohio) and Jean Clobert (CNRS, France) have shown an alarming high rate of local population extinctions in Mexico over the last 35 years. Based on these observed real-time extinctions and biophysical modelling of lizard body temperatures in the field (based on experiments), they conclude that the rate of climate change and increasing temperatures are too high for the lizards to have time to adapt.

Lizards and other ectotherms are constrained in their foraging time in hot climates because they must avoid overheating. In particular, viviparous lizards suffer, since pregnant females are especially sensitive: they carry embryos in their bodies which easily die at high temperatures. It is therefore not surprising that most viviparous lizard populations are found at higher latitudes and altitudes, i. e. in colder climates. As temperatures increase dramatically in these environments, these viviparous lizards face a significantly higher extinction risk compared to oviparous lizards, largely because their foraging time becomes drastically reduced and they have to spend a larger part of the day in the shade, to avoid overheating.

The research team estimate, conservatively, that about 20 % of all lizard species on Earth run a significant and serious risk of becoming extinct before 2080, unless the current global warming trend is reversed. This is rather alarming, as lizards is only one of several organismal groups that are likely to have passed the "extinction" threshold determined by anthroprogenic global warming. Other groups that have been discussed are amphibians.

Perhaps we are now entering the next (the sixth) massextinction, which will also drag humans away from this planet? Keep in mind that one of the most famous massextinctions about 251 milllion years ago (The Perman-Triassic massextinction event) killed between 70 and 96 % of all living species, and happened after a global temperature increase of about 6 degrees. Incidentally, a six-degree temperature increase is one of the scenarios outlined by UN:s intergovernmental panel IPCC in their most pessimistic scenario over the coming 100 years (although some climate scientists consider this a rather realistic scenario). If so, Homo sapiens might not have long time left on this planet.

You can read more about this lizard study here, and do not forget to watch the video, where Jean Clobert and Barry Sinervo discuss their findings.