Population and comparative genetics of thermotolerance divergence between yeast species

Many familiar traits in the natural world—from lions’ manes to the longevity of bristlecone pine trees—arose in the distant past, and have long since fixed in their respective species. A key challenge in evolutionary genetics is to figure out how and why species-defining traits have come to be. We used the thermotolerance growth advantage of the yeast Saccharomyces cerevisiae over its sister species Saccharomyces paradoxus as a model for addressing these questions. Analyzing loci at which the S. cerevisiae allele promotes thermotolerance, we detected robust evidence for positive selection, including amino acid divergence between the species and conservation within S. cerevisiae populations. Since such signatures were particularly strong at the chromosome segregation gene ESP1, we used this locus as a case study for focused mechanistic follow-up. Experiments revealed that, in culture at high temperature, the S. paradoxus ESP1 allele conferred a qualitative defect in biomass accumulation and cell division relative to the S. cerevisiae allele. Only genetic divergence in the ESP1 coding region mattered phenotypically, with no functional impact detectable from the promoter. Together, these data support a model in which an ancient ancestor of S. cerevisiae, under selection to boost viability at high temperature, acquired amino acid variants at ESP1 and many other loci, which have been constrained since then. Complex adaptations of this type hold promise as a paradigm for interspecies genetics, especially in deeply diverged traits that may have taken millions of years to evolve.

A millennium-long 'Blue Ring' chronology from Bristlecone Pine as a record of volcanic forcing on climate

<p>'Blue Rings' (BRs) are distinct wood anatomical anomalies recently discovered in several tree species. Previous studies connect their occurrence to lower than normal temperatures during the cell wall lignification phase of xylogenesis. Cell wall lignification usually continues after radial growth is completed, after the growth season. Therefore, systematic analysis of blue rings can add another level of time resolution to dendroclimatic studies. Additionally, BRs are more sensitive temperature recorders than frost rings which require freezing temperatures to form. We  present a continuous chronology of blue rings in North American bristlecone pine covering the last millennium and their connections to volcanic eruptions known both from historic and ice core records. Most recorded BR years coincide with cooling following large volcanic eruptions. The three most prominent events during the last 1000 years, with the highest share of blue rings in bristlecone pine from the White Mountains of California are at: 1453, 1601 and 1884CE (83%, 91%, 69% of blue rings respectively), attributed to known eruptions of Kuawe (attribution still debated) 1452CE -Vanuatu, Huaynaputina 1600CE – Peru, and Krakatoa 1883CE - Indonesia. Fourth most prominent event with 58% of blue rings is noted in 1200CE. Acidity peak in 1200CE is so far recorded only in Greenland ice-cores suggesting northern hemisphere high latitude eruption, but strong BR signal would suggest a broader climatic significance of this event. It is interesting to note that BRs were indicated in 69% of samples in 1884, relating to the known eruption and associated climate impact of Krakatoa (1883), yet no BRs were observed in 1816, the so-called year without a summer which followed the largest historically noted and well described eruption of Tambora, Indonesia (1815). We did find a strong BR signal in 1809 (with BRs continuing in 1810 and 1811) following an unidentified but prominent eruption seen in ice core records. The 1809 and 1815 eruptions are thought to be responsible for the cold decade from 1810 to 1819 thought by some to be the coldest decade of the last 500ys. The source of 1809 eruption remains unknown and scientific debate over the scale of the eruption continues, but bipolar acidity peaks in ice cores point to a tropical eruption with widespread sulfate distribution to both hemispheres and tephra in ice cores points to two coinciding high latitude eruptions of only regional prominence. The BR record supports 1809 CE as an event of global climatic significance illustrating the capacity for BRs  to capture cooling events with even higher time resolution (after the radial growth is completed) and of smaller magnitude than frost rings, TRW or MXD studies to help better investigate and understand the impacts of volcanism on climate and society.</p>

Maximum latewood density records of Great Basin Bristlecone pine (Pinus Longaeva) from the White Mountains, California

<p>Great Basin Bristlecone pine (Pinus longaeva) is known for its longevity. The longest continuous tree-ring width chronology covers more than 9000 years. Tree-ring width of upper treeline bristlecone pine trees is influenced by summer temperature variability at decadal to centennial scales, but to infer a temperature signal on interannual scales, Maximum Latewood Density (MXD) is a better proxy. Here, we present a preliminary MXD chronology to investigate the temperature signal in upper treeline and lower elevation bristlecone pines. MXD was measured with an X-ray Computed Tomography toolchain in 24 dated cores,  with the oldest sample dating back to 776 CE. Ring and fibre angles were corrected and two MXD chronologies for different elevations were developed, which will be used to study climate-growth relationships and the effect of elevation on them. Future scanning will allow constructing a 5000+ year-long MXD chronology from upper treeline sites, which will provide an annual-resolution North American temperature record covering the mid-to-late Holocene.</p>