Engineering evolution to study speciation in yeasts

The Saccharomyces 'sensu stricto' yeasts are a group of species that will mate with one another, but interspecific pairings produce sterile hybrids. A retrospective analysis of their genomes revealed that translocations between the chromosomes of these species do not correlate with the group's sequence-based phylogeny (that is, translocations do not drive the process of speciation). However, that analysis was unable to infer what contribution such rearrangements make to reproductive isolation between these organisms. Here, we report experiments that take an interventionist, rather than a retrospective approach to studying speciation, by reconfiguring the Saccharomyces cerevisiae genome so that it is collinear with that of Saccharomyces mikatae. We demonstrate that this imposed genomic collinearity allows the generation of interspecific hybrids that produce a large proportion of spores that are viable, but extensively aneuploid. We obtained similar results in crosses between wild-type S. cerevisiae and the naturally collinear species Saccharomyces paradoxus, but not with non-collinear crosses. This controlled comparison of the effect of chromosomal translocation on species barriers suggests a mechanism for the generation of redundancy in the S. cerevisiae genome.     

Continental mantle signature of Bushveld magmas and coeval diamonds

The emplacement of the 2.05-billion-year-old Bushveld complex, the world's largest layered intrusion and platinum-group element (PGE) repository, is a singular event in the history of the Kaapvaal craton of southern Africa, one of Earth's earliest surviving continental nuclei. In the prevailing model for the complex's mineralization, the radiogenic strontium and osmium isotope signatures of Bushveld PGE ores are attributed to continental crustal contamination of the host magmas. The scale of the intrusion and lateral homogeneity of the PGE-enriched layers, however, have long been problematical for the crustal contamination model, given the typically heterogeneous nature of continental crust. Furthermore, the distribution of Bushveld magmatism matches that of seismically anomalous underlying mantle, implying significant interaction before emplacement in the crust. Mineral samples of the ancient 200-km-deep craton keel, encapsulated in macrodiamonds and entrained by proximal kimberlites, reveal the nature of continental mantle potentially incorporated by Bushveld magmas. Here we show that sulphide inclusions in approximately 2-billion-year-old diamonds from the 0.5-billion-year-old Venetia and 1.2-billion-year-old Premier kimberlites (on opposite sides of the complex) have initial osmium isotope ratios even more radiogenic than those of Bushveld sulphide ore minerals. Sulphide Re-Os and silicate Sm-Nd and Rb-Sr isotope compositions indicate that continental mantle harzburgite and eclogite components, in addition to the original convecting mantle magma, most probably contributed to the genesis of both the diamonds and the Bushveld complex. Coeval diamonds provide key evidence that the main source of Bushveld PGEs is the mantle rather than the crust.     

The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans

Choanoflagellates are the closest known relatives of metazoans. To discover potential molecular mechanisms underlying the evolution of metazoan multicellularity, we sequenced and analysed the genome of the unicellular choanoflagellate Monosiga brevicollis. The genome contains approximately 9,200 intron-rich genes, including a number that encode cell adhesion and signalling protein domains that are otherwise restricted to metazoans. Here we show that the physical linkages among protein domains often differ between M. brevicollis and metazoans, suggesting that abundant domain shuffling followed the separation of the choanoflagellate and metazoan lineages. The completion of the M. brevicollis genome allows us to reconstruct with increasing resolution the genomic changes that accompanied the origin of metazoans.     

The decline and fate of an iron-induced subarctic phytoplankton bloom

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.     

Haematopoietic stem cells do not transdifferentiate into cardiac myocytes in myocardial infarcts

The mammalian heart has a very limited regenerative capacity and, hence, heals by scar formation. Recent reports suggest that haematopoietic stem cells can transdifferentiate into unexpected phenotypes such as skeletal muscle, hepatocytes, epithelial cells, neurons, endothelial cells and cardiomyocytes, in response to tissue injury or placement in a new environment. Furthermore, transplanted human hearts contain myocytes derived from extra-cardiac progenitor cells, which may have originated from bone marrow. Although most studies suggest that transdifferentiation is extremely rare under physiological conditions, extensive regeneration of myocardial infarcts was reported recently after direct stem cell injection, prompting several clinical trials. Here, we used both cardiomyocyte-restricted and ubiquitously expressed reporter transgenes to track the fate of haematopoietic stem cells after 145 transplants into normal and injured adult mouse hearts. No transdifferentiation into cardiomyocytes was detectable when using these genetic techniques to follow cell fate, and stem-cell-engrafted hearts showed no overt increase in cardiomyocytes compared to sham-engrafted hearts. These results indicate that haematopoietic stem cells do not readily acquire a cardiac phenotype, and raise a cautionary note for clinical studies of infarct repair.