Since the completion of the genome sequence of Saccharomyces cerevisiae in 19961,2, there has been an exponential increase in complete genome sequences accompanied by great advances in our understanding of genome evolution. Although little is known about the natural and life histories of yeasts in the wild, there are an increasing number of studies looking at ecological and geographic distributions3,4, population structure5-8, and sexual versus asexual reproduction9,10. Less well understood at the whole genome level are the evolutionary processes acting within populations and species leading to adaptation to different environments, phenotypic differences and reproductive isolation. Here we present one- to four-fold or more coverage of the genome sequences of over seventy isolates of the baker's yeast, S. cerevisiae, and its closest relative, S. paradoxus. We examine variation in gene content, SNPs, indels, copy numbers and transposable elements. We find that phenotypic variation broadly correlates with global genome-wide phylogenetic relationships. Interestingly, S. paradoxus populations are well delineated along geographic boundaries while the variation among worldwide S. cerevisiae isolates shows less differentiation and is comparable to a single S. paradoxus population. Rather than one or two domestication events leading to the extant baker's yeasts, the population structure of S. cerevisiae consists of a few well-defined geographically isolated lineages and many different mosaics of these lineages, supporting the idea that human influence provided the opportunity for cross-breeding and production of new combinations of pre-existing variation.

Studies of human speech processing have provided evidence for a segmentation strategy in the perception of continuous speech, whereby a word boundary is postulated, and a lexical access procedure initiated, at each metrically strong syllable. The likely success of this strategy was here estimated against the characteristics of the English vocabulary. Two computerized dictionaries were found to list approximately three times as many words beginning with strong syllables (i.e. syllables containing a full vowel) as beginning with weak syllables (i.e. syllables containing a reduced vowel). Consideration of frequency of lexical word occurrence reveals that words beginning with strong syllables occur on average more often than words beginning with weak syllables. Together, these findings motivate an estimate for everyday speech recognition that approximately 85% of lexical words (i.e. excluding function words) will begin with strong syllables. This estimate was tested against a corpus of 190 000 words of spontaneous British English conversion. In this corpus, 90% of lexical words were found to begin with strong syllables. This suggests that a strategy of postulating word boundaries at the onset of strong syllables would have a high success rate in that few actual lexical word onsets would be missed.

Ribosomal DNA (rDNA) plays a key role in ribosome biogenesis, encoding genes for the structural RNA components of this important cellular organelle. These genes are vital for efficient functioning of the cellular protein synthesis machinery and as such are highly conserved and normally present in high copy numbers. In the baker's yeast Saccharomyces cerevisiae, there are more than 100 rDNA repeats located at a single locus on chromosome XII. Stability and sequence homogeneity of the rDNA array is essential for function, and this is achieved primarily by the mechanism of gene conversion. Detecting variation within these arrays is extremely problematic due to their large size and repetitive structure. In an attempt to address this, we have analyzed over 35 Mbp of rDNA sequence obtained from whole-genome shotgun sequencing (WGSS) of 34 strains of S. cerevisiae. Contrary to expectation, we find significant rDNA sequence variation exists within individual genomes. Many of the detected polymorphisms are not fully resolved. For this type of sequence variation, we introduce the term partial single nucleotide polymorphism, or pSNP. Comparative analysis of the complete data set reveals that different S. cerevisiae genomes possess different patterns of rDNA polymorphism, with much of the variation located within the rapidly evolving nontranscribed intergenic spacer (IGS) region. Furthermore, we find that strains known to have either structured or mosaic/hybrid genomes can be distinguished from one another based on rDNA pSNP number, indicating that pSNP dynamics may provide a reliable new measure of genome origin and stability.

Size exclusion chromatography (SEC) has been used extensively in the characterization of coal‐derived liquids to estimate molecular masses (MM), indicating the presence of considerably greater upper mass ranges compared to mass spectrometric methods based on heated probes and field ionization. In the present study, the use of tetrahydrofuran (THF), a solvent commonly used in SEC, has been shown to lead to partial loss of sample on column packings and to allow some separation by an adsorption mechanism rather than by size exclusion. In this paper, we have compared these results with data from SEC, where 1‐methyl 2‐pyrrolidinone was used as the mobile phase, indicating that the largest molecules (not observed in THF‐based work) appear at the exclusion limit, with a continuum of material down to the total permeation limit of the column. In terms of polystyrene standard masses, this is equivalent to a range from 100 u, at the lower limit, up to in excess of 30–40 000 u and possibly over 2 000 000 u. Our results from SEC in NMP suggest that limitations exist in much solution‐based work attempting to identify distributions of MMs in coal‐derived liquids. Matrix‐assisted laser desorption/ionization (MALDI) spectra of the fractions show similar features to those seen by SEC in NMP, with a continuum from high to low mass, and a significant shift to higher mass in the insoluble fraction compared with the soluble fraction. The new data are in line with observations by laser desorption mass spectrometry (LD‐MS) and MALDI‐MS, which have shown much greater upper‐MM limits than either SEC (in THF) or mass spectrometric techniques relying on thermal evaporation of coal‐derived materials from heated probes. The work would appear to have implications for other polydispersed natural polymer systems.

A century ago, stewards were responsible for managing estates or for keeping order at public events.

No abstract

No abstract

No abstract