We describe the draft genome of the microcrustacean Daphnia pulex, which is only 200 Mb and contains at least 30,907 genes. The high gene count is a consequence of an elevated rate of gene duplication resulting in tandem gene clusters. More than 1/3 of Daphnia’s genes have no detectable homologs in any other available proteome, and the most amplified gene families are specific to the Daphnia lineage. The co-expansion of gene families interacting within metabolic pathways suggests that the maintenance of duplicated genes is not random, and the analysis of gene expression under different environmental conditions reveals that numerous paralogs acquire divergent expression patterns soon after duplication. Daphnia-specific genes – including many additional loci within sequenced regions that are otherwise devoid of annotations – are the most responsive genes to ecological challenges.

Sequencing of eukaryotic genomes allows one to address major evolutionary problems, such as the evolution of gene structure. We compared the intron positions in 684 orthologous gene sets from 8 complete genomes of animals, plants, fungi, and protists and constructed parsimonious scenarios of evolution of the exon-intron structure for the respective genes. Approximately one-third of the introns in the malaria parasite Plasmodium falciparum are shared with at least one crown group eukaryote; this number indicates that these introns have been conserved through >1.5 billion years of evolution that separate Plasmodium from the crown group. Paradoxically, humans share many more introns with the plant Arabidopsis thaliana than with the fly or nematode. The inferred evolutionary scenario holds that the common ancestor of Plasmodium and the crown group and, especially, the common ancestor of animals, plants, and fungi had numerous introns. Most of these ancestral introns, which are retained in the genomes of vertebrates and plants, have been lost in fungi, nematodes, arthropods, and probably Plasmodium. In addition, numerous introns have been inserted into vertebrate and plant genes, whereas, in other lineages, intron gain was much less prominent.

Deinococcus radiodurans R1 (DEIRA) is a bacterium best known for its extreme resistance to the lethal effects of ionizing radiation, but the molecular mechanisms underlying this phenotype remain poorly understood. To define the repertoire of DEIRA genes responding to acute irradiation (15 kGy), transcriptome dynamics were examined in cells representing early, middle, and late phases of recovery by using DNA microarrays covering Ϸ94% of its predicted genes. At least at one time point during DEIRA recovery, 832 genes (28% of the genome) were induced and 451 genes (15%) were repressed 2-fold or more. The expression patterns of the majority of the induced genes resemble the previously characterized expression profile of recA after irradiation. DEIRA recA, which is central to genomic restoration after irradiation, is substantially up-regulated on DNA damage (early phase) and down-regulated before the onset of exponential growth (late phase). Many other genes were expressed later in recovery, displaying a growth-related pattern of induction. Genes induced in the early phase of recovery included those involved in DNA replication, repair, and recombination, cell wall metabolism, cellular transport, and many encoding uncharacterized proteins. Collectively, the microarray data suggest that DEIRA cells efficiently coordinate their recovery by a complex network, within which both DNA repair and metabolic functions play critical roles. Components of this network include a predicted distinct ATP-dependent DNA ligase and metabolic pathway switching that could prevent additional genomic damage elicited by metabolism-induced free radicals.T he Gram-positive aerobic bacterium Deinococcus radiodurans R1 (DEIRA) has an extraordinary resistance to ␥-radiation and a wide range of other DNA-damaging conditions, including desiccation and oxidizing agents (1, 2). Ionizing radiation induces DNA double-stranded breaks (DSBs) that are the most lethal form of DNA damage (3). After acute exposures to 10 kGy, early stationary phase (ESP) DEIRA can reassemble its 3.285-Mbp genome, which consists of four haploid genomic copies per cell (4), from hundreds of DNA DSB fragments without lethality or induced mutagenesis (5, 6). Also remarkable is DEIRA's ability to grow at 60 Gy͞h without any discernable effect on its growth rate (7). Because most organisms, generally, can tolerate so few DSBs (8), radiationinduced DSBs and their repair have been difficult to study. In DEIRA, however, there are so many DSBs in fully viable irradiated cells after high-dose irradiation that the steps in DSB repair can be monitored directly in mass culture (5, 9-11). This characteristic has been exploited and used to examine the timing of DNA recombination (5, 10, 12) after high-dose irradiation and has revealed the sequential action of RecA-independent and -dependent pathways during repair (11).Comparative genomic and experimental analyses support the view that DEIRA's extreme radiation resistance phenotype is complex, likely determined collectively by an assortment o...

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A comprehensive evolutionary classification of proteins encoded in complete eukaryotic genomes Sequencing the genomes of multiple, taxonomically diverse eukaryotes enables in-depth comparative-genomic analysis which is expected to help in reconstructing ancestral eukaryotic genomes and major events in eukaryotic evolution and in making functional predictions for currently uncharacterized conserved genes. AbstractBackground: Sequencing the genomes of multiple, taxonomically diverse eukaryotes enables in-depth comparative-genomic analysis which is expected to help in reconstructing ancestral eukaryotic genomes and major events in eukaryotic evolution and in making functional predictions for currently uncharacterized conserved genes.

Lineage-specific gene loss, to a large extent, accounts for the differences in gene repertoires between genomes, particularly among eukaryotes. We derived a parsimonious scenario ofgene losses for eukaryotic orthologous groups (KOGs) from seven complete eukaryotic genomes. The scenario involves substantial gene loss in fungi, nematodes, and insects. Based on this evolutionary scenario and estimates of the divergence times between major eukaryotic phyla, we introduce a numerical measure, the propensity for gene loss (PGL). We explore the connection among the propensity of a gene to be lost in evolution (PGL value), protein sequence divergence, the effect of gene knockout on fitness, the number of protein-protein interactions, and expression level for the genes in KOGs. Significant correlations between PGL and each of these variables were detected. Genes that have a lower propensity to be lost in eukaryotic evolution accumulate fewer substitutions in their protein sequences and tend to be essential for the organism viability, tend to be highly expressed, and have many interaction partners. The dependence between PGL and gene dispensability and interactivity is much stronger than that for sequence evolution rate. Thus, propensity of a gene to be lost during evolution seems to be a direct reflection of its biological importance.

Prochlorococcus marinus, the dominant photosynthetic organism in the ocean, is found in two main ecological forms: high-light-adapted genotypes in the upper part of the water column and low-lightadapted genotypes at the bottom of the illuminated layer. P. marinus SS120, the complete genome sequence reported here, is an extremely low-light-adapted form. The genome of P. marinus SS120 is composed of a single circular chromosome of 1,751,080 bp with an average G؉C content of 36.4%. It contains 1,884 predicted protein-coding genes with an average size of 825 bp, a single rRNA operon, and 40 tRNA genes. Together with the 1.66-Mbp genome of P. marinus MED4, the genome of P. marinus SS120 is one of the two smallest genomes of a photosynthetic organism known to date. It lacks many genes that are involved in photosynthesis, DNA repair, solute uptake, intermediary metabolism, motility, phototaxis, and other functions that are conserved among other cyanobacteria. Systems of signal transduction and environmental stress response show a particularly drastic reduction in the number of components, even taking into account the small size of the SS120 genome. In contrast, housekeeping genes, which encode enzymes of amino acid, nucleotide, cofactor, and cell wall biosynthesis, are all present. Because of its remarkable compactness, the genome of P. marinus SS120 might approximate the minimal gene complement of a photosynthetic organism.

The variable lymphocyte receptors (VLRs) of jawless vertebrates such as lamprey and hagfish are composed of highly diverse modular leucine-rich repeats. Each lymphocyte assembles a unique VLR by rearrangement of the germline gene. In the lamprey genome, we identify here about 850 distinct cassettes encoding leucine-rich repeat modules that serve as sequence templates for the hypervariable VLR repertoires. The data indicate a gene conversion-like process in VLR diversification. Genomic analysis suggested a link between the VLR and platelet glycoprotein receptors. Lamprey lymphocytes express two putative deaminases of the AID-APOBEC family that may be involved in VLR diversification, as indicated by in vitro mutagenesis and recombination assays. Vertebrate acquired immunity could have therefore originated from lymphocyte receptor diversification by an ancestral AID-like DNA cytosine deaminase.