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    block this user Ahmed Moustafa

    Assistant Professor

    American University in Cairo

    EST Analysis of Ostreococcus lucimarinus, the Most Compact Eukaryotic Genome, Shows an Excess of Introns in Highly Expressed Genes

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    Background: The genome of the pico-eukaryotic (bacterial-sized) prasinophyte green alga Ostreococcus lucimarinus has one of the highest gene densities known in eukaryotes, yet it contains many introns. Phylogenetic studies suggest this unusually compact genome (13.2 Mb) is an evolutionarily derived state among prasinophytes. The presence of introns in the highly reduced O. lucimarinus genome appears to be in opposition to simple explanations of genome evolution based on unidirectional tendencies, either neutral or selective. Therefore, patterns of intron retention in this species can potentially provide insights into the forces governing intron evolution. Methodology/Principal Findings: Here we studied intron features and levels of expression in O. lucimarinus using expressed sequence tags (ESTs) to annotate the current genome assembly. ESTs were assembled into unigene clusters that were mapped back to the O. lucimarinus Build 2.0 assembly using BLAST and the level of gene expression was inferred from the number of ESTs in each cluster. We find a positive correlation between expression levels and both intron number (R=+0.0893, p=<0.0005) and intron density (number of introns/kb of CDS; R=+0.0753, p=<0.005). Conclusions/Significance: In a species with a genome that has been recently subjected to a great reduction of non-coding DNA, these results imply the existence of selective/functional roles for introns that are principally detectable in highly expressed genes. In these cases, introns are likely maintained by balancing the selective forces favoring their maintenance with other mutational and/or selective forces acting on genome size.

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    Description

    Title : EST Analysis of Ostreococcus lucimarinus, the Most Compact Eukaryotic Genome, Shows an Excess of Introns in Highly Expressed Genes
    Author(s) : William Lanier, Ahmed Moustafa, Debashish Bhattacharya, Josep M. Comeron
    Abstract : Background: The genome of the pico-eukaryotic (bacterial-sized) prasinophyte green alga Ostreococcus lucimarinus has one of the highest gene densities known in eukaryotes, yet it contains many introns. Phylogenetic studies suggest this unusually compact genome (13.2 Mb) is an evolutionarily derived state among prasinophytes. The presence of introns in the highly reduced O. lucimarinus genome appears to be in opposition to simple explanations of genome evolution based on unidirectional tendencies, either neutral or selective. Therefore, patterns of intron retention in this species can potentially provide insights into the forces governing intron evolution. Methodology/Principal Findings: Here we studied intron features and levels of expression in O. lucimarinus using expressed sequence tags (ESTs) to annotate the current genome assembly. ESTs were assembled into unigene clusters that were mapped back to the O. lucimarinus Build 2.0 assembly using BLAST and the level of gene expression was inferred from the number of ESTs in each cluster. We find a positive correlation between expression levels and both intron number (R=+0.0893, p=<0.0005) and intron density (number of introns/kb of CDS; R=+0.0753, p=<0.005). Conclusions/Significance: In a species with a genome that has been recently subjected to a great reduction of non-coding DNA, these results imply the existence of selective/functional roles for introns that are principally detectable in highly expressed genes. In these cases, introns are likely maintained by balancing the selective forces favoring their maintenance with other mutational and/or selective forces acting on genome size.
    Subject : unspecified
    Area : Other
    Language : English
    Year : 2008

    Affiliations American University in Cairo
    Editors : Robert DeSalle
    Journal : PLoS ONE
    Volume : 3
    Issue : 5
    Publisher : Public Library of Science
    Pages : e2171 -
    Url : http://dx.plos.org/10.1371/journal.pone.0002171
    Doi : 10.1371/journal.pone.0002171

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