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    The genetic foundation and evolutionary origin of oleaginous traits in microalgae

    Dose expansion and evolutionary origin of genes involved in TAG biosynthesis in Nannochloropsis. (A) Copy numbers (indicated by the diameters of circles) of  genes involved in TAG biosynthesis in Nannochloropsis and Chlamydomonas. Purple indicates the proportion of genes that might be acquired via horizontal gene transfer. (B) DGAT genes that might originated from multiple genomes including the secondary endosymbiosis host and the engulfed green and red algae.

        Photosynthetic microalgae are of interest as potential feedstock for fuels because of their high yield of oil in cell, high growth rate, tolerance of diverse environmental conditions and capability to grow on non-arable land using brackish water and wastewater. However, the genetic and evolutionary mechanism of accumulating oil in microalgae remains elusive.

        Nannochloropsis is a wild oleaginous microalgae. It’s widely distributed in marine and is cultivated in large scale. Currently, Dr. Dongmei Wang, Dr. Kang Ning, Dr. Jing Li and their colleagues from the Single Cell Center of Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Chinese Academy of Sciences, carried out a comprehensive genomic analysis of six Nannochloropsis genomes to reveal the genetic foundation of oleaginousness of Nannochloropsis. They found that the six genomes share key oleaginous traits, such as the gene dose expansion of selected lipid biosynthesis genes compared to green algae Chlamydomonas (Fig.1A).

        The most prominent example of gene dose expansion is DGAT, which catalyzes the last step of TAG synthesis. There’re 13 DGAT genes in Nannochloropsis, representing the highest gene dose in known genomes. No evidence were found indicating horizontal gene transfer (HGT) events, gene duplication and transposition. Through a comprehensive phylogenetic analysis, researchers proposed that among the 11 DGAT-2s, one gene might originate from the red algae related secondary endosymbiont, four from green algae related endosymbiont, and the other six from the eukaryotic host genome (Fig. 1B). In addition, large proportion (15.3%) of TAG biosynthesis related genes were acquired by Nannochloropsis via HGT) from bacteria (Fig. 1A. Therefore, multiple genome pooling and horizontal genetic exchange, together with selective inheritance of lipid synthesis genes and species-specific gene loss, have led to the enormous genetic apparatus for oleaginousness and the wide genomic divergence among present-day Nannochloropsis.

        This work was led by Prof. XU Jian from Single Cell Center of Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Prof. Qiang Hu’s team from Arizona State University and Prof. Feng Chen’s team from University of Maryland also participated in the study. This work was published online in PLoS Genetics (2014) 


    Nannochloropsis Genomes Reveal Evolution of Microalgal Oleaginous Traits

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