Functional and Phylogenetic Analysis of the Glutathione Transferase Gene Family in Poplar

Gene duplication, whether genome-wide or local, plays a major role in plant evolution (reviewed in Flagel and Wendel, 2009). Over the course of evolution, many, perhaps most, duplicate genes (paralogs) are short-lived, losing functionality and ultimately being re- moved by deletion. However, some dupli- cates persist, either retaining their full ancestral function, a subset of their ancestral functions (subfunctionalization), or acquiring new functions (neofunctionalization) (reviewed in Demuth and Hahn, 2009). A particular challenge is to understand whether the ca- pacity for sub- and neofunctionalization dif- fers among genes and whether this might explain why only some genes diversify into large gene families. In this issue, Lan et al. (pages nnn) combine thorough molecular evolutionary analysis (reconstruction of the history of gene duplication and assessment of signatures of selection) with expression and functional assays for members of a large gene family, the glutathione S-transferase (GST) gene family in poplar (Populus trichocarpa). GST proteins are thought to function in detoxification of xenobiotics and in response to biotic and abiotic stress. Given the diversity of potential xenobiotics and stressors, it is reasonable to suppose that having diverse GST functions is adaptive. But how did this diversity arise? To investigate this question, Lan et al. examined the phylogenetic relation- ships among the 81 GST family members in Populus and their chromosomal locations, gene expression patterns, and protein struc- tural features (see figure). The authors con- structed a hypothetical evolutionary history via genome duplication, tandem duplication, and genomic rearrangement. They found a number of gene fragments and pseudo- genes showing that, even in the GST gene family, many paralogs fail to persist. Exami- nation of GST gene expression in response to abiotic stress and in different tissues showed divergence in gene expression patterns, in- cluding cases of divergence between close paralogs. In addition, to test protein function, the authors determined the substrate specificity and specific activity of recombinant GSTs, finding divergence in activity toward different substrates among and within GST subfam- ilies. Many closely related paralogs showed overlapping specificities and similar activity, but even the most recent duplicates diverged in one or more characteristics of expression, specificity, and activity. Thus, by examining multiple characteristics, the authors revealed that this large gene family shows extensive subfunctionalization (and occasional neofunc- tionalization), even among highly related pa- ralogs. It seems that gene families tend to grow when they have structural and/or func- tional features that allow rapid subfunctional- ization.