Genomic organization of the glutathione S-transferase family in insects

Abstract

Cytosolic glutathione S-transferases (GSTs) are a large and diverse gene family in insects. They are classified into six major subclasses. Sigma, Omega, Zeta, and Theta have representatives across Metazoa while Delta and Epsilon are specific to Insecta and Holometabola, respectively. In this study, GSTs are assigned to a subclass by a combination of literature, phylogenetic, and genomic evidence. Moreover, it is confirmed that GSTs frequently cluster by genomic position as a result of recent gene expansions. These expansions are largely explained by the number of protein-coding genes in the genome, although life history is another contributing factor.

Introduction

Glutathione S-transferases (GSTs) are a gene family of enzymes involved in detoxifying cells of natural and artificial molecules (Lumjuan et al., 2005, Ranson et al., 2000, Rogers et al., 1999, Tang and Tu, 1994). Examples include GST-D1 in Drosophila melanogaster and an unidentified number of GSTs in Pediculus humanus which are associated with resistance to insecticides (Barrios et al., 2010, Tang and Tu, 1994). GSTs mainly metabolize these substances by making them more water soluble, aiding in their excretion from the organism (Habig et al., 1974).

In insects, there are two major classes of GSTs, microsomal and cytosolic. The membrane-bound microsomal type is structurally and evolutionarily distinct from the cytosolic type (Enayati et al., 2005). The microsomal class contains few gene duplicates – such as D. melanogaster with only one member (Ranson et al., 2002, Toba and Aigaki, 2000).

This study is focused on the larger family of cytosolic GSTs. The cytosolic type has an active site of catalytic activity and typically forms a dimeric complex. These GSTs are characterized by two domains (Pfam domains PS50404 and PS50405; Hulo et al., 2006), a N-terminal domain of ∼80 amino acid sites and a C-terminal domain of ∼120 sites. The N-terminal domain contains a site which interacts with a glutathione molecule. The C-terminal domain contains a site which interacts with the substrate (review by Enayati et al. (2005)).

This highly diverse insect gene family is divided into six major subclasses: Delta, Epsilon, Sigma, Omega, Theta, and Zeta. Among the currently available genomes, the dipterans consistently have large expansions of these genes. For instance, Ding et al. (2003) identified 37 putative cytosolic GSTs in D. melanogaster and 28 in Anopheles gambiae in contrast to the hymenopteran Apis mellifera with only 11 GSTs (Claudianos et al., 2006, Honeybee Genome Sequence Consortium, 2006). Similar to the eusocial hymenopteran, the exopterogote parasite P. humanus has 11 GSTs which span five of the subclasses (Kirkness et al., 2010). However, this pattern is not a trend in exopterogotes as the free-living aphid Acyrthosiphon pisum contains 18 putative GSTs (Ramsey et al., 2010, Aphid Genomics Consortium, 2010). The current sample of insect genomes is currently insufficient to distinguish whether the GST expansion pattern is mainly driven by shared ancestry or by ecological adaptation to a niche (i.e., parasitism).

The bulk of the GST expansions are in the Delta and Epsilon subclasses. These are Insecta-specific while the other four subclasses have a broader taxonomic distribution (Low et al., 2007, Ranson et al., 2001, Ranson et al., 2002, Sawicki et al., 2003). The overall high rate of turnover of GST genes is especially apparent from studies of the 12 drosophilid genomes (Low et al., 2007, Drosophila 12 Genomes Consortium., 2007). The turnover does not occur abruptly over time but instead conforms to a somewhat uniform rate in the loss and gain of genes. This is not surprising since Hahn et al. (2007) estimated that ∼40% of drosophilid gene families differed in size among lineages and ∼20% of gene families were lost altogether in one or more lineages.

Moreover, Severson et al. (2004) suggested that gene order across 50 kilobases has a one-half probability of being conserved between D. melanogaster and Culicidae. Therefore, genetic linkage of GSTs was examined across each genome (Fig. 1). In this study, a diverse set of insect genomes was examined and expands upon previous studies of GSTs which generally were restricted to a genus-level analysis. This set includes: four Diptera, two Hymenoptera, one Coleopteran, one Lepidopteran, and two outgroup species of Exopterogota. The aim was to gather evidence across Insecta of gene duplication and linkage patterns in this large and diverse gene family.