Gene Duplication and Subsequent Differentiation of Esterases in Cactophilic Drosophila Species

Abstract

Phytophagous insects are excellent model systems to study the genetic and ecological bases of adaptation and population differentiation because the host plant constitutes an immediate environmental factor that can affect the early stages of the life cycle (Matzkin, 2005; Matzkin et al., 2006). New host plant exploitation can result in genetic and biochemical adjustments to the new resource and to chemically distinct niches, which can include potentially toxic compounds, new mating environments, parasitoids, bacteria and fungi (Kircher, 1982; Fogleman & Abril, 1990; Via, 1990; Fogleman & Danielson, 2001). These adjustments are the result of a number of physiological changes, including those related to biochemical systems associated with adaptation to the new environment. The species of the Drosophila repleta group occupy different habitats, but their common feature is that they are phytophagous; that is, they lay eggs in rotting cacti cladodes. The developing larvae feed on the yeast that are part of the rotting process (Starmer & Gilbert, 1982; Pereira et al., 1983; Starmer et al., 1986), according to the cactus-Drosophila-yeast system; therefore, they are considered specialists. However, adults are generalists because they visit other food sources in their environment (Morais et al., 1994). This ecological specificity of cactophilic Drosophila directly influences species distribution, as they are always associated with the host cactus distribution (Tidon-Sklorz & Sene, 1995; Manfrin & Sene, 2006; Mateus and Sene, 2007). Drosophila has been used as a research model for more than a century, and the first report of gene duplication was described by Bridges for the Bar locus in D. melanogaster over 70 years ago (Bridges, 1936). Since that time, mainly after the advent of biochemical and molecular biology techniques, several other examples of duplicated genes have been presented, and pathways of evolution by gene duplication have been proposed (for example, Stephens, 1951; Nei, 1969). These pathways were thoroughly discussed in 1970 in Ohno’s book “Evolution by gene duplication” (Ohno, 1970). Subsequently, several other works have reviewed the mechanisms and roles of gene duplication in the evolutionary process (A. Wagner, 2002; Kondrashov et al., 2002; and Zhang, 2003). Currently, the genomes of twelve Drosophila species have been completely sequenced (Tweedie et al., 2009), but many aspects of the functional divergence of the products of a