Characterization and molecular phylogenetic analysis of mariner elements from wild and domesticated species of silkmoths

Abstract

Transposable elements are powerful tools for studying molecular genetics as they serve as agents for chromosomal insertions, deletions, or rearrangements and are found to be maintained in a variety of the genomes. The mariner like elements (MLEs), first isolated from Drosophila mauritiana (Haymer and Marsh, 1986; Jacobson et al., 1986), are now known to be present in a wide range of animal species (see review Hartl et al., 1997b), and plants (Feschotte and Wessler, 2002; Jarvik and Lark, 1998). MLEs are characterized by the presence of an ORF coding for a transposase of about 350 amino acids, short inverted terminal repeats at the ends, and a TA duplication at the insertion site (Lohe et al., 1996). The MLE transposase contains two highly conserved motifs WVPHEL and YSPDLAP separated by about 150 amino acid motifs, as well as a specific D,D(34)D signature motif (Doak et al., 1994; Robertson, 1993). The MLEs have been classified into several distinct subfamilies according to sequence similarities; elements from different subfamilies are typically 40–56% identical at the nucleotide level (Robertson and MacLeod, 1993). Gene transfer between species, a phenomenon known as horizontal gene transfer, appears to have played an important role in the evolution of MLEs. Horizontal transmission of MLEs is inferred from the occurrence of very similar transposon sequences in distantly related species, and from presence of different subfamilies of mariner elements in any particular species (Robertson, 1993). The extremely broad host range of MLEs, indicative of the host independence of the transposition process, has attracted interests because of the potential use of MLEs for genetic manipulations with insect species, with special emphasis on insects of economic importance (Kidwell, 1993). Recent studies demonstrated the potential of mariner-based transformation vectors for introducing exogenous DNA into the wide range of hosts, including fruitfly, mouse, chicken, mosquito, zebrafish, and leishmania (for recent review see Plasterk et al., 1999). MLEs also present important issues from an evolutionary point of view. The vast majority of MLEs are not functional, because they contain multiple inactivating mutations such as deletions, insertions, and nucleotide substitutions (Lohe et al., 1997; Maruyama et al., 1991; Robertson, 1993). The only mariner elements demonstrated to be autonomous are the MosI from D. mauritiana (Medhora et al., 1991) and closely related elements from Drosophila simulans (Capy et al., 1992). This apparent predominance of inactive MLEs prompted the presumption that mutational inactivation is an important part of the MLE life cycle within the species, which follows the initial invasion by horizontal transmission. These processes, along with the stochastic loss of inactive MLEs by random genetic drift, have been implicated as possible mechanisms underlying the curious distribution of MLEs among species (Lohe et al., 1995; Hartl et al., 1997a). To explore the evolutionary biology and dynamics of the MLEs, we undertook a study of MLEs in the genomes of diverse silkmoths collected from various parts of the world. The species used for mariner analysis included representatives of the domesticated silkmoth Bombyx mori (chromosome no. 28, mariner nomenclature, Bmmar), (Indian polyvoltine strain, Nistari) and its wild progenitor, Bombyx mandarina (27, Bmamar) and wild silkmoths of the Saturniidae family [Antheraea mylitta Molecular Phylogenetics and Evolution 25 (2002) 210–217 MOLECULAR PHYLOGENETICS AND EVOLUTION