ABSTRACT
One ofthe most important tools for studying the biology of organisms at the molecular level is the ability to manipulate their genomes by transformation. A critical component of a transformation system is the choice of markers for the detection of the transgenic organism. Markers can be either selei.table or scorabie. Selectable markers, such as antibiotic resistance, are useful in detecting transformants in prokaryotic and unicellular eukaryotic systems. However, in con~piex muiticel!ular organisms. the use of seiectabie markers to detect transformanrs is often confoui~ded by the existence of inherent, variable levels of resistance to the selection agent in the population of the organism targeted for transformation. In addition, selection itself can often be biased toward the identification of transformed individuals with multiple copies of the resistance marker. This results from multiple integrations of the gene vector in the initial trailsforlnation event or because of subsequent selection in transformed lines for individuals in which the selects-ble marker is amplified. Scorable markers: usually representing the presence or absence of an easily delectable phenotype. are thus prefened as markers for the detection of transformation in complex organisms such as insects. There is no selection pressure that can result in changes in the number of marker copies in established transformed lines, and such markers often produce phenotypes that can be used to distinguish individuals that are heterozygous from those that are hon;orygous for the marker.
