Introduction

As previous chapters in this book have demonstrated, considerable progress has been achieved during the last decade in the study of a variety of developmentally regulated processes that play key roles in lepidopteran insect cell differentiation and organismic development. Recent advances in recombinant DNA methodology have permitted the cloning of many new structural and several regulatory genes encoding DNA binding proteins that may control the transcriptional properties of other genes. In addition, the adaptation of functional assays, such as cell transfection and in vitro transcription, to lepidopteran systems has resulted in the acquisition of significant new information regarding the functional properties of cloned genes. Despite this progress, however, our understanding of the regulatory processes that control differential gene function during cell differentiation or the precise role that specific gene products assume in complex physiological functions that determine normal growth and development remains sketchy. This has been due to two main reasons: first, lack of expression systems that could allow introduction of cloned genes into insect cells in vivo and subsequent assessments of the consequences of normal or abnormal gene expression during the lifespan of the organism; and second, difficulty in obtaining pure, physiologically relevant proteins in quantities that can permit the undertaking of extensive biochemical studies.

The importance of in vivo expression systems can be best exemplified by the progress that has been achieved in the fruit fly, Drosophila melanogaster, following the development of P element-based embryo transformation methods (Rubin and Spradling, 1982).