Introduction

The enormous size and surprising fluidity of eukaryotic genomes have long been a fascinating, highly active area of investigation. One of the more remarkable examples of this fluidity is found in the genomes of insects, which can be as small as 5 X 107 base pairs (bp), one-sixtieth the size of the human genome, to 1.2 X 1010 bp, four times the size of the human genome (for a review, see John and Miklos, 1988). The initial steps in the molecular dissection of eukaryotic genomes began with a series of experiments that took advantage of the dependence of DNA:DNA reassociation rates on nucleotide sequence complexity (Britten and Kohne, 1968). These reassociation kinetic studies indicated that all eukaryotic genomes could be divided into repetitive sequences, present hundreds to millions of times per genome, and non–repetitive sequences, present once or at most only a few times per genome. The most highly repeated DNA sequences are usually short, from a few nucleotides to 200 nucleotides in length, arranged as large tandem clusters on the chromosomes (John and Miklos, 1979; Brutlag, 1980; Singer, 1982a). This tandemly repeated DNA is usually referred to as satellite DNA and is typically located at the heterochromatic centromeres of chromosomes. It has been argued that satellite DNA may play a simple structural role by preventing genes from being too near the centromere, or more functional roles such as assisting in chromosome pairing and disjunction during mitosis or meiosis (John and Miklos, 1988; Wu, True, and Johnson, 1989).