Rapidly evolving Bkm DNA is associated with hypervariable domains

Abstract

Bkm sequences were first found on the W sex chromosomes of snakes were they exist in amounts large enough to appear as a satellite DNA fraction in females (Singh, Purdom & Jones 1976). In situ hybridization shows that the sequences are interspersed throughout the sex-determining W chromosome (Singh, Purdom & Jones 1980). They are also present in lesser abundance throughout the genomes of all snakes and of all eukaryotes so far surveyed (Jones & Singh 1981a). Concentrations of Bkm sequences occur on sex chromosomes in many other vertebrate species (Jones & Singh 1981b). This ubiquitous distribution does not necessarily indicate functional conservation but could equally reflect processes which generate novel nucleotide sequences. Hybridization of Bkm probes has so far failed to show related sequences in prokaryotes (Jones, Singh & Phillips 1983), suggesting either that the sequences are subject to negative selection or that the chromosomal processes which generate them may be absent.