Patterns of Gene Activity in Larval Tissues of the Blowfly, Calliphora

It is now widely recognised that the giant polytene chromosomes found in terminally differentiated tissues of certain organisms, most notably amongst the Diptera, offer unique opportunities for correlated cytological and biochemical approaches to the study of gene activity and its regulation (Ashburner, 1970). Unfortunately, until the advent of mass isolation techniques (e.g. Boyd et al, 1968; Zweidler and Cohen, 1971), tissues with polytene chromosomes have been better known to cytologists than to biochemists. One consequence has sometimes been the selection of experimental material for studies of gene activity on the basis of favourable chromosome banding and chromosome continuity. Such studies have proved especially helpful in revealing the relation of local chromosome dispersion (“puffing”) to gene activity (Beermann, 1952; Ashburner, 1970). On the other hand, the occurrence and functional significance of larger scale chromosome dispersion, involving whole blocks of band-interband sequences, may escape attention if a criterion which ensures a relatively low level of gene read-out is employed in choosing tissues for investigation (Thomson, 1969).