On the Problem of Genetic Similarity between Sibling Species-Puffing Patterns in Drosophila melanogaster and Drosophila simulans

The recent paper of Hubby and Throckmorton (1968) published in this journal prompts me to communicate some observations arising from a study of puffing patterns of salivary gland chromosomes in Drosophila melanogaster and D. simulans. Hubby and Throckmorton (1968) studied the electrophoretic mobility of various proteins within triads of Drosophila species, each triad being composed of two sibling species and one nonsibling species from the same species group. For example, one such triad was the sibling species pair .D. melanogaster and D. simulans and the morphologically dissimilar species D. takahashii. They found that within sibling pairs an average of 50%o of the proteins selected for study showed identical electrophoretic mobilities on acrylamide gels, while between either one of the sibling pair and the third member of the triad only 18%o (on average) of the proteins had identical mobilities. Hubby and Throckmorton draw the reasonable conclusion from their data that the morphological similarity of sibling species reflects their genetic similarity-a conclusion that contrasts with the view of Mayr (1963). Patterns of puffing activity of polytene chromosomes reflect patterns of active gene loci (see Beermann, 1956). During the larval development of Diptera, the patterns of puffing of salivary gland chromosomes change in a regular and characteristic manner. In D. melanogaster I have found (Ashburner, 1967, unpublished) that during the late third instar larval and prepupal development he activity of 105 puff loci is closely dependent upon the physiological age of the animals. Comparison of the sequence of puffing changes in different strains of D. melanogaster showed that there is variation both in the spectrum of puff sites that are potentially active and in the timing and/or magnitude of activity at those sites which are common to the strains (Ashburner, unpublished). For example, two strains of D. melanogaster, Oregon-R and Thoday's vg6, share 82/83 of their developmentally specific autosomal puffs. The activity of 53 (64%o of the common puff loci) of these is more or less identical in the two strains, 10 (12%o ) are of different magnitudes in the two strains, 16 (19%0 ) differ in their timing, and 4 (5%o) are different in both their size and their timing. In fact, the similarity of the puffing patterns of Oregon-R and vg6 is greater than these figures might suggest: 69 (83%o) of the common puffs have similar mean sizes, irrespective of their timing, and 60 (76%o) have similar timing of their active periods, irrespective of their size.