THE MUSCLE OF LAWRENCE IN DROSOPHILA - A CASE OF REPEATED EVOLUTIONARY LOSS

The muscle of Lawrence (MOL) is a bilaterally symmetrical muscle spann ing the tergite of the fifth abdominal segment of adult male Drosophil a melanogaster. It is not, however, a general feature of male-specific development within the subfamily Drosophilinae. Of 95 species surveye d within this subfamily, 67 exist with no MOL at all, By drawing compa risons with published cladograms of species relatedness, three conclus ions regarding the evolutionary history of the MOL are made: (i) The M OL predates the major radiations of the genus Drosophila, given its pr esence in earlier-branching Chymomyza and Scaptodrosophila; the MOL ha s been subsequently excluded in at least one present species of each o f these two primitive genera. (ii) Within the genus Drosophila the MOL is present sporadically in the radiation of the subgenus Sophophora, showing repetitive loss even in very close evolutionary lineages, (iii ) The MOL may have been entirely excluded from the prolific radiation of the subgenus Drosophila. Thus the MOL shows a uniquely incongruous pattern of presence or absence relative to accepted drosophilid phylog eny.