The muscle of Lawrence in Drosophila: A case of repeated evolutionary loss

The muscle of Lawrence (MOL) is a bilaterally symmetrical muscle spanning the tergite of the fifth abdominal segment of adult male Drosophila melanogaster. It is not, however, a general feature of male-specific development within the subfamily Drosophilinae. Of 95 species surveyed within this subfamily, 67 exist with no MOL at all. By drawing comparisons with published cladograms of species relatedness, three conclusions regarding the evolutionary history of the MOL are made: (i) The MOL predates the major radiations of the genus Drosophila, given its presence in earlier-branching Chymomyza and Scaptodrosophila; the MOL has been subsequently excluded in at least one present species of each of 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 phylogeny.

劳伦斯肌（muscle of Lawrence，MOL）是一种两侧对称的肌肉，横跨成年雄性黑腹果蝇（Drosophila melanogaster）第五腹节的背板。然而，该肌肉并非果蝇亚科（Drosophilinae）内雄性特异性发育的普遍特征。在该亚科的95个被调查物种中，有67个物种完全不具备MOL。通过与已发表的物种亲缘关系分支系统图进行比对，我们可就MOL的演化历史得出三点结论：(i) 鉴于该肌肉在较早分化的酪蝇属（Chymomyza）和盾果蝇属（Scaptodrosophila）中均有存在，可知MOL的起源早于果蝇属（Drosophila）的主要辐射演化；随后这两个原始属的至少一个现生物种中已丢失了该肌肉。(ii) 在果蝇属内，MOL在雅果蝇亚属（Sophophora）的辐射演化类群中呈零星分布，即便在亲缘关系极近的演化支系中也反复出现丢失现象。(iii) MOL可能完全未出现在果蝇亚属（Drosophila）的大量辐射演化类群中。因此，相较于学界公认的果蝇科系统发育关系，MOL的有无分布模式呈现出独特的不一致性。