Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3204, USA.
Genetics. 2010 Dec;186(4):1321-36. doi: 10.1534/genetics.110.121202. Epub 2010 Sep 13.
In Drosophila melanogaster, the gene Sex-lethal (Sxl) controls all aspects of female development. Since melanogaster males lacking Sxl appear wild type, Sxl would seem to be functionally female specific. Nevertheless, in insects as diverse as honeybees and houseflies, Sxl seems not to determine sex or to be functionally female specific. Here we describe three lines of work that address the questions of how, when, and even whether the ancestor of melanogaster Sxl ever shed its non-female-specific functions. First, to test the hypothesis that the birth of Sxl's closest paralog allowed Sxl to lose essential ancestral non-female-specific functions, we determined the CG3056 null phenotype. That phenotype failed to support this hypothesis. Second, to define when Sxl might have lost ancestral non-female-specific functions, we isolated and characterized Sxl mutations in D. virilis, a species distant from melanogaster and notable for the large amount of Sxl protein expression in males. We found no change in Sxl regulation or functioning in the 40+ MY since these two species diverged. Finally, we discovered conserved non-sex-specific Sxl mRNAs containing a previously unknown, potentially translation-initiating exon, and we identified a conserved open reading frame starting in Sxl male-specific exon 3. We conclude that Drosophila Sxl may appear functionally female specific not because it lost non-female-specific functions, but because those functions are nonessential in the laboratory. The potential evolutionary relevance of these nonessential functions is discussed.
在黑腹果蝇中，基因 Sex-lethal（Sxl）控制着所有雌性发育的方面。由于缺乏 Sxl 的雄性果蝇看起来是野生型的，因此 Sxl 似乎在功能上是特异性雌性的。然而，在蜜蜂和家蝇等多种多样的昆虫中，Sxl 似乎并不决定性别，也不是功能上特异性雌性的。在这里，我们描述了三条解决问题的工作线，即 Sxl 的祖先何时以及是否甚至丧失了其非特异性雌性功能。首先，为了检验 Sxl 最接近的直系同源基因的诞生使 Sxl 丧失了必要的祖先非特异性雌性功能的假说，我们确定了 CG3056 缺失表型。该表型未能支持这一假说。其次，为了确定 Sxl 何时可能丧失了祖先的非特异性雌性功能，我们分离并鉴定了 D. virilis 中的 Sxl 突变，D. virilis 是一种与黑腹果蝇相距较远的物种，雄性中 Sxl 蛋白表达量很大。我们发现，自这两个物种分化以来的 4000 多万年中，Sxl 的调控或功能没有发生变化。最后，我们发现了含有以前未知的、可能起始翻译的外显子的保守的非性特异性 Sxl mRNA，并鉴定了一个从 Sxl 雄性特异性外显子 3 开始的保守开放阅读框。我们得出的结论是，果蝇 Sxl 似乎在功能上是特异性雌性的，不是因为它失去了非特异性雌性功能，而是因为这些功能在实验室中是不必要的。这些非必要功能的潜在进化相关性将在文中进行讨论。
