PLoS Biology：科学家发现果蝇性别决定新机制

佚名

生物谷综合：美国科学家最近研究一个关于果蝇的基本问题——性别决定机制，却出乎意料地得到了新的结论。相关论文12月27日在线发表于《公共科学图书馆.生物学》（PLoS  Biology）上。

人们知道，在哺乳动物中，性别往往取决于Y染色体是否存在，Y染色体中的雄性化基因会令携带者成长为雄性。不过，其它一些物种却进化出了不同的方式。此前的一些研究发现，决定果蝇雌雄性别的是X染色体与常染色体的比例。

在最新的研究中，美国德州农工大学的James  Erickson和Jerome  Quintero通过研究黑腹果蝇（Drosophila  melanogaster）发现，果蝇X染色体的数量要比上述比例更能决定其性别。

尽管雌雄性果蝇的染色体也是XX和XY，但果蝇和哺乳动物的性染色体机制是分别独立进化出现的，二者存在很大不同。在果蝇中，只有两个X染色体才能产生足够的雌性化信号，从而令胚胎在某一特定而短暂的阶段朝雌性方向发展。（科学网  任霄鹏/编译）

原始出处：

Received: July 2, 2007; Accepted: November 9, 2007; Published: December 27, 2007

Indirect Effects of Ploidy Suggest X Chromosome Dose, Not the X:A Ratio, Signals Sex in Drosophila

James W. Erickson*, Jerome J. Quintero¤

1 Department of Biology, Texas A&M University, College Station, Texas, United States of America

In the textbook view, the ratio of X chromosomes to autosome sets, X:A, is the primary signal specifying sexual fate in Drosophila. An alternative idea is that X chromosome number signals sex through the direct actions of several X-encoded signal element (XSE) proteins. In this alternative, the influence of autosome dose on X chromosome counting is largely indirect. Haploids (1X;1A), which possess the male number of X chromosomes but the female X:A of 1.0, and triploid intersexes (XX;AAA), which possess a female dose of two X chromosomes and the ambiguous X:A ratio of 0.67, represent critical tests of these hypotheses. To directly address the effects of ploidy in primary sex determination, we compared the responses of the signal target, the female-specific SxlPe promoter of the switch gene Sex-lethal, in haploid, diploid, and triploid embryos. We found that haploids activate SxlPe because an extra precellular nuclear division elevates total X chromosome numbers and XSE levels beyond those in diploid males. Conversely, triploid embryos cellularize one cycle earlier than diploids, causing premature cessation of SxlPe expression. This prevents XX;AAA embryos from fully engaging the autoregulatory mechanism that maintains subsequent Sxl expression, causing them to develop as sexual mosaics. We conclude that the X:A ratio predicts sexual fate, but does not actively specify it. Instead, the instructive X chromosome signal is more appropriately seen as collective XSE dose in the early embryo. Our findings reiterate that correlations between X:A ratios and cell fates in other organisms need not implicate the value of the ratio as an active signal.

Figure 1.Delayed Onset of Sxl RNA Synthesis in Haploids

Haploid 1X;1A (top row) and diploid XX;AA (bottom row) embryos were stained following in situ hybridization. Dots represent nascent transcripts from SxlPe in surface nuclei. Nuclear cycles (12–15) are indicated; e and m denote early (5 min) and mid (20 min) stages of the cellularization cycles. Haploid embryos cellularize during nuclear cycle 15 and diploids during cycle 14. Cycle 12 and haploid cycle 13 embryos were illuminated with visible and UV light to highlight DAPI-stained nuclei. Embryos were progeny of sibling mh¹/mh¹or mh¹/FM3 females and mh¹/Y males.