生物谷綜合：美國科學(xué)家最近研究一個(gè)關(guān)于果蠅的基本問題——性別決定機(jī)制，卻出乎意料地得到了新的結(jié)論,。相關(guān)論文12月27日在線發(fā)表于《公共科學(xué)圖書館.生物學(xué)》（PLoS Biology）上,。
人們知道,，在哺乳動(dòng)物中，性別往往取決于Y染色體是否存在,，Y染色體中的雄性化基因會(huì)令攜帶者成長為雄性,。不過，其它一些物種卻進(jìn)化出了不同的方式,。此前的一些研究發(fā)現(xiàn),，決定果蠅雌雄性別的是X染色體與常染色體的比例。
在最新的研究中,，美國德州農(nóng)工大學(xué)的James Erickson和Jerome Quintero通過研究黑腹果蠅（Drosophila melanogaster）發(fā)現(xiàn),，果蠅X染色體的數(shù)量要比上述比例更能決定其性別。
盡管雌雄性果蠅的染色體也是XX和XY,，但果蠅和哺乳動(dòng)物的性染色體機(jī)制是分別獨(dú)立進(jìn)化出現(xiàn)的,，二者存在很大不同。在果蠅中,，只有兩個(gè)X染色體才能產(chǎn)生足夠的雌性化信號(hào),，從而令胚胎在某一特定而短暫的階段朝雌性方向發(fā)展。（科學(xué)網(wǎng) 任霄鵬/編譯）
原始出處：
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.
