细胞期刊(Cell)2014-05-09 1:56 PM

鸟类进化过程中阴茎衰减的发展基础 Developmental Basis of Phallus Reduction during Bird Evolution

论文要点
•鸟类在进化过程中大多数谱系的阴茎发生了减弱或消失 

•鸡类变得拥有更小的阴茎,但其原始生长机制仍得到保留

•派生的BMP4区域导致鸡胚阴茎发生细胞凋亡和衰退
  
•基本鸟类和鸭子缺乏远端BMP4表达并发展出了增长的阴茎
Highlights
•During bird evolution, the phallus was reduced or lost in most lineages
•Chickens form a reduced phallus, but primitive outgrowth mechanisms are conserved
•A derived Bmp4 domain causes apoptosis and regression of the embryonic chick phallus
•Basal birds and ducks lack distal Bmp4 expression and develop elongated phalluses

总结
Summary
研究背景
进化过程中最令人费解的事件是鸟类阴茎的退化和消失。所有鸟类都通过体内受精繁殖,但只有大约3%的鸟类保留了具有插入能力的阴茎。一些有关导致阴茎退化进化机制的假设已被提出; 然而我们对其基础发育机制仍然未知。
Background
One of the most puzzling events in evolution is the reduction and loss of the phallus in birds. All birds reproduce by internal fertilization, but only ∼3% of birds have retained a phallus capable of intromission. A number of hypotheses have been proposed for the evolutionary mechanisms that drove phallus reduction; however, the underlying developmental mechanisms are unknown.

研究结果
我们调查了鸟类两个姊妹分支的生殖器发育,即鸡形目(陆禽),其中大部分缺乏具有插入能力的阴茎;以及雁形目(水禽),具有发达的阴茎; 此外还调查了两个外类群的生殖器发育情况,即古鳄总目(鸸鹋)和鳄目(短吻鳄)。鸡形目胚胎经历了生殖结节的隐秘发育,即阴茎的初期形成,但是在此之后经历了细胞凋亡,这导致了结节的退化。从分子水平来看,BMP4表达的派生模式在雏鸡(雉类)外阴结节中被确定。雏鸡生殖器中Bmp信号的抑制避免了细胞凋亡和阴茎退化,且在此的能刺激鸭类(雁形目)生殖器的BMP活化诱导了细胞凋亡的类雉类模式。因此,远端bmp活化是诱导鸡雁小纲中细胞凋亡的充要条件。 
Results
We investigated genital development in two sister clades of birds, Galliformes (land fowl), most of which lack an intromittent phallus, and Anseriformes (waterfowl), which have well developed phalluses; and in two outgroups, Paleognathae (emus) and Crocodilia (alligators). Galliform embryos undergo cryptic development of a genital tubercle, the precursor of the phallus, but this later undergoes apoptosis, leading to regression of the tubercle. At the molecular level, a derived pattern of Bmp4 expression was identified in chick (a galliform) genital tubercles. Inhibition of Bmp signaling in chick genitalia rescues cells from apoptosis and prevents phallus regression, whereas activation of Bmp signaling in duck (an anseriform) genitalia induces a galliform-like pattern of apoptosis. Thus, distal Bmp activity is necessary and sufficient to induce apoptosis in Galloanserae genital tubercles.

研究结论
我们的研究结果表明,雉类具有插入能力阴茎的退化不是因为生长信号中断导致而是由于生殖结节中BMP4使得细胞死亡发生初始活化所导致的。这些有关Bmps在喙形,羽毛,无齿进化中 作用的研究发现和探索表明,Bmps基因调控的调制在禽流感形态的进化中发挥了重要作用。
Conclusions
Our results indicate that evolutionary reduction of the intromittent phallus in galliform birds occurred not by disruption of outgrowth signals but by de novo activation of cell death by Bmp4 in the genital tubercle. These findings, together with discoveries implicating Bmps in evolution of beak shape, feathers, and toothlessness, suggest that modulation of Bmp gene regulation played a major role in the evolution of avian morphology.

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