Nature Discovery: 人类的阴茎是如何丢掉“刺”变得光滑的？

How the penis lost its spikes？

-Humans ditched DNA to evolve smooth penises and bigger brains.

   猩猩是人类的表兄弟，是目前地前地球上和人类关系最近的亲戚。人和猩猩的基因组差异极小（perfect identity with 96%, 96% 完全一样），就男人和女人之间的差别要将近2%呢，毕竟男人和女人有一条X染色体的差别啊。

    与猩猩相比，人的大脑容量更大；还有一个更有意思的差别，那就是阴茎，结构和形态十分不同。猩猩的阴茎都有“倒刺”，而人没有，它们的阴茎都是像矛一样，而人的却是均匀粗细，顶端有柔软的蘑菇盖样结构。大脑容量变大的作用毋庸置疑，阴茎的这些变化也带来进化上的优越性（生殖优越性）。在进化选择上，性取向也是一个重要的因素。另外，一夫一妻制是人类采取的主要生殖策略（这与猩猩不同），这样，一方面生殖行为必须有双方快乐的驱动力，而不是一种强迫的行为;另外,雄性还要防止外来基因的入侵（第三者插入），除了武力驱赶外，还通过阴茎的特殊构造清除外来入侵者的基因（精子）。这都是用来解释人的阴茎与猩猩的这种结构区别。

问题是，这种变化时怎么来的？

斯坦福大学的科学家们发现了这种变化的分子机制。他们通过比较基因组的序列，发现，人丢失了一些DNA片段，这些片段大都是非编码序列，其中有的是在雄激素受体基因的旁侧；通过动物实验，把这段序列插入到小鼠的雄激素受体基因对应部位，发现老鼠的阴茎也长出刺样结构。

“我们祖先失去阴茎“倒刺”倒是我们今天的今天的收获，全世界的每一对夫妻都会为祖先丢失这段DNA心生感激” （...our ancestors' loss of penile spines is our gain today."Couples everywhere can be thankful that this particular piece of DNA was ditched," ）

    通过同样的方法，它们也鉴定了一些可能导致人脑容量变大的基因片段。上述结果发表在最新一期的Nature上（McLean, C. Y. et al. Nature 471, 216-219 (2011)）。

看来，进化中的不光是靠得到新基因和新功能，还需要失去一些基因和功能啊。

全文 http://www.nature.com/nature/journal/v471/n7337/full/nature09774.html

象鼻虫的阴茎“倒刺”

How the penis lost its spikes

Humans ditched DNA to evolve smooth penises and bigger brains.

Zoë Corbyn

Sex would be a very different proposition for humans if — like some animals including chimpanzees, macaques and mice — men had penises studded with small, hard spines.

Now researchers at Stanford University in California have found a molecular mechanism for how the human penis could have evolved to be so distinctly spine-free. They have pinpointed it as the loss of a particular chunk of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.

"It is a small but fascinating part of a bigger picture about the evolution of human-specific traits," said Gill Bejerano, a developmental biologist at Stanford who led the work along with colleague David Kingsley. "We add a molecular perspective to a discussion that has been going on for several decades at least."

Published in Nature today1, the research also suggests a molecular mechanism for how we evolved bigger brains than chimpanzees and lost the small sensory whiskers that the apes — who are amongst our closest relatives and with whom it has been estimated we share 96% of our DNA — have on their face.

Monogamous strategy

It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known.

The researchers did not set out to study penile spines. Rather, they were looking for chunks of DNA that had been lost from the human genome but not the chimp genome, so they could then try to pinpoint what those chunks did.

The approach differs from that in most studies, explain Bejerano and Kingsley, in looking at what has been deleted from the human genome rather than what is present. "In the case of our study, had you started from the human genome, there would be nothing there to see," says Bejerano.

They first systematically identified 510 DNA sequences missing in humans and present in chimps, finding that those sequences were almost exclusively from the non-coding regions of the genome, between genes. They then homed in on two sequences whose absence in humans they thought might be interesting — one from near the androgen receptor (AR) gene and one from near a gene involved in tumour suppression (GADD45G).

Inserting the chimpanzee sequences into mouse embryos revealed that the former sequence produced both the hard penile spines and sensory whiskers present in some animals. The latter sequence acted as a kind of brake on the growth of specific brain regions — with the removal of its function appearing to have paved the way for the evolution of the larger human brain.

"The goal of the project was to find molecular lesions [losses] that underlie human evolutionary traits, with the examples illustrating different aspects of the principle," says Kingsley.

"Until we looked at where the DNA was expressed, we had no idea which switch — if any — it would actually control," adds Bejerano.

Other molecular biologists praised the work for its clever approach and said it would open up new avenues of inquiry, particularly for those working on the evolution of the human brain.

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 "It is detective work and a great reminder that, in the course of evolution, information is both gained and lost," said Sean Carroll, an expert in animal genetics and evolution at the University of Wisconsin, Madison.

"As so often with very good ideas, it seems almost obvious in hindsight," said Svante Pääbo, who directs the genetics department of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and was part of the team that recently sequenced the Neanderthal genome. "Since two of the almost 500 deleted sequences they identified turn out to be interesting, I am sure that several other ones on their list will turn out to be interesting too," he added. The researchers are continuing to analyse the remaining 508 DNA sequences.

David Haussler, who studies the molecular evolution of the human genome at the University of California, Santa Cruz, added that our ancestors' loss of penile spines is our gain today."Couples everywhere can be thankful that this particular piece of DNA was ditched," he says.