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来源: 英语作文网 时间: 2016-06-22 阅读:
Early mammals evolved in a burst during the Jurassic period, adapting a nocturnal lifestyle when dinosaurs1 were the dominant2 daytime predator3. How these early mammals evolved night vision to find food and survive has been a mystery, but a new study publishing June 20 in Developmental Cell suggests that rods in the mammalian eye, extremely sensitive to light, developed from color-detecting cone4 cells during this time to give mammals an edge in low-light conditions. Cone cells are specialized5 for certain wavelengths6 of light to help animals detect color, while rods can detect even a single photon and are specialized for low-light vision. "The majority of mammals have rod-dominant retinas, but if you look at fish, frogs, or birds, the vast majority are cone-dominated--so the evolutionary7 question has always been, 'What happened?'" says Anand Swaroop, a retina biologist at the National Eye Institute, part of the National Institutes of Health. "We've been working for a long time to understand the fundamental mechanisms8 behind rod and cone development."
Previous work done by Swaroop and his colleagues showed that a transcription factor called NRL pushes cells in the retina toward maturing into rods by suppressing genes9 involved in cone development. "We began to wonder if, somehow, the short-wavelength cones10 were converted into rods during evolution," says Swaroop.
To investigate the origin of rods in mammals, Swaroop and his team examined rod and cone cells taken from mice at different stages of development. Details of an organism's embryonic11 development often reveal traits carried by its evolutionary ancestors; consider, for instance, how human embryos12 initially13 develop gill-like slits14 and a tail.
The researchers saw that in early stages, two days after the mice were born, developing rod cells expressed genes normally seen in mature short-wavelength cones (which are used in other animals to detect ultraviolet light). When the researchers examined the epigenetics of purified rod cells from mice, they saw that these aspects became repressed by histone and DNA15 methylations later in development, ten days after the mice were born.
In zebrafish, which are diurnal16 and cone-dominated, another set of experiments showed that the rod cells didn't resemble cones at all. To investigate when the mammalian elements that turn cones to rods might have originated, the researchers reviewed genomic sequences from a variety of vertebrate animals. The team discovered that the genes responsible for the regulation of NRL became more refined in the placental mammals as the modern retina evolved and were lost in several non-mammalian groups. The origin of this regulatory system appeared to coincide with the evolution of nocturnality in early mammals.

1 dinosaurs     
n.恐龙( dinosaur的名词复数 );守旧落伍的人,过时落后的东西
  • The brontosaurus was one of the largest of all dinosaurs. 雷龙是所有恐龙中最大的一种。 来自《简明英汉词典》
  • Dinosaurs have been extinct for millions of years. 恐龙绝种已有几百万年了。 来自《简明英汉词典》

  • 2 dominant     
  • The British were formerly dominant in India.英国人从前统治印度。
  • She was a dominant figure in the French film industry.她在法国电影界是个举足轻重的人物

  • 3 predator     
  • The final part of this chapter was devoted to a brief summary of predator species.本章最后部分简要总结了食肉动物。
  • Komodo dragon is the largest living lizard and a fearsome predator.科摩多龙是目前存在的最大蜥蜴,它是一种令人恐惧的捕食性动物。

  • 4 cone     
  • Saw-dust piled up in a great cone.锯屑堆积如山。
  • The police have sectioned off part of the road with traffic cone.警察用锥形路标把部分路面分隔开来。

  • 5 specialized     
  • There are many specialized agencies in the United Nations.联合国有许多专门机构。
  • These tools are very specialized.这些是专用工具。

  • 6 wavelengths     
    n.波长( wavelength的名词复数 );具有相同的/不同的思路;合拍;不合拍
  • I find him difficult to talk to—we're on completely different wavelengths. 我没法和他谈话,因为我们俩完全不对路。 来自《简明英汉词典》
  • Sunlight consists of different wavelengths of radiation. 阳光由几种不同波长的射线组成。 来自辞典例句

  • 7 evolutionary     
  • Life has its own evolutionary process.生命有其自身的进化过程。
  • These are fascinating questions to be resolved by the evolutionary studies of plants.这些十分吸引人的问题将在研究植物进化过程中得以解决。

  • 8 mechanisms     
    n.机械( mechanism的名词复数 );机械装置;[生物学] 机制;机械作用
  • The research will provide direct insight into molecular mechanisms. 这项研究将使人能够直接地了解分子的机理。 来自《简明英汉词典》
  • He explained how the two mechanisms worked. 他解释这两台机械装置是如何工作的。 来自《简明英汉词典》

  • 9 genes     
    n.基因( gene的名词复数 )
  • You have good genes from your parents, so you should live a long time. 你从父母那儿获得优良的基因,所以能够活得很长。 来自《简明英汉词典》
  • Differences will help to reveal the functions of the genes. 它们间的差异将会帮助我们揭开基因多种功能。 来自英汉非文学 - 生命科学 - 生物技术的世纪

  • 10 cones     
    n.(人眼)圆锥细胞;圆锥体( cone的名词复数 );球果;圆锥形东西;(盛冰淇淋的)锥形蛋卷筒
  • In the pines squirrels commonly chew off and drop entire cones. 松树上的松鼠通常咬掉和弄落整个球果。 来自辞典例句
  • Many children would rather eat ice cream from cones than from dishes. 许多小孩喜欢吃蛋卷冰淇淋胜过盘装冰淇淋。 来自辞典例句

  • 11 embryonic     
  • It is still in an embryonic stage.它还处于萌芽阶段。
  • The plan,as yet,only exists in embryonic form.这个计划迄今为止还只是在酝酿之中。

  • 12 embryos     
    n.晶胚;胚,胚胎( embryo的名词复数 )
  • Somatic cells of angiosperms enter a regenerative phase and behave like embryos. 被子植物体细胞进入一个生殖阶段,而且其行为象胚。 来自辞典例句
  • Evolution can explain why human embryos look like gilled fishes. 进化论能够解释为什么人类的胚胎看起来象除去了内脏的鱼一样。 来自辞典例句

  • 13 initially     
  • The ban was initially opposed by the US.这一禁令首先遭到美国的反对。
  • Feathers initially developed from insect scales.羽毛最初由昆虫的翅瓣演化而来。

  • 14 slits     
    n.狭长的口子,裂缝( slit的名词复数 )v.切开,撕开( slit的第三人称单数 );在…上开狭长口子
  • He appears to have two slits for eyes. 他眯着两眼。
  • "You go to--Halifax,'she said tensely, her green eyes slits of rage. "你给我滚----滚到远远的地方去!" 她恶狠狠地说,那双绿眼睛冒出了怒火。

  • 15 DNA     
    (缩)deoxyribonucleic acid 脱氧核糖核酸
  • DNA is stored in the nucleus of a cell.脱氧核糖核酸储存于细胞的细胞核里。
  • Gene mutations are alterations in the DNA code.基因突变是指DNA密码的改变。

  • 16 diurnal     
  • Kangaroos are diurnal animals.袋鼠是日间活动的动物。
  • Over water the diurnal change in refraction is likely to be small. 在水面上,折光的周日变化可能是很小的。

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