英语听力文摘 English Digest 569、为何有些狗身上有斑点？

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　　In dogs with multicolored coats, white areas have no pigment¹ cells. Black, brown or yellow areas, on the other hand, get their color from special pigment cells.

 

　　When a dog is just an embryo², these pigment cells travel from the spinal³ cord to the rest of the body. A single cell then clones itself in a certain area, forming a group of cells to color the skin and fur there. In a solid black dog, groups of cloned pigment cells fit together like pieces of a jigsaw⁴ puzzle to form the black coat.

 

　　How do the pieces fit so well?

 

　　There’s experimental evidence that pigment cells space themselves out equidistantly from each other, then divide and grow to fill the areas in between.

 

　　Spotted⁵ dogs like Beagles and Fox Terriers, also known as piebalds, have no pigment cells in the white part of their coats. Either some of their pigment cells never developed, or they divided more slowly than usual.

 

　　Dogs with white just on the feet, tip of the tail and belly⁶ have a milder piebald effect. That’s because these parts of the body are farthest from the spinal cord and the pigment cells didn’t quite get that far. As adults, dogs born with this pattern may lose most of the white, because the pigment cells continue to divide and migrate slowly out to the tips of the toes and tail.

 

　　Environmental influences on the embryo like radiation or chemicals can slow the movement of pigment cells and result in spotting. However, as dog breeders know, Spot’s spots more likely are determined⁷ by Spot’s genes⁸.

 

 

　　为何有些狗身上有斑点？

 

　　杂色狗身上的白色部分没有色素细胞，而黑色，棕色或黄色部分都有特殊的色素细胞。

 

　　狗在胚胎期时，色素细胞从脊髓游离到身体其他部位。一个细胞在某个部位可以复制成一个细胞群，为该部位的皮毛着色。纯黑色的狗身上，克隆形成的色素细胞群像拼图玩具一样组合在一起，形成狗的黑色皮毛。

 

　　这些部分怎么组合得这么完美呢？

 

　　实验证明，色素细胞相互之间会等距散开，接着再分裂成长填充中间部位。

 

　　像米格鲁猎犬和猎狐犬这种有斑点的狗也叫花斑狗。它们身上的白色部位没有色素细胞。这可能是因为它们体内的一些色素细胞没有分裂生长，也可能是这些色素细胞分裂得比较慢。

 

　　有些狗只有爪子，尾稍和腹部是白色的，这是轻微的花斑。产生这种现象是因为这些部位离脊髓最远，色素细胞没有扩散那么远。出生时带有轻微花斑的狗成年后白色部位可能就不见了，因为色素细胞会继续分裂并缓慢地移动到脚爪和尾巴末端。

 

　　环境对胚胎的影响，比如辐射和化学物质，会减缓色素细胞的移动，造成花斑。但是，像饲犬者所说，花斑狗身上的斑点更多地是由基因决定的。

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