小家鼠與黑腹絨鼠毛色基因之演化遺傳分析

Abstract

遺傳多樣性為生物多樣性研究之重要一環，實驗動物-小家鼠(Mus musculus)及其近親鼠類的毛色基因，因毛色變化多端，為研究遺傳多樣性的良好材料。 我們使用Munsell土壤比色卡描述亞洲小家鼠的毛色變異，發現符合Gloger’s rule：恆溫動物在潮溼環境的個體毛色偏黑，在乾燥的棲地毛色偏淡。小家鼠的背部毛色從黃色、褐色到黑色間變化，腹部毛色從白色至黑色間變化。背部毛色的變異較腹部毛色小，且在野外族群發現的毛色種類，遠不如實驗室純品系老鼠的各式毛色變化。小家鼠背部毛色的明暗和雨量有顯著相關，植被茂密且潮溼的環境(環境背景色較深)，背部毛色偏黑，在植被稀疏且乾燥的環境(環境背景色較淡)，背部毛色偏淡，暗示背部毛色可能是受保護色的天擇作用。 分析小家鼠Agouti基因，發現在intron 1的位置上有endogenous retroviruses (ERVs)和long terminal repeats (LTRs)的插入，插入的比率不低，同時插入的位置非常保守。沒有找到插入的突變和毛色或體重的關聯性，暗示插入的突變對個體沒有不利的影響，因此在野生族群中能維持如此比率的突變基因型。 另一個基因melanocortin 1 receptor (Mc1r)亦被發現會影響許多野生動物的毛色，然而有關它的調節區研究卻很少。我們首先取得黑腹絨鼠Mc1r基因上游的序列，結果顯示無論是調節區或編碼區的序列變異，均與毛色變異無關，表示黑腹絨鼠的毛色應是受其它基因的作用影響。

The genetic diversity is an important research area for biodiversity. The candidate genes for highly varied coat color of the laboratory mouse (house mouse, Mus musculus) and other rodents are promising subjects for the research of genetic diversity in wild populations.

We described the coat color variation of house mice in Asia by using Munsell Soil Color Charts and found it is consistent with Gloger’s rule, i.e., individuals of endothermic animals are darker in humid habitats than those in drier habitats. Dorsal coat color ranged from yellow through brown to black, whereas ventral coat color ranged from white to black. Dorsal coat color varied less than the ventral color. In our samples, the variation in coat color in natural populations was far less than that has been observed in the laboratory. We found a significant correlation between the lightness variable of dorsal coat color and precipitation. Mice with dark coat color were observed in more humid and closed habitats (darker background color), and pale coat color in drier, more open habitats (lighter background color). This result might imply the role of concealment as a selective force affecting dorsal coat color in house mice. We screened insertional mutations in intron 1 of a coat color candidate gene – Agouti, in wild mouse populations. These insertions were found to be caused by transposition of endogenous retroviruses (ERVs). The frequency of retrotransposition in Agouti’s intron 1 is not low and the insertion site is very conserved. No association between the retrotransposition and the variation of coat color or body weight was found. The insertional mutations may be non-deleterious alleles, therefore maintaining a certain frequency in wild populations. On the other hand, the molecular genetic changes associated with adaptive morphologies remain an interesting puzzle in evolutionary biology. Previous studies have shown that mutations in the coding regions of another coat color candidate gene, melanocortin 1 receptor (Mc1r) underlies coat color variation in a wide range of animal species. However, the effects of regulatory regions of Mc1r on coat color variation still remains unclear. In this study, we obtained the upstream sequences of the Mc1r gene from Pére David’s vole (Eothenomys melanogaster). No association was found between the coat color variation and the polymorphisms in either regulatory or coding sequences. This implies that there may be other genes, acting alone or in concert with Mc1r, underlying coat color variation in Pére David’s vole.