Gruhl M, Kao W Y, Bergtrom G
Department of Biological Sciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA.
J Mol Evol. 1997 Nov;45(5):499-508. doi: 10.1007/pl00006254.
While globin genes ctt-2beta and ctt-9.1 in Chironomus thummi thummi each have a single intron, all of the other insect globin genes reported so far are intronless. We analyzed four globin genes linked to the two intron-bearing genes in C. th. thummi. Three have a single intron at the same position as ctt-2beta and ctt-9.1; the fourth is intronless and lies between intron bearing genes. Finally, in addition to its intron, one gene (ctt-13RT) was recently interrupted by retrotransposition. Phylogenetic analyses show that the six genes in C. th. thummi share common ancestry with five globin genes in the distantly related species C. tentans, and that a 5-gene ancestral cluster predates the divergence of the two species. One gene in the ancestral cluster gave rise to ctn-ORFB in C. tentans, and duplicated in C. th. thummi to create ctt-11 and ctt-12. From parsimonious calculations of evolutionary distances since speciation, ctt-11, ctt-12, and ctn-ORFB evolved rapidly, while ctn-ORFE in C. tentans evolved slowly compared to other globin genes in the clusters. While these four globins are under selective pressure, we suggest that most chironomid globin genes were not selected for their unique function. Instead, we propose that high gene copy number itself was selected because conditions favored organisms that could synthesize more hemoglobin. High gene copy number selection to produce more of a useful product may be the basis of forming multigene families, all of whose members initially accumulate neutral substitutions while retaining essential function. Maintenance of a large family of globin genes not only ensured high levels of hemoglobin production, but may have facilitated the extensive divergence of chironomids into as many as 5000 species.
在深红眼摇蚊（Chironomus thummi thummi）中，球蛋白基因ctt - 2β和ctt - 9.1各有一个内含子，而迄今为止报道的所有其他昆虫球蛋白基因都没有内含子。我们分析了与深红眼摇蚊中两个含内含子基因相连的四个球蛋白基因。其中三个在与ctt - 2β和ctt - 9.1相同的位置有一个内含子；第四个没有内含子，位于含内含子基因之间。最后，除了其内含子外，一个基因（ctt - 13RT）最近因反转录转座而中断。系统发育分析表明，深红眼摇蚊中的这六个基因与远缘物种细摇蚊（C. tentans）中的五个球蛋白基因有共同的祖先，并且一个包含五个基因的祖先簇早于这两个物种的分化。祖先簇中的一个基因产生了细摇蚊中的ctn - ORFB，并在深红眼摇蚊中复制形成了ctt - 11和ctt - 12。从物种形成后进化距离的简约计算来看，ctt - 11、ctt - 12和ctn - ORFB进化迅速，而细摇蚊中的ctn - ORFE与簇中的其他球蛋白基因相比进化缓慢。虽然这四种球蛋白受到选择压力，但我们认为大多数摇蚊球蛋白基因并非因其独特功能而被选择。相反，我们提出选择高基因拷贝数本身是因为环境有利于能够合成更多血红蛋白的生物体。选择高基因拷贝数来产生更多有用产物可能是形成多基因家族的基础，其所有成员最初在保留基本功能的同时积累中性替换。维持一大类球蛋白基因不仅确保了高水平的血红蛋白产生，而且可能促进了摇蚊广泛分化成多达5000个物种。
