Patel Vidushi S, Cooper Steven J B, Deakin Janine E, Fulton Bob, Graves Tina, Warren Wesley C, Wilson Richard K, Graves Jennifer A M
The ARC Centre for Kangaroo Genomics, Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia.
BMC Biol. 2008 Jul 25;6:34. doi: 10.1186/1741-7007-6-34.
Vertebrate alpha (alpha)- and beta (beta)-globin gene families exemplify the way in which genomes evolve to produce functional complexity. From tandem duplication of a single globin locus, the alpha- and beta-globin clusters expanded, and then were separated onto different chromosomes. The previous finding of a fossil beta-globin gene (omega) in the marsupial alpha-cluster, however, suggested that duplication of the alpha-beta cluster onto two chromosomes, followed by lineage-specific gene loss and duplication, produced paralogous alpha- and beta-globin clusters in birds and mammals. Here we analyse genomic data from an egg-laying monotreme mammal, the platypus (Ornithorhynchus anatinus), to explore haemoglobin evolution at the stem of the mammalian radiation.
The platypus alpha-globin cluster (chromosome 21) contains embryonic and adult alpha- globin genes, a beta-like omega-globin gene, and the GBY globin gene with homology to cytoglobin, arranged as 5'-zeta-zeta'-alphaD-alpha3-alpha2-alpha1-omega-GBY-3'. The platypus beta-globin cluster (chromosome 2) contains single embryonic and adult globin genes arranged as 5'-epsilon-beta-3'. Surprisingly, all of these globin genes were expressed in some adult tissues. Comparison of flanking sequences revealed that all jawed vertebrate alpha-globin clusters are flanked by MPG-C16orf35 and LUC7L, whereas all bird and mammal beta-globin clusters are embedded in olfactory genes. Thus, the mammalian alpha- and beta-globin clusters are orthologous to the bird alpha- and beta-globin clusters respectively.
We propose that alpha- and beta-globin clusters evolved from an ancient MPG-C16orf35-alpha-beta-GBY-LUC7L arrangement 410 million years ago. A copy of the original beta (represented by omega in marsupials and monotremes) was inserted into an array of olfactory genes before the amniote radiation (>315 million years ago), then duplicated and diverged to form orthologous clusters of beta-globin genes with different expression profiles in different lineages.
脊椎动物的α-和β-珠蛋白基因家族体现了基因组进化产生功能复杂性的方式。从单个珠蛋白基因座的串联重复开始，α-和β-珠蛋白基因簇不断扩展，随后被分隔到不同的染色体上。然而，先前在有袋类动物的α-基因簇中发现了一个化石β-珠蛋白基因（ω），这表明α-β基因簇复制到两条染色体上，随后发生谱系特异性的基因丢失和复制，在鸟类和哺乳动物中产生了旁系同源的α-和β-珠蛋白基因簇。在此，我们分析了产卵单孔目哺乳动物鸭嘴兽（Ornithorhynchus anatinus）的基因组数据，以探究哺乳动物辐射起源时血红蛋白的进化情况。
鸭嘴兽的α-珠蛋白基因簇（第21号染色体）包含胚胎型和成年型α-珠蛋白基因、一个类β-ω珠蛋白基因以及与细胞珠蛋白具有同源性的GBY珠蛋白基因，排列顺序为5'-ζ-ζ'-αD-α3-α2-α1-ω-GBY-3'。鸭嘴兽的β-珠蛋白基因簇（第2号染色体）包含单个胚胎型和成年型珠蛋白基因，排列顺序为5'-ε-β-3'。令人惊讶的是，所有这些珠蛋白基因在某些成年组织中均有表达。侧翼序列比较显示，所有有颌脊椎动物的α-珠蛋白基因簇侧翼均为MPG-C16orf35和LUC7L，而所有鸟类和哺乳动物的β-珠蛋白基因簇均嵌入嗅觉基因中。因此，哺乳动物的α-和β-珠蛋白基因簇分别与鸟类的α-和β-珠蛋白基因簇直系同源。
我们提出α-和β-珠蛋白基因簇在4.1亿年前从古老的MPG-C16orf35-α-β-GBY-LUC7L排列形式进化而来。原始β基因的一个拷贝（在有袋类动物和单孔目动物中由ω代表）在羊膜动物辐射（>3.15亿年前）之前插入到一系列嗅觉基因中，随后复制并分化，形成了在不同谱系中具有不同表达谱的β-珠蛋白基因直系同源簇。
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