基因倍增和脊椎动物起源

有机体基因复制导致基因复杂性增加及其和脊椎动物起源的关系已经成为进化生物学研究的热点。20世纪70年代由Ohno提出后经Holland等修正的原始脊索动物经两轮基因组复制产生脊椎动物的假设目前已被广泛接受。脊椎动物起源和进化过程中发生过两轮基因组复制的主要证据有三点：（1）据估计脊椎动物基因组内编码基因数目大约相当于果蝇、海鞘等无脊椎动物的4倍；原口动物如果蝇和后口动物如头索动物文昌鱼的基因组大都只有单拷贝的基因，而脊椎动物的基因组则通常有4个同属于一个家族的基因。（2）无脊椎动物如节肢动物、海胆和头索动物文昌鱼都只有一个Hox基因簇，而脊椎动物除鱼类外，有7个具有Hox基因簇，其余都具有4个Hox基因簇。（3）基因作图证明，不但在鱼类和哺乳动物染色体广大片段上基因顺序相似，而且有证据显示哺乳动物基因组不同染色体之间存在相似性。据认为第一次基因倍增发生在脊椎动物与头索动物分开之后，第二次基因倍增发生在有颌类脊椎动物和无颌类脊椎动物分开以后。但是，基因是逐个发生倍增，还是通过基因组内某些DNA片段抑或整个基因组的加倍而实现的，目前还颇有争议。

Gene duplication and origin of vertebrates

The connection between increasing genomic complexity by gene duplication and origin of vertebrates is of great interest to evolutionary biologists. The most widely accepted hypothesis is that the origin of vertebrates correlated with two rounds of genome duplication early in vertebrate evolution, which is originally suggested by Ohno in 1970 and later modified by Holland et al in 1994. Three important observations have been marshaled in line with this hypothesis. First, the number of coding genes in vertebrates is roughly four times the number estimated for invertebrates such as Drosophila and ascidian. Moreover, for many genes in the invertebrates such as Drosophila and amphioxus, there are typically four putative vertebrate orthologs. Second, there exists one Hox gene cluster in invertebrates such as arthropods and echinoderms and cephochordates such as amphioxus and four clusters in vertebrates, although teleost fishes are exceptional with the recent discovery of seven Hox gene clusters. Third, large sections of chromosomes have similar gene order between fishes and mammals, and there is evidence of paralogy of chromosomes in mammalian genomes. It is proposed that the first round of gene duplication occurred after the diversification of vertebrates from cephalochordate, and the second round after the diversification of jawed vertebrates from jawless vertebrates. However, it remains open whether gene duplication early in vertebrate evolution is small-scale (regional) DNA duplication or large-scale (genomic) duplication.