Babin Patrick J
Génomique et Physiologie des Poissons, Université Bordeaux 1, UMR NuAGe, 33405 Talence, France.
Gene. 2008 Apr 30;413(1-2):76-82. doi: 10.1016/j.gene.2008.02.001. Epub 2008 Feb 9.
Vitellogenin (Vtg) derivatives are the main egg-yolk proteins in most oviparous animal species, and are, therefore, key players in reproduction and embryo development. Conserved synteny and phylogeny were used to identify a Vtg gene cluster (VGC) that had been evolutionarily conserved in most oviparous vertebrates, encompassing the three linked Vtgs on chicken (Gallus gallus) chromosome 8. Tandem arranged homologs to chicken VtgII and VtgIII were retrieved in similar locations in Xenopus (Xenopus tropicalis) and homologous transcribed inverted genes were found in medaka (Oryzias latipes), stickleback (Gasterosteus aculeatus), pufferfish (Takifugu rubripes), and Tetrahodon (Tetraodon nigroviridis), while zebrafish (Danio rerio) Vtg3 may represent a residual trace of VGC in this genome. Vtgs were not conserved in the paralogous chromosomal segment attributed to a whole-genome duplication event in the ancestor of teleosts, while tandem duplicated forms have survived the recent African clawed frog (Xenopus laevis) tetraploidization. Orthologs to chicken VtgI were found in similar locations in teleost fish, as well as in the platypus (Ornithorhynchus anatinus). Additional Vtg fragments found suggested that VGC had been conserved in this egg-laying mammal. A low ratio of nonsynonymous-to-synonymous substitution values and the paucity of pseudogene features suggest functional platypus Vtg products. Genomic identification of Vtgs, Apob, and Mtp in this genome, together with maximum likelihood and Bayesian inference phylogenetic analyses, support the existence of these three large lipid transfer protein superfamily members at the base of the mammalian lineage. In conclusion, the establishment of a VGC in the vertebrate lineage predates the divergence of ray-finned fish and tetrapods and the shift in reproductive and developmental strategy observed between prototherians and therians may be associated with its loss, as shown by its absence from the genomic resources currently available from therians.
卵黄蛋白原（Vtg）衍生物是大多数卵生动物物种中的主要蛋黄蛋白，因此是生殖和胚胎发育的关键参与者。利用保守的同线性和系统发育来鉴定一个在大多数卵生脊椎动物中进化保守的Vtg基因簇（VGC），该基因簇包含鸡（原鸡）8号染色体上的三个连锁Vtgs。在非洲爪蟾（热带爪蟾）的相似位置检索到与鸡VtgII和VtgIII串联排列的同源物，并且在青鳉、棘鱼、河豚和四齿鲀中发现了同源转录的反向基因，而斑马鱼的Vtg3可能代表该基因组中VGC的残余痕迹。在硬骨鱼祖先的全基因组复制事件所导致的旁系染色体区段中，Vtgs并不保守，而串联重复形式在最近的非洲爪蟾四倍体化过程中得以保留。在硬骨鱼以及鸭嘴兽的相似位置发现了与鸡VtgI的直系同源物。发现的其他Vtg片段表明VGC在这种产卵哺乳动物中是保守的。非同义替换与同义替换值的低比率以及假基因特征的缺乏表明鸭嘴兽Vtg产物具有功能。对该基因组中Vtgs、载脂蛋白B（Apob）和微粒体甘油三酯转运蛋白（Mtp）的基因组鉴定，以及最大似然法和贝叶斯推断系统发育分析，支持了这三个大的脂质转运蛋白超家族成员在哺乳动物谱系基部的存在。总之，脊椎动物谱系中VGC的建立早于辐鳍鱼和四足动物的分化，并且原兽类和兽类之间观察到的生殖和发育策略的转变可能与其丢失有关，正如目前兽类可用的基因组资源中没有VGC所表明的那样。
