Miska Katarzyna B, Harrison Gavan A, Hellman Lars, Miller Robert D
Department of Biology, University of New Mexico, Albuquerque 87131, USA.
Immunogenetics. 2002 Sep;54(6):381-93. doi: 10.1007/s00251-002-0484-2. Epub 2002 Jul 16.
We report the isolation and characterization of cDNA clones of expressed, functional major histocompatibility complex class-I ( Mhc-I) genes from two species of monotremes: the duck-billed platypus and the short-beaked echidna. The cDNA clones were isolated from libraries constructed from spleen RNA, clearly establishing their expression in at least this one peripheral lymphoid organ. From the presence of conserved amino acid residues, it appears the expressed sequences encode molecules that likely function as classical Mhc-I. These clones were isolated using monotreme Mhc-I processed pseudogenes as probes. These processed pseudogenes were isolated from genomic DNA and, based on their structure, are likely independently derived in the platypus and echidna. When all the monotreme sequences were included in phylogenetic analyses, we found no apparent orthologous relationships between the platypus and echidna Mhc-I. Analyses that included a large number of Mhc-I sequences from other taxa support a separate monotreme Mhc-I clade, basal to a therian Mhc-I clade that is comprised of sequences from marsupial and placental mammals. The phylogenies also support the hypothesis that Mhc-I genes of placental mammals, marsupials, and monotremes are derived from three separate lineages of Mhc-I genes, best explained by two rounds of duplications and deletions. The first round would have occurred prior to the divergence of monotremes and therians, and the second prior to the divergence of marsupials and placental mammals. The sequences described here represent the first reported functional monotreme Mhc-I, as well as the first processed pseudogenes of any type from monotremes.
我们报告了从两种单孔目动物（鸭嘴兽和短鼻针鼹）中分离并鉴定出的表达功能性主要组织相容性复合体I类（Mhc-I）基因的cDNA克隆。这些cDNA克隆是从由脾脏RNA构建的文库中分离得到的，清楚地证明了它们至少在这一个外周淋巴器官中的表达。从保守氨基酸残基的存在情况来看，这些表达序列似乎编码了可能作为经典Mhc-I发挥功能的分子。这些克隆是使用单孔目Mhc-I加工假基因作为探针分离得到的。这些加工假基因是从基因组DNA中分离出来的，基于它们的结构，可能在鸭嘴兽和针鼹中是独立衍生的。当所有单孔目序列都纳入系统发育分析时，我们发现鸭嘴兽和针鼹的Mhc-I之间没有明显的直系同源关系。包含来自其他分类群的大量Mhc-I序列的分析支持了一个单独的单孔目Mhc-I进化枝，它位于有袋类和胎盘哺乳动物序列组成的兽亚纲Mhc-I进化枝的基部。系统发育分析也支持这样的假设，即胎盘哺乳动物、有袋类动物和单孔目动物的Mhc-I基因源自Mhc-I基因的三个独立谱系，最好的解释是经过两轮重复和缺失。第一轮重复和缺失可能发生在单孔目动物和兽亚纲动物分化之前，第二轮发生在有袋类动物和胎盘哺乳动物分化之前。这里描述的序列代表了首次报道的功能性单孔目Mhc-I，以及单孔目动物的任何类型的首个加工假基因。
