Katz Laura A, Snoeyenbos-West Oona, Doerder F Paul
Department of Biological Sciences, Smith College, Northampton.
Mol Biol Evol. 2006 Mar;23(3):608-14. doi: 10.1093/molbev/msj067. Epub 2005 Nov 24.
High levels of synonymous substitutions among alleles of the surface antigen SerH led to the hypothesis that Tetrahymena thermophila has a tremendously large effective population size, one that is greater than estimated for many prokaryotes (Lynch, M., and J. S. Conery. 2003. Science 302:1401-1404.). Here we show that SerH is unusual as there are substantially lower levels of synonymous variation at five additional loci (four nuclear and one mitochondrial) characterized from T. thermophila populations. Hence, the effective population size of T. thermophila, a model single-celled eukaryote, is lower and more consistent with estimates from other microbial eukaryotes. Moreover, reanalysis of SerH polymorphism data indicates that this protein evolves through a combination of vertical transmission of alleles and concerted evolution of repeat units within alleles. SerH may be under balancing selection due to a mechanism analogous to the maintenance of antigenic variation in vertebrate immune systems. Finally, the dual nature of ciliate genomes and particularly the amitotic divisions of processed macronuclear genomes may make it difficult to estimate accurately effective population size from synonymous polymorphisms. This is because selection and drift operate on processed chromosomes in macronuclei, where assortment of alleles, disruption of linkage groups, and recombination can alter the genetic landscape relative to more canonical eukaryotic genomes.
表面抗原SerH的等位基因间存在高水平的同义替换，这使得人们推测嗜热四膜虫具有极大的有效种群大小，大于许多原核生物的估计值（林奇，M.，和J. S. 科纳里。2003年。《科学》302:1401 - 1404）。在此我们表明，SerH是不同寻常的，因为在另外五个从嗜热四膜虫种群中鉴定出的位点（四个核位点和一个线粒体位点），同义变异水平显著更低。因此，作为模式单细胞真核生物的嗜热四膜虫的有效种群大小更低，并且与其他微生物真核生物的估计值更为一致。此外，对SerH多态性数据的重新分析表明，该蛋白质通过等位基因的垂直传递和等位基因内重复单元的协同进化相结合的方式进化。由于一种类似于脊椎动物免疫系统中维持抗原变异的机制，SerH可能处于平衡选择之下。最后，纤毛虫基因组的双重性质，尤其是经加工的大核基因组的无丝分裂，可能使得从同义多态性准确估计有效种群大小变得困难。这是因为选择和漂变作用于大核中的经加工染色体，在那里等位基因的分配、连锁群的破坏和重组会改变相对于更典型真核基因组的遗传格局。
