Glass Shiona K, Moszczynska Anna, Crease Teresa J
Department of Integrative Biology, University of Guelph, Guelph, ON N1G2W1 Canada.
Genome. 2008 Dec;51(12):988-1000. doi: 10.1139/G08-092.
The goal of this study was to determine the impact of breeding system and the presence of the transposon Pokey on intraindividual variation in 28S rRNA genes. We PCR-amplified, cloned, and sequenced 1000 nucleotides downstream of the Pokey insertion site in genes with and without insertions from 10 obligately and 10 cyclically parthenogenetic isolates of Daphnia pulex. Variation among genes with Pokey insertions was higher than variation among genes without insertions in both cyclic and obligate isolates. Although the differences were not quite significant (p = 0.06 in both cases), the results suggest that Pokey insertions are likely to inhibit the homogenization of their host genes to some extent. We also observed that the complement of 28S rRNA alleles differed between genes with and without inserts in some isolates, suggesting that a particular inserted gene can persist for substantial periods of time and even spread within the rDNA array, despite the fact that insertions are deleterious. This apparently contradictory pattern can be explained if homogenization of rRNA genes occurs primarily by gene conversion, but copies with Pokey inserts can occasionally increase in frequency within arrays owing to unequal crossing over events that do not originate in the inserted genes themselves.
本研究的目的是确定繁殖系统以及转座子Pokey的存在对28S rRNA基因个体内变异的影响。我们对来自10个孤雌生殖型和10个周期性孤雌生殖型的蚤状溞分离株中带有和不带有插入片段的基因，在Pokey插入位点下游1000个核苷酸处进行了PCR扩增、克隆和测序。在周期性和孤雌生殖型分离株中，带有Pokey插入的基因之间的变异高于不带有插入的基因之间的变异。尽管差异不太显著（两种情况下p = 0.06），但结果表明Pokey插入可能在一定程度上抑制其宿主基因的同质化。我们还观察到，在一些分离株中，带有和不带有插入片段的基因之间28S rRNA等位基因的互补情况不同，这表明尽管插入是有害的，但特定的插入基因可以持续存在相当长的时间，甚至在rDNA阵列中传播。如果rRNA基因的同质化主要通过基因转换发生，但带有Pokey插入的拷贝偶尔会由于并非起源于插入基因本身的不等交换事件而在阵列中频率增加，那么这种明显矛盾的模式就可以得到解释。
