Department of Life Sciences, Imperial College London, Silwood Park Campus, Ascot, Berkshire, SL5 7PY, UK.
BMC Evol Biol. 2012 Aug 17;12:148. doi: 10.1186/1471-2148-12-148.
Bdelloid rotifers are microscopic animals that have apparently survived without sex for millions of years and are able to survive desiccation at all life stages through a process called anhydrobiosis. Both of these characteristics are believed to have played a role in shaping several unusual features of bdelloid genomes discovered in recent years. Studies into the impact of asexuality and anhydrobiosis on bdelloid genomes have focused on understanding gene copy number. Here we investigate copy number and sequence divergence in alpha tubulin. Alpha tubulin is conserved and normally present in low copy numbers in animals, but multiplication of alpha tubulin copies has occurred in animals adapted to extreme environments, such as cold-adapted Antarctic fish. Using cloning and sequencing we compared alpha tubulin copy variation in four species of bdelloid rotifers and four species of monogonont rotifers, which are facultatively sexual and cannot survive desiccation as adults. Results were verified using transcriptome data from one bdelloid species, Adineta ricciae.
In common with the typical pattern for animals, monogonont rotifers contain either one or two copies of alpha tubulin, but bdelloid species contain between 11 and 13 different copies, distributed across five classes. Approximately half of the copies form a highly conserved group that vary by only 1.1% amino acid pairwise divergence with each other and with the monogonont copies. The other copies have divergent amino acid sequences that evolved significantly faster between classes than within them, relative to synonymous changes, and vary in predicted biochemical properties. Copies of each class were expressed under the laboratory conditions used to construct the transcriptome.
Our findings are consistent with recent evidence that bdelloids are degenerate tetraploids and that functional divergence of ancestral copies of genes has occurred, but show how further duplication events in the ancestor of bdelloids led to proliferation in both conserved and functionally divergent copies of this gene.
蛭形轮虫是一种微小的动物，显然已经有数百万年没有进行有性繁殖了，并且能够在所有生命阶段通过一种叫做脱水休眠的过程来耐受干燥。这两个特性被认为在塑造近年来发现的蛭形轮虫基因组的几个不寻常特征方面发挥了作用。对无性繁殖和脱水休眠对蛭形轮虫基因组影响的研究集中在理解基因拷贝数上。在这里，我们研究了α微管蛋白的拷贝数和序列差异。α微管蛋白在动物中是保守的，通常以低拷贝数存在，但在适应极端环境的动物中，如适应寒冷的南极鱼类，α微管蛋白的拷贝数会增加。我们使用克隆和测序技术比较了四种蛭形轮虫和四种单轮轮虫的α微管蛋白拷贝数变异，单轮轮虫是兼性有性繁殖的，不能在成体时耐受干燥。结果使用一种蛭形轮虫，Adineta ricciae 的转录组数据进行了验证。
与动物的典型模式一致，单轮轮虫只含有一个或两个α微管蛋白拷贝，但蛭形轮虫物种含有 11 到 13 个不同的拷贝，分布在五个类别中。大约一半的拷贝形成一个高度保守的群体，彼此之间以及与单轮轮虫的拷贝之间的氨基酸差异仅为 1.1%。其他拷贝具有不同的氨基酸序列，与类内相比，它们在类间的进化速度显著更快，相对于同义突变，并且在预测的生化特性上也有所不同。每个类别的拷贝都在构建转录组时使用的实验室条件下表达。
我们的发现与最近的证据一致，即蛭形轮虫是退化的四倍体，并且基因的祖先拷贝发生了功能分化，但也表明蛭形轮虫的祖先经历了进一步的复制事件，导致了该基因的保守和功能分化拷贝的增殖。
