Van Doninck Karine, Mandigo Morgan L, Hur Jae H, Wang Peter, Guglielmini Julien, Milinkovitch Michel C, Lane William S, Meselson Matthew
Department of Biology, University of Namur, Namur, Belgium.
PLoS Genet. 2009 Mar;5(3):e1000401. doi: 10.1371/journal.pgen.1000401. Epub 2009 Mar 6.
Rotifers of Class Bdelloidea are remarkable in having evolved for millions of years, apparently without males and meiosis. In addition, they are unusually resistant to desiccation and ionizing radiation and are able to repair hundreds of radiation-induced DNA double-strand breaks per genome with little effect on viability or reproduction. Because specific histone H2A variants are involved in DSB repair and certain meiotic processes in other eukaryotes, we investigated the histone H2A genes and proteins of two bdelloid species. Genomic libraries were built and probed to identify histone H2A genes in Adineta vaga and Philodina roseola, species representing two different bdelloid families. The expressed H2A proteins were visualized on SDS-PAGE gels and identified by tandem mass spectrometry. We find that neither the core histone H2A, present in nearly all other eukaryotes, nor the H2AX variant, a ubiquitous component of the eukaryotic DSB repair machinery, are present in bdelloid rotifers. Instead, they are replaced by unusual histone H2A variants of higher mass. In contrast, a species of rotifer belonging to the facultatively sexual, desiccation- and radiation-intolerant sister class of bdelloid rotifers, the monogononts, contains a canonical core histone H2A and appears to lack the bdelloid H2A variant genes. Applying phylogenetic tools, we demonstrate that the bdelloid-specific H2A variants arose as distinct lineages from canonical H2A separate from those leading to the H2AX and H2AZ variants. The replacement of core H2A and H2AX in bdelloid rotifers by previously uncharacterized H2A variants with extended carboxy-terminal tails is further evidence for evolutionary diversity within this class of histone H2A genes and may represent adaptation to unusual features specific to bdelloid rotifers.
蛭形轮虫纲的轮虫非常引人注目，它们已经进化了数百万年，显然没有雄性个体，也不进行减数分裂。此外，它们对干燥和电离辐射具有异常的抵抗力，并且能够修复每个基因组中数百个辐射诱导的DNA双链断裂，而对生存能力或繁殖几乎没有影响。由于特定的组蛋白H2A变体参与其他真核生物的双链断裂修复和某些减数分裂过程，我们研究了两种蛭形轮虫的组蛋白H2A基因和蛋白质。构建了基因组文库并进行探测，以鉴定vagina艾氏轮虫和玫瑰旋轮虫中的组蛋白H2A基因，这两个物种代表了两个不同的蛭形轮虫科。在SDS-PAGE凝胶上可视化表达的H2A蛋白，并通过串联质谱法进行鉴定。我们发现，几乎所有其他真核生物中都存在的核心组蛋白H2A，以及真核生物双链断裂修复机制中普遍存在的H2AX变体，在蛭形轮虫中都不存在。相反，它们被质量更高的异常组蛋白H2A变体所取代。相比之下，属于蛭形轮虫兼性有性、不耐干燥和辐射的姐妹纲——单巢纲的一种轮虫，含有典型的核心组蛋白H2A，并且似乎缺乏蛭形轮虫的H2A变体基因。应用系统发育工具，我们证明蛭形轮虫特有的H2A变体作为与典型H2A不同的谱系出现，与导致H2AX和H2AZ变体的谱系分开。蛭形轮虫中核心H2A和H2AX被具有延长羧基末端尾巴的先前未表征的H2A变体所取代，这进一步证明了这类组蛋白H2A基因的进化多样性，并且可能代表了对蛭形轮虫特有的异常特征的适应。
