Chen Xuejie, Xu Shiqi, Yu Zhijun, Guo Lida, Yang Shujie, Liu Limeng, Yang Xiaolong, Liu Jingze
Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology of Hebei Province, College of Life Sciences, Hebei Normal University, Shijiazhuang 050024, China.
Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology of Hebei Province, College of Life Sciences, Hebei Normal University, Shijiazhuang 050024, China; College of Basic Medicine, Chengde Medical University, Chengde 067000, China.
Infect Genet Evol. 2014 Jan;21:308-14. doi: 10.1016/j.meegid.2013.12.002. Epub 2013 Dec 6.
As an obligate hematophagous ectoparasite, the hard tick Haemaphysalis longicornis exhibits two reproductive strategies, bisexual reproduction, and obligate parthenogenesis, which have attracted a widespread attention. However, the speciation of parthenogenetic population remained ambiguous due to its similarity in morphology but the remarkable differences in cytogenetics as compared with those of the bisexual ones. In the present study, we explored several new lines of genetic evidence to resolve this controversial issue. The number of the chromosomes in two lineages was checked by classical methods and their total DNA levels were determined utilizing flowcytometry. In addition, the sequences of 12S rDNA, 16S rDNA, cytochrome c oxidase I and II (COI, COII) and internal transcribed spacer-2 (ITS-2) genes were used to assess their phylogenetic relationship. We observed that the chromosome ploidy of bisexual and parthenogenetic H. longicornis collected by our laboratory was diploid and triploid, respectively. Flowcytometry analysis indicated a ratio close to 2:3 in the DNA contents of bisexual to parthenogenetic H. longicornis. Although the chromosome ploidy is different, their gene sequences are extremely similar. Analogous to the intra-species genetic difference of other invertebrates, sequence differences of all loci examined are below 2%. Phylogenetic trees constructed from 12S rDNA, 16S rDNA, COI, and ITS-2 genes revealed that they were all in the same monophyletic clade instead of splitting independently into evolutional branches. Moreover, according to 4× Rule, the K/θ ratio of two reproductive populations calculated based on COI was much smaller than four, strongly supporting that they belong to the same species. Therefore, we conclude that the evolutionary process just disturbs the chromosome ploidy and the sexual determination of parthenogenetic population and that it would be better to consider parthenogenetic H. longicornis as a metapopulation rather than a cryptic species.
长角血蜱作为一种专性吸血的体外寄生虫，具有两性生殖和专性孤雌生殖两种生殖策略，这引起了广泛关注。然而，孤雌生殖种群的物种形成仍不明确，因为其形态与两性生殖种群相似，但细胞遗传学存在显著差异。在本研究中，我们探索了几条新的遗传证据线索来解决这一有争议的问题。通过经典方法检查了两个谱系的染色体数量，并利用流式细胞术测定了它们的总DNA水平。此外，使用12S rDNA、16S rDNA、细胞色素c氧化酶I和II（COI、COII）以及内部转录间隔区2（ITS-2）基因的序列来评估它们的系统发育关系。我们观察到，本实验室采集的两性生殖和孤雌生殖长角血蜱的染色体倍性分别为二倍体和三倍体。流式细胞术分析表明，两性生殖与孤雌生殖长角血蜱的DNA含量之比接近2:3。尽管染色体倍性不同，但它们的基因序列极为相似。与其他无脊椎动物的种内遗传差异类似，所有检测位点的序列差异均低于2%。由12S rDNA、16S rDNA、COI和ITS-2基因构建的系统发育树表明，它们都处于同一个单系类群中，而不是独立分裂成进化分支。此外，根据4×规则，基于COI计算的两个生殖种群的K/θ比值远小于4，有力地支持它们属于同一物种。因此，我们得出结论，进化过程只是扰乱了孤雌生殖种群的染色体倍性和性别决定，将孤雌生殖的长角血蜱视为一个复合种群而非一个隐存种更为合适。
