蜱是一類在世界范圍內(nèi)廣泛分布,，專性吸食脊椎動(dòng)物血液為生的體外寄生蟲(chóng),。它們是細(xì)菌、病毒,、蠕蟲(chóng)等病原體的傳播者,，能夠?qū)е露喾N人類和動(dòng)物疾病，因此在動(dòng)物流行病學(xué)上具有重要意義,。蜱主要分為軟蜱和硬蜱,，它們具有不同的吸血策略，軟蜱是短時(shí)程吸血（幾分鐘到幾小時(shí)）,，而硬蜱是長(zhǎng)時(shí)程吸血（幾天甚至上周）,。
Kunitz/BPTI蛋白大量存在于蜱類唾液腺中并在吸血過(guò)程中發(fā)揮著重要作用,。在軟蜱和硬蜱中,，Kunitz/BPTI蛋白具有不同的功能和結(jié)構(gòu)域組成。Kunitz/BPTI蛋白的這種功能分化和結(jié)構(gòu)域差異是怎樣形成的,，該家族的進(jìn)化是否與軟蜱和硬蜱中不同的吸血策略相關(guān),，至今還不清楚。
昆明動(dòng)物研究所遺傳資源與進(jìn)化國(guó)家重點(diǎn)實(shí)驗(yàn)室生物信息學(xué)和系統(tǒng)生物學(xué)研究組的博士生代紹興和張愛(ài)娣,，在黃京飛研究員的指導(dǎo)下,，系統(tǒng)地研究了在肩突硬蜱（Ixodes scapularis）中Kunitz/BPTI基因家族的進(jìn)化，擴(kuò)張和表達(dá),。研究揭示了Kunitz/BPTI基因家族在軟蜱和硬蜱中存在的巨大差異,。多結(jié)構(gòu)域蛋白是在軟蜱和硬蜱分化后才在硬蜱中進(jìn)化而來(lái),。Group II and III 基因只存在于硬蜱屬（Ixodes)中，并且在硬蜱的長(zhǎng)時(shí)程吸血過(guò)程中表達(dá)上調(diào),。正選擇驅(qū)動(dòng)了Kunitz/BPTI基因家族的進(jìn)化,，在這過(guò)程中g(shù)roup II基因進(jìn)化出了調(diào)控離子通道的新功能，通過(guò)增強(qiáng)硬蜱的吸血效率而在蜱類吸血過(guò)程中發(fā)揮作用,。因此這項(xiàng)研究結(jié)果提示Kunitz/BPTI基因家族在軟蜱和硬蜱中存在的巨大差異與硬蜱進(jìn)化出的長(zhǎng)時(shí)程吸血行為相關(guān),。最后，我們鑒定出來(lái)的六個(gè)差異表達(dá)基因可以作為控制蜱類的候選靶基因,。
該研究結(jié)果目前發(fā)表于BMC Evolutionary Biology (該研究得到了科技部973項(xiàng)目,，國(guó)家自然科學(xué)基金等的資助。（生物谷 Bioon）
doi:10.1186/1471-2148-12-4
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Evolution, expansion and expression of the Kunitz/BPTI gene family associated with long-term blood feeding in Ixodes Scapularis
Shao-Xing Dai, Ai-Di Zhang, and Jing-Fei Huang
Background
Recent studies of the tick saliva transcriptome have revealed the profound role of salivary proteins in blood feeding. Kunitz/BPTI proteins are abundant in the salivary glands of ticks and perform multiple functions in blood feeding, such as inhibiting blood coagulation, regulating host blood supply and disrupting host angiogenesis. However, Kunitz/BPTI proteins in soft and hard ticks have different functions and molecular mechanisms. How these differences emerged and whether they are associated with the evolution of long-term blood feeding in hard ticks remain unknown.
Results
In this study, the evolution, expansion and expression of Kunitz/BPTI family in Ixodes scapularis were investigated. Single- and multi-domain Kunitz/BPTI proteins have similar gene structures. Single-domain proteins were classified into three groups (groups I, II and III) based on their cysteine patterns. Group I represents the ancestral branch of the Kunitz/BPTI family, and members of this group function as serine protease inhibitors. The group I domain was used as a module to create multi-domain proteins in hard ticks after the split between hard and soft ticks. However, groups II and III, which evolved from group I, are only present and expanded in the genus Ixodes. These lineage-specific expanded genes exhibit significantly higher expression during long-term blood feeding in Ixodes scapularis. Interestingly, functional site analysis suggested that group II proteins lost the ability to inhibit serine proteases and evolved a new function of modulating ion channels. Finally, evolutionary analyses revealed that the expansion and diversification of the Kunitz/BPTI family in the genus Ixodes were driven by positive selection.
Conclusions
These results suggest that the differences in the Kunitz/BPTI family between soft and hard ticks may be linked to the evolution of long-term blood feeding in hard ticks. In Ixodes, the lineage-specific expanded genes (Group II and III) lost the ancient function of inhibiting serine proteases and evolved new functions to adapt to long-term blood feeding. Therefore, these genes may play a profound role in the long-term blood feeding of hard ticks. Based our analysis, we propose that the six genes identified in our study may be candidate target genes for tick control.
