新型入侵紅火蟻病毒之特徵鑑定及親緣關係

Abstract

入侵紅火蟻 (Solenopsi.invicta)，於近十年入侵台灣、香港以及中國大陸後，也發現入侵紅火蟻病毒 (Solenopsis invicta virus-1, SINV-1) 伴隨其寄主一起入侵。由於病毒複製時間短，繁殖速度快，因此病原體的快速演化相比於寄主的基因體具有更高的遺傳多樣性，對寄主的族群動態變化，提供了更加詳細而靈敏的證據。以往的研究顯示，該病毒藉由其極高的突變及演化速率，可成為另一種遺傳標誌，用來彌補寄主在入侵時，因族群量少而造成的基因體變異較少的缺陷。本研究採用四種在 3’ ORF （open reading frame）之病毒蛋白基因序列 (VP1、VP2、VP3 以及 VP4)，比對其序列變異，闡明在近十年來入侵亞洲之大部分紅火蟻病毒，皆和現今所發表的病毒基因型皆不同，此可能是因為火蟻入侵新地區後，發生適應選汰的現象。這些病毒蛋白基因序列皆顯示，在近期入侵的區域所出現的紅火蟻病毒基因型，也可在美國當地的蟻巢中發現，顯示入侵紅火蟻最有可能從美國入侵到台灣。為了更加了解這種新發現的基因型火蟻病毒，本研究進行亞洲型入侵紅火蟻病毒之全基因體解序，證實其為紅火蟻病毒的另一基因型，且將之命名為 SINV-1-Asia。原先存在於美國的 SINV-1-Asia 基因型既然可於新入侵地區發現，其他 SINV-1 基因型也應有機會發現。藉由此想法，於台灣樣本 (編號 138) 之 VP4 片段上，偵測到類似 SINV-1 (TX5) 序列，這意味著 SINV-1 以及 SINV-1A 也有機會在新入侵地出現。因此未來若該病毒任何一基因型真能於防治上有所幫助，新入侵地將可直接實行。

Rapidly evolving pathogens provide a powerful approach to address population dynamic of the host, under several circumstances, in a more detailed fashion than the genetic marker from host’s genome. Previous studies have shown that Solenopsis invicta virus-1 (SINV-1) often accompanies with the recent invasions of its host, the red imported fire ant (S. invicta), into Taiwan, Hong Kong and China in last decades. This virus, with its extremely high mutation and fast evolution, may therefore serve as an additional genetic marker that potentially compensates reduced host’s genetic variation associated with small propagule size during invasion. The present study therefore employs sequences of four viral protein genes (VP1, VP2, VP3 and VP4) in ORF2 to reveal that most of recently introduced SINV-1 genotypes in Asia are different from those previously published, suggesting the possible involvement of adaptive selection. Sequence analyses from those viral genes indicate that this SINV-1 genotype identified in the recently introduced areas can also be found in the USA, suggesting that SINV-1 is introduced from the US to Taiwan. For better understanding of this newly-discovered genotype, the whole genome sequence of the genotype of this virus was obtained, and viral phylogenomic analysis confirms it a derived variant of SINV-1, and called it SINV-1-Asia. Other SINV-1 genotypes might present in those recently introduced areas because SINV-1-Asia originated from the US can also be found in Asia. Based on this idea, I did find SINV-1 (TX5)-like sequences at VP4 in sample (number 138) collected in Taiwan. It therefore seems possible to find SINV-1 and SINV-1A in newly introduced areas.