Kelley Scott T, Cassirer E Frances, Weiser Glen C, Safaee Shirin
Department of Biology, 5500 Campanile Drive, San Diego State University, San Diego, CA 92182, United States.
Infect Genet Evol. 2007 Jan;7(1):13-23. doi: 10.1016/j.meegid.2006.03.005. Epub 2006 Apr 25.
Wild and domestic animal populations are known to be sources and reservoirs of emerging diseases. There is also a growing recognition that horizontal genetic transfer (HGT) plays an important role in bacterial pathogenesis. We used molecular phylogenetic methods to assess diversity and cross-transmission rates of Pasteurellaceae bacteria in populations of bighorn sheep, Dall's sheep, domestic sheep and domestic goats. Members of the Pasteurellaceae cause an array of deadly illnesses including bacterial pneumonia known as "pasteurellosis", a particularly devastating disease for bighorn sheep. A phylogenetic analysis of a combined dataset of two RNA genes (16S ribosomal RNA and RNAse P RNA) revealed remarkable evolutionary diversity among Pasteurella trehalosi and Mannheimia (Pasteurella) haemolytica bacteria isolated from sheep and goats. Several phylotypes appeared to associate with particular host species, though we found numerous instances of apparent cross-transmission among species and populations. Statistical analyses revealed that host species, geographic locale and biovariant classification, but not virulence, correlated strongly with Pasteurellaceae phylogeny. Sheep host species correlated with P. trehalosi isolates phylogeny (PTP test; P=0.002), but not with the phylogeny of M. haemolytica isolates, suggesting that P. trehalosi bacteria may be more host specific. With regards to populations within species, we also discovered a strong correlation between geographic locale and isolate phylogeny in the Rocky Mountain bighorn sheep (PTP test; P=0.001). We also investigated the potential for HGT of the leukotoxin A (lktA) gene, which produces a toxin that plays an integral role in causing disease. Comparative analysis of the combined RNA gene phylogeny and the lktA phylogenies revealed considerable incongruence between the phylogenies, suggestive of HGT. Furthermore, we found identical lktA alleles in unrelated bacterial species, some of which had been isolated from sheep in distantly removed populations. For example, lktA sequences from P. trehalosi isolated from remote Alaskan Dall's sheep were 100% identical over a 900-nucleotide stretch to sequences determined from M. haemolytica isolated from domestic sheep in the UK. This extremely high degree of sequence similarity of lktA sequences among distinct bacterial species suggests that HGT has played a role in the evolution of lktA in wild hosts.
已知野生动物和家畜种群是新出现疾病的来源和宿主。人们也越来越认识到水平基因转移（HGT）在细菌致病过程中起着重要作用。我们使用分子系统发育方法来评估大角羊、多尔氏羊、家羊和家山羊种群中巴斯德氏菌科细菌的多样性和交叉传播率。巴斯德氏菌科的成员会引发一系列致命疾病，包括被称为“巴氏杆菌病”的细菌性肺炎，这对大角羊来说是一种特别具有毁灭性的疾病。对两个RNA基因（16S核糖体RNA和RNA酶P RNA）的组合数据集进行的系统发育分析显示，从绵羊和山羊分离出的海藻糖巴斯德氏菌和溶血曼氏杆菌（巴斯德氏菌属）细菌之间存在显著的进化多样性。尽管我们发现了物种和种群之间明显的交叉传播实例，但有几种系统发育型似乎与特定宿主物种相关。统计分析表明，宿主物种、地理位置和生物变种分类与巴斯德氏菌科系统发育密切相关，但毒力与之无关。绵羊宿主物种与海藻糖巴斯德氏菌分离株的系统发育相关（PTP检验；P = 0.002），但与溶血曼氏杆菌分离株的系统发育无关，这表明海藻糖巴斯德氏菌可能更具宿主特异性。关于物种内的种群，我们还发现落基山大角羊的地理位置与分离株系统发育之间存在很强的相关性（PTP检验；P = 0.001）。我们还研究了白细胞毒素A（lktA）基因水平基因转移的可能性，该基因产生的毒素在致病过程中起着不可或缺的作用。对组合RNA基因系统发育和lktA系统发育的比较分析显示，系统发育之间存在相当大的不一致，这表明存在水平基因转移。此外，我们在不相关的细菌物种中发现了相同的lktA等位基因，其中一些是从相距遥远的种群中的绵羊分离出来的。例如，从偏远的阿拉斯加多尔氏羊分离出的海藻糖巴斯德氏菌的lktA序列在900个核苷酸片段上与从英国家羊分离出的溶血曼氏杆菌确定的序列100%相同。不同细菌物种之间lktA序列的这种极高程度的序列相似性表明，水平基因转移在野生宿主中lktA的进化中发挥了作用。
