Genetic Analysis of Host Specificity in Schistosoma mansoni

描述（由申请人提供）：寄生虫对特定宿主表现出很强的特异性，但这种宿主特异性的遗传基础尚不清楚。在人类感染的三种重要医学血吸虫之一的曼氏血吸虫中，优雅的实验工作表明，对中间蜗牛宿主的化学识别和蜗牛体内寄生虫的生存都遵循简单的孟德尔遗传模式。虽然埃及寄生虫对同域蜗牛生物phalaria alexandrina表现出强烈的化学识别，但巴西寄生虫没有特异性，甚至被非媒介蜗牛宿主所吸引。同样，虽然埃及和巴西的寄生虫都在它们的同域蜗牛宿主中感染和繁殖，但F1杂交种只在南美光螺中发育。新的分子工具使我们能够确定在宿主-寄生虫系统中决定宿主特异性的寄生虫基因，为了解寄生虫与蜗牛载体之间的关键分子相互作用提供了一种手段。利用R21基金，我们(a)开发了mansoni的5 cM连锁图谱，(b)通过鉴定一个强QTL （LOD = 21）证明了连锁图谱的效用，该QTL对chr 6的短区域具有奥氨喹抗性。我们现在建议利用遗传图谱，连同最近发表的mansoni s .基因组序列，确定mansoni s . - Biomphalaria系统中宿主特异性的基因组区域。寄主特异性包括两个组成部分(a) miracidia对蜗牛的化学定位和(b)血吸虫幼虫在蜗牛体内的渗透和克隆增殖。对于这两个性状，我们将在巴西和埃及曼索尼血吸虫之间进行遗传杂交，利用单一miracidia的蜗牛感染在蜗牛中产生单一基因型感染。我们将量化单个F2 miracidia对光光白僵菌和alexandrina白僵菌的化学识别行为，然后利用mansoni基因组中间隔约4 cM （2Mb）的snp对单个miracidia进行基因分型，以确定该性状的qtl。由于不能在蜗牛体内生长的寄生虫无法进行基因分型，因此使用经典的连锁作图方法无法对蜗牛宿主内的存活和克隆增殖进行遗传作图。因此，我们将使用由疟疾和酵母遗传学家开发的极端QTL （X-QTL）方法来检测从B. glabrata或B. alexandrina蜗牛中出现的F2尾蚴的等位基因频率。在致病位点上，我们预计来自埃及曼氏丝虫病亲本的等位基因在亚历山大白蝇出现的F2尾蚴中相对于光斑白蝇出现的等位基因过多。因此，与正常孟德尔分离的位点特异性偏差允许QTL定位。由于这两个性状的QTL区域都有很好的定位，我们将使用RNAi破坏基因功能或基于逆转录病毒的转染来帮助鉴定致病位点。了解mansoni - Biomphalaria系统中宿主特异性的遗传和分子基础对于旨在破坏寄生虫生命周期中这一步骤的控制工作至关重要，并且为理解最重要的人类蠕虫寄生虫的宿主特异性进化提供了关键。

DESCRIPTION (provided by applicant): Parasites characteristically show strong specificity to particular hosts, but the genetic basis for this host specificity is poorly understood. In Schistosoma mansoni, one of three medically important schistosome species infecting humans, elegant experimental work demonstrates that both chemical recognition of the intermediate snail host and survival of parasites within snails follow a simple Mendelian pattern of inheritance. While Egyptian parasites show strong chemical recognition of the sympatric snail Biomphalaria alexandrina, Brazilian parasites show no specificity and are attracted to even non-vector snail hosts. Similarly, while both Egyptian and Brazilian parasites infect and proliferate in their sympatric snail hosts, F1 hybrids develop only in S. American B. glabrata snails. New molecular tools allow us to determine the parasite genes that determine host specificity in this host-parasite system, providing a means to understand key molecular interactions between parasites and snail vector. Using R21 funding we have (a) developed a 5 cM linkage map for S. mansoni, and (b) demonstrated the utility of linkage mapping by identifying a strong QTL (LOD = 21) for oxamniquine resistance to a short region of chr 6. We now propose to exploit the genetic map, together with the recently published genome sequence of S. mansoni identify the genome region(s) that underlie host specificity in the S. mansoni - Biomphalaria system. Host specificity involves two components (a) chemical location of snails by miracidia and (b) penetration and clonal proliferation of schistosome larvae within snails. For both traits, we will conduct genetic crosses between Brazilian and Egyptian S. mansoni, using snail infections with single miracidia to generate single genotype infections in snails. We will quantify chemical recognition behavior of single F2 miracidia to both B. glabrata and B. alexandrina, and then genotype individual miracidia using SNPs spaced at ~4 cM (2Mb) intervals across the S. mansoni genome to identify QTLs for this trait. Genetic mapping of survival and clonal proliferation within the snail host is not possible using classical linkage mapping methods, because parasites that do not grow within snails cannot be genotyped. We will therefore use extreme QTL (X-QTL) methods, developed by malaria and yeast geneticists, to examine allele frequencies of F2 cercariae emerging from either B. glabrata or B. alexandrina snails. At the causative loci, we expect alleles from the Egyptian S. mansoni parent to be overrepresented in F2 cercariae emerging from B. alexandrina relative to those emerging from B. glabrata. Hence locus specific deviation from normal Mendelian segregation allows QTL location. Having fine mapped QTL regions for both traits, we will use RNAi disruption of gene function or retroviral based transfection to aid identification of causative loci. Understanding the genetic and molecular basis of host specificity in the S. mansoni - Biomphalaria system is critical for control efforts that aim to disrupt this step in the parasite lifecycle and provides a key to understanding evolution of host specificity in the most important of the human helminth parasites.