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K. Parasitism 寄生

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寄生即两种生物在一起生活,一方受益, 另一方受害,后者给前者提供营养物质和居 住场所,这种生物的关系称为寄生

K1 THE
Key Notes 要点
The diversity of parasites
寄生物的多样性

NATURE OF PARASITISM 寄生的性质

Parasites are a subgroup of predators (which consume the tissue of another living organism) that live in close association with their host. Parasites can be classified into two broad groups: (i) microparasites, which multiply within, or on the surface of ,the host, and (ii) macroparasites which grow in or on the host, but do not multiply. The main microparasites are viruses, bacteria, fungi and protozoans. Helminth worms and insects are important macroparasites. A large group of insect macroparasites are parasitoids, which lay eggs in or on the body of their insect host, and usually cause the death of the host. 寄生物是摄取其他生物的组织、与其寄主紧密相联而生的捕食 者的子群。寄生物可以分为两大类(i)微寄生物,在寄主体 内或表面繁殖,(ii)大寄生物,在寄主体内或表面生长,但 不繁殖。主要的微寄生物有病毒、细菌、真菌和原生动物。寄 生蠕虫和昆虫是重要的大寄生物。一大类昆虫大寄生物是拟寄 生生物,它们在昆虫寄主身上或体内产卵,通常导致寄主死亡。

K1. The nature of parasitism
The diversity of parasites Microparasites and Macroparasites Microparasites微寄生 直接在宿主体内或表面繁殖 Macroparasites大型寄生 在寄主体内体表生长,但不繁殖, Parasitoids 拟寄生 主要是指昆虫产卵于宿主体内
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The main microparasites are viruses,bacteria,fungi and protozoans. ? 主要的微寄生物有病毒、细菌、真菌和原生 动物。
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埃博拉病毒

细菌

真菌

原生动物

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Macroparasites of plants and animals are dominated by invertebrates.In animals,helminth worms are particularly important,whilst insects are the main macroparasites of plants(particularly butterfly and moth larvae and beetles),though other plants(e.g. mistletoes) maybe important. 动植物的大寄生物主要是无脊椎动物。在动物中,寄生蠕虫特别重要,而 昆虫是植物的主要的大寄生物(特别是蝴蝶和蛾的幼虫以及甲虫),尽管 其他植物(如榭寄生)也可能重要。

蝴蝶幼虫

蛾幼虫
榭寄生是一种寄生植物,可以从寄主植物上吸取 水分和无机物,进行光合作用制造养分,但养分 还是不够的。所以当寄主植物枯萎的时候,榭寄 生也会跟着枯萎掉。当然,离开寄主植物的榭寄 生没过多久也会枯萎,不过据说这下来的榭寄生 存放几个月后,树枝会逐渐变成金黄色。 一般,榭寄生会被当作圣诞树上的装饰,或者做 成圈圈挂在门口。西方视它为一种神圣的植物, 常用来装饰圣诞树。据说在榭寄生下亲吻是很吉 祥的,传说在榭寄生下亲吻的情侣,会厮守到永
远的。榭寄生象征的是爱,和平和宽恕。

甲虫

Wasps寄生蜂

flies寄生蝇

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Most parasites are biotrophs,only surviving on living tissue,but some(such as the sheep blowfly,Lucilia cuprina,and the plant fungal disease Pythium,which causes’dampingoff’)continue to live on the host after causing its death.These are necrotrophs. 大多数寄生物是食生物者,仅在活组织上生活,但 一些寄生物在其寄主死后仍能继续存活在寄主上, 如丝光绿蝇( Lucilia cuprina)和引起植物幼苗腐 烂的植物真菌病(Pythium),这些称作食尸动物。

Biotrophs and necrotrophs Biotrophs 活养寄生 寄生在活组织上 necrotrophs 尸养寄生 宿主死后,仍可以在宿主体内体表存活,如 大苍蝇 ? Modes of transmission vertical horizontal
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Modes of transmission 传播方式

Parasite transmission can be either horizontal (among members of a population) or, less commonly, vertical (passed from mother to offspring). Horizontal transmission may either be direct or indirect, mediated by a vector (e.g. a mosquito) or an alternate host. 寄生物可以横向传播(在种群个体之间),或在少数 情况下,纵向传播(从母体到后代)。横向传播或直 接或间接,由传病媒介(如蚊子)或中间寄主做中介。

Host response to disease
寄主对疾病的 反应

In vertebrates, infection by microparasites results in a strong immunological response. There are two components to this response: (i) the cellular immune response, where specialized cells directly attack pathogen cells, and (ii) the B-cell immune response, which gives rise to antibodies. After the first infection by a pathogen, immunological memory creates a raid response to future attacks by the same organism, resulting in immunity. Invertebrates and plants also may suppress infections, but by less sophisticated, and usually less specific mechanisms. The loss of potential hosts which immunity engenders accentuates the boom-and-bust strategy of microparasites (see Topic K2).

脊椎动物被微寄生物感染后产生强烈的免疫反应。这种反应主要有两种成

分:(i)细胞免疫反应,特定细胞直接攻击病原体细胞,和(ii)B-细胞
免疫反应,使抗体量上升。初次被病原体感染后,免疫记忆对未来同样的 侵袭产生快速反应,导致免疫。无脊椎动物和植物也可以抑制感染,但方

式较简单,通常较少特化机制。免疫造成的潜在寄主减少,使微寄生物的
繁荣-和-破产对策加强(见K2)

Host response immunological response Eg:the cellular immune response 如吞噬细胞, 脊椎动物被微寄生物感染后会产生强烈的免 疫反应,这种反应有两种明显成份: ①细胞免疫反应,吞噬细胞(如白血细胞-T淋巴 细胞)攻击并吞没病原体细胞。 ②B-细胞免疫反应,以特定蛋白(或抗体)的生 产为基础,由B淋巴细胞结合到病原体表面。 如果再次遭遇同样病原体(或抗原),免疫记 忆会快速生产特异抗体,提高免疫力。
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Behavioral strategies Eg: preening 鸟用嘴除去体表寄生虫 migration localized cell death 植物局部细胞死 亡

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Denying chickens the opportunity to preen by debeaking changed the louse load from less than 50 per individual to 1600 per individual. Caribou avoid intense mosquito attacks by migration to higher altitudes in the summer months. 将鸡喙剥下,使其不能整理,虱子的感染率从少于 每只鸡50个变为每只鸡1600个。北美产驯鹿通过在 夏季迁移到更高处来躲避大量蚊子的进攻。

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When tobacco leaves are infected with tobacco mosaic virus,the plant responds by killing the cells in the locality ,thus depriving the parasite of its food source. ? 烟草叶子被烟草花叶病毒感染后,植物会杀 死感染部细胞,这样夺走寄生物的食物资源。
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Many parasites obligately switch between two or three host Complex life cycles species in the course of their life cycle. Three different explanations have been advanced to explain such complex life 复杂的生活史 cycles: (i) alternate hosts are vectors which have been attacked by the parasite (this cannot apply to immobile hosts which cannot be vectors); (ii)optimal habitat use is occurring, where different species are the optimal resource in different seasons, or for different life cycle stage; (iii) the pattern is due to evolutionary constraint, as parasites become highly adapted to one host for part of their life cycle, they are unable to leave it even though the rewards are higher elsewhere. 许多寄生物在其生活史中不得不转换二或三种寄主。对这种 复杂的生活史有三种不同的解释:(i)交替寄生是被寄生 物感染的传病媒介(不适用于不可能成为传病媒介的不动寄 主);(ii)出现最适生境利用,在这种环境中,不同寄主 种是不同季节或不同生活史阶段的最佳资源;(iii)这种模 式起因于进化约束,因为寄生物在其生活史的某阶段中高度 适应一种寄主,所以它们不能离开该寄主,即使在其他地方 能获得更高的利益。

Complex life cycles 许多寄生生物在生命周期中会有两个甚至更多 寄主。原因: vectors带菌媒介,如蚊子 optimal habitat use Different morphs(变体)-different hosts(寄主)
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Thus,it has been suggested that host-alternating aphids use the hosts on which phloem sap quality (on which aphids feed) is highest when plants are growing most rapidly,which is in trees in the spring and herbaceous plants in the summer. 因而,有人认为寄主交替的蚜虫总是利用韧皮部汁 液(蚜虫以此为食)质量最高的寄主。而韧皮部汁 液质量最高与植物生长最快的时期相对应,树木在 春季生长最快,而草本植物则在夏季生长最快。

Social parasites 社会性寄生物

A completely different form of relationship is found between’ social parasites’ and their hosts. Social

parasites gain benefit from their animal hosts not by
feeding on their tissues but by coercing them to provide food or other benefits. Such relationships are found in cuckoos which lay their eggs in the nests of other bird species, which then undertake the rearing of the young,and in some ant species which coerce the workers of another species to provision their brood. “社会性寄生物”与其寄主之间有一种完全不同的关 系。社会性寄生物不通过摄食寄主组织获益,而是通 过强迫寄主提供食物或其他利益而获利。这种关系发 现在杜鹃,它们将蛋下在别种鸟的巢中,让这种鸟饲

育其幼鸟。这种关系还发现在一些蚂蚁种类,它们迫
使其他种工蚁给它们的幼体食品或其他利益。

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Social parasites Eg: brood parasitism 幼体寄生,窝寄生 intraspecific brood parasitism: 如鸭子常把蛋下在其他鸭子的窝里,这样宿主 鸭子就会少下蛋 interspecific brood parasitism: 如布谷鸟把蛋产在棕鸟巢里,并把棕鸟巢里的 蛋挪走。 ant and wasp-species

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Some species,such as the ant Lasius regina,have a worker caste and are able to rear their own broods but may coerce other species to undertake this task,whilst other ,obligate parasites,are workless and depend entirely on other species to rear their young.In either case ,colony takeover usually occurs by the parasite queen invading the nest and killing or dominating the resident queen.The resident workers continue to supply food and servies to the brood,to which the parasite queen adds her own eggs. 一些种类,如蚂蚁(Lasius regina),具有工蚁,可以饲育 其幼体,但它们可能强迫其他种来担当这一工作。另一些专 性寄生物,没有工蚁,完全依赖其他种来饲育其幼体。在这 两个实例中,通常都是通过寄生物蚁王侵袭其他蚁巢并杀死 或控制土著蚁王而发生殖民霸占。土著工蚁继续为其窝中幼 体提供食物和服务,而寄生物蚁王向窝中加入了其自己的卵。

K2 THE
dynamics 寄生物寄主动态

Key Notes 要 Vertebrate hosts commonly acquire immunity to parasites. These Parasite-host 点

DYNAMICS OF PARASITISM 寄生的动态

immune hosts are no longer available to the parasites, and the size of the susceptible population is reduced, This results in a fall in the incidence of disease. However, as new susceptible hosts enter the population (for example, by birth), the disease will increase in incidence again. Thus, there is a tendency of diseases to cycle, rising as the number of new susceptibles increases and falling as the level of immunity rises. As the influx of new susceptibles increases and falling as the level of immunity rises. As the reproductive rate of a disease falls ,the response of disease incidence to the influx of suscetibles is slower and the dynamics change from clear, synchronous cycles with a short period between peaks to reduced synchrony and longer periodicity until finally no cycles are observable. 脊椎动物寄主一般对寄生产生免疫。这些免疫寄主不能再被寄生物利 用,因而易感种群的大小减少。这使疾病的传染力降低。然而,随着 新的易感寄主加入种群(如新个体出生),疾病的传染力会再次增加。 因此,传染病有循环的趋势,在新的易感个体增加时上升,在免疫水 平上升时下降。当一种致病寄生物繁殖率下降时,传染力对易感个体 加入的反应较慢,动态从两峰之间短间隔的明显的同步循环变为同步 性减弱,间隔加长,直到最后观察不到循环。

Host- parasite evolution 寄主—— 寄生物进化

The close association between parasites and their hosts
often results in evolutionary interactions, or coevolution. Coevolution may give rise to defence mechanisms in the host and routes to overcome these defences in the parasite

– so called ‘arms races’. Not all coevolution between host
and parasite is escalatory, however. For example, coevolution may lead to the reduction in the virulence of a parasite. 寄生物与其寄主间紧密的关联经常导致二者在进化上

的相互作用,或协同进化。协同进化使寄主的防御机
制提高,而给寄生物克服这些防御的通路——这叫做 “军备竞赛”。但是,寄主与寄生物之间的协同进化 并不都是提高性的。例如,协同进化会降低寄生物的 毒性。

K2. The dynamics of parasitism
Host-parasites evolution 在某种程度上,与捕食者和猎物的相互作 用相似。
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A rather different example of coevolution comes from the social parasite(see Topic K1),the cuckoo,Cuculus canorus.The cuckoo has several common host species,in particular the reed warbler,meadow pipit, pied wagail and dunnock.Each of these hosts is parasitized by a different race of cuckoo,the femaleof which lays an egg which mimics that of its host in color and pattern(with the marked exception of the dunnock egg).(Fig.3).Hosts are generally likely to evict eggs which are unlike their own,which provides a clear benefit to the evolution of a mimic egg.Some apparently potential host species are particularly acute in their discriminatory powers and reject 100% of strange eggs.It has been suggested that these are species which have ‘won’the evolutionary arms race against the cuckoo.The dunnock,on the other hand,appears to have rather low powers of discrimination,possibly because the association with the cuckoo has only occurred relatively recently,and the proportion of the dunnock population affected by cuckoo parasitism is relatively small,reducing the evolutionary impact of parasitism. 一个非常不同的协同进化的例子来自社会寄生者(见K1),大杜鹃。大杜鹃有几个常见的寄 主种,特别是芦莺、草地鹨、白鹡鸰和林岩鹨。这些寄主每一种都被一种不同的杜鹃亚种 所寄生,其雌体下一枚颜色和模式都模仿寄主卵的蛋(林岩鹨的蛋是显著例外)。寄主一 般逐出不像其卵的蛋,这为模仿蛋的进化提供了明显利益。一些明显的潜在寄主种辨别能 力特别敏锐,排除100%的异样卵。有人认为这些是在进化竞赛中“赢了”杜鹃的种类。 另一方面,林岩鹨似乎辨别能力很低,可能因为其与杜鹃的关联发生在相对近代,被杜鹃 寄生影响的林岩鹨种群比例相对小小,降低了寄生的进化影响。 图3

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A model of microparasite disease 微寄生物传染病 模型

The reproductive rate of a microparasite (Rp ) can be expressed as the number of mew cases that will arise from an infected host : Rp =βSd. If Rp is less than one ,then the disease incidence is falling in the host population, whilst if Rp is greater than one ,the level of disease is increasing. This model can predict (i) the evolution of reduced virulence when host deaths occur rapidly, (ii) altered host behavior to maximize parasite fitness, (iii) that there is a threshold density of hosts for a given parasite, below which the parasite will not survive, and (iv) that diseases with short periods of infectivity should not persist in small populations.
微寄生物的繁殖率(Rp)可表示为由一个感染寄主所传染的新 个体数:Rp =βSd。如果Rp小于1,则病症传染力在寄主种群 中下降,而如果Rp大于1,疾病传染上升。该模型可预测(i) 寄主死亡快速发生时降低毒性的进化,(ii)转换寄主使寄生 的适合度最大,(iii)对 某一寄生物来说有一寄主密度阈值, 低于该阈值寄生物不能存活,和(iv)易传染时间短的疾病不 能在小种群中持续。

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A model of microparasites disease

S-the number of susceptible hosts易感寄主 数量 d-the duration of the infectious period感染持 续时间 Rp-the microparasite’s basic reproductive rate 基础繁殖率

? -the transmission rate传播率

Rp ? ? ? S ? d

利用模型可以进行简单的预测 ? Rp<1,疾病发生率下降 ? Rp>1,疾病发生率上升 1. ? 、s、d均不能太小,否则疾病发生率降低 2. 延长时间 如果时间短, Rp就会小,寄主就会降低毒性, 使时间变长, Rp变大 3. 增加 ? sneezing,伤风病毒传播的有效途径 4. Rp=1, S ? 1
T

?d

T

S -寄主能持续生存的宿主的极限密度

S< ST , Rp<1,疾病发生率下降 Parasites: 增加 ? ,增加d,使得 ST 降 低。 ? Vaccination 接种疫苗 降低s, Rp降低,herd immunity群体免 疫力上升。
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Heterogeneity in host populations
寄生物种群的异质 性

Whilst some diseases are stable, others have evolved ‘escapes’ from host defences by variation. The ‘childhood’ diseases of humans – measles, mumps chickenpox etc. are largely restricted to children because they are antigenically stable. In contrast, parasites such as influenza, rhinoviruses (the cause of the common cold) and Salmonella have multitudes of strains which are constantly being added to. Hosts therefore have a constantly varying army of attacking parasites to contend with. 一些疾病是稳定的,而另一些进化了通过变异“逃离”寄主 防御的手段。人类“孩童时期”的疾病——麻疹、腮腺炎、 水痘等大部分局限于孩子,因为它们的抗原稳定。与此相对, 寄生物如流感病毒、鼻病毒(普通感冒原)和沙门氏菌 (Salmonella)具有可经常加入的很多变异型。寄主因此要 经常与变化的寄生物军队的进攻而进行战斗。

Heterogeneity in host populations

寄主种群的异质 性

Individuals within a host population are very rarely equally at risk of being successfully attacked by a given parasite. The age, behavior, state of health, proximity of the infected individuals, and, of particular importance, the genetic predisposition of an individual will all influence the outcome. 寄主种群内个体在被寄生物感染的风险上很少是均等的。个 体的年龄、行为、健康状况、与被感染个体的接近程度以及 特别重要的个体的遗传性易患病程度,都会影响结果。

Parasites as a reason for sex – the ‘Red Queen’ 寄生作为有性 繁殖 的原因:“红皇后”

The costs of sexual reproduction are high compared to asexual reproduction, as males produce no offspring, so population growth is slow. Therefore, there must be some balancing benefit. It has been suggested that the main benefit of sex is to produce genetic variation to overcome the ubiquitous and dynamic attack of parasites.

有性繁殖的代价高于无性繁殖,因为雄性不产后代,
所以种群增长慢。因此,一定存在一些可平衡利益。 据认为有性繁殖的主要利益是产生遗传变异,来对付

寄生物普遍存在的和变化着的攻击。

This idea of the need for continuous evolutionary change has been termed the ‘Red Queen’effect,from the character in Lewis Caroll’s Through the Looking Glass.The Red Queen says to Alice ‘it takes all the running you can do ,to keep in the same place’. 这种需要持续进化变异的观念称作“红皇后”效应, 来自Lewis Carroll的通过镜子中的人物。红皇后对 爱丽斯说“尽你所能奔跑来保持在原地”。
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