Williams Bryony A P, Keeling Patrick J
Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, 3529-6270 University Boulevard, Vancouver, BC V6T 1Z4, Canada.
Adv Parasitol. 2003;54:9-68. doi: 10.1016/s0065-308x(03)54001-5.
A number of parasitic protists and fungi have adopted extremely specialised characteristics of morphology, biochemistry, and molecular biology, sometimes making it difficult to discern their evolutionary origins. One aspect of several parasitic groups that reflects this is their metabolic organelles, mitochondria and plastids. These organelles are derived from endosymbiosis with an alpha-proteobacterium and a cyanobacterium respectively, and are home to a variety of core metabolic processes. As parasites adapted, new demands, or perhaps a relaxation of demands, frequently led to significant changes in these organelles. At the extreme, the organelles are degenerated and transformed beyond recognition, and are referred to as "cryptic". Generally, there is no prior cytological evidence for a cryptic organelle, and its presence is only discovered through phylogenetic analysis of molecular relicts followed by their localisation to organelle-like structures. Since the organelles are derived from eubacteria, the genes for proteins and RNAs associated with them are generally easily recognisable, and since the metabolic activities retained in these organelles are prokaryotic, or at least very unusual, they often serve as an important target for therapeutics. Cryptic mitochondria are now known in several protist and fungal parasites. In some cases (e.g., Trichomonas), well characterised but evolutionarily enigmatic organelles called hydrogenosomes were shown to be derived from mitochondria. In other cases (e.g., Entamoeba and microsporidia), "amitochondriate" parasites have been shown to harbour a previously undetected mitochondrial organelle. Typically, little is known about the functions of these newly discovered organelles, but recent progress in several groups has revealed a number of potential functions. Cryptic plastids have now been found in a small number of parasites that were not previously suspected to have algal ancestors. One recent case is the discovery that helicosporidian parasites are really highly adapted green alga, but the most spectacular case is the discovery of a plastid in the Apicomplexa. Apicomplexa are very well-studied parasites that include the malaria parasite, Plasmodium, so the discovery of a cryptic plastid in Apicomplexa came as quite a surprise. The apicomplexan plastid is now very well characterised and has been shown to function in the biosynthesis of fatty acids, isopentenyl diphosphate and heme, activities also found in photosynthetic plastids.
许多寄生原生生物和真菌呈现出形态学、生物化学和分子生物学方面极其特殊的特征，有时很难辨别它们的进化起源。几个寄生群体反映这一点的一个方面是它们的代谢细胞器，即线粒体和质体。这些细胞器分别源自与α-变形菌和蓝细菌的内共生，是各种核心代谢过程的发生场所。随着寄生虫的适应，新的需求，或者也许是需求的放松，经常导致这些细胞器发生重大变化。在极端情况下，这些细胞器会退化并转变得面目全非，被称为“隐蔽的”。一般来说，对于隐蔽细胞器没有先前的细胞学证据，其存在仅通过对分子遗迹的系统发育分析，然后将它们定位到类似细胞器的结构中才被发现。由于这些细胞器源自真细菌，与它们相关的蛋白质和RNA的基因通常很容易识别，并且由于保留在这些细胞器中的代谢活动是原核的，或者至少非常不寻常，它们常常成为治疗的重要靶点。现在在几种原生生物和真菌寄生虫中已知有隐蔽线粒体。在某些情况下（如阴道毛滴虫），特征明确但进化上神秘的细胞器——氢化酶体，已被证明源自线粒体。在其他情况下（如溶组织内阿米巴和微孢子虫），“无线粒体”寄生虫已被证明含有以前未被检测到的线粒体细胞器。通常，对于这些新发现的细胞器的功能知之甚少，但几个群体最近的进展揭示了许多潜在功能。现在在少数以前不被怀疑有藻类祖先的寄生虫中发现了隐蔽质体。最近的一个例子是发现螺旋孢子虫寄生虫实际上是高度适应的绿藻，但最引人注目的例子是在顶复门中发现了质体。顶复门是研究得非常透彻的寄生虫，包括疟原虫属的疟原虫，所以在顶复门中发现隐蔽质体相当令人惊讶。现在顶复门质体已得到很好地表征，并已证明其在脂肪酸、异戊烯基二磷酸和血红素的生物合成中起作用，这些活动在光合质体中也能找到。
