Sexual reproduction in trypanosomes

锥虫病是一种主要的家畜疾病，通过破坏耕作和生产动物，历史上使撒哈拉以南非洲的农业发展陷入瘫痪。这种采采蝇传播疾病继续严重限制非洲许多国家的畜牧业生产，因为它非常普遍，影响到大多数主要牲畜物种，包括牛、绵羊、山羊、猪、马、驴和骆驼。兽医几乎没有可用于预防或治疗的药物，耐药性正在增加和普遍存在；没有疫苗。气候变化可能会加剧动物锥虫病的问题，这种疾病已经影响到温带以及热带和亚热带地区。这种疾病是由微小的单细胞生物锥虫引起的，这种微生物被称为锥虫，在受影响牲畜的血液中发现。我们研究的目标是找出锥虫是如何在自己之间交换遗传物质的，以及这是否会产生具有新特征的新型寄生虫。基因交换使基因能够通过细胞间的转移在细胞群体中迅速传播。对于病原微生物来说，这一点尤其重要，因为一旦出现耐药基因，它就会迅速传播，使之前有效的药物失效。因此，有性周期的微生物可能比无性繁殖的微生物更难控制。在这里，我们将调查家畜病原体布鲁氏锥虫和弓形锥虫的有性繁殖。这两个锥虫都是由采采蝇携带的，但有显著的区别：布鲁氏锥虫在苍蝇的唾液腺中发育，这是已知进行有性繁殖的地方，形成了新的杂交菌株；贡氏锥虫在苍蝇的咬人和进食器官--鼻子中发育，在我们最近的工作中，我们已经证明它在那里也进行了基因交换，但到目前为止还没有关于细节的信息。在人类中，配子--卵子和精子--是由一种称为减数分裂的特殊形式的细胞分裂产生的。卵子和精子结合形成一个称为受精卵的单一杂交细胞，然后发育成胚胎。受精卵的基因组由细胞核中的一组母体和父体染色体加上母体线粒体DNA组成。我们将研究锥虫如何产生配子，以及配子如何结合形成受精卵。我们已经通过搜索表达减数分裂特异蛋白的细胞来鉴定正在进行减数分裂的锥虫，并将跟踪这些减数分裂细胞，看看它们是如何产生配子的。我们已经证明了配子融合在一起并交换细胞质，但还不知道它们是如何结合DNA的。像人类一样，锥虫既有由成对染色体组成的核DNA，也有线粒体DNA；然而，锥虫线粒体DNA被紧密地包装在一个称为动泡体的细胞室中。通过对杂交后代的分析，我们知道核基因组和动泡体基因组都是从双亲锥虫中遗传来的。我们的初步实验表明，该方法是可行的，并将产生可解释的数据。到项目结束时，我们将阐明锥虫交换遗传物质的过程。从长远来看，这将为控制牲畜锥虫病的战略提供参考，因为我们将了解有害基因通过寄生虫种群传播的限制，以及出现牲畜以前从未遇到过的新菌株的可能性。

Trypanosomiasis is a major livestock disease that historically crippled the development of agriculture in sub-Saharan Africa by destroying both draught and production animals. This tsetse fly-transmitted disease continues to severely constrain livestock production in many countries in Africa, because it is very widespread and affects most of the major livestock species, including cattle, sheep, goats, pigs, horses, donkeys and camels. Vets have few drugs available for prophylaxis or treatment, and drug resistance is increasing and widespread; there is no vaccine. Climate change is likely to exacerbate the problem of animal trypanosomiasis, which already affects temperate as well as tropical and subtropical regions.The disease is caused by microscopic, single-celled organisms called trypanosomes, which are found in the blood of affected livestock. The goal of our research is to find out how trypanosomes swap their genetic material among themselves and whether this generates novel types of parasite with new features. Genetic exchange enables the rapid spread of genes through a cell population by transfer from cell to cell. This is particularly important in the case of pathogenic microbes, because once a gene for drug resistance has arisen, it can quickly spread, rendering a previously effective drug useless. A microbe with a sexual cycle is therefore potentially more difficult to control than one that reproduces asexually. Here we will investigate sexual reproduction in the livestock pathogens Trypanosoma brucei and T. congolense. Both these trypanosomes are carried by tsetse flies, but with significant differences: T. brucei develops in the fly salivary glands and this is where it is known to undergo sexual reproduction with formation of novel hybrid strains; T. congolense develops in the fly's biting and feeding apparatus, the proboscis, and in our recent work we have shown it also undergoes genetic exchange there, but as yet have no information on the details. In humans, the gametes - egg and sperm - are produced by a special form of cell division called meiosis. Egg and sperm combine to form a single hybrid cell called the zygote, which then develops into the embryo. The genome of the zygote consists of one set of maternal and paternal chromosomes in the cell nucleus, plus maternal mitochondrial DNA. We will investigate how trypanosomes produce gametes and how the gametes combine to form a zygote. We have identified trypanosomes undergoing meiosis by searching for cells expressing meiosis-specific proteins tagged by fluorescence and will follow these meiotic cells to see how they produce gametes. We have shown that the gametes fuse together and exchange cytoplasm, but do not yet understand how they combine their DNA. Like humans, trypanosomes have both nuclear DNA consisting of paired chromosomes and mitochondrial DNA; however trypanosome mitochondrial DNA is tightly packaged into a cellular compartment called the kinetoplast. We know from analysis of hybrid progeny that both the nuclear and kinetoplast genomes are inherited from both parental trypanosomes.Our preliminary experiments show that the approaches are feasible and will yield interpretable data. By the end of the project we will have elucidated the process by which trypanosomes exchange genetic material. In the long term, this will inform strategies to control livestock trypanosomiasis, because we will understand the limitations to spread of harmful genes through the parasite population and the likelihood of new strains arising that have never been encountered previously by livestock.