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Stone CM, Chitnis N. Implications of heterogeneous biting exposure and animal hosts on Trypanosomiasis brucei gambiense transmission and control. PLoS Comput Biol. Download references. We thank the International Atomic Energy Agency, Vienna for their generous supply of tsetse flies and are grateful to Mrs Vanessa Ferris for expert technical help.
All authors read and approved the final manuscript. You can also search for this author in PubMed Google Scholar. Correspondence to Wendy Gibson. Immune cells are one of the main targets of extracellular vesicles.
Several types of extracellular vesicles are promoters of the innate and acquired immune response and defined as types of pathogen-associated molecular patterns PAMPs which could be formed by a wide range of macro-molecules such as lipids, proteins, carbohydrates, or nucleic acids and are recognized by pattern recognition receptors PPRs such as toll-like receptors TLRs present in leukocytes and various non-immune cells, which will in turn initiate a signaling cascade that leads to the activation of an immune response against the pathogen [ 94 ].
Studies on T. The life cycle of T. The differentiation of epimastigotes, the non-infective dividing forms found in the digestive tract of the invertebrate host into metacyclic trypomastigotes metacyclogenesis occur in an amino-acid-rich and carbohydrate-poor medium. In vertebrate, the trypomastigotes differentiate into the dividing forms called amastigotes occur in a medium poor in free glucose.
Trypanosomatids can use either glucose or amino acids as main carbon and energy source, although one cannot rule out the use of fatty acids as well. Amino acids, especially l -proline and l -glutamine which are abundant in the hemolymph and tissue fluids of the blood sucking vector are the main source of carbon and energy in the insect stages. Several amino acids such as proline, aspartate and glutamate are actively transported and oxidized by T.
Also, the presence of at least 60 genes belonging to a single family of amino acid transporter in T. While some trypanosomatids metabolically prefer glucose to amino acid when grown in a medium rich in glucose and amino acid, as seen in proline and glucose metabolism in T. Amino acids are crucial nutrients during the T. Arginine metabolism is linked to T.
Certain amino acids such as proline, glutamate, and aspartate are essentials in the process of metacyclogenesis. Apart from being involved in the process of metacyclogenesis, proline and glutamate seems to have a broad variety of functions.
While proline is involved in fulfilling the energy requirements for host cell invasion, differentiation from the intracellular transient epimastigote-like stage to trypomastigote forms and resistance to oxidative stress, glutamate is directly involved in osmoregulation and cell volume control. As a result of changing environments during the life cycle, T. In order to adapt to the different environment they find within one or the other host species, T.
All these changes are orchestrated by the differential expression of stage-specific genes. Cellular differentiation is controlled at multiple levels including, for most eukaryotic cells, initiation of gene transcription. The discriminatory mechanisms for the initiation of transcription at individual loci is largely absent in trypanosomatids and most protein-coding genes lack promoters and are transcribed as long polycistronic units that are processed into individual mRNAs.
Consequently, trypanosomes rely on post-transcriptional processes such as translational efficiency, mRNA stability and post-translational modification to coordinate developmental transitions and other adaptive responses encountered throughout their complex life cycles.
In eukaryotes, protein-coding genes are transcribed into monocistronic pre-mRNA transcripts containing coding sequences exons and non-coding sequences introns that are processed into mature mRNAs through cis -splicing reactions. In trypanosomatids, however, transcription is polycistronic, there are no introns and, therefore, no cis -splicing reactions.
Processing of pre-mRNA into single gene units is effected by trans - splicing reactions, a process that has been found to operate only in trypanosomatids and other organisms like Euglena , nematode and trematode worms.
Granules of mRNA such as processing bodies P bodies and stress granules SGs are involved in post-transcriptional regulation of gene expression.
P bodies are constitutively present in the cell and can grow in size and number when cells are perturbed while SGs only arise under cellular stress. P bodies were initially thought to be the place where mRNA was recruited to be degraded and recently, a function as mRNA storage depots has been assigned to P bodies. By contrast, SGs are stalled 43S translation pre-initiation complexes, mainly composed of mRNA, translation initiation factors and 40S ribosomal proteins.
SGs are thought to function as mRNA triage centers during stress. Trypanosoma cruzi , the parasite responsible of Chagas disease has a complex life cycle including intracellular and extracellular forms, which alternate between invertebrate insect vectors and vertebrate mammalian hosts. During their life cycles, they alternate between a mammalian host and an insect vector and undergo profound biochemical and morphological transformations in order to adapt to the different environments changes orchestrated by precise gene regulation programs.
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Login to your personal dashboard for more detailed statistics on your publications. Edited by Wanderley De Souza. Edited by Mohammad Manjur Shah. We are IntechOpen, the world's leading publisher of Open Access books. The research could eventually lead to new approaches for controlling sleeping sickness in humans and wasting diseases in livestock which are caused by trypanosomes carried by the bloodsucking tsetse fly. Biologists believe that sexual reproduction evolved very early and is now ubiquitous in organisms with complex cell structure the eukaryotes, essentially all living organisms except bacteria.
However, real evidence is lacking for a large section of the evolutionary tree. Trypanosomes represent an early and very distant branch of the eukaryote tree of life and until now it was unclear whether they do indeed reproduce sexually.
Offspring that result from sexual reproduction inherit half their genetic material from each parent. At the core of this process is meiosis, the cellular division that shuffles the parental genes and deals them out in new combinations to the offspring. In organisms which cause diseases, sexual reproduction can spread genes which make them more virulent, or resistant to drugs used for treatment, as well as creating completely new strains with combinations of genes not previously encountered.
Some time ago it was shown that genetic shuffling could occur when two different trypanosome strains were mixed in the tsetse fly, but it was far from clear that this was true sexual reproduction. Direct visualization of the process was difficult because it happened inside the insect. Using a conservative algorithm to detect chromosomal segments derived from meiotic recombination, we identified recombined segments ranging in size from 10kb to kb S2 Fig.
Evidence of meiotic recombination events were detected on 69 of the 80 Markers that are polymorphic in the population can be heterozygous green or homozygous in each individual yellow representing a site that is homozygous for one base, blue representing individuals homozygous for the alternative base. A Independent assortment of chromosomes and subsequent fusion of gametes has resulted in a region I that is homozygous in some individuals and heterozygous in others across a kb region containing polymorphic markers on an kb contig.
In this region, individuals can be homozygous for either set of linked polymorphic markers. Interestingly, genotypes are geographically clustered, indicating identity by descent.
Runs of identical color indicate identical sequence. B Meiotic recombination has resulted in a region II of a kb contig that partially retain the linkage patterns of both parental chromosomes while the rest retains the linkage pattern of only one parental chromosome, showing how crossing over can affect parts of chromosomes.
In this region, there is evidence of at least three independent meiotic recombination events. C Sexual reproduction after a recent colonization of a city block has resulted in the genetic divergence of closely related samples over a run of polymorphic markers. For example, a meiotic recombination event is apparent in region II in one sampled individual on Block 7. The remaining contigs S1 Fig contain near-complete sequence similarity among all clones isolated from Block 7 suggesting that all individuals on this block share a recent common ancestor and the meiotic event occurring in region II occurred after the Block 7 was colonized.
Similarly, a meiotic recombination event in a region III of a kb contig occurred in one lineage inhabiting Blocks 1—3 in Mariano Melgar. The regions shown here contain only some of the meiotic recombination events distributed across contigs, with each detected meiotic recombination event spanning from 10kb to nearly kb.
The relative positions of each contig within the genome are unknown. For data spanning the whole genome, see S1 Fig. The conservative criteria used to identify these meiotic recombination events are unlikely to be met on contigs shorter than kb S3 Fig. Isolates derived from geographically-proximal locations commonly have the same recombination events, suggesting identity-by-descent S4 and S5 Figs. Samples collected in the same year or from the same host, as opposed to collections from geographically similar areas, do not form monophyletic clades S5 Fig.
These data suggest that geography is the most important predictor of phylogenetic relationship. Of those markers in Hardy-Weinberg equilibrium, only one 0. All polymorphic markers contain only two character states. The evolutionary advantages of sex for eukaryotic microparasites are numerous and include the potential to increase the diversity of immune evasion genes within individuals where diversity is paramount for survival within hosts.
Yet many populations of eukaryotic microparasites that have the capacity for sexual reproduction exhibit a clonal population structure suggesting that sexual reproduction either does not occur with sufficient regularity to disrupt clonal population structures in nature or that sexual reproduction occurs primarily among closely related individuals. The genome sequence data analyzed here supports the hypothesis that T.
The clonal population structure observed in this and other T. The meiotic processes necessary for gamete formation and sexual reproduction create novel associations of polymorphic markers through recombination between homologous chromosomes. Fusion of a gamete containing a recombined chromosome with a gamete from the same individual containing one of the parental chromosomes during sexual reproduction will result in homozygosity throughout one large region of the chromosome while the sites that were heterozygous in the parent will remain heterozygous in the offspring in the other chromosomal region.
Here, we looked for evidence of recombined chromosomes derived from meiotic processes to detect evidence of sexual recombination. Among the genome sequences from T. Sexual reproduction occurs with sufficient regularity to observe meiotic recombination events within subpopulations that have only recently established in blocks within the city Fig 3C.
These types of meiotic recombination events are present throughout the T. The population structure of T. Despite evidence of multiple meiosis and fertilization events in this relatively young population, asexual reproduction remains the dominant form of T. The dominance of asexual reproduction has maintained both the higher than expected levels of heterozygosity observed at the majority of polymorphic markers as well as the high levels of linkage disequilibrium among the polymorphic markers within the observed linkage blocks on many contigs.
Although heterozygosity is expected to decay with repeated inbreeding, the limited number of sexual reproduction events experienced in this population since the heterozygous common ancestor have resulted in only a moderate reduction in heterozygosity among the extant descendants. The geographic structure of this population, where subpopulations cluster within city blocks S4 and S5 Figs , may prevent gene flow, and thus recombination, between subpopulations in different geographic regions [ 22 ].
This phenomenon, known as the Wahlund effect [ 32 ], is expected to cause deviations from the expected Hardy-Weinberg equilibrium and level of linkage disequilibrium, even when sexual recombination is occurring within geographically-isolated subpopulations.
These data also suggest that the common ancestor of all sampled T. While the data suggest that the common ancestor was likely heterozygous at most polymorphic sites identified here, heterozygous sites likely comprised a very small percentage of the ancestral genome because less than 0.
Variation in the number of chromosomes within each T. Although the proportion of base counts derived from sequencing depth across heterozygous sites suggests this population is primarily diploid S6 Fig , the conclusions presented here regarding the number and location of sexual recombination events observed are robust to variable ploidy levels. That is, many contigs contain internal recombination breakpoints that are detected in multiple isolates Fig 3B , suggesting that these chromosomes have experienced a meiotic recombination event, regardless of the number of other chromosomes in the strains where these contigs were identified.
High levels of inbreeding also cause the population structure of this T. Inbreeding limits the exchange of diverse alleles among individuals within the population resulting in multiple independently evolving lineages.
Further, inbreeding reduces the benefits of sexual reproduction as it results in the continuous decay of diversity within each lineage until the absorbing boundary of complete homozygosity is attained Fig 4. That is, only half of the offspring that result from self-mating, where gametes fuse only with gametes of the same parent, in a heterozygote lineage will be heterozygous while all offspring of homozygotes would be homozygous, thus reducing the proportion of heterozygotes in the population by half with each generation.
No evidence of outcrossing was detected in the current dataset Fig 3A. It is unclear, however, if the apparent inbreeding results from obligatory selfing or if outbreeding occurs but cannot be observed due to the limited genome-wide diversity in Arequipa. The obligatory selfing hypothesis is supported by published investigations in T. Future studies are necessary to assess the population or molecular mechanisms that result in high levels of selfing in areas with opportunities for outcrossing.
After one generation of inbreeding, half of the offspring retain the heterozygosity found in the parent while half become homozygous. As diversity cannot be restored in homozygous regions in the absence of outcrossing and heterozygosity decays by half each generation, population-level heterozygosity continually declines in inbreeding populations until all lineages are homozygous. The biological mechanism of sexual reproduction in T.
The location in what host or host tissue , frequency, and mechanism affecting the probability of outcrossing in T. By contrast, experimental evidence suggests that T. Haploid gametes have not yet been detected in T.
Parasexual reproduction, which occurs in several eukaryotic microbes, is the creation of a tetraploid cell through the fusion of diploid cells, followed by chromosomal crossing over during mitosis and stochastic chromosome loss to return to a diploid state [ 23 , 36 , 37 ].
The data presented here are broadly consistent with both parasexual recombination and meiosis as both mechanisms result in the exchange of large genomic segments [ 36 ].
However, the stochastic chromosome loss that occurs during parasexual reproduction is expected to generate considerable aneuploidy which was not detected in the current dataset S6 Fig. Sexual reproduction in parasites like T.
The strong selection pressure generated by the host immune system has resulted in the extensive and diverse immune evasion gene family in T. Sexual reproduction can maintain immune evasion gene diversity in the face of homogenization events that commonly occur among paralogs within genomes by redistributing variation among trans-sialidase genes within a population through chromosome sorting or cross-over events [ 42 — 44 ]. While we did not specifically investigate trans-sialidase genes, it is possible that even in populations where inbreeding is common, low frequencies of sex can increase trans-sialidase diversity within individuals, thus increasing the probability of survival within a host.
Future studies could assess whether recombination hotspots are conserved across T. Since the T. Thus, a substantial proportion of the crossing over detected here must have occurred prior to isolation. Although the ubiquitous observations of clonal population structures have resulted in the hypothesis that the frequency of sexual reproduction is insufficient to disrupt the clonal population structure in natural T. Sexual reproduction occurred repeatedly in the recent evolutionary history of this population and has occurred within subpopulations that have only recently established on city blocks Fig 3C.
The number of meiotic recombination events reported here is conservative, as events occurring between homozygous regions cannot be detected and our criteria for identifying recombined regions was strict. Nevertheless, the extant linkage disequilibrium across large linkage blocks suggests that asexual reproduction has been the most common mode of reproduction in this population.
This mixed life-history strategy has important medical and evolutionary implications. Sexual reproduction may allow for rapid diversification of antigens which may contribute to the variability in serological diagnostics [ 46 — 48 ] and has the potential to generate genetic and phenotypic diversity [ 49 , 50 ] in pathogenicity [ 51 , 52 ], host and vector propensity [ 53 , 54 ], and vulnerability to drugs [ 55 , 56 ] if outcrossing is common in populations with greater diversity.
Primarily asexual reproduction coupled with periodic sexual inbreeding, on the other hand, will result in clonal population structures that maximize the diversity among lineages and minimize the possibility that virulence factors or drug resistance will introgress into other lineages. Primarily asexual reproduction coupled with periodic sexual outcrossing may be significant for the adaptive evolution in novel environments and may be particularly important for invading urbanizing environments where rapid adaptation may be at a premium.
Genomic DNA from T. Uninfected T. For all samples lab- and naturally-infected vectors , T. Six T. Low quality bases were trimmed from raw reads using trimmomatic Phylogenetic analyses demonstrate that the reference genome most closely related to the Arequipan isolates is the discrete typing unit 1 DTU1 genome TcJR clone 4 S7 Fig.
Only the contigs longer than 10kb were used for the assembly to avoid spurious alignments to short contigs, for a total genome assembly that includes nearly 28Mbp S2 Table. This assembly largely excluded the extensive repeat regions found throughout the T.
Duplicate reads were removed using Picard MarkDuplicates [ 61 ]. Indels were excluded. Only loci for which all samples achieved a minimum depth of 20 and a Genotype Quality score GQ greater than 40 were included.
These filters maximized the number of polymorphic sites identified while ensuring that duplicate and triplicate sequences resulted in identical SNP datasets. The final consensus SNP panel included polymorphic sites. To assess ploidy in these isolates, the proportion of reads containing each nucleotide was calculated for all heterozygous sites in all isolates.
Cross-over events during meiosis create a novel combination of polymorphic markers by combining large genetic regions of homologous chromosomes Fig 1C. During meiosis, non-sister chromatids of homologous chromosomes can exchange genetic material resulting in gametes containing chromosomes that differ from either parental chromosome.
The chromosomes that result from a meiotic cross-over event in gametes are comprised of two sections, one retaining the linkage association among markers found on one of the parental chromosomes joined with another chromosomal segment with the linkage associations found on the other parental chromosome.
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