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Low nuclear genetic variation in a globally widespread avian malaria parasite Plasmodium relictum

Kutzer, Megan (2012) BION34 20121
Degree Projects in Biology
Abstract
Abstract
Plasmodium relictum is a generalist avian malaria parasite found to infect approximately 400 species of bird on every continent except Antarctica. The two P. relictum isolates studied, SGS1 and GRW4, have been found in 55 species of passeriform bird. A parasite with a broad geographic range combined with lack of vector- and host-specificity is likely to contain multiple genetic variants. Here, a partially sequenced transcriptome of P. relictum (lineage SGS1) was used as a reference sequence to amplify four coding regions and one non-coding region of the nuclear genome to look more closely at withinspecies variation. The screen included infected birds of five species of passerines collected in Europe, Africa, North America and... (More)
Abstract
Plasmodium relictum is a generalist avian malaria parasite found to infect approximately 400 species of bird on every continent except Antarctica. The two P. relictum isolates studied, SGS1 and GRW4, have been found in 55 species of passeriform bird. A parasite with a broad geographic range combined with lack of vector- and host-specificity is likely to contain multiple genetic variants. Here, a partially sequenced transcriptome of P. relictum (lineage SGS1) was used as a reference sequence to amplify four coding regions and one non-coding region of the nuclear genome to look more closely at withinspecies variation. The screen included infected birds of five species of passerines collected in Europe, Africa, North America and Hawaii. Nuclear genes are particularly difficult to amplify in avian malaria parasites because host erythrocytes are nucleated, so the concentration of host DNA in blood is greater than that of the parasite. The study was limited to five nuclear loci (1156 bp). Nevertheless, results were striking. Within each of the mitochondrial lineages (SGS1 and GRW4) it was found that bird species within continents were infected by the same parasite nuclear haplotype. Contrary to predictions, the mitochondrial lineages exhibited limited haplotype and allelic diversity, which suggests a recent, rapid range expansion of P. relictum.


Popular science summary:

Low nuclear genetic variation in an avian malaria parasite

Avian malaria parasites (Plasmodium spp.) are widespread, occurring on every continent, except Antarctica, and are capable of infecting virtually all-avian taxa. There is considerable genetic diversity and phylogenetic complexity among parasites, suggesting that the number of parasite species may approach the number of host species, making multihost infections common. While most pathogens are capable of infecting multiple hosts, the evolutionary mechanisms and adaptations required by true host generalists are poorly understood.

In this study, a partially sequenced transcriptome of Plasmodium relictum (lineage SGS1) obtained from the blood of an experimentally infected crossbill (Loxia curvirostra) was used as a reference sequence to amplify coding and non-coding regions of the nuclear genome, allowing me to look more closely at within species variation. To examine variation within P. relictum isolates SGS1 and GRW4, we chose to amplify nuclear genes used in studies of human malaria that should be under selection pressure to avoid recognition by the host’s immune system, as well as genes with conserved housekeeping functions. In a parasite species with a broad geographic range, we expect that there should be some population structure and differentiation among isolates, particularly in the nuclear loci involved in immune evasion. On the other hand, low genetic diversity within the population may suggest a recent and rapid range expansion of P. relictum.

In spite of the wide geographic and host species distribution observed in the individual parasite lineages, there was limited nuclear genetic variation within lineages and moderate variation between lineages. Although sample sizes were low, it is interesting that lineages were limited geographically and nucleotide diversity was low overall. Malignant malaria (Plasmodium falciparum) exhibits similarly low diversity among isolates, which has been proposed as evidence of a recent population bottleneck, despite widespread polymorphisms in antigenic genes.

While little variation was observed in this study, a large-scale genetic screen that includes isolates from all over the world will perhaps help to resolve the extent of variation in the population. Studies of nuclear gene loci will add to our existing knowledge of P. relictum populations and allow us to study how genetic structure, structural diversity and variation within and between populations influence the evolution of parasite life history.

Advisors: Staffan Bensch and Olof Hellgren
Master´s Degree Project: 45 credits in Molecular Ecology, 2012.
Department of Biology, Lund University (Less)
Please use this url to cite or link to this publication:
author
Kutzer, Megan
supervisor
organization
course
BION34 20121
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
3564013
date added to LUP
2013-03-13 16:14:35
date last changed
2013-03-26 13:45:53
@misc{3564013,
  abstract     = {Abstract
Plasmodium relictum is a generalist avian malaria parasite found to infect approximately 400 species of bird on every continent except Antarctica. The two P. relictum isolates studied, SGS1 and GRW4, have been found in 55 species of passeriform bird. A parasite with a broad geographic range combined with lack of vector- and host-specificity is likely to contain multiple genetic variants. Here, a partially sequenced transcriptome of P. relictum (lineage SGS1) was used as a reference sequence to amplify four coding regions and one non-coding region of the nuclear genome to look more closely at withinspecies variation. The screen included infected birds of five species of passerines collected in Europe, Africa, North America and Hawaii. Nuclear genes are particularly difficult to amplify in avian malaria parasites because host erythrocytes are nucleated, so the concentration of host DNA in blood is greater than that of the parasite. The study was limited to five nuclear loci (1156 bp). Nevertheless, results were striking. Within each of the mitochondrial lineages (SGS1 and GRW4) it was found that bird species within continents were infected by the same parasite nuclear haplotype. Contrary to predictions, the mitochondrial lineages exhibited limited haplotype and allelic diversity, which suggests a recent, rapid range expansion of P. relictum.


Popular science summary:

Low nuclear genetic variation in an avian malaria parasite

Avian malaria parasites (Plasmodium spp.) are widespread, occurring on every continent, except Antarctica, and are capable of infecting virtually all-avian taxa. There is considerable genetic diversity and phylogenetic complexity among parasites, suggesting that the number of parasite species may approach the number of host species, making multihost infections common. While most pathogens are capable of infecting multiple hosts, the evolutionary mechanisms and adaptations required by true host generalists are poorly understood. 

In this study, a partially sequenced transcriptome of Plasmodium relictum (lineage SGS1) obtained from the blood of an experimentally infected crossbill (Loxia curvirostra) was used as a reference sequence to amplify coding and non-coding regions of the nuclear genome, allowing me to look more closely at within species variation. To examine variation within P. relictum isolates SGS1 and GRW4, we chose to amplify nuclear genes used in studies of human malaria that should be under selection pressure to avoid recognition by the host’s immune system, as well as genes with conserved housekeeping functions. In a parasite species with a broad geographic range, we expect that there should be some population structure and differentiation among isolates, particularly in the nuclear loci involved in immune evasion. On the other hand, low genetic diversity within the population may suggest a recent and rapid range expansion of P. relictum. 

In spite of the wide geographic and host species distribution observed in the individual parasite lineages, there was limited nuclear genetic variation within lineages and moderate variation between lineages. Although sample sizes were low, it is interesting that lineages were limited geographically and nucleotide diversity was low overall. Malignant malaria (Plasmodium falciparum) exhibits similarly low diversity among isolates, which has been proposed as evidence of a recent population bottleneck, despite widespread polymorphisms in antigenic genes. 

While little variation was observed in this study, a large-scale genetic screen that includes isolates from all over the world will perhaps help to resolve the extent of variation in the population. Studies of nuclear gene loci will add to our existing knowledge of P. relictum populations and allow us to study how genetic structure, structural diversity and variation within and between populations influence the evolution of parasite life history. 

Advisors: Staffan Bensch and Olof Hellgren
Master´s Degree Project: 45 credits in Molecular Ecology, 2012. 
Department of Biology, Lund University},
  author       = {Kutzer, Megan},
  language     = {eng},
  note         = {Student Paper},
  title        = {Low nuclear genetic variation in a globally widespread avian malaria parasite Plasmodium relictum},
  year         = {2012},
}