| Genetics |
| Malaria parasites are, like all members of the Apicomplexa, haploid for almost their entire life cycle. An exception is the zygote stage, which is diploid prior to the meiotic division that results in the production of sporozoites. The parasite possesses three individual genomes; an extra-chromosomal mitochondrial genome, known as the 6kb element, a 35kb circular genome associated with the apicoplast, and a large nuclear genome. The mitochondrial genome encodes two truncated ribosomal RNAs and three components of respiration (Cytochrome c oxidase subunits I and II, and cytochrome b (Funes et al., 2004). The apicoplast genome encodes 30 proteins which are primarily involved in gene expression (Funes et al., 2004; Gardner et al., 2002). It has been shown, however, that about 10% of the genes encoded by the nuclear genome are targeted to the apicoplast, supplementing its proteome via post-translational targeting into the organelle (Gardner et al., 2002; Roos et al., 1999b; Zuegge et al., 2001). The exact role of the apicoplast remains unclear, but it is known to be involved in the anabolic synthesis of fatty acids, isoprenoids and haem. (Gardner et al., 2002).
The nuclear genome of P. falciparum consists of 14 chromosomes and encodes about 5,300 genes with a total genome size of 22.8 megabases. It is extremely (A+T)-rich, with an overall (A+T) content of 80.6%, rising to 90% in intronic and intergenic regions (Gardner et al., 2002). There is considerable chromosomal size polymorphism between strains of parasites (Corcoran and Kemp, 1986), which could be due to unequal crossing-over of homologous chromosomes during meiosis, or non-meiotic chromosome breaking and healing events (Babiker et al., 1994; Gardner et al., 2002; HernandezRivas et al., 1996; Scherf et al., 1992). Malaria parasites exhibit extensive genetic polymorphism between species and between strains within species. Some of these genetic polymorphisms manifest themselves as phenotypic differences between strains for quantifiable traits such as virulence and susceptibility to drugs. This genetic diversity is subject to natural selection, which is most intuitively apparent in the way that drug-resistance appears and spreads under strong drug pressure. This diversity arises both by spontaneous genetic mutations, and by novel genotypes arising through recombination. The meiotic division and recombination which the parasites undergo in the mosquito can result in independent assortment of genes on different chromosomes, crossing-over events between linked genes on homologous chromosomes (both producing novel genotypes), and intragenic recombination (resulting in novel alleles of genes) (Walliker et al., 1998). Meiosis occurs after the fusing of the micro- and macrogametocytes to form a zygote. If gametocytes from two genetically distinct parasites are taken up by the mosquito, then crossing resulting in heterozygous zygotes can occur. Each heterozygote will, through the process of meiosis, produce 4 genetically distinct haploid daughter cells (recombinant progeny). If there are equal numbers of male and female gametes present from each parental strain in the mosquito, then selfing will occur 50% of the time, resulting in 25% of the zygotes being genetically identical to one parental strain, and 25% identical to the other. The remaining 50% will be hybrid between the two parentals (see below). |
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| References
Funes,S., Reyes-Prieto,A., Perez-Martinez,X., and Gonzalez-Halphen,D. (2004). On the evolutionary origins of apicoplasts: revisiting the rhodophyte vs. chlorophyte controversy. Microbes and Infection, 6, 305-311. Gardner,M.J., et al (2002). Genome sequence of the human malaria parasite Plasmodium falciparum. Nature, 419, 498-511. Roos,D.S., Crawford,M.J., Donald,R.G.K., Kissinger,J.C., Klimczak,L.J., and Striepen,B. (1999b). Origin, targeting, and function of the apicomplexan plastid. Current Opinion in Microbiology, 2, 426-432. Zuegge,J., Ralph,S., Schmuker,M., McFadden,G.I., and Schneider,G. (2001). Deciphering apicoplast targeting signals - feature extraction from nuclear-encoded precursors of Plasmodium falciparum apicoplast proteins. Gene, 280, 19-26. Corcoran,L.M. and Kemp,D.J. (1986). Chromosomes of Plasmodium-Falciparum. Papua New Guinea Medical Journal, 29, 95-101. Scherf,A., Carter,R., Petersen,C., Alano,P., Nelson,R., Aikawa,M., Mattei,D., Dasilva,L.P., and Leech,J. (1992). Gene Inactivation of Pf11-1 of Plasmodium-Falciparum by Chromosome Breakage and Healing - Identification of A Gametocyte-Specific Protein with A Potential Role in Gametogenesis. Embo Journal, 11, 2293-2301. Babiker,H.A., Ranfordcartwright,L.C., Currie,D., Charlwood,J.D., Billingsley,P., Teuscher,T., and Walliker,D. (1994). Random Mating in A Natural-Population of the Malaria Parasite Plasmodium-Falciparum. Parasitology, 109, 413-421. HernandezRivas,R., Hinterberg,K., and Scherf,A. (1996). Compartmentalization of genes coding for immunodominant antigens to fragile chromosome ends leads to dispersed subtelomeric gene families and rapid gene evolution in Plasmodium falciparum. Molecular and Biochemical Parasitology, 78, 137-148. Walliker,D. (2000). Malaria. In Thompson,R.C.A. (Ed.), Molecular Epidemiology of Infectious Diseases, . Arnold, London, pp. 93-112. Walliker,D., Babiker,A., and Ranford-Cartwright,L. (1998). The Genetic Structure of Malaria Parasite Populations. In Sherman,I.W. (Ed.), Malaria:Parasite biology, Pathenogenesis and Protection, . ASM Press, Washinton DC. Walliker,D., Carter,R., and Morgan,S. (1971). Genetic Recombination in Malaria Parasites. Nature, 232, 561-&. Walliker,D., Carter,R., and Sanderson,A. (1975). Genetic Studies on Plasmodium-Chabaudi - Recombination Between Enzyme Markers. Parasitology, 70, 19-24. Ranford-Cartwright,L.C., Balfe,P., Carter,R., and Walliker,D. (1993). Frequency of Cross-Fertilization in the Human Malaria Parasite Plasmodium falciparum. Parasitology, 107, 11-18. Kerr,P.J., Ranfordcartwright,L.C., and Walliker,D. (1994). Proof of Intragenic Recombination in Plasmodium-Falciparum. Molecular and Biochemical Parasitology, 66, 241-248. Rich,S.M., Hudson,R.R., and Ayala,F.J. (1997). Plasmodium falciparum antigenic diversity: Evidence of clonal population structure. Proceedings of the National Academy of Sciences of the United States of America, 94, 13040-13045. Creasey,A., Mendis,K., Carlton,J., Williamson,D., Wilson,I., and Carter,R. (1994). Maternal Inheritance of Extrachromosomal Dna in Malaria Parasites. Molecular and Biochemical Parasitology, 65, 95-98. Creasey,A.M., Ranfordcartwright,L.C., Moore,D.J., Williamson,D.H., Wilson,R.J.M., Walliker,D., and Carter,R. (1993). Uniparental Inheritance of the Mitochondrial Gene Cytochrome-B in Plasmodium-Falciparum. Current Genetics, 23, 360-364. |
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| Recombination between malaria parasites was first formally proven with genetic crossing experiments conducted with the rodent parasites Plasmodium yoelii (Walliker et al., 1971), and Plasmodium chabaudi (Walliker et al., 1975a). In the P. chabaudi experiments, two cloned parasites which differed in their response to the anti-malarial drug pyrimethamine and in the electrophoretic patterns of two enzymes (6-phosphogluconate dehydrogenase (6-PGD) and lactate dehydrogenase (LDH)) were mixed in mosquitoes and the resulting progeny were cloned and characterised for their enzyme type and their phenotypic response to pyrimethamine. It was found that not only had the two enzyme isoforms recombined, but that pyrimethamine susceptibility segregated independently (Walliker et al., 1975a). This showed that crossing between gametes, and recombination between the parental characters had occurred.
The production of heterozygotes (in the oocyst or zygote stage in the mosquito) between two heterologous malaria parasites has been shown to occur experimentally with P. falciparum (Ranford-Cartwright et al., 1993). This study involved the typing of alleles of msp-1 and msp-2 from individual oocysts dissected from mosquitoes that had fed on a mixed blood infection of clones 3D7 and HB3. It was found that some oocysts contained alleles exclusively from HB3, some contained alleles only from 3D7, and the remainder of the oocysts contained alleles from both parents, and were therefore hybrids - the products of fertilisation between the two different parental strains. The proportion of the homozygous and heterozygous forms was consistent with random fertilisation between parents (Ranford-Cartwright et al., 1993). Evidence for intragenic recombination resulting in novel alleles of genes was demonstrated by the production of a novel msp-1 gene resulting from a crossing over event in a conserved region of the gene (Kerr et al., 1994). The possibility of intragenic recombination during mitosis has also been proposed, based on studies of the gene encoding circumsporozoite protein (csp) (Rich et al., 1997). The inheritance of the 6 kb element appears to follow the same pattern as other mitochondrial genomes in eukaryotes – it is inherited from the female parent only (1994; Creasey et al., 1993). The complete genome of Plasmodium falciparum (clone 3D7) was published in 2002, and heralded the “post-genomic” age of malariology (Gardner et al., 2002). The genome presents researchers with a powerful new resource with which to work toward new drug and vaccine development. |
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