Reproductive Strategies of the Malaria Parasite

Abstract

In spite of its tiny size and its apparent simply life cycle, several recent studies have shown that malaria parasites and their close relatives are able to modify in a relatively complex way their reproductive strategies. Malaria parasites are able to change population sex ratios to optimise reproductive success according to postulates of ‘local mate competition’ theory but also to assure fertilisation following predictions of ‘fertility insurance’ hypothesis. In addition, other potential reproductive strategies of these parasites, as multiple erythrocytic infections, are more probably a defensive mechanism of hosts than a parasitic adaptation. Malaria parasites adapt to environmental circumstances including hosts defences and number of competing parasite lineages present in the host to maximise reproductive and transmission success. The knowledge of these adaptive mechanisms will allow advances in the fight against malaria parasites.

Key Concepts:

  • Malaria parasites adapt to hosts responses and parasite competition by changing sex ratios in a way that maximise reproductive and transmission success.

  • Multiple erythrocytic invasions are not a parasite reproductive strategy but could be a defensive mechanism of hosts.

Keywords: fertility insurance hypothesis; Haemoproteus; local mate competition; multiple invasions; Plasmodium; sex ratio

Figure 1.

Life cycle of Plasmodium parasites. Parasite stages in the vertebrate host: A, sporozoites; B, schizont (infected liver cell); C, rupture of schizont; D, throphozoite; E, schizont; F, macrogametocyte and G, microgametocyte. Parasite stages in the invertebrate host (vector): H, macrogamete; I, microgametes (exflagellation); J, ookinete; K, oocyst and L, release of sporozoites.

Figure 2.

(a) Plasmodium falciparum parasites growing inside host erythrocytes. Multiple infections in the same erythrocyte are common. (b) The malaria‐like parasite Haemoproteus infecting bird erythrocytes. The male (up) and female (down) gametocytes are easily recognised due to staining and structural differences. Bird erythrocytes are nucleated.

Figure 3.

Ornithophilic biting midges Culicoides. These insects are considered the main vectors of Haemoproteus parasites.

Figure 4.

Three different hypotheses have been proposed to explain the occurrence of multiple gametocyte invasions (MGIs) in malarial parasites and related haemosporidians. (a) According to the parasite adaptive hypothesis, parasites favour the occurrence of MGIs composed by gametocytes of different sexes (represented using different colours and forms in the figure) to facilitate parasite encounter and fertilisation in the insect vector. (b) According to the traditional or ‘random’ hypothesis, the occurrence of MGIs is due to a random process associated with the parasite density of infection. (c) According to the host adaptive hypothesis, host immune system is capable to favour the occurrence of MGIs by crosslink of parasites by antibodies. In all cases, red blood cells are represented as empty circles. Adapted from Martínez‐de la Puente and Merino .

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Further Reading

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Merino, Santiago, and Martínez‐de la Puente, Josué(Oct 2010) Reproductive Strategies of the Malaria Parasite. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022860]