The parasite goes away, the parasite comes back!

When we think about parasites, the initial image from conventional wisdom is about linear relationships between two or three organisms, usually the parasite itself, the intermediate and the final hosts. The life story of trypanosomatid parasites belonging to the genera Trypanosoma and Leishmania are archetypical of these events: they cycle back and forth through invertebrate and vertebrate hosts. The concept of a parasitic world would be less complex if the relationships between parasites and hosts were based on organisms with linear behaviour and constant physiological status, but voilá!, there come the plants and their wealth of metabolites, versatile molecules and above all the plentiful and unpredictable interactions with the environment. To further complicate matters, protozoan and fungal parasites may interact simultaneously with both plants and animals and each of these hosts can induce a different and striking response to the invasion and subsequent perturbation of the metabolic equilibrium by the parasite.

Two papers in the February 2015 issue [110(1)] of Memórias do Instituto Oswaldo Cruz contribute with more details on these complex parasite relationships: in the first one by, Medina et al. (2015), a trypanosomatid plant parasite of the genus Phytomonas is challenged by a defensive response from an infected tomato plant (Solanum lycopersicum). This plant species is one of several economically important crops that Phytomonas sp. have been infecting for hundreds of millions of years since the split from a common ancestor with Leishmania, according to a dated phylogenetic tree of the Trypanosomatidae family (Lukeš et al. 2014). The paper is concerned with the role played by secondary metabolites from the plant in the possible modulation of this evolutionary game: the higher the parasite bet (successful infection, replication and survival) the greater the prospect of a positive return rate for the plant defenses (more biochemical innovation through specialised bio-synthetic routes).

In the other paper, a practical consequence of the concatenated network of a host-parasite interaction is exposed through research on plant-fungi parasite relationships and their secondary metabolites (Campos et al. 2015). In this work, a promising anti-Trypanosoma drug candidate is isolated from a fungi that develops its life cycle in a complex organism (Caesalpinia echinata Lam., a.k.a pau brasil or Brazilwood, the plant species whose reddish trunk inspired the name of the newly discovered land by the Portuguese explorers in 1500 and that also was the “first commodity” to be exported from Brazil in the second half of the XVI century).

These results suggest that the analytical approach of a biological system in which a parasite that lives on another organism that depends on other parasites is akin to the theoretical framework of the hypercycle principle, which was proposed by Eigen and Schuster (1977).

Adeilton Alves Brandão | Editor


Campos FF, Sales Jr PA, Romanha AJ, Araújo MSS, Siqueira EP, Resende JM, Alves TMA, Martins-Filho AO, dos Santos VL, Rosa CA, Zani CL, Cota BB 2015. Bioactive endophytic fungi isolated from Caesalpinia echinata Lam. (Brazilwood) and identification of beauvericin as a trypanocidal metabolite from Fusarium sp. Mem Inst Oswaldo Cruz 110: 65-74.

Eigen M, Schuster P 1977. The hypercycle. A principle of natural self-organization. Part A: emergence of the hypercycle. Naturwissenschaften 64: 541-565.

Lukeš J, Skalický T, Týč J, Votýpka J, Yurchenko V 2014. Evolution of parasitism in kinetoplastid flagellates. Mol Biochem Parasitol 195: 115-122.

Medina JM, Rodrigues JCF, Moreira OC, Atella G, de Souza W, Barrabin H 2015. Mechanisms of growth inhibition of Phytomonas serpens by the alkaloids tomatine and tomatidine. Mem Inst Oswaldo Cruz 110: 48-55.