Geneviève Lanotte (retired), Jean-Antoine Rioux (retired), and Jérémy Bouyer from CIRAD recount that cellular fusion in Leishmania was filmed in the 80s and provide the sex movie to the scientific community.
The title of our comment refers to the humorous title of the Review by Rougeron et al. (2010) on the sexuality of Leishmania: Everything You Always Wanted to Know about Sex (but Were Afraid to Ask)” in Leishmania after Two Decades of Laboratory and Field Analyses. In this paper, the authors present recent works challenging the dogmatic hypothesis of “dominant and ancient clonal reproductive mode”. Amazing animations already exist to illustrate the clonal reproduction cycle of Leishmania. Rougeron et al. (2010) refer to population genetics and experimental works but point out the “lack of clear evidence (experimental or biological confirmation) of sexuality in Leishmania parasites” and suggest using high-resolution imaging to capture a mating event. But what if a sex movie was already available?
Before satisfying our curiosity, let’s revisit some concepts. A species is a transient state resulting from an equilibrium between two diversities, genetic (intrinsic) and environmental (extrinsic). It is a tradeoff between stability and evolution, conditioning its adaptability to environmental variations, a real dilemma for naturalists. For most systematicians, sexuality –in the sense of sex– is seen as the major driver for speciation, i.e. for creating and maintaining this “long duration stasis” that we call a species. The process starts with the fusion of gametes ensuring the genetic mixing of genes within the population.
Coming back to Leishmania, where clonality is the dominant paradigm, the representation of the biological species becomes more complex, hence the interest to identify conjugation sequences, i.e. a sexual cycle. Demonstrating a cellular mating process, punctual or total, is not that easy in Leishmania and can become an obsession… Already known in other Protista, the cells concerned should behave like gametes i.e. own their structural and functional characteristics. Eventually, the observation would lead to the observation of a single fusional cell, i.e. a “zygote”. At the end of the 80s, G. Lanotte accomplished this extraordinary observation! How was it possible?
First, it was necessary to remind the ancestral parasitological stage of Kinetoplastida, which probably had a monoxenic cycle, in a phytophageous or saprophageous Arthropod and with an oro-fecal transmission of promastigotes or infesting cysts originating from the fecal bulb and evacuated in the post-prandial drop. But these primitive Kinetoplastida evolved with the apparition of hematophagy, and some promastigotes migrated from their rectal location to the middle gut, oesophagus and finally the mouth parts. Their transformation into infecting (Meta) promastigotes was concurrent of a break in their replication, at least in the Arthropod. In the cutaneous tissue of their vertebrate host however, the pathogenic amastigotes started their iterative replication again, after penetrating a macrophage. But in this complex cycle, if there is a cellular fusion somewhere in Leishmania, it is in the mid gut of the vector and from gametogenic promastigotes… It was very difficult (but still possible) to observe cellular fusion L. infantum strains isolated from vertebrates (dogs and humans). But things changed when looking at strains of L. tropica isolated from sandflies (Phlebotomus sergenti) captured at Foum Djemma, Azilal province, Morroco (ISER/MA/89/LEM/1685): observing a mating event then became common. The movie presented here is a less than two min extract of a total sequence of cellular fusion that lasted 55 min. Details on the conditions allowing to film these events are available in Lanotte & Rioux (1990).
Let’s describe this sex party. For an expert eye, promastigotes that are about to merge are very specific, with a pyriform shape, the insertion of the flagellum at the slender side of the body and, above all, thanks to their saccadic moves. Because of these moves, they were irreverently named “fornicomastigotes”. The two partners look very similar but on the side opposite to the flagellum, one is flat whereas the other presents a protuberance that might serve to fix the inermous partner. When the cellular contact is established, the protuberance disappears, the movements of the flagellum accelerate and the fusion starts. A hyaline layer appears between the cells, thickening progressively while short parallel spans appear to fix to cell membranes. After some minutes, the cell membranes in contact are lysed to form one cell while the flagellums keep moving. The fusion of fornicomastigotes generate a zygotic biflagellous cell that is called a “zygomastigote”. The movements of the flagellums then slow down and they thicken in loops so that the zygomastigote take a funny “mickey mouse” shape.
Yes, Leishmania has a sexual life: enjoy! But what is the evolution of this zygomastigote and what is the importance of sexuality in Leishmania? It remains to be discovered…
Dr. Geneviève Lanotte and Pr. Jean-Antoine Rioux are retired specialists of Leishmania and their sandflies vectors. Pr. Jean-Antoine Rioux was the director of the “Jardin des Plantes de Montpellier” from 1977 to 1994. Dr. Jérémy Bouyer is a specialist of tsetse flies control, vectors of African trypanosomes, working at UMR CMAEE CIRAD-INRA “Control of exotic and emerging animal diseases”, Montpellier, France and based at ISRA-LNERV, Dakar, Sénégal.