An infectious topic in reticulate evolution: introgression and hybridization in animal parasites

Abstract

Little attention has been given to the role that introgression and hybridization have played in the evolution of parasites. Most studies are host-centric and ask if the hybrid of a free-living species is more or less susceptible to parasite infection. Here we focus on what is known about how introgression and hybridization have influenced the evolution of protozoan and helminth parasites of animals. There are reports of genome or gene introgression from distantly related taxa into apicomplexans and filarial nematodes. Most common are genetic based reports of potential hybridization among congeneric taxa, but in several cases, more work is needed to definitively conclude current hybridization. In the medically important Trypanosoma it is clear that some clonal lineages are the product of past hybridization events. Similarly, strong evidence exists for current hybridization in human helminths such as Schistosoma and Ascaris. There remain topics that warrant further examination such as the potential hybrid origin of polyploid platyhelminths. Furthermore, little work has investigated the phenotype or fitness, and even less the epidemiological significance of hybrid parasites.

Figure 1

Fasciola specimens collected from goats in a province (Yen Bai, YB) of North Vietnam. The first three flukes (left to right) show the normal shape of Fasciola gigantica while the last individual resembles both the hybrid and F. hepatica. The figure shows that hybrids can look like one of the parents, thus illustrating the point that it is difficult to use parasite morphology to identify hybrids. Reprinted from Thanh et al. [32] with permission from Elsevier.

Figure 2

Demonstrating that fluorescence microscopy can identify trypanosome hybrids. Fluorescent proteins indicate the hybrid (yellow) or parental status (red and green) of trypanosome parasites. (a) Close proximity of red and green parental trypanosomes in salivary glands at early establishment. Flies dissected at 20 days. (b) Yellow hybrids with red and green parental trypanosomes in salivary glands. Flies dissected at 27 days. Trypanosomes are 20–30 μm in length. Reprinted from Gibson et al. [63], originally published by BioMed Central.

Figure 3

Development of Leishmania hybrids in Lutzomyia longipalpis and Phlebotomus paptasi. Infection rates and density of Leishmania major (MA), Leishmania infantum (IN), hybrid LEM4891 (H1) and hybrid LEM4833 (H3) in sand fly midgut on days 2, 7 and 10 p.i. Infections were classified into three categories: heavy (more than 1000 promastigotes per gut) – black bars, moderate (100-1000) – grey bars, light (1-100) – white bars. Numbers above the bars indicate the number of dissected females. (a) Development in L. longipalpis: the infection rate and the intensity of infection did not differ between Leishmania strains studied. (b) Development in P. papatasi: on days 7 and 10 p.i., the infection rate and the intensity of infection significantly differed between L. major, hybrids and L. infantum. Reprinted from Volf et al. [68] with permission from Elsevier.