Disentangling hybridization and host colonization in parasitic roundworms of humans and pigs

Abstract

Knowledge of cross-transmission and hybridization between parasites of humans and reservoir hosts is critical for understanding the evolution of the parasite and for implementing control programmes. There is now a consensus that populations of pig and human Ascaris (roundworms) show significant genetic subdivision. However, it is unclear whether this has resulted from a single or multiple host shift(s). Furthermore, previous molecular data have not been sufficient to determine whether sympatric populations of human and pig Ascaris can exchange genes. To disentangle patterns of host colonization and hybridization, we used 23 microsatellite loci to conduct Bayesian clustering analyses of individual worms collected from pigs and humans. We observed strong differentiation between populations which was primarily driven by geography, with secondary differentiation resulting from host affiliation within locations. This pattern is consistent with multiple host colonization events. However, there is low support for the short internal branches of the dendrograms. In part, the relationships among clusters may result from current hybridization among sympatric human and pig roundworms. Indeed, congruence in three Bayesian methods indicated that 4 and 7% of roundworms sampled from Guatemala and China, respectively, were hybrids. These results indicate that there is contemporary cross-transmission between populations of human and pig Ascaris.

Estimation of population structure in coastal Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. menziesii] using allozyme and microsatellite markers

“…They are independent of the amount of genetic variation and therefore suitable for comparisons between studies that employ different genetic markers. Although these measures are relatively new, they were already used in a few recently published studies that, together with our study, validated their use (e.g., Criscione et al 2007;Kettle et al 2007;Langergraber et al 2007;Ryynänen et al 2007;Leinonen et al 2008). Standardized F ST was usually several times higher than unstandardized F ST for microsatellite loci (e.g., Criscione et al 2007;Jordan and Snell 2008;González-Pérez et al 2009).…”

“…Although these measures are relatively new, they were already used in a few recently published studies that, together with our study, validated their use (e.g., Criscione et al 2007;Kettle et al 2007;Langergraber et al 2007;Ryynänen et al 2007;Leinonen et al 2008). Standardized F ST was usually several times higher than unstandardized F ST for microsatellite loci (e.g., Criscione et al 2007;Jordan and Snell 2008;González-Pérez et al 2009). Standardizing of F ST became now a required procedure when different markers are compared or combined for analysis (e.g., Langergraber et al 2007;Ryynänen et al 2007).…”

Assessing the zoonotic potential ofAscaris suumandTrichuris suis: looking to the future from an analysis of the past

“…The close phylogeny between Ascaris in humans and pigs may reflect the complex evolutionary history of Ascaris (Loreille & Bouchet, 2003) and may be due to multiple host colonization events, as suggested by Criscione et al (2007) who found that worms assort first by geography (Nepal, China and Guatemala) and then by host origin. Sequence analysis of the cox1 gene (mitochondrial) indicates that Ascaris does not assort into two different monophyletic groups based on host origin, but instead splits into three or more groups, all but one of which include worms from both hosts (Nejsum et al, 2010;Betson et al, 2011;Zhou et al, 2011).…”

“…More recently, detailed genetic analysis (23 microsatellite loci) identified 4 and 7% of worms from China and Guatemala, respectively, as being hybrids (Criscione et al, 2007), suggesting that cross-infections and interbreeding can take place between the pig and human worm populations in areas where humans and pigs live in close proximity. Even though this study only included a limited number of worms (n ¼ 129), more research applying fine-scale genetic analysis to sympatric Ascaris populations is warranted in order to illuminate the zoonotic potential of A. suum in these settings.…”

Modelling the Effects of Mass Drug Administration on the Molecular Epidemiology of Schistosomes

“…Examples of where genetic data has already informed public health policy include a study on Ascaris in pigs and humans in Guatemala, where even though infections were sympatric, there appeared to be, using the molecular tools available at the time, little gene flow between the parasite populations indicating no transmission between the two host species (Anderson et al, 1993). Conversely two other more recent studies have indicated cross transmission between the Ascaris lumbricoides and A. suum species, with, furthermore, up to 4% and 7% of Ascaris appearing as hybrids, which raises a number of potential implications for long-term evolutionary dynamics (Criscione et al, 2007). Models have already been used in directly transmitted pathogens, including sexually transmitted diseases, where contact tracing data may not be complete, but where genome data can inform on infection networks (e.g.…”