Population genetic diversity of Schistosoma japonicum arises from the host switching in the life cycle

Abstract

Background: Schistosoma japonicum is a multi-host parasite, including asexual amplification in snail hosts and sexual reproduction in mammalian hosts. The genetic diversity of S. japonicum by host switching is less understood, which could help elucidate the genetic evolution of S. japonicum under host pressure and provide instruction for host sampling and the infection pattern to make S. japonicum infection models.

Methods: Different developmental stages of S. japonicum were collected and genotyped with 24 microsatellite loci, including 345 cercariae from naturally infected snails and 472 and 540 adult worms from artificially infected mice and rabbits, separately. The genetic distribution of S. japonicum within and among hosts by different sampling was assessed, and the genetic diversity and population structure were calculated at different population levels during host switching.

Results: Seven cercariae were the minimum sample size to retrieve 85% of alleles for S. japonicum in each snail, and meanwhile, sampling parasites from 19 snails could recover 85% of the total Na of S. japonicum in all snails in this study. After infection in mice and rabbits, 8 worms per mouse and 76 worms per rabbit were the minimum samplings to retrieve 90% of alleles from each corresponding definitive host. Further, 16 mice and 2 rabbits were the least sampling size to recover 85% of the total Na of S. japonicum in all mice and rabbits, respectively. Although no significant difference was shown for S. japonicum between mice and rabbits at the suprapopulation level, it is clear that the genetic diversity of worms from 20 (or 40) mice was significantly higher than that from 1 (or 2) rabbits, especially when the host sampling was not sufficiently enough. The differentiation of worms at the infrapopulation level among mice is less than among rabbits. In addition, genetic differentiation was shown between cercaria and adult worms, which was considered to relate to allele loss after host switching.

Conclusions: The population genetic diversity of S. japonicum differs in different developmental stages. Host species and sampling number significantly affect the distribution pattern of alleles and the genetic structure of S. japonicum at the suprapopulation level.

Fig 1. The number of sampling cercariae and snails affected the coverage of alleles.

(a) Thirty-seven curves represent Na of cercariae in corresponding 37 snails. The red dotted line shows 85% of the total Na for all 37 snails, and the black dotted line shows 50%. Ten cercariae per snail were detected, except 7 cercariae in 5 individual snails because of the sample limitation. S33-S82, the serial number of the infected snail we collected. (b) Twenty-four curves represent Na of cercariae on corresponding 24 loci. The red dotted line shows 85% of the total Na for 23 loci (except p6), which reached the plateau; the black dotted line shows 50% of the total Na. The standard deviation of Na among replicates for each locus progressively decreased with the increased number of snails.

Fig 2. The sampling size of definitive hosts and worms affects the coverage of alleles.

(a) Effect of worm sampling size in each mouse. M97-M150, the serial number of the mouse we infected. Forty-eight curves represent Na of worms of 48 mice. The red dotted line shows 90% of the total Na, and the black dotted line shows 50% of the total Na in each mouse. Ten worms per mouse were detected, except 7 worms in 1 mouse (M141) because of the sample limitation. (b) Effect of mouse sampling size in each locus. Twenty-four curves represent Na of worms on corresponding 24 loci. The red dotted line shows 85% of the total Na and the plateau, and the black dotted line shows 50%. (c) Effect of worm sampling size in each rabbit. R4-R6, the serial number of the rabbit we infected. Three curves represent the Na of worms from the corresponding 3 rabbits. The red dotted line shows 90% of the total Na in each rabbit, and the black dotted line shows 50% of the total Na. (d) Effect of rabbit sampling size in each locus. Twenty-four curves represent Na of worms from rabbits on corresponding 24 loci. The dotted line shows 85% of the total Na for all loci except p14, p60, and p2.

Fig 3. Genetic diversity of worms from different mice and rabbits sampling sizes.

1 R, one rabbit was selected; 2 R, two rabbits were selected; 20 M, 20 mice; 40 M, 40 mice.

Fig 4. Genetic differentiation of S. japonicum from different hosts.

(a) Genetic diversity of S. japonicum from different hosts. Each dot represents the genetic diversity of S. japonicum on each locus. The horizontal line in the middle represents the average value. (b) AMOVA analyses of different populations. “Between populations” represents molecular variance between two schistosome populations from corresponding hosts, “Among Individual hosts” represents molecular variance among individual hosts for the same schistosome population, and “Within Individual hosts” represents the molecular variance of S. japonicum infrapopulation within individual hosts.

Fig 5. Clustering for all samples using STRUCTURE.

(a) The ln likelihood [Pr (XIK)] for a given number of K over 10 times. (b) ΔK trend. (c) Population genetic structure of all samples when the estimated number of clusters K=2, 3, or 4.

Fig 6. Cluster analysis of S. japonicum from different hosts.

(a) PCoA analyses of S. japonicum from different hosts. (b) UPGMA tree of S. japonicum from different hosts based on Nei unbiased genetic distance. S33-S82, the serial number of the infected snail; M97-M150, the serial number of the mouse infected; R4-R6, the serial number of the rabbit infected.

Fig 7. Private alleles in cercariae and adult worms from different species of hosts.

(a) Private alleles of cercariae but not in worms from mice and rabbits with no base deviation. Each dot represents a private allele. “cercariae vs adults in mice” means the alleles present in cercariae but not in the adult worms from mice; “cercariae vs adults in rabbits” means the alleles present in the cercariae but not in adult worms from rabbits; “overlap” means the alleles present in cercaria but both lost in adult worms from mice and rabbits. (b) Private alleles of cercariae but not in worms from mice and rabbits with the permission of one repeat deviation of microsatellite locus. (c) Private alleles of cercariae but not in worms from mice and rabbits with the permission of two repeats deviation. (d) Number of private alleles of adult worms among different sizes of hosts with the permission of no base, one repeat, and two repeats deviations. Each dot represents the number of private alleles on a locus.