Contrasting the distribution of phenotypic and molecular variation in the freshwater snail Biomphalaria pfeifferi, the intermediate host of Schistosoma mansoni

Abstract

Population differentiation was investigated by confronting phenotypic and molecular variation in the highly selfing freshwater snail Biomphalaria pfeifferi, the intermediate host of Schistosoma mansoni. We sampled seven natural populations separated by a few kilometers, and characterized by different habitat regimes (permanent/temporary) and openness (open/closed). A genetic analysis based on five microsatellite markers confirms that B. pfeifferi is a selfer (s≈0.9) and exhibits limited variation within populations. Most pairwise FST were significant indicating marked population structure, though no isolation by distance was detected. Families from the seven populations were monitored under laboratory conditions over two generations (G1 and G2), allowing to record several life-history traits, including growth, fecundity and survival, over 25 weeks. Marked differences were detected among populations for traits expressed early in the life cycle (up to sexual maturity). Age and size at first reproduction had high heritability values, but such a trend was not found for early reproductive traits. In most populations, G1 snails matured later and at a larger size than G2 individuals. Individuals from permanent habitats matured at a smaller size and were more fecund than those from temporary habitats. The mean phenotypic differentiation over all populations (QST) was lower than the mean genetic differentiation (FST), suggesting stabilizing selection. However, no difference was detected between QST and FST for both habitat regime and habitat openness.

Figure 1

Location of the seven populations of B. pfeifferi sampled in the Man area, western Côte d'Ivoire. The populations sampled in the Man district are indicated in the enlarged panel.

Figure 2

Schematic diagram of the experimental protocol conducted in the seven populations studied (more details in text). G_i refers to generation. Seven to twenty field-caught G₀ individuals per population were isolated. Sixty G₁ offspring were chosen at random (from 3 to 10 G₁ depending on sample size in natural populations), and set in three groups of 20 individuals on week 3. At week 6, 10 individuals were sampled per group (30 G₁ individuals per population), and isolated for 25 weeks (week 7–31) during which fecundity, growth and survival were monitored. At week 18, egg capsules were collected to monitor the hatching rate of G₂ individuals. One week after hatching (10–13 days), two G₂ were randomly chosen per G₁ snail, and they were isolated in boxes for 17 weeks and monitored up to sexual maturity in order to quantify some life-history traits (size and fecundity).