0851996159

Secondly, the multiplication in the mollusc is, as far as we know, clonal.

This means that, while an individual larval stage has a unique chance

for meeting the right host, the genome it contains has thousands of

possibilities to ensure its transmission either at the same time or several

days or weeks later. For instance, the unique genome of a schistosome

miracidium can be spread in tropical water bodies for weeks (an infected

Biomphalariamollusc may shed some 100,000 cercariae over a period

of 2–3 months). A potential cost of this strategy is that, if several larval

stages having the same genome succeed in infecting the same host,

this may significantly reduce the genetic diversity within parasite

4 C. Combeset al.

Host time

Host space

USHDSH

USH

DSH

Host time

Host-Space

USHDSH

USH

DSH

Host time

Host space

USH DSH USH DSH

USH

DSH

Host time

Host space

USH

DSH

A B

C D

P

P

P

P

Fig. 1.1. Selective pressures accounted for behaviour of infective stages of

trematodes (USH, upstream host; DSH, downstream host). USH host time and USH

host space do not obviously coincide with DSH host time and DSH host space.

The figure shows the four possible selective pressures that may act on parasite

behaviour. In (A), there is no need for a particular behaviour. In (B), the infective

stages (P) are selected for to move into DSH host space. In (C), the infective stages

are selected for to move into DSH host time. In (D), the infective stages are selected

for to move into both DSH host space and DSH host time.