phenotypes’, can be used to symbolize the state of the two arms races.
Behavioural adaptations may reinforce host specificity, e.g. by eliminat-
ing those host species whose behaviour or other characteristics do not
correspond with those of the infective stages of the parasite. For example,
each species ofSchistosomahas a shedding pattern that is adapted to
the behaviour of the host. A stable polymorphism can be established
when the genetic variability of hosts and parasites permits lateral transfer
(e.g. in Schistosoma mansoni in humans and black rats); and this
may represent the beginning of reproductive isolation and alloxenic
speciation. Combes and Théron (2000) also suggest that gene flow may
be restricted between parasites inhabiting different microhabitats of the
same host species (synxenic speciation), for example between congeneric
trematodes living in different parts of the digestive tract of the same host
species.
Concerning the second mechanism, habitat compatibility, Combes
and Théron (2000) emphasize that parasites will find it difficult to invest
in evasion mechanisms against several different host defences, which
may explain adaptation to certain hosts. An example is the association
between the parasitoid wasp Leptophilina houlardi and Drosophila
melanogasterin different regions. In an area where the wasp attacks five
instead of a single species ofDrosophila, the defence reactions of the flies
to the parasitoid are much more effective.
The question of why some parasites are much more specific than
closely related species cannot be answered. Reasons may be different
durations of the parasite–host association or the extreme sensitivity of the
balance between benefit and constraint to slight changes in selective
pressures.
Most of the arguments given by Combes and Théron (2000) are
convincing. However, it must be stressed that evidence for synxenic
speciation of parasites occupying different microhabitats on or in
vertebrates is not available. In a study using molecular techniques,
Littlewoodet al. (1997) found that polystome monogeneans inhabiting
the same microhabitat (urinary bladder/cloaca or pharynx/mouth cavity)
of different host species are more closely related to each other than
polystomes infecting different sites in the same host species. This suggests
that speciation has been alloxenic. On the other hand, the studies by
Tauber and Tauber (1977a,b) and Tauberet al. (1977) on plant-parasitic
insects have demonstrated reproductive isolation as the result of few
mutations and habitat/seasonal isolation, lending support to the view that
sympatric speciation occurs.
The mating hypothesis can be further generalized by including
additional niche parameters, such as seasonality, host age, sex of hosts,
ecological specificity (adaptations to macroenvironmental factors of the
host) and geographical isolation.188 K. Rohde