Parasitoid
wasps are good for examining the ecological and evolutionary patterns that
spring from close interspecific interaction. Currently we are studying
the phylogenetic pattern of cryptic (sibling) species. That is, groups of
closely
related
species
that
do not
interbreed,
but are morphologically and ecologically very similar. Based on molecular and
morphological phylogenetic analysis as well as behavioral studies, we have
found about 15 species of Cotesia parasitoids
exclusively using the checkerspot butterfly genera Melitaea and Euphydryas in
Eurasia and North America. Most of the Cotesia fall into two cryptic species
groups.
Kankare,
M., Stefanescu, C., van Nouhuys, S., Shaw, M. R. 2005. Host
specialization by Cotesia wasps
(Hymenoptera: Braconidae) parasitising species-rich Melitaeini (Lepidoptera:
Nymphalidae) communities in north-eastern Spain. The Biological Journal
of the Linnaean Society, 86: 45-65 PDF
Kankare, M., van Nouhuys, S. Hanski, I. 2005. Genetic
divergence among host-specific cryptic species in Cotesia melitaearum agg.,
a parasitoid of checkerspot butterflies. Annals of the Entomological Society
of America, 98: 382-394 PDF
Two
radiations of Cotesia on checkerspot hosts For organisms
such as parasitoids, that have strong interactions with their hosts,
formation
of cryptic species groups may be driven by competitive exclusion.
Evidence of this is observation that no Cotesia within a cryptic
species group share a host species. Because the ancestors of the two cryptic
species
groups probably colonized the checkerspot butterflies independently, comparative
studies may allow us to get at the process of species group formation.
Mechanisms
of co-existence
Two very similar species cannot share a limiting resource, which may explain
why we have found no examples of Cotesia from a single cryptic
species group using the same host butterfly species. However, there are
three examples of
two Cotesia species from different cryptic species groups sharing
a host butterfly (3 butterfly species out of 18), even in the same locale.
This leads us to
ask what allows their coexistence. We are conducting laboratory competition
experiments using two contrasting species pairs. Their coexistence can
differ if the wasps differ in immature aggression,
phenology, voltinism, host immune
response,
and control
of host phenology. Additionally, each of the two Cotesia groups
is primarily associated with host butterflies feeding on plants with slightly
different
chemical defense. We will test the hypothesis, using laboratory experiments,
that each group is better adapted to hosts sequestering one or the other
class of plant defensive chemicals
