My
research focuses on the origins of insect species and the processes by
which they diverge. I am interested mainly in the great diversity
of parasitic insects and the role of their highly specific host
associations in their speciation.
The main
focus of my PhD research has beenDiachasma alloeum, a
parasitoid wasp (left) that lays its eggs in
the developing larvae of Rhagoletis
pomonella. Wasps in this genus attack
several different R. pomonella
fly
populations, including those that infest apples, hawthorns, blueberries
and snowberries. The apple and hawthorn populations of R.pomonella
are thought to be in the process of splitting into two species under
sympatric geographic conditions, and the other fly hosts are thought to
have undergone the same process in the past. My research asks
the following question: has the divergence of these
wasps' fly hosts
driven
a sequential, or "cascading" host race formation in D.
alloeum? Several pieces of evidence suggest that the
answer is yes:
First, eclosion
of wasps in each host-associated population closely tracks eclosion of
their respective fly hosts, meaning that the wasps are partially
allochronically
isolated from one another. Second, evidence suggests that the two
populations of
wasps respond differently to fruit odors, both showing preference for
their natal fruit odor and (in some cases) avoiding nonnatal odors.
Third, my preliminary data suggests that significant
microsatellite frequency differences define host-associated populations.
Finally, measurements of a number of morphological characters suggest
that certain features, such as ovipositor length, may be responding to
selection based on fruit size, fruit hardness and larval depth.
In
collaboration with Dietmar Schwarz, I am also investigating the origins
and current status of Diachasma
wasps attacking the hybrid-origin "Lonicera fly," another
relatively young species (or host race) of Rhagoletis.
Rhagoletis
pomonella and habitat avoidance
Rhagoletis
pomonella
is a "true" fruit fly that has formed host races in response
to the
introduction of a novel host. Host races represent a first step
towards
formation of a new species.
Adult R.
pomonella flies
(shown at right on an apple)
locate hosts using visual and olfactory cues, mate on or near the host
fruit, and oviposit
eggs under the fruit's skin. Maggots
develop, crawl out of the abscised fruit, and overwinter in the
soil. The
ancestral host of R. pomonella was the fruit of hawthorn
trees (Craetagus
spp.). However,
with the introduction of domestic
apples (Malus pumila) from Europe more than 200 years ago, some R. pomonella individuals began to
oviposit into this novel host. Over time, this shift has led the
formation of host races.
The apple race of R. pomonella
is different from the hawthorn race
in several ways. First, eclosion of apple flies is
temporally earlier in the year, a difference that reflects the earlier
fruiting phenology of apple trees. Second, allozyme data show
frequency differences between the races at several sites. Third,
apple and hawthorn
flies respond positively to the odor of chemical volatiles emanating
from the surface of their natal fruit and are actually antagonized by
non-natal volatiles, responses which may serve to keep the two races
from interbreeding.
My current Rhagoletis pomonella
research continues to explore the role of these chemical volatiles in
host
recognition and the speciation process. Flight tunnel tests
suggest that F1 hybrids may not respond to any natural volatile signals
and therefore may have a lowered fitness with respect to the parental
generation. One plan, therefore, is to examine host choice by F1
hybrid flies in nature by way of mark-release-recapture studies.
Dr. Feder and I have also developed our findings on odor recognition
into mathematical models relating habitat choice to sympatic
speciation.
My work has also looked into the responses of resident fly
populations to different combinations of olfactory and visual cues,
which may advance
our understanding of the ecological forces keeping the two races
apart.
Apple maggot genomics
...is in its infancy. Despite the pressing need for a
genome project that will one day assist in demonstrating the genetic
controls for diapause and odor recognition in R. pomonella, we are not yet to
that point. We have, however, begun contruction of dual sequence
libraries that will help to get us closer to these goals: a
454-sequencing-based library and an EST-based approach. This
project is part of a collaboration with Stewart
Berlocher and Dietmar
Schwarz of the University of Illinois.
Other
Feder lab people include Tom
Powell (host race formation of R.pomonella
infesting southern
hawthorn species), Sheina Sim (Rhagoletis
species comparisions) and Tracy Arcella (population genetics and
conservation biology of the Rusty Crayfish, Orconectes rusticus). And
Jeff is always looking for fresh blood
(read: grad students) so apply if interested.
Please feel free to contact me
with any questions you may have: aforbes at nd.edu.
Andrew
A. Forbes
Adult R.
pomonella flies
(shown at right on an apple)
locate hosts using visual and olfactory cues, mate on or near the host
fruit, and oviposit
eggs under the fruit's skin. Maggots
develop, crawl out of the abscised fruit, and overwinter in the
soil. The
ancestral host of R. pomonella was the fruit of hawthorn
trees (Craetagus
spp.). However,
with the introduction of domestic
apples (Malus pumila) from Europe more than 200 years ago, some R. pomonella individuals began to
oviposit into this novel host. Over time, this shift has led the
formation of host races.