Typically, we think of environmental fluctuations occurring on two
scales: short-term, such as day-to-night transitions or weather
variations, or long-term, such as seasonal transitions. However,
insects are also subject to spatial variation, and can experience
rapid environmental changes simply by moving about their habitat. In
the case of more immobile insects, we might expect to see two
populations living within a particular area to experience vastly
different environmental conditions just due to variation that exists
between microhabitats. Together, seasonal and habitat fluctuations
interact over an insect’s lifespan affecting everything from
developmental timing to various aspects of their physiology, such as
acute stress response and nutrient acquisition and utilization. In
this dissertation, I address how seasonal and environmental factors
affect the development, physiology, and stress-response of a
temperate and a polar insect.
The metabolic gene phosphoenolpyruvate carboxykinase (Pepck)
has been routinely associated as one of the underlying genetic
components of insect diapause and stress response. Previously, it was
unclear which isoform of Pepck, mitochondrial (Pepck-M)
or cytosolic (Pepck-C), was participating in diapause and in
response to stress. In Chapter 2, I cloned cDNA sequences for both
isoforms, and using duplex qPCR measured gene expression throughout
development and in response to the stresses of cold shock and
starvation in adults of the temperate flesh fly, Sarcophaga
bullata. I found that each isoform exhibits unique expression
patterns throughout development and during diapause, as well as in
response to stress. Pepck-M expression was highest in
wandering 3rd instar larvae and in diapausing pupae, and
was unresponsive to stress. Pepck-C had its highest expression
levels in diapausing pupae, as well as in response to cold shock and
starvation. From this, I concluded that Pepck isoforms have
distinct roles in meeting the metabolic demands of a developing
insect, and in response to acute stress.
In Chapter 3, I set out to elucidate which environmental cues program
seasonal metabolic depression in the Antarctic midge, Belgica
antarctica. Individuals were collected from a single field
population, and upon being brought into the lab were separated into
several treatment groups corresponding to a particular environmental
cue: photoperiod, hydration state, and temperature. Over the course
of 3 months I measured the metabolic rates of all treatment groups
and the original field population. During the final two weeks of
measurement, all treatment groups and the field population entered a
metabolically depressed state, irrespective of the environmental cues
they were receiving. While most insects rely on external
environmental cues to enter metabolic depression prior to winter, my
data indicates that B. antarctica is instead utilizing an
as-of-yet unknown internal cue in order to trigger seasonal metabolic
depression.
Chapter 4 addresses how microhabitat influences the development and
physiology of populations of B. antarctica. For 5 populations
(4 of which were on the same island), I recorded microhabitat
temperature, larval metabolic rates, larval mass, adult emergence,
and larval macronutrient content over the course of 4 months.
Microhabitat temperature was vastly different between sites,
resulting in differential accumulation of degree days, a measure of
accumulated heat units, between the 5 populations. Adult emergences
were asynchronous between most sites sampled, in some cases showing
no overlap with other sites, indicating differential developmental
rates. Metabolic rate and mass were also different between sites,
suggesting an effect of microhabitat on larval size and providing
further evidence for varying developmental rates among populations.
Macronutrient content was also dissimilar among sites, with
populations accumulating different amounts of lipid, carbohydrate,
and protein. Although we often group individuals and populations of a
species into one larger population, I show here that for immobile
insects like B. antarctica there exist significant differences
between populations that are a product of interaction with their
microhabitat.