The enormous morphological and life history diversity represented within the insects is astounding and we use a variety of physiological, biochemical, and evolutionary ecology techniques to understand the mechanisms generating this diversity. Current topics of interest in the lab include:
- the physiology of phenotypic plasticity, particularly with respect to diapause and reproduction,
- the physiological regulation of nutrient metabolism and storage and the impacts of storage on life history traits such as survival and fecundity,
- insects as model systems to study the physiology of human diseases such as diabetes, obesity, and infertilit
Some examples of current and past projects appear below; contact Dan Hahn for more information.
Regulation of nutrient storage and metabolism in diapausing flesh flies
Above: One of our favorite beasts, the flesh fly Sarcophaga crassipalpis. The interesting diapause physiology of this species makes it a nice alternative to the traditional rodent models of diabetes and obesity pictured below.
Almost all insects face environmental hardships such as extremes in temperature, humidity, or food limitation that make some portion of the year unsuitable for growth or reproduction, and most insects deal with these unsuitable periods by entering an environmentally-induced state of dormancy known as diapause. Diapausing insects are in a state of developmental arrest and often have depressed metabolic rates. However, diapasue periods can be long and food scarce or unavailable. Therefore, some insects accumulate additional nutrient reserves, including fat, amino acids, and carbohydrates, to sustain themselves through the diapause period. The facultative induction of diapasue and the associated shift in nutrient storage and metabolism by environmental cues makes diapasue an excellent model system to study both the physiology of phenotypic plasticity and the physiology of nutrient storage and metabolism. In addition, because many basic metabolic processes are broadly conserved between insects and vertebrates, diapause-associated plasticity in nutrient storage in insects is a useful alternative to the traditional vertebrate models used to study basic physiological processes in diabetes and obesity.
We are currently studying diapause-associated plasticity in fat and protein reserves in the flesh fly, Sarcophaga crassipalpis. Day-length is the primary cue these flesh flies use to determine whether they will enter diapause. Larvae reared under short-day conditions accumulate significantly greater lipid reserves in preparation for the pupal diapause period and we have shown that they do so by growing both larger and fatter. We are exploring how entering the diapause program causes these individuals to alter their body size and fat storage set points. We would also like to understand how size and fat storage affects survival during diapasue and post-diapause reproduction. Of particular interest is the roles played by insulin and glucagon-like adipokinetic hormone signaling both in producing larger and fatter diapause-destined individuals during larval development and in the utilization of stores during diapause.
Diapause physiology and speciation in the apple maggot
The apple maggot, Rhagoletis pomonella, has been a premier model system for studying speciation for decades. Hawthorns are the historical host plants for R. pomonella where adults mate and lay their eggs on the fruits and the larvae develop within. However, sometime after the introduction of apples into the United States in the 1800’s this species underwent a host shift onto apples followed by significant genetic divergence between populations using apples and hawthorns. In addition, the life histories of the two populations have diverged synchronizing each population with the fruiting phenology of their host plant. At sympatric sites apples fruit several weeks earlier than hawthorns and individuals from the apple host race both eclose and complete development significantly earlier than the hawthorn race. Both populations of R. pomonella are functionally univoltine and undergo an obligate diapause every winter. Therefore, apple race individuals enter diapause earlier in the summer than hawthorn race individuals and are exposed to higher temperatures for a longer time prior to the onset of winter. This difference in pre-winter conditions has lead to the evolution of a deeper or more refractory early diapause in the apple race. In collaboration with Jeff Feder’s lab at Notre Dame and Dave Denlinger’s lab at Ohio State, we are examining the physiology of the early diapause period in the two races. We predict that the apple race has evolved one of three strategies to deal with the hotter, more metabolically demanding conditions they experience early on in the diapause period:
- apple individuals store more reserves,
- apple individuals utilize their stores more economically by greater metabolic rate suppression,
- a combination of both. If the two host races differ in nutrient storage or metabolism, we will examine the mechanisms underlying these differences.
Our lab has a long-standing interest in ants and phytophagous insects, particularly grasshoppers and moths, as model systems for physiology and evolutionary ecology. Some examples of past work are: the evolutionary ecology of diet preferences in tropical arboreal and ground-nesting ants (Hahn and Wheeler 2002), protein storage and colony founding strategies in desert harvester ants (Hahn et al. 2004), the evolution of fat storage tactics in desert carpenter ants, and feeding nutrient storage and utilization in the grasshopper Schistocerca americana (Hahn and Wheeler 2004).