The University of Texas at Austin
Laboratory of Mary Ann Rankin
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Our current research focuses on the physiological relationships between two life history characters, migration and reproduction, in an economically significant migratory grasshopper Melanoplus sanguinipes. Previous work has established a genetic basis for migratory behavior in this species, and has also revealed relationships between flight and reproduction that are dependent upon the performance of, not just the propensity for, migratory flight. In particular, long-duration flight accelerates onset of reproduction by ten days or more and enhances fecundity 2-20 times (mean of 6 times) over the entire life of the insect, an observation that challenges the conventional assumption that migration necessarily involves a reproductive cost. Our work should provide a deeper understanding of the endocrine and other physiological mechanisms behind these relationships and should clarify the adaptations that have made this species a successful colonizer and virulent agricultural pest. This work should also expand understanding of the potential for such adaptations in general.

The research addresses two general objectives:

1. To understand how performance of long flight interacts with the endocrine system to accelerate reproduction and enhance reproductive success over the entire life of the insect. The hypotheses to be tested under this objective are:

a. Long-duration flight causes the release or synthesis of neurohormones and/or neuromodulators that in turn cause changes in the pattern and/or level of juvenile hormone (JH) release and/or tissue sensitivity to JH thereby enhancing reproductive output

b. Neuropeptides released as a consequence of long flight directly affect fat body or ovary to accelerate and enhance oogenesis

c. Flight stimulates lipid mobilization for ovaries as well as flight muscle

d. Adipokinetic hormone (AKH) released during flight accelerates the final stages of oocyte development

2. To determine whether performance of long flight alters nutrient acquisition via increased mating (males contribute nutrient to females at mating via spermatophore transfer with sperm), or digestive efficiency. The hypotheses to be tested under this objective are:

a. Performance of long-duration flight causes an increase in mating behavior, possibly via increased JH titers or another neuroendocrine stimulus following long flight

b. Performance of long-duration flight improves food utilization, possibly via endocrine stimulation

Experiments will include monitoring levels of neurohormones produced in the brain neurosecretory cells and corpora cardiaca of M. sanguinipes after long-duration tethered flight using matrix assisted laser desorption ionization or high performance liquid chromatography with tandem electrospray ionization mass spectroscopy; JH, 20-hydroxyecdysone (20HE) and AKH titer determinations using radioimmunoassay; in vitro culture experiments; behavioral experiments; JH receptor binding experiments and experiments to determine the extent of food utilization.

Phenotypic correlations between life history characters can indicate shared physiological mechanisms and may reflect genetic and evolutionary relationships as well. A long-term goal of this research is to analyze the consequences and limits of the physiological relationships through genetic analysis. These experiments are proposed for future work because our first priority is to clarify the complex physiological links between flight and reproduction under standard conditions before trying to manipulate the system via selection or comparison of sedentary vs. migratory populations. Once the goals of this research are accomplished, a basis will exist for experiments to directly address the evolution of a colonizing life history. The results of this work should be of interest to population biologists studying life history theory and evolution, physiologists interested in understanding endocrine relationships in insects and physiological ecologists interested in resource allocation and coordination of life history characters.

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