Phenotypic Engineering
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Phenotypic Engineering Age at Maturity

leptin injectionleptin pellet implant
Leptin Injection                                                    Leptin Pellet Implants

Variation in the age at which an individual begins reproducing (age at maturity, %) has important fitness consequences, and has been the focus of much life history theory. Traditionally, life history theory assumes that variation in life history phenotypes has a genetic basis and is the product of local adaptation by natural selection. However, in many ectotherms, variation in age at maturity is easily induced by environmental variation (phenotypic modulation or phenotypic plasticity). Because the genetic basis of phenotypic variation in age at maturity is rarely known, it can be impossible to distinguish among alternative hypotheses, especially those consistent with observed variation. An obvious way out of this dilemma is to test the central prediction of the adaptive hypothesis: experimentally manipulate age at maturity and test whether the local phenotype has a higher fitness than alternative phenotypes. This powerful approach not only tests that adaptive hypothesis, but also provides an opportunity to study natural selection on age at maturity. Indeed, Sinervo and Huey (1992) and Ketterson and Nolan (1992) have pioneered experimental manipulation of life history phenotypes (allometric or phenotypic engineering), providing new insights to natural selection and the evolution of life history traits not obtainable with comparative approaches.

We have data to suggest that phenotypic engineering age at maturity in Eastern Fence Lizards, Sceloporus undulatus may be possible using the hormone leptin. Leptin is a lipostatic hormone which regulates energy metabolism and age at maturity in mammals. Phenotypic engineering of age at maturity has not previously been possible, but fence lizards in our pilot experiment that received leptin injections showed virtually all of the short term effects of leptin observed in mice, including higher body temperatures, metabolic rates, and activity rates, and lower feeding rates. We are conducting comprehensive laboratory and field studies designed to test the feasibility of using leptin to test adaptive hypotheses about geographic variation in age at maturity. We also want to measure leptin concentrations among unmanipulated free-ranging lizards to test the hypothesis that natural variation in leptin is correlated with differences in age at maturity.

Our phenotypic engineering experiment provides an opportunity to investigate leptin's action at not only the individual and population levels, but also at the cell and molecular levels. We will employ molecular techniques (cloning , sequencing , recombinant protein expression) to: (1) provide a more potent engineering tool, and, (2) contribute to ongoing structure/function studies of this important regulatory hormone. Results from our studies will significantly contribute to research programs at three different levels of biological organization: molecular evolution of leptin, fitness consequences for individual lizards of phenotypic engineering with leptin, and evolution of life history traits.

Poster Describing Results of Leptin Injection
(now in press, Journal of Experimental Biology, 1999
Niewiarowski, P.H., Balk, M.L., and Londraville, R.L.)