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Evolutionary/Endocrine-genetics of life history variation and trade-offs. The functional causes of variation in life history traits (e.g. age at which reproduction begins, age-specific patterns of fecundity, longevity), and trade-offs between traits, has been an important topic in evolutionary biology for over five decades. Thus far, nearly all work on this problem has focused on energetics (e.g. variation in energy reserves between phenotypes with different life histories). Despite considerable interest, little is known about regulatory (e.g. endocrine) causes of life history variation and trade-offs. This is surprising since many of the classic life history traits are known to be hormonally regulated. My laboratory has undertaken the first, and currently the only, detailed analysis of the relationship between naturally-occurring genetic variation for hormone titers and life history variation and trade-offs. The model system that I am using to address this topic is wing polymorphism in crickets of the genus Gryllus [see Zera and Harshman (2001) for review]
Wing polymorphism, occurs commonly in insects, and has evolved independently, in many insect orders [see Zera and Denno (1997) for review]. The polymorphism involves dramatic, discontinuous variation in dispersal capability (wing and flight muscle development) and reproductive traits (onset of ovarian growth; total fecundity). Ovarian growth is negatively associated with flight capability, and is a classic example of a life history trade-off. Using genetic stocks selected for the long-winged (LW, flight-capable) or short-winged morph (SW, flightless), my lab has documented genetic variation in the titers of the key gonadotropins, juvenile hormone (JH) and ecdysone (see Zera and Bottsford, 2001). The JH titer is correlated with both ovarian growth and flight capability (e.g. flight muscle histolysis in adults). Experimental manipulation of the JH titer in several species alters ovarian growth and flight muscle mass. Thus, observed endocrine-organ correlations appear to represent functional rather than spurious relationships. To my knowledge, this is the first demonstration that phenotypes that differ genetically in an important life history trait also differ genetically in the titer of a hormone that potentially regulates that trait.
One intriguing and unexpected finding of these studies was the morph-specific temporal variation in the JH titer. The titer of this hormone changes 50-100-fold during the day in the flight-capable morph, but is temporally constant in the flightless morph. This is the first example of genetically-based differences in temporal fluctuation in the titer of a hormone in natural populations and indicates a potentially novel mechanism by which a single hormone may regulate multiple, antagonistic traits (see Zera and Cisper, 2001). The short-term elevation in the JH titer in the dispersing morph may be of sufficient duration to regulate nocturnal flight behavior, but not of sufficient duration to initiate ovarian growth, which would be maladaptive with respect to flight capability. A newly funded NSF grant (7/02-6/05) will focus on the proximate endocrine mechanisms that control the morph-specific temporal fluctuation in the JH titer and the adaptive significance of this phenomenon in both the laboratory and in the field. These studies will constitute the first endocrine studies of insects undertaken in the field. Genetic variation in hormone receptors in target tissues (ecdysteroid and JH receptors in ovaries and flight muscles; vitellogenin receptors in ovaries), and the endocrine-genetics of flight muscle development will also be foci of future studies.