Direct and Correlated Responses to Selection

Hemolymph juvenile hormone esterase (JHE) activity on the third day of the last stadium in the cricket, Gryllus assirnilis, exhibited a significant response to selection in each of six replicate lines. Mean realized heritability was 0.26 t 0.04. The response was due to changes in whole-organism enzyme activity as well as to changes in the proportion of enzyme allocated to the hemolymph compartment. I n vivo juvenile hormone metabolism differed between some lines selected for high us. low enzyme activity. Only minimal differences were observed between lines with respect to hemolymph protein concentration or whole-cricket activity of juvenile hormone epoxide hydrolase, the other major JHdegrading enzyme. Dramatic correlated responses to selection, equal in magnitude to the direct response, were observed for JHE activity on each of three other days of the last juvenile stadium. In contrast, no correlated responses in JHE activity were observed in adults. This indicates that JHE activities throughout the last stadium will evolve as a highly correlated unit independent of adult activities and the evolution of endocrine mechanisms regulating juvenile development can be decoupled from those controlling adult reproduction. This study represents the first quantitative-genetic analysis of naturally occurring e docrine . . variation in an insect species. D URING the past three decades a tremendous amount of chemical, biochemical and physiological information has been obtained on insect endocrine characteristics (DOWNER and LAUFER 1983; KERKUT and GILBERT 1985; GUPTA 1990). However, genetic aspects of insect endocrinology, especially population-genetic aspects, have been much less studied. Except for a few rare cases (see below), there are currently no published data on the amount, characteristics or degree of interaction among naturally occurring, genetically based endocrine variations. Because of this paucity of populationgenetic information on hormone titers, hormone receptors or activities of enzymes involved in hormone biosynthesis or degradation, our understanding of the microevolutionary processes that modify or constrain the insect endocrine system is limited. Most physiological traits in either natural or unselected, outbred laboratory populations appear to have a polygenic mode of inheritance. These range from whole-organism traits such as locomotor activity and desiccation tolerance to enzyme activities and concentrations of energy reserves (LAURIE-AHLBERG et al. 1980, 1982; ARNOLD 1987; CLARK and KEITH 1988; GARLAND 1988; BENNETT and HUEY 1990; GARLAND and CARTER 1994). A polygenic mode of inheritance also appears to be common for unselected endocrine variation in laboratory strains of domestic stocks of vertebrates Correspond2ngauthor:AnthonyJ. Zera, School of Biological Sciences, University of Nebraska, Lincoln, NE 68588. E-mail: [email protected] Genetics 141: 1125-1134 (November, 1995) (SHIRE 1979). Quantitativegenetic methodologies are the most appropriate for analyzing this type of genetic variation. In the present study, we used a conventional artificial selection experiment to investigate genetic variation and covariation in a model insect endocrine character, juvenile hormone esterase (JHE) activity, in the cricket Gryllus assimilis. JHE is a hydrolytic enzyme that degrades the key developmental and reproductive hormone, juvenile hormone (JH) (HAMMOCK 1985; ROE and VENKATESH 1990). The activity of this enzyme increases dramatically during the last stadium when a reduction in the JH titer to very low levels is required for metamorphosis to proceed. Numerous biochemical and physiological studies have implicated a role for JHE in the regulation of the JH titer (HAMMOCK 1985; ZERA and TIEBEL 1989; ROE and VENKATESH 1990; ZERA and HOLTMEIER 1992). We focused on JHE activity in G. assirnilis for a variety of reasons. First, as described above, JHE is functionally important by virtue of its role in modulating the JH titer. Second, in contrast to all other insect endocrine traits, a large data base is available on naturally occurring genetic variation in JHE activity in the closely related cricket, G. rubens. G. rubens is dimorphic for dispersal capability, and JHE has been extensively studied in the context of the regulation of the JH titer and morph determination (ZERA and TIEBEL 1989; ZERA et al. 1993 and references therein). This background information allows interpretation of the JHE selection experiments in G assimilis to a degree not possible for other less-studied endocrine characters. Finally, JHE 1126 A. J. Zera and C. Zhang