Evolutionary genetics of juvenile hormone and ecdysteroid regulation in Gryllus: a case study in the microevolution of endocrine regulation

During the past 15 years the first detailed synthesis of endocrinology and population genetics has begun, in which natural genetic variations for endocrine regulators have been characterized, almost exclusively in species of the cricket genus Gryllus. Artificial selection studies have documented that regulators of the juvenile hormone titer can rapidly evolve and exhibit levels of genetic variability similar to other physiological traits. Strong genetic correlations exist between some but not all regulators of the JH titer during the juvenile stage. No genetic correlation exists between regulators functioning in juvenile and adult stages, and thus, endocrine regulation can evolve independently in these stages. Genetic variation in the JH titer, the ecdysteroid titer, and JHE activity, in adult and juvenile stages, have been documented in genetic stocks of wing-polymorphic crickets; morph-specific differences in these endocrine traits are potentially responsible for genetically based differences in aspects of wing and flight muscle development, adult egg production, and adult dispersal. An unexpected morph-specific, genetic polymorphism for a circadian rhythm for the JH titer was observed in both the laboratory and field. Few comparable studies exist in non-Gryllus species, in which in vivo endocrine-genetic variation has been directly quantified using reliable analytical methods; many reported cases of endocrine variation in these species have been obtained using an inappropriate method and thus should be considered unsubstantiated. Obtaining basic information on the characteristics of natural genetic variation for endocrine regulators still remains one of the most important tasks of the fledgling subdiscipline of evolutionary endocrinology. Single gene endocrine mutants in Drosophila are promising candidates for investigating molecular-genetic variation in natural populations. Future studies should also focus on endocrine traits studied in the field and geographic variation in endocrine regulation.