Endocrine analysis in evolutionary‐developmental studies of insect polymorphism: hormone manipulation versus direct measurement of hormonal regulators

Abstract

"Hormone manipulation" is being used increasingly in evo-devo studies as the sole or primary technique to investigate the regulation of insect polymorphism by hormones, most notably juvenile hormone (JH). This manuscript critically evaluates the limitations and strengths of this indirect method for inferring aspects of endocrine regulation, and conclusions derived from recent endocrine studies of evolution and development in which data have been obtained primarily or exclusively by this method. The main conclusions of this critique are as follows: first, when used alone, or as the primary empirical technique, hormone manipulation is a superficial method that is fraught with problems with respect to identifying a hormone that regulates developmental-morphological variation, let alone identifying its mode of action. Second, conclusions reported in studies using this technique as the exclusive, or nearly exclusive experimental approach, most notably recent studies of JH regulation of horn polymorphism in dung beetles, and some studies of wing polymorphism should be considered, at best, weakly supported until substantiated by well-validated, direct methods. Finally, there are many reliable and well-validated techniques that can be used to directly and accurately quantify JH levels, and activities of JH regulators, in many insects, even in small, nonmodel species. Some of the most important of these assays will be briefly described and their strengths and weaknesses will be discussed.

Population dependent effects of photoperiod on diapause related physiological traits in an invasive beetle (Leptinotarsa decemlineata)

“…Since these traits are integrated, multilevel systems, with polygenic quantitative genetic backgrounds and whose expression often is plastic, the study of mechanism is challenging (Flatt & Heyland 2011). Studies aiming to elucidate the genetic, molecular or physiological mechanisms behind patterns of evolution in life-history traits therefore often focus on more specific traits or components of life-history traits (Roff 1992, Zera 2007, Flatt & Heyland 2011.…”

“…Therefore I studied population dependent differences in expression profiles of genes belonging to the circadian clock and the insulin signaling pathway (study III). Also hormones could be likely candidates in driving population dependent divergence, since they sit on key regulatory points in the pathways producing differing phenotypes (Zera 2007, Oostra et al 2010. Additionally, I study whether there are population dependent differences in susceptibility to manipulations of hemolymph JH titers by manipulating them positively or negatively through topical applications (study IV).…”

“…Since populations differ in respect to CPP (study I) and it has been suggested that life-history differences between individuals within a population or between populations could be mediated through hormones (see Zera 2007), my interest was to probe for potential population dependent differences in receptivity to manipulation of JH titers in adult beetles. JH titers were manipulated with topical applications of three substances.…”

“…I therefore investigated population dependent differences in the receptivity to manipulation of JH titers. Differences in receptivity could indicate that population divergence in response to photoperiod is driven by changes in the endocrinological machinery inherently driving life-history switches (Zera 2007, Flatt and Heyland 2011, Oostra 2013. Manipulation of the JH titer with pyriproxyfen (a juvenile hormone analogue, JHA) led to increased egg production and decreased burrowing propensity but there was no difference among populations (except for Polish beetles reared under SD conditions).…”

“…I therefore conclude that even though a slight population dependent difference was found in H17 receptivity concerning egg production, the overall small differences between the populations indicate that latitudinal variation seen in life-history and physiological traits (Danilevski 1965, de Kort 1990, Boman et al 2008, Study I, II, III, V) are probably not driven by local adaptation in the mechanisms directly related to JH reception. Before drawing final conclusions it would however be important to assess JH titers directly (Zera 2007) to see whether populations differ in baseline JH values (which could explain why they differ in receptivity as well).…”