THE ENDOCRINE GENETICS OF WING POLYMORPHISM IN GRYLLUS: CRITIQUE OF RECENT STUDIES AND STATE OF THE ART.

In a series of papers, the most recent of which was published in Evolution, quantitative-genetic experiments were undertaken on reproductive and physiological correlates of wing polymorphism in the sand cricket, Gryllusfirmnus (Fairbairn 1994; Fairbairn and Yadlowski 1997; Roff et al. 1997). A goal of these studies was to determine the physiological causes underlying: (1) genetic variation for an ecologically important threshold trait (wing polymorphism); and (2) genetic correlations between wing morph and other reproductive and migratory features, such as fecundity and flight muscle histolysis. These authors concluded that genetic variation for the activity of the endocrine regulator, juvenile hormone esterase (JHE), is "causally related to the production of alternate morphs" in G. firmus (Roff et al. 1997, p. 1917). One of these studies (Fairbairn and Yadlowski 1997) has recently been elevated to a textbook example of the physiological mechanisms regulating the expression of quantitative-genetic variation (Schlichting and Pigliucci 1998). I take issue with many of the conclusions concerning the endocrine basis of wing polymorphism stated in papers by Roff, Fairbairn, and coworkers (Fairbairn 1994; Fairbairn and Yadlowski 1997; Roff et al. 1997). These authors clearly have demonstrated a correlation between the activity of JHE during the last nymphal stadium and the subsequent molt to either a long-winged or short-winged adult. However, the studies lack any functional information on the role of JHE in regulating alternate morph development or reproduction in G. firmus. Therefore, their conclusions concerning a causal relationship between genetic variation for the activity of this enzyme and alternate wing morph development or reproduction in G. firmus are premature. Furthermore, these papers present an inaccurate account of data and discussion of the role of JHE in wing morph development in the congener, Gryllus rubens, reported by myself and colleagues (e.g., Zera and Tiebel 1989; Zera et al. 1989; Zera and Holtmeier 1992; Zera and Denno 1997). Accounts by Fairbairn and Yadlowski (1997) and Roff et al. (1997) give the impression that the regulatory role of JHE in wing morph development in Gryllus is understood to a much greater degree than is actually the case, and therefore are counterproductive to future research on this topic. Wing polymorphism is becoming an increasingly useful model for investigating the physiological mechanisms underlying genetically based variation in morphology, development, and life-history (Roff 1986; Zera and Denno 1997; Zera et al. 1998). Future progress on this model is critically dependent upon an accurate assessment of which physiological hypotheses are well supported and thus can serve as strong foundations for subsequent research. The main goals of this commentary are: (1) to point out serious errors concerning the endocrine regulation of wing polymorphism in Giyllus that are contained in papers of Roff, Fairbairn, and colleagues; and (2) to establish which aspects of the endocrine regulation of wing polymorphism are well supported and which are not. I also highlight what I consider to be pitfalls and appropriate methods in the analysis of endocrine variation which I hope will be helpful to nonphysiologists. I will first review basic aspects of wing polymorphism and its endocrine regulation to provide background for the issues raised in this commentary.