The Endocrine Regulation of Wing Polymorphism in Insects: State of the Art, Recent Surprises, and Future Directions

The endocrine mechanisms controlling the development and reproduction of flight-capable (long-winged) and flightless (short-winged or wingless) morphs of wing-polymorphic insects have been intensively investigated. The classical model, put forward in the early 1960s, postulates that morph-specific differences in development and reproduction are caused by variation in the titers of juvenile hormone (JH) and/or ecdysone. Despite decades of study, the importance of these hormones in regulating wing polymorphism in aphids and planthoppers remains uncertain. This uncertainly is largely a consequence of technical and size constraints which have severely limited the types of endocrine approaches that can be used in these insects. Recent studies in wing-polymorphic crickets (Gryllus) have provided the first direct evidence that the in vivo blood titers of juvenile hormone and ecdysone, and especially the activity of the JH regulator, juvenile hormone esterase, differ between nascent morphs. Morph differences are largely consistent with the classical model, although some types of data are problematic, and other explanations are possible. Adult morphs differ dramatically in the JH titer but titer differences are more complex than those proposed by the classical model. Detailed endocrine information is thus far available only for a few species of crickets, and the hormonal control of wing polymorphism for insects as a whole remains poorly understood. Future studies should continue to investigate the role of JH and ecdysteroids in morph development and reproduction, and should expand to include studies of morph-specific differences in hormone receptors and neurohormones

Intermediary Metabolism and Life History Trade-offs: Lipid Metabolism in Lines of the Wing-polymorphic Cricket, \u3ci\u3eGryllus firmus,\u3c/i\u3e Selected for Flight Capability vs. Early Age Reproduction

The extent to which modifications in intermediary metabolism contribute to life history variation and trade-offs is an important but poorly understood aspect of life history evolution. Artificial selection was used to produce replicate genetic stocks of the wing-polymorphic cricket, Gryllus firmus, that were nearly pure-breeding for either the flight-capable (LW[f]) morph, which delays ovarian growth, or the flightless (SW) morph, which exhibits enhanced early-age fecundity. LW(f) lines accumulated substantially more triglyceride, the main flight fuel in Gryllus, compared with SW-selected lines, and enhanced accumulation of triglyceride was strongly associated with reduced ovarian growth. Increased triglyceride accumulation in LW(f) lines resulted from elevated de novo biosynthesis of fatty acid and two morph-specific trade-offs: (1) greater proportional utilization of fatty acid for glyceride biosynthesis vs. oxidation, and (2) a greater diversion of fatty acids into triglyceride vs. phospholipid biosynthesis. Even though SW lines produced less total lipid and triglyceride, they produced more phospholipid (important in egg development) than did LW(f) lines. Differences between LW(f) and SW morphs in lipid biosynthesis resulted from substantial alterations in the activities of all studied lipogenic enzymes, a result that is consistent with expectations of Metabolic Control Theory. Finally, application of a juvenile hormone analogue to LW(f) females produced a striking SW phenocopy with respect to all aspects of lipid metabolism studied. Global alterations of lipid metabolism, most likely produced by alterations in endocrine regulation, underlie morph specializations for flight vs. early-age fecundity in G. firmus. Modification of the endocrine control of intermediary metabolism is likely to be an important mechanism by which intermediary metabolism evolves and contributes to life history evolution

Morph-associated JH titer diel rhythm in \u3ci\u3eGryllus firmus\u3c/i\u3e: Experimental verification of its circadian basis and cycle characterization in artificially selected lines raised in the field

Previous studies demonstrated a high-amplitude, diel cycle for the hemolymph JH titer in the wing-polymorphic cricket, Gryllus firmus. The JH titer rose and fell in the flight-capable morph (long-winged, LW(f)) above and below the relatively temporally invariant JH titer in the flightless (short-winged, SW) morph. The morph-specific JH titer cycle appeared to be primarily driven by a morph-specific diel cycle in the rate of JH biosynthesis. In the present study, cycles of the JH titer and rate of JH biosynthesis in the LW(f) morph persisted in the laboratory under constant darkness with an approximate 24 h periodicity. The JH titer cycle also shifted in concert with a shift in the onset of the scotophase, was temperature compensated in constant darkness, and became arrhythmic under constant light. These results provide strong support for the circadian basis of the morph-specific diel rhythm of the JH titer and JH biosynthetic rate. Persistence of the JH titer cycle under constant darkness in multiple LW-selected and SW-selected stocks also provides support for the genetic basis of the morph-associated circadian rhythm. The morph-specific JH titer cycle was observed in these stocks raised in the field, in both males and females, in each of 3 years studied. The onset of the cycle in the LW(f) morph, a few hours before sunset, correlated well with the onset of the cycle, a few hours before lights-off, in the laboratory. The morph-specific JH titer cycle is a general feature of G. firmus, under a variety of environmental conditions, and is not an artifact of specific laboratory conditions or specific genetic stocks. It is a powerful experimental model to investigate the mechanisms underlying endocrine circadian rhythms, their evolution, and their impact on life history evolution

Morph-associated JH titer diel rhythm in \u3ci\u3eGryllus firmus\u3c/i\u3e: Experimental verification of its circadian basis and cycle characterization in artificially selected lines raised in the field

Previous studies demonstrated a high-amplitude, diel cycle for the hemolymph JH titer in the wing-polymorphic cricket, Gryllus firmus. The JH titer rose and fell in the flight-capable morph (long-winged, LW(f)) above and below the relatively temporally invariant JH titer in the flightless (short-winged, SW) morph. The morph-specific JH titer cycle appeared to be primarily driven by a morph-specific diel cycle in the rate of JH biosynthesis. In the present study, cycles of the JH titer and rate of JH biosynthesis in the LW(f) morph persisted in the laboratory under constant darkness with an approximate 24 h periodicity. The JH titer cycle also shifted in concert with a shift in the onset of the scotophase, was temperature compensated in constant darkness, and became arrhythmic under constant light. These results provide strong support for the circadian basis of the morph-specific diel rhythm of the JH titer and JH biosynthetic rate. Persistence of the JH titer cycle under constant darkness in multiple LW-selected and SW-selected stocks also provides support for the genetic basis of the morph-associated circadian rhythm. The morph-specific JH titer cycle was observed in these stocks raised in the field, in both males and females, in each of 3 years studied. The onset of the cycle in the LW(f) morph, a few hours before sunset, correlated well with the onset of the cycle, a few hours before lights-off, in the laboratory. The morph-specific JH titer cycle is a general feature of G. firmus, under a variety of environmental conditions, and is not an artifact of specific laboratory conditions or specific genetic stocks. It is a powerful experimental model to investigate the mechanisms underlying endocrine circadian rhythms, their evolution, and their impact on life history evolution

Locomotion in Response to Shifting Climate Zones: Not So Fast

Although a species’ locomotor capacity is suggestive of its ability to escape global climate change, such a suggestion is not necessarily straightforward. Species vary substantially in locomotor capacity, both ontogenetically and within/among populations, and much of this variation has a genetic basis. Accordingly, locomotor capacity can and does evolve rapidly, as selection experiments demonstrate. Importantly, even though this evolution of locomotor capacity may be rapid enough to escape changing climate, genetic correlations among traits (often due to pleiotropy) are such that successful or rapid dispersers are often limited in colonization or reproductive ability, which may be viewed as a trade-off. The nuanced assessment of this variation and evolution is reviewed for well-studied models: salmon, flying versus flightless insects, rodents undergoing experimental evolution, and metapopulations of butterflies. This work reveals how integration of physiology with population biology and functional genomics can be especially informative

Effect of a Juvenile Hormone Analogue on Lipid Metabolism in a Wing-Polymorphic Cricket: Implications for the Endocrine-Biochemical Bases of Life-History Trade-Offs

The wing-polymorphic cricket, Gryllus firmus, has a flight-capable morph (LW[f]: long winged with functional flight muscles) and a flightless morph (SW: short winged with reduced nonfunctional flight muscles) that differ genetically in many aspects of lipid metabolism. To determine whether these differences result from genetically based alterations in endocrine regulation, the juvenile hormone mimic, methoprene, was applied to the LW(f) morph. This hormone manipulation converted the LW(f) morph into a SW phenocopy with respect to all aspects of lipid metabolism studied; that is, methoprene application decreased in vivo biosynthesis of total lipid and triglyceride, increased absolute and relative biosynthesis of phospholipid, increased oxidation of fatty acids, and decreased in vitro specific activities of each of six lipogenic enzymes and a transaminase. Furthermore, methoprene increased ovarian growth and decreased fat body mass and flight muscle mass in the LW(f) morph. Differences in each of these biochemical, morphological, or reproductive traits between hormone-treated and control LW(f) females were similar in magnitude to differences between unmanipulated LW(f) and SW females. Variation in endocrine regulation contributes significantly to genetically based differences in lipid metabolism between LW(f) and SW females. This is the first evidence for endocrine regulation of a genetically based life-history trade-off operating via hormonal effects on specific metabolic pathways and enzymes of intermediary metabolism

Biochemical basis of specialization for dispersal vs. reproduction in a wing-polymorphic cricket: Morph-specific metabolism of amino acids

The biochemical basis of specializations for dispersal vs. reproduction is an understudied aspect of dispersal polymorphism in insects. Using a radiolabelled amino acid, we quantified differences in in vivo amino acid metabolism between morphs of the wing-polymorphic cricket, Gryllus firmus, that trade-off early age reproduction and dispersal capability. Studies were conducted in crickets fed a variety of diets expected to influence amino acid and lipid metabolism. On the day of molt to adulthood, prior to the morph-specific trade-off between ovarian growth and biochemical preparation for flight (e.g. biosynthesis of triglyceride flight fuel), morphs did not differ in any aspect of amino acid metabolism. However, on day 5 of adulthood, when the morph-specific trade-off between ovarian growth and flight fuel production was manifest, the morphs differed substantially in each of the three aspects of amino acid metabolism studied: conversion to protein, oxidation, and conversion to lipid. Morphs also differed in degree of allocation of products of amino acid metabolism to ovaries vs. the soma. Most importantly, morphs differed in the relative metabolism of radiolabelled glycine through these pathways (i.e. biochemical trade-offs), and in the relative allocation of end products of amino acid metabolism to the soma vs. ovaries (allocation trade-offs). A functionally important interaction between amino acid and lipid metabolism was noted: greater oxidation of amino acids in the flight-capable morph spared fatty acids for enhanced conversion into triglyceride flight fuel. By contrast, greater oxidation of fatty acids by the flightless morph spared amino acids for enhanced conversion into ovarian protein. Diet significantly affected amino acid metabolism. However, MORPH×DIET interactions were rare and morphs differed in amino acid metabolism to a similar degree under the range of diets tested

Differential Mating Success of Male Wing Morphs of the Cricket, \u3ci\u3eGryllus rubens\u3c/i\u3e

Genetically marked individuals were used to study differential mating success between male wing morphs of the cricket, Gryllus rubens. Previous studies of Gryllus rubens and other wing-dimorphic insects have documented that flightless short-winged or wingless females typically attain reproductive maturity earlier and oviposit more eggs relative to their long-winged counterparts. This study was done to determine if flightless males also exhibit enhanced reproductive characteristics. Segregation analyses documented the genetic basis of allozymes used to infer paternity in subsequent experiments. Control experiments documented the absence of effects on mating success independent of wing morph due to (1) the genetic stock from which males were taken; (2) male size; or (3) female wing morph. Mating trials involving a long-winged male, a short-winged male and a female of either wing morph documented no significant differences in the number of progeny sired by male wing morphs. This pattern was true for both the first group of offspring and for the entire set of offspring produced during a 20-day period. Thus, in contrast to females, we observed no increase in reproductive output in males resulting from the loss of the flight apparatus. However, substantial variance in mating success was observed between males independent of wing morph. This result was likely due to the existence of a dominance hierarchy and the increased mating success of the dominant male

Differential Mating Success of Male Wing Morphs of the Cricket, \u3ci\u3eGryllus rubens\u3c/i\u3e

Genetically marked individuals were used to study differential mating success between male wing morphs of the cricket, Gryllus rubens. Previous studies of Gryllus rubens and other wing-dimorphic insects have documented that flightless short-winged or wingless females typically attain reproductive maturity earlier and oviposit more eggs relative to their long-winged counterparts. This study was done to determine if flightless males also exhibit enhanced reproductive characteristics. Segregation analyses documented the genetic basis of allozymes used to infer paternity in subsequent experiments. Control experiments documented the absence of effects on mating success independent of wing morph due to (1) the genetic stock from which males were taken; (2) male size; or (3) female wing morph. Mating trials involving a long-winged male, a short-winged male and a female of either wing morph documented no significant differences in the number of progeny sired by male wing morphs. This pattern was true for both the first group of offspring and for the entire set of offspring produced during a 20-day period. Thus, in contrast to females, we observed no increase in reproductive output in males resulting from the loss of the flight apparatus. However, substantial variance in mating success was observed between males independent of wing morph. This result was likely due to the existence of a dominance hierarchy and the increased mating success of the dominant male

Tissue and stage-specific juvenile hormone esterase (JHE) and epoxide hydrolase (JHEH) enzyme activities and \u3ci\u3eJhe \u3c/i\u3etranscript abundance in lines of the cricket\u3ci\u3e Gryllus assimilis \u3c/i\u3eartificially selected for plasma JHE activity: Implications for JHE microevolution

Fat body and midgut juvenile hormone esterase (JHE) and juvenile hormone epoxide hydrolase (JHEH) specific activities, and plasma JHE activity, were measured throughout the last stadium in two pairs (blocks) of lines of the cricket Gryllus assimilis, each pair of which had been artificially selected for high- or low-plasma JHE activity. Highly significant differences were observed between high- and low-activity lines of each block on most days for fat body JHE, and on one day for midgut JHE activity. In each block, line differences in developmental profiles for fat body JHE activity paralleled line differences in plasma JHE activity during the early-mid stadium, but not during the latter part of the stadium. The developmental profile of midgut JHE activity differed from that of plasma and fat body JHE activity, exhibiting peaks during the early and latter parts of the stadium. Midgut and fat body JHEH activities exhibited a mid-stadium peak in all lines, but activities were very similar in all lines. Fat body JHE appears to be a more significant contributor to plasma JHE than is midgut JHE. During the middle of the last stadium (day 4), Jhe transcript abundance was significantly higher in fat body or midgut of highvs. low-JHE-activity lines. Jhe transcript abundance was positively correlated with JHE enzyme activity in either fat body or midgut, and with plasma JHE activity. Natural populations of G. assimilis harbor genetic variation for Jhe gene expression which appears to contribute to genetic variation in JHE specific activity in fat body and midgut. These genes appear to have been the targets of artificial selection that resulted in lines that differ dramatically in high- or low-plasma JHE activity. These genes appear to have little, if any, pleiotropic effects on JHEH specific activity