Morph Determination Research Articles

Male morph determination in Rhizoglyphus echinopus (Acaridae).

In Rhizoglyphus echinopus (Fumouze and Robin) two male morphs occur: heteromorphs, with a thickened and sharply terminated third pair of legs that serve as a weapon in intrasexual conflicts, and homeomorphs, with unmodified legs. This study investigated the system of male morph determination. No significant heritability of male morph was found, but cues emanating from a dense colony were found to suppress the production of heteromorphs. Developmental plasticity was retained throughout the protonymphal stage. Diet did not influence morph expression, but lowered temperature decreased the proportion of heteromorphs emerging.

Population dynamics of the bird cherry‐oat aphid, Rhopalosiphum padi (L.), during the autumn and winter: a modelling approach

Abstract1 A simulation model was developed to investigate the inter‐relationship of factors influencing the population dynamics of the bird cherry‐oat aphid (Rhopalosiphum padi (L.)) in barley crops during the autumn and winter.2 The model incorporated algorithms describing alate immigration, development and survival of adults and nymphs, fecundity and morph determination in newly born nymphs.3 The model was validated against pest outbreaks in barley fields in south‐east England.4 It simulated accurately the size of the outbreaks with predictions of peak aphid populations within 20% of the observed in all but one case. Similarly, all but one of the year‐sowing date combination predictions of timing of peak abundance fell within 14 days of the observed.5 A sensitivity analysis of the model highlighted the relative importance of various population processes in determining simulated aphid population dynamics; decreasing mortality rates of apterous nymphs by as little as 5% over the autumn and winter increased peak densities by as much as 60‐fold, whereas increasing daily temperatures by only 1 °C more than doubled peak aphid abundance.6 The model identified our understanding of the mechanisms of aphid mortality as a limiting factor in the accurate prediction of R. padi outbreaks in the field.

Untersuchungen über Fichtengallenläuse, Dreyfusia spp. (Hom., Adelgidae) und deren Predatoren in der Nord‐Türkei

Abstract: In the East Pontic mountains at Giresun (Black Sea shore) three species of the genus Dreyfusia have been established: D. nordmannianae Eckst. and D. prelli Grosm. on Abies nordmanniana Link. and Picea orientalis Link. and Dreyfusia nov. spec. on A. nordmanniana. The latter species, earlier known from Greece and the Balkans, but not yet described is most probably unholocyctic on fir.In the Zigana mountains between Trabzon and Gümüsane a species producing plenty galls on young P. orientalis trees and closely resembling D. merkeri Eichh. has been observed. Further investigations are necessary to evaluate whether or not it lives unholocyclic on spruce.In the West Pontic mountains near Bolu D. nordmannianae was intensively studied in twig infestations on A. bornmülleriana Mattf. The pattern of attack differs greatly from the one experienced in central Europe in that almost exclusively the current year’s shoots are attacked whereas the leader and the older twig axis remain free of attack. In Europe all parts of the trees are colonized.Considering the known facts about generation‐ and morph sequences, morph determination and phenological behaviour of D. nordmannianae the characteristics of their population dynamics in Turkey and the interactions of the curious mode of attack and the extraordinary efficiency of their special predators have been completely evaluated.

The effect of temperature on the photoperiodic ‘counters’ for female morph and sex determination in two clones of the black bean aphid, Aphis fabae

AbstractExperiments were performed on two clones of the black bean aphid, Aphis fabae Scopoli – one from Aberdeen, Scotland (57°N), the other from Cambridge, England (52°N) − to determine the number of long‐ or short‐night cycles required for 50% induction of winged versus wingless females on the one hand and males versus females on the other (i.e. required day number, RDN), at three temperatures, 12.5, 15 and 17.5°C. In the case of female morph determination, the RDN for long‐night cycles was temperature compensated, whereas that for short‐night cycles was highly temperature dependent. For sex determination, the RDN for long‐night cycles was again temperature compensated, whereas, due to the mechanism of sex determination, male production was close to 100% in our protocol, even with a maximal number of short‐night cycles, and the RDN could therefore not be assessed. Model‐generated response curves, using the recently developed ‘double circadian oscillator model’ for photoperiodic time measurement in insects and mites, closely resembled the observations. It could also be shown that differences observed between response curves of female morph and sex determination in the Scottish clone were due, according to the model, to differences in their photoperiodic ‘counters’, rather than to differences in their clocks.

Escaping an ecological dead‐end: asexual overwintering and morph determination in the lettuce root aphid Pemphigus bursarius L.

Summary1. Pemphigus bursarius exhibits a host‐alternating life‐cycle, overwintering as eggs on the primary host plant (poplar) before migrating in summer to the secondary host plant (mainly annual Compositae). A proportion of the population does not produce return migrants (sexuparae) in the autumn but remains in the soil and overwinters as asexual apterae, even after the annual plants have died in early winter. Laboratory and field experiments investigated the biology and overwintering success of these asexual apterae.2. Both temperature and photoperiod are important in morph determination. The highest number of apterae (approximately 50% of progeny) was produced in a 20 °C, LD 18 : 6 h regime. Very few apterae were produced in an alatae‐inducing environment (15 °C, LD 12 : 12 h), but the proportion increased over successive generations.3. Apterae remaining in the soil in the autumn overwintered successfully in large numbers and were able to reinfest directly the root systems of newly planted lettuce grown in the same field in the following growing season.4. Overwintered asexual populations also produced alates in July, which were able to colonise other lettuce plants, indicating that they were not sexuparae. Hence, P. bursarius can avoid the ecological dead‐end that would occur through local patch extinction. Clones can therefore persist indefinitely as both asexual apterae and alatae without the need to return to poplar and undergo the sexual phase of the life‐cycle.

Coevolution of traits determining migratory tendency: correlated response of a critical enzyme, juvenile hormone esterase, to selection on wing morphology

AbstractMigratory tendency in insects is a complex trait, composed of a suite of correlated behavioural, physiological, morphological and life history traits. We investigate the genetic and physiological basis of the coevolution of this suite of traits using laboratory lines of the wing dimorphic cricket, Gryllus firmus, selected for increasing and decreasing incidence of macroptery. Selection on wing morphology has produced strong direct responses in proportion macropterous as well as correlated (indirect) responses in wing muscle histolysis, flight propensity and fecundity. We investigate the hypothesis that these responses have been mediated by changes in the metabolism of juvenile hormone (JH) during the final nymphal stadium (the critical period for wing morph determination). Previous studies of Gryllus sp. have established that JH titre in this period is determined primarily by the activity of the degradative enzyme, juvenile hormone esterase (JHE). Assays of JHE activity in the final nymphal stadium of the replicated control and selected lines demonstrate highly significant differences in both mean activity and the probability of macroptery for a given level of activity (i.e., the threshold activity required to induce wing formation). These correlated responses in JH metabolism support the general hypothesis that the correlations among traits determining migratory tendency result at least in part from the common influence of JH during the final nymphal stadium. We discuss these results in the context of the quantitative genetic model for the evolution of polygenic, dichotomous traits (the threshold model), and present four general predictions concerning the coevolution of traits associated with ecological (i.e., trophic, life history, behavioural) dimorphisms.

The role of juvenile hormone and juvenile hormone esterase in wing morph determination in Modicogryllus confirmatus

The role of juvenile hormone (JH) and juvenile hormone esterase (JHE) in regulating wing morph determination was studied in the cricket Modicogryllus confirmatus. JHE activities were significantly higher in nascent long-winged (LW) vs short-winged (SW) crickets during the latter half but not during the first half of the last stadium. The magnitude and direction of the activity differences were similar to those previously documented between wing morphs of the cricket, Gryllus rubens. In contrast, activities of general esterase, an enzyme or group of enzymes with no demonstrated role in regulating the JH titer in insects, showed no or only minor differences between morphs. The magnitude and direction of the JHE activity variation is consistent with a regulatory role for this enzyme in some aspect of wing dimorphism. However, the timing of the differences (exclusively during the last half of the last stadium) argue against a role in regulating wing length development per se. Single or multiple applications of juvenile hormone-III to nascent LW individuals during the first few days of the last stadium significantly redirected development from long to short wings. Multiple applications of acetone, by itself, also increased the production of short-winged adults. For most treatments, all individuals with shortened wings also had undeveloped flight muscles. These data suggest that JH may play a role in wing morph determination in M. confirmatus but that it affects a different aspect of the polymorphism from JHE.

Male morph determination in two species of acarid mites

Male morph was found to be determined in different ways in two species of acarid mites within which morphologically distinct fighter and nonfighter males occur. Fighter males sired significantly larger proportions of fighters in comparison to nonfighter males in Rhizoglyphus robinii, but not in Caloglyphus berlesei. The genetic mechanism of male morph determination in R. robinii remains unclear: neither single-locus nor polygenic models fit the results of the crosses. Mites fed on a poor diet during development were less likely to develop into fighters in both species. However, increased density inhibited the development of fighters in C. berlesei, but not in R. robinii.

Characterization of juvenile hormone esterase from genetically-determined wing morphs of the cricket, Gryllus rubens

Long-winged (LW) vs short-winged (SW) genetic stocks of the cricket Gryllus rubens differ in plasma juvenile hormone esterase (JHE) activity during the last stadium. These activity differences may be important in morph determination. In the present study, plasma JHEs from the LW vs SW stocks were characterized with respect to a variety of kinetic and physical characteristics. Gel permeation chromatography of LW or SW plasma each resulted in a single JHE peak of high molecular weight (190 kDa). This molecular weight is about twice as high as that of JHEs from most other insects. The apparent Michaelis constant for JH III ranged from 47 to 81 nM. Like JHEs from other insects, the enzyme from G. rubens was inhibited strongly by trifluoropropanone transition-state analogs and weakly by the general esterase inhibitors, eserine and DFP. JHEs from LW and SW plasma exhibited no significant differences in K M, inhibition by trifluoropropanone or general esterase inhibitors, thermal denaturation profiles or pH profiles. The absence of K M differences between LW and SW JHEs indicate that the 2–4 fold higher enzyme activity in LW plasma, previous documented in assays employing saturating substrate concentration, will exist under physiological substrate concentrations. Two isoforms (pI = 5.1, 4.2−4.1) were identified in SW plasma but only the more acidic form was observed in LW plasma. This is the first documentation of genetically-determined differences in JHE isozymes in any insect species. However, the functional significance of these isoform differences, if any, remains to be established. These results provide no evidence that the plasma JHE activity differences between LW and SW stocks results from allozymes or isozymes with altered kinetic or stability characteristics.

POLYPHENISM IN SPADEFOOT TOAD TADPOLES AS A LOCALLY ADJUSTED EVOLUTIONARILY STABLE STRATEGY.

I examined the evolutionary factors maintaining two environmentally induced morphs in ponds of variable duration. Larvae of New Mexico spadefoot toads (Scaphiopus multiplicatus) often occur in the same pond as a large, rapidly developing carnivorous morph and as a smaller, more slowly developing omnivorous morph. Previous studies revealed that carnivores can be induced by feeding tadpoles live fairy shrimp and that morph determination is reversible. Field and laboratory experiments indicated that the ability of an individual to become a carnivore or an omnivore is maintained evolutionarily as a response to variability in pond longevity and food abundance. Carnivores survived better in highly ephemeral artificial ponds, because they developed faster. Omnivores survived better in longer-duration artificial ponds, because their larger fat reserves enhanced postmetamorphic survival. The two morphs also occupy different trophic niches. Experimental manipulations of morph frequency in ponds of intermediate duration revealed that increased competition for food among individuals of the more common morph made the rarer form more successful. Morph frequency within each pond was stabilized at an equilibrium by frequency-dependent morph reversal, which reflected frequency-dependent natural selection on size at metamorphosis: larger metamorphs had higher survival, and individuals reared at a frequency above the pond's equilibrium frequency were smaller at metamorphosis than were individuals of that morph reared at a frequency below the pond's equilibrium. Because neighboring ponds often differed in pond longevity and food abundance, each pond possessed a unique equilibrium morph frequency. This implies that morph determination in Scaphiopus is a locally adjusted evolutionarily stable strategy (ESS).

In vivo and in vitro degradation of juvenile hormone-III in presumptive long-winged and short-winged Gryllus rubens

Degradation of exogenous juvenile hormone-III was compared between long-winged and short-winged morphs of the cricket Gryllus rubens during the last stadium. This was done to determine if morph-specific differences in haemolymph juvenile hormone esterase activity, which are thought to be important in morph determination, result in corresponding differences in in vivo hormone degradation. Percentage degradation of racemic or (10-R)-juvenile hormone-III was significantly higher in plasma from the long-winged vs the short-winged morph during the mid-stadium while no differences between the morphs were observed early in the stadium. These results paralleled morph-specific differences in plasma juvenile hormone esterase activity. Similar patterns were obtained in vivo, except for early-stadium, short-winged crickets where hormone degradation was much higher than expected based on haemolymph juvenile hormone esterase activity. When esterase activity in long-winged individuals was inhibited in vivo by a trifluoropropanone derivative, there was a corresponding decrease in in vivo hormone degradation. No morph-specific differences were observed in degree of hormone binding by haemolymph components during either the early or mid-stadium, indicating that variation in hormone binders cannot account for differences between morphs in degree of hormone degradation. Importantly, differences between morphs in hormone degradation were much less than differences in haemolymph juvenile hormone esterase activity. These results collectively document morph-specific covariation in haemolymph juvenile hormone esterase activity and in vivo hormone degradation. However, because differences between morphs in hormone degradation are not large, their functional significance with respect to morph determination remains uncertain.

The adaptive significance of an environmentally-cued developmental switch in an anuran tadpole.

This study investigated the proximate basis of bimodally-distributed, environmentally-induced variation that occurs in natural populations of spade-foot toad tadpoles (Scaphiopus multiplicatus). Most individuals in most populations occur as a small, slowly-developing omnivore morph. In some of these same populations, a varying number of individuals occur as a large, rapidly-developing carnivore morph (Pfennig 1989). Censuses of 37 different natural ponds revealed that the frequency of the faster-developing carnivore morph correlated significantly positively with fairy shrimp density (their chief prey) and pond drying rate. By simultaneously varying two diet components and pond drying regime in artificial pools I found that only fairy shrimp density significantly affected the proportion of carnivores. Separate experiments established that the extent to which tadpoles developed the carnivore morphology correlated with shrimp density, and that morph determination depended on the ingestion of shrimp, not simply their presence. If a critical number of shrimp were ingested, the tadpole developed into a carnivore; if not, the tadpole developed by default into an omnivore. Thus a single cue - shrimp ingestion - triggers alternative ontogenetic trajectories. Using shrimp density to induce morph differentiation enables tadpoles to respond to their environment adaptively as shrimp are most abundant in highly ephemeral ponds, where the faster developing carnivores are favored.

Juvenile hormone-III biosynthesis in presumptive long-winged and short-winged Gryllus rubens: Implications for the endocrine regulation of wing dimorphism

In vitro rates of juvenile hormone-III biosynthesis were quantified for corpora allata from nascent long-winged and short-winged Gryllus rubens. Rates were measured during the penultimate and last stadia, two juvenile-hormone-sensitive periods of wing morph determination. This was done to assess the contribution of differences in rates of hormone biosynthesis to juvenile-hormone-mediated morph determination. Corpora allata from individuals of the same morph, sex and stage of development exhibited low variability in juvenile hormone-III biosynthetic rates. Developmental profiles of these biosynthetic rates for each sex and morph were U-shaped during each stadium. Rates varied from 1 to 5 and 0 to 5 pmol juvenile hormone-III/h/pair corpora allata during the penultimate and last stadium, respectively. Juvenile hormone-III biosynthetic rates did not differ significantly between morphs of the same sex on any day in development, except for a few cases at the beginning or end of each stadium. In general, developmental patterns of juvenile hormone-III biosynthesis were similar between the sexes of each morph. These data (1) provide no evidence that morph-specific differences in rates of juvenile hormone-III biosynthesis underlie juvenile-hormone-mediated morph determination, and (2) further implicate previously documented variation in juvenile hormone esterase activity in morph determination. Data on juvenile hormone biosynthesis, degradation and titre also collectively implicate cessation of biosynthesis rather than elevation of juvenile hormone esterase activity in producing the steep decline in the juvenile hormone titre early in the last stadium; this decline presumably initiates metamorphosis. Alternate morph development appears to result at least in part from differential clearance of low levels of juvenile hormone following the steep decline.

The Relation of Adult Wing-Form in the Brown Planthopper, Nilaparvata lugens STÅL (Homoptera : Delphacidae) to Wing-Pad Length of Last Instar Nymphs

Relationships between adult wing-form and wing-pad length in Nilaparvata lugens were examined under laboratory conditions (25±1°C). Most of the last instar female nymphs with wing-pads shorter than 0.94 mm developed to brachypterous adults, but female nymphs with wing-pads longer than 0.94 mm became both brachypterous and macropterous adults. Thus the brachypterous females of this insect consisted of short wing-pad brachypters and long wing-pad brachypters. In females, with the increase of rearing density during the nymphal stage, short wing-pad brachypters decreased with the increase of long wing-pad brachypters. This was followed by an increase of macropters. Male nymphs with shorter wing-pads tended to become brachypterous adults, but a critical wing-pad size for induction of macropters was not detected. Male nymphs emerged as macropterous adults over the wide density range studied. Relationships between weight and wing-pad length of last instar nymphs were different between sexes. Body weight tended to be heavier in female nymphs with short wing-pads than in those with long wing-pads. The wing morph determination mechanism in N. lugens females was hypothesized to consist of 2 sequential processes. The first process may affect development of wing-pads, while the second process influences development of adult wings.

Wing dimorphism in Gryllus rubens: genetic basis of morph determination and fertility differences between morphs.

The genetic basis of wing morph determination and fertility differences between wing morphs were studied in the wing-dimorphic cricket, Gryllus rubens. Using pair corsses, a significant effect of genotype on morph determination was documented in F3 progeny of field-collected crickets. The effect of genotype was significantly stronger in females than in males. Results are consistent with an earlier study (Zera and Tiebel 1988) showing that wing development is more strongly buffered from environmental variation in females. Segregation patterns were consistent with a polygenic mode of inheritance and provided no evidence for the existence of genes of major effect, maternal effects, or sex linkage. Only a weak morph x sex association was observed. These results contrast those of Walker (1987) where sex-linked loci of major effect on morph determination were identified in crosses between long-wing and short-wing-selected strains of G. rubens. Short-winged female G. rubens began ovipositing earlier and oviposited significantly more eggs than long-winged females during the first 24 days after adult eclosion. The greater reproductive output of the short-winged morph was due entirely to greater oviposition during the first two weeks after adult eclosion. Preliminary results indicate that flight may further accentuate the reduced reproductive output of long-winged versus short-winged females. These data provide the foundation for investigating the endocrine basis of morph-associated fertility variation in G. rubens and its mechanistic relationship with morph determination.

Phenological studies on the wing dimorphism of a semi‐aquatic bug, Microvelia douglasi (Heteroptera: Veliidae)

SummarySeasonal changes in the percentage of macropterous individuals in natural population of Microvelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.

‘Bistability’ experiments and photoperiodic morph determination in the aphid Aphis fabae

Abstract. Newly born larvae (presumptive gynoparae) of the aphid Aphis fabae were exposed to ‘symmetrical skeleton’ photoperiods, consisting of two equal pulses of light per 24 h. The general form of the photoperiods tested was LDLD 1:10:1:12 h. This regime is open to two ‘interpretations’, LDLD 1:10:1:12 h (PPs12) and LDLD 1:12:1:10 h (PPs14). As both light pulses are close to 12 h apart, it is expected that the interpretations result in two distinct steady state phase relationships, i.e. this regime would show bistability. If this is true and both interpretations could be adopted, PPs12 is expected to result in a low and PPs14 in a high percentage of ‘apterized’ insects.Experiments undertaken with the aphid showed that bistability did not occur either at 15d̀C or 20d̀C, and the results could be entirely explained on the basis of an ‘instantly’ damping circadian oscillator clock.

Juvenile hormone and ecdysteroid titres during critical periods of wing morph determination in Gryllus rubens

Juvenile hormone and ecdysteroid titre developmental profiles were compared between Gryllus rubens that were genetically-determined to develop into long-winged vs short-winged adults. Hormone titres were measured during the penultimate and last stadia, two periods during which wing length development is sensitive to exogenous juvenile hormone. No large differences in juvenile hormone titres were observed between nascent wing morphs during any stage in development. Slightly elevated titres were observed in long-wing vs short-wing-destined crickets during the middle third of the penultimate stadium. However, titre differences were opposite those inferred from earlier experiments involving topical application of juvenoids. An earlier decline in the juvenile hormone titre was also suggested in presumptive long-winged crickets during the last stadium. The earlier decline in macropters is consistent with previous juvenile hormone topical application experiments as well as with morph specific differences in juvenile hormone esterase activity. Peak ecdysteroid titres were higher in presumptive macropterous vs brachypterous males, while an elevated ecdysteroid titre occurred for a longer duration in macropterous females during the penultimate stadium. In both sexes, ecdysteroid titres were higher in presumptive long-winged vs short-winged morphs throughout the last stadium; peak titres were significantly higher in macropterous vs brachypterous females but not males. Both the longer duration of elevated ecdysteroid titres and the higher hormone titres in presumptive long-winged crickets are consistent with a regulatory role of ecdysteroids in wing morph determination.

Adaptation, compromise, and constraint: the development, morphometrics, and behavioral basis of a fighter-flier polymorphism in male Hoplothrips karnyi (Insecta: Thysanoptera)

Males of the colonial, wing-polymorphic thrips Hoplothrips karnyi (Hood) fight each other with their forelegs in defense of communal female oviposition areas. In this study, males were reared individually under varying conditions of food deprivation to investigate the developmental cues used in morph determination and the relationships between wing morph, developmental time in each instar, propupal weight, and five adult morphological characters associated with fighting ability and dispersal ability. Males deprived of food for five days midway through the second (final) larval instar had smaller propupal weights and were more likely to develop wings than males deprived of food in the first instar or control males. However, the mean propupal weight of all males that developed wings was not significantly less than that of wingless males. Wing morph of female parents had no measurable effect on this character in the offspring. Wingless males possess relatively larger fore-femora and prothoraces than do winged males, but winged males possess relatively larger pterothoraces (Fig. 1). Behavioral observations of wingless and winged males of similar weight as propupae showed that wingless males won fights and became dominant in oviposition areas. Thus, a trade-off exists between characters associated with male fighting and dispersal ability. The cost of wings, in terms of fore-femora size and prothorax size, increased with propupal weight. Wingless males that developed in the experimental treatment that produced a high proportion of winged males were relatively small in size, and were intermediate in body shape with respect to winged males and other wingless males (Fig. 2). This shape intermediacy indicates that there may be developmental constraints on alternative tactics of resource allocation. Total developmental time varied between wing morphs, but was not correlated with propupal weight or adult morphological characters of winged or wingless males. For wingless males that developed in the treatment that produced a high proportion of winged males, adult morphological characters were negatively correlated with the duration of the second instar. This correlation suggests that the development of small wingless males involves a compromise between the benefits of large adult size and the costs of prolonging the second instar to increase the probability of becoming larger.

EVOLUTIONARY REDUCTION OF COMPLEX LIFE CYCLES: LOSS OF HOST-ALTERNATION IN PEMPHIGUS (HOMOPTERA: APHIDIDAE).

In a Utah canyon, the aphid, Pemphigus betae, exhibits two life cycles: a cycle involving host-alternation between cottonwood trees and roots of herbaceous plants and a secondarily reduced cycle, in which the cottonwood generations are eliminated so that wingless forms live year round on roots. Relative frequencies of the two life-cycle types vary along a 30-km stretch of the canyon, with the reduced cycle predominating at upper sites. Factors underlying this life-cycle variation were examined with common-garden and transfer experiments. Results showed 1) a facultative increase in production of alternating forms in response to crowding in root colonies, 2) a genetic component to both within- and between-site variation in tendency to produce alternating morphs, and 3) site-specific environmental effects on level of investment in the reduced versus alternating life cycles. Thus, the variation in frequency of life-cycle reduction in this aphid is dependent on a complex of interdependent factors. These include adaptive phenotypic plasticity, microgeographically variable cues affecting mechanisms of morph determination, and genetically based variation in tendency to show reduction versus alternation. Genetic variation between sites corresponds to microgeographic variation in success of life-cycle phases. Where cottonwood hosts are absent (lower elevations) or where the cottonwood phase has low survival (upper elevations), clones tend to produce fewer migrating morphs, as compared to clones from middle elevations, where the cottonwood phase is relatively favorable. Such independence between conditions in alternate phases is a general feature of complex life cycles and can generate strong site-specific selection for permanent life-cycle reduction. Such life-cycle shifts have sometimes been followed by extensive radiations in aphids and other groups.