Phenotypic Plasticity in Response to the Heterogeneous Water Supplyin the Rhizomatous Grass Species, Calamagrostis epigejosin the Mu Us Sandy Land of China

Calamagrostis epigejos (L.) Roth. is a perennial grass with slender and long rhizome segments between interconnected neighbor ramets. To investigate the phenotypic plasticity in response to the heterogeneous soil water supply, ramet pairs of the species were subjected to heterogeneous water supply by which either mother ramets or daughter ramets were in high or low soil water supply, respectively, in the Maowusu (Mu Us) Sandy Land of Nei Mongol. The results showed that the phenotypic characteristics of the individual ramets of C epigejos were greatly influenced by the heterogeneous water supply. The ramets treated with high water supply significantly produced more new rhizomes and more offspring (ramets), and accumulated more shoot biomass, and allocated more biomass to their shoots than those treated with low water supply. In comparison with the daughter ramets in homogeneous soil water supply, phenotypic characteristics, in terms of new rhizome growth, the production of new offspring, and the biomass allocation pattern, of the daughter ramets within the pairs of the species were not significantly changed, no matter that high or low soil water supply to mother ramets. The phenotypic responses of mother ramets to soil water supply were similar to those of daughter ramets. From these results, it is inferred that the interconnected ramets of C epigejos response phenotypically to their local soil water rather than to the soil water experienced by the interconnected ramets. The interconnected ramets of C epigejos might be independent of each other in water relationship, although they are physically interconnected with rhizome segments. The physiological independence of interconnected ramets might facilitate the risk spreading and thus enhance the genet survivorship under the frequent drought stresses in Mu Us Sandland.

Phenotypic plasticity in response to soil moisture availability in the clonal plant Eremosparton songoricum (Litv.) Vass.

Eremosparton songoricum （Litv.） Vass is a dwarf clonal shrubby legume developed on the windward slopes of mobile or semi-fixed sand dunes of Central Asia. It is assumed that E. songoricum must possess a high degree of phenotypic plasticity for such a heterogeneous habitat. The variations of E. songoricum plants growing in two typical microhabitats （the upper slope and the lower slope of semi-mobile dune） were investigated. The morphological characteristics and the biomass allocation patterns were measured and compared at the clonal fragment level. Compared with the clonal fragment on the lower slope of dune, the clonal fragment on the upper slope of the dune （a） declined in total biomass and ramet height, （b） increased the length of rhizomes and the number of roots, （c） increased the degree of asym- metry, and （d） heightened allocation to the belowground biomass. Our results confirmed the hypothesis of high phenotypic adjustment capacity of E. songoricum to habitat moisture availability. Phenotypic plasticity of E. songoricum contributed to reduce the damage risk, led to an environmentally induced specialization in function of resources exploitation, resulted in its persistence in heterogeneous environments and was adaptive in sand dune environment.

The first cavefish in the Dinaric Karst?Cave colonization made possible by phenotypic plasticity in Telestes karsticus

Cave animals are an excellent model system for studying adaptive evolution.At present,however,little is known about the mechanisms that enable surface colonizers to survive in the challenging environment of caves.One possibility is that these species have the necessary genetic background to respond with plastic changes to the pressures of underground habitats.To gain insight into this process,we conducted a comparative study with the fish species Telestes karsticus,which occurs in a hydrological system consisting of an interconnected stream and a cave.Results showed that T.karsticus resided year-round and spawned in Sušik cave,making it the first known cavefish in the Dinaric Karst.Cave and surface populations differed in morphological and physiological characteristics,as well as in patterns of gene expression without any evidence of genetic divergence.To test whether observed trait differences were plastic or genetic,we placed adult fish from both populations under light/dark or constant dark conditions.Common laboratory conditions erased all morphometric differences between the two morphs,suggesting phenotypic plasticity is driving the divergence of shape and size in wild fish.Lighter pigmentation and increased fat deposition exhibited by cave individuals were also observed in surface fish kept in the dark in the laboratory.Our study also revealed that specialized cave traits were not solely attributed to developmental plasticity,but also arose from adult responses,including acclimatization.Thus,we conclude that T.karsticus can adapt to cave conditions,with phenotypic plasticity playing an important role in the process of cave colonization.

Phenotypic plasticity of floral traits and pollination adaption in an alpine plant Pedicularis siphonantha D. Don when transplanted from higher to lower elevation in Eastern Himalaya

Phenotypic plasticity and/or pollinatormediated selection may be responsible for the changes in floral traits of plants when they are forced to live in new conditions. Although the two events could be independent, we hypothesized that phenotypic plasticity in floral traits might help to coordinate plant-pollinator interactions and enhance plant reproductive success in changing habitats. To test this hypothesis, we investigated floral traits and pollination on three natural populations of a lousewort(Pedicularis siphonantha) ranging at different elevations, as well as two downward transplanted populations in Shangeri-La County and Deqin County, northwest Yunnan, China. The results indicated that floral traits, i.e. phenology, longevity,display size, corolla tube length and pollen production differed significantly among populations. Moreover,or the two transplanted populations, floral traits diverged from their original populations, but converged to their host populations. Although the phenotypic plasticity in floral traits might be a rapid response to abiotic factor such as warmer environment, the changes in floral traits were found to be well adapted to pollination environment of the host population. Compared with plants of their original habitats in higher elevation, the transplanted individuals advanced flowering time, shortened flower longevity, reduced floral display size and pollen production, received higher visiting frequency and yielded more seeds. These findings suggested that phenotypic plasticity of floral traits might help plants adjust their resource allocation strategy between preand post-pollination stages in response to harsh or temperate conditions, which might correspondingly meet a pollinator-poor or hyphen rich environment.This would be beneficial for the widely-distributed species to adapt to various environmental changes.

Manipulation of photosensory and circadian signaling restricts phenotypic plasticity in response to changing environmental conditions in Arabidopsis

Plants exploit phenotypic plasticity to adapt their growth and development to prevailing environmental conditions.Interpretation of light and temperature signals is aided by the circadian system,which provides a temporal context.Phenotypic plasticity provides a selective and competitive advantage in nature but is obstructive during large-scale,intensive agricultural practices since economically important traits(including vegetative growth and flowering time)can vary widely depending on local environmental condi-tions.This prevents accurate prediction of harvesting times and produces a variable crop.In this study,we sought to restrict phenotypic plasticity and circadian regulation by manipulating signaling systems that govern plants'responses to environmental signals.Mathematical modeling of plant growth and develop-ment predicted reduced plant responses to changing environments when circadian and light signaling pathways were manipulated.We tested this prediction by utilizing a constitutively active allele of the plant photoreceptor phytochrome B,along with disruption of the circadian systemvia mutation of EARLYFLOW-ERING3.We found that these manipulations produced plants that are less responsive to light and temper-ature cues and thus fail to anticipate dawn.These engineered plants have uniform vegetative growth and flowering time,demonstrating how phenotypic plasticity can be limited while maintaining plant productiv-ity.This has significant implications for future agriculture in both open fields and controlled environments.

Phenotypic Plasticity of Life History Characteristics: Quantitative Analysis of Delayed Reproduction of Green Foxtail (Setaria viridis) in the Songnen Plain of China

Green foxtail （Setaria viridis L.） is a common weed species in temperate regions. Research on the effect of delayed reproduction on the phenotypic plasticity and regularity of the vegetative and reproductive growth is of vital significance for understanding population regulation and control of the weed in the growing season. Green foxtail seeds were sown every 10 days from 25 June to 24 August of 2004. The growth and production metrics were measured via harvesting tufts and statistical analysis was carried out. The results showed that the reproductive tillers, seed number, seed biomass and one thousand-seed weight of plants at the first sowing （25 June） approximately increased 28.8, 7 827.0, 1 104,0 and 12.3 times compared with that at the last sowing （24 August）, respectively. Total tillers, reproductive tillers and height increased linearly as the reproductive period delayed, however, biomass increased exponentially. Quadratic equations best explained the relationships between the delayed reproductive period and seed number, inflorescence length, one thousand-seed weight, seed biomass. Based on the quantity and quality of seed production, weeding young seedlings emerging before July can be the most effective weed-control strategy in the Songnen Plain.

The evolution of phenotypic plasticity in fish swimming

Fish have a remarkable amount of variation in their swimming performance, from within species dif- ferences to diversity among major taxonomic groups. Fish swimming is a complex, integrative phenotype and has the ability to plastically respond to a myriad of environmental changes. The plasticity of fish swimming has been observed on whole-organismal traits such as burst speed or critical swimming speed, as well as underlying phenotypes such as muscle fiber types, kinematics, cardiovascular system, and neuronal processes. Whether the plastic responses of fish swimming are beneficial seems to depend on the environmental variable that is changing. For example, because of the effects of temperature on biochemical processes, alterations of fish swimming in response to tem- perature do not seem to be beneficial. In contrast, changes in fish swimming in response to variation in flow may benefit the fish to maintain position in the water column. In this paper, we examine how this plasticity in fish swimming might evolve, focusing on environmental variables that have received the most attention： temperature, habitat, dissolved oxygen, and carbon dioxide variation. Using examples from previous research, we highlight many of the ways fish swimming can plastic- ally respond to environmental variation and discuss potential avenues of future research aimed at understanding how plasticity of fish swimming might evolve. We consider the direct and indirect ef- fects of environmental variation on swimming performance, including changes in swimming kine- matics and suborganismal traits thought to predict swimming performance. We also discuss the role of the evolution of plasticity in shaping macroevolutionary patterns of diversity in fish swimming.

RPL1, a Gene Involved in Epigenetic Processes Regulates Phenotypic Plasticity in Rice

Organisms can adjust their phenotype in response to changing environmental conditions. This phenomenon is termed phenotypic plasticity. Despite its ubiquitous occurrence, there has been very little study on the molecular mechanism of phenotypic plasticity. In this study, we isolated a rice （Oryza sativa L.） mutant, rice plasticity 1 （rpl1）, that displayed increased environment-dependent phenotypic variations. RPL1 was expressed in all tissues examined. The protein was localized in the nucleus and its distribution in the nucleus overlapped with heterochromatin. The rpll mutation led to an increase in DNA methylation on repetitive sequences and a decrease in overall histone acetylation. In addition, the mutation affected responses of the rice plant to phytohormones such as brassinosteroid, gibberellin, and cytokinin. Analysis of the putative rice brassinosteroid receptor OsBRI1, a key hormone signaling gene, indicated that RPL1 may be involved in the regulation of epigenomic modification of the gene. These data suggest that RPL1 regulated phenotypic plasticity likely through its involvement in epigenetic processes affecting responses of the plant to phytohormones.

Modeling the influence of phenotypic plasticity on maize hybrid performance

Phenotypic plasticity,the ability of an individual to alter its phenotype in response to changes in the environment,has been proposed as a target for breeding crop varieties with high environmental fitness.Here,we used phenotypic and genotypic data from multiple maize(Zea mays L.)populations to mathematically model phenotypic plasticity in response to the environment(PPRE)in inbred and hybrid lines.PPRE can be simply described by a linear model in which the two main parameters,intercept a and slope b,reflect two classes of genes responsive to endogenous(class A)and exogenous(class B)signals that coordinate plant development.Together,class A and class B genes contribute to the phenotypic plasticity of an individual in response to the environment.We also made connections between phenotypic plasticity and hybrid performance or general combining ability(GCA)of yield using 30 F_(1) hybrid populations generated by crossing the same maternal line with 30 paternal lines from different maize heterotic groups.We show that the parameters a and b from two given parental lines must be concordant to reach an ideal GCA of F_(1) yield.We hypothesize that coordinated regulation of the two classes of genes in the F_(1) hybrid genome is the basis for high GCA.Based on this theory,we built a series of predictive models to evaluate GCA in silico between parental lines of different heterotic groups.

Phenotypic plasticity Gasterosteus aculeatus ence in captivity of the threespine stickleback telencephalon in response to experi-

Threespine stickleback were used to examine phenotypic plasticity of telencephala in relation to inferred ecology. Fish from derived, allopatric, freshwater populations were sampled from three shallow, structurally complex lakes with ben- thic-foraging stickleback （benthics） and from three deep, structurally simple lakes with planktivores （limnetics）. The telencepha- Ion of specimens preserved immediately after capture （field-preserved）, field-caught fish held in aquaria for 90 days （lab-held）, and lab-bred fish from crosses and raised in aquaria were compared. Field-preserved sea-run （ancestral） stickleback were col- lected from two separate sites, and parents of lab-bred sea-run stickleback were collected from one of these sites. In field-preserved and lab-held fish, the telencephala of limnetics exhibited triangular dorsal shape, while those of benthics and sea-run fish had rounder shapes. No such pattern was detected in lab-bred fish. Within each treatment type, benthics had larger relative telencephalon sizes, using overall brain size as the covariate, than limnetics. Among field-preserved samples, sea-run fish had smaller telencephalon sizes than lake fish. Intra-population analyses of lake samples showed that field-preserved fish consis- tently had larger relative telencephalon sizes than lab-bred fish. The opposite was true of the sea-run population. In a separate study using one benthic population and one limnetic population, samples were preserved in the field immediately or held in the lab for 30, 60, and 90 days before they were sacrificed. In both populations, the telencephalon shapes of lab-held fish were similar to those of field-preserved fish but became progressively more like lab-bred ones over 90 days. In contrast, relative telencephalon size decreased dramatically by 30 days after which there was littie change. In freshwater threespine stickleback, the telencephalon exhibits considerable phenotypic plasticity, which was probably present in the ancestor [Current Zoology 58 （1）： 189-210, 2012].

Phenotypic plasticity in the monoclonal marbled crayfish is associated with very low genetic diversity but pronounced epigenetic diversity

Clonal organisms are particularly useful to investigate the contribution of epigenetics to phenotypic plasticity,because confounding effects of genetic variation are negligible.In the last decade,the apomictic parthenogenetic marbled crayfish,Procambarus virginalis,has been developed as a model to investigate the relationships between phenotypic plasticity and genetic and epigenetic diversity in detail.This crayfish originated about 30 years ago by autotriploidy from a single slough crayfish Procambarus fallax.As the result of human releases and active spreading,marbled crayfish has established numerous populations in very diverse habitats in 22 countries from the tropics to cold temperate regions.Studies in the laboratory and field revealed considerable plasticity in coloration,spination,morphometric parameters,growth,food preference,population structure,trophic position,and niche width.Illumina and PacBio whole-genome sequencing of marbled crayfish from representatives of 19 populations in Europe and Madagascar demonstrated extremely low genetic diversity within and among populations,indicating that the observed phenotypic diversity and ability to live in strikingly different environments are not due to adaptation by selection on genetic variation.In contrast,considerable differences were found between populations in the DNA methylation patterns of hundreds of genes,suggesting that the environmentally induced phenotypic plasticity is mediated by epigenetic mechanisms and corresponding changes in gene expression.Specific DNA methylation fingerprints persisted in local populations over successive years indicating the existence of epigenetic ecotypes,but there is presently no information as to whether these epigenetic signatures are transgenerationally inherited or established anew in each generation and whether the recorded phenotypic plasticity is adaptive or nonadaptive.

Role of phenotypic plasticity in morphological differentiation between watersnake populations

An individual’s morphology is shaped by the environmental pressures it experiences,and the resulting morphological response is the culmination of both genetic factors and environmental(non-genetic)conditions experienced early in its life(i.e.phenotypic plasticity).The role that phenotypic plasticity plays in shaping phenotypes is important,but evidence for its influence is often mixed.We exposed female neonate diamond-backed watersnakes(Nerodia rhombifer)from populations experiencing different prey-size regimes to different feeding treatments to test the influence of phenotypic plasticity in shaping trophic morphology.We found that snakes in a large-prey treatment from a population frequently encountering large prey exhibited a higher growth rate in body size than individuals in a small-prey treatment from the same population.This pattern was not observed in snakes from a population that regularly encounters small prey.We also found that regardless of treatment,snakes from the smallprey population were smaller at birth than snakes from the large-prey population and remained so throughout the study.These results suggest that the ability to plastically respond to environmental pressures may be populationspecific.These results also indicate a genetic predisposition towards larger body sizes in a population where large prey items are more common.

Significance of differential allelic expression in phenotypic plasticity and evolutionary potential of microbial eukaryotes

Background:Differential allelic expression(DAE)plays a key role in the regularion of many biological processes,and it may also play a role in adaptive evolution.Recently,environment-dependent DAE has been observed in species of marine phytoplankton,and most remarkably,alleles that showed the highest level of DAE also showed the fastest rate of evolution.Methods:To better understand the role of DAE in adaptive evolution and phenotypic plasticity,we developed a 2-D cellular automata model“DAEsy-World”that builds on the classical Daisyworld model.Results:Simulations show that DAE delineates the evolution of alternative alleles of a gene,enabling the two alleles to adapt to different environmental conditions and sub-functionalize.With DAE,the build-up of genetic polymorphisms within genes is driven by positive selection rather than strict neutral evolution,and this can enhance phenotypic plasticity.Moreover,in sexually reproducing organisms,DAE also increased the standing genetic variation,augmenting a species’adaptive evolutionary potential and ability to respond to fluctuating and/or changing conditions(cf,genetic assimilation).We furthermore show that DAE is likely to evolve in fluctuating environmental conditions.Conclusions:DAE increases the adaptive evolutionary potential of both sexual and asexually reproducing organisms,and it may affect the pattern of nucleotide substitutions of genes.

Genetic interrogation of phenotypic plasticity informs genome-enabled breeding in cotton

Phenotypic plasticity, or the ability to adapt to and thrive in changing climates and variable environments, is essential for developmental programs in plants. Despite its importance, the genetic underpinnings of phenotypic plasticity for key agronomic traits remain poorly understood in many crops. In this study, we aim to fill this gap by using genome-wide association studies to identify genetic variations associated with phenotypic plasticity in upland cotton (Gossypium hirsutum L.). We identified 73 additive quantitative trait loci (QTLs), 32 dominant QTLs, and 6799 epistatic QTLs associated with 20 traits. We also identified 117 additive QTLs, 28 dominant QTLs, and 4691 epistatic QTLs associated with phenotypic plasticity in 19 traits. Our findings reveal new genetic factors, including additive, dominant, and epistatic QTLs, that are linked to phenotypic plasticity and agronomic traits. Meanwhile, we find that the genetic factors controlling the mean phenotype and phenotypic plasticity are largely independent in upland cotton, indicating the potential for simultaneous improvement. Additionally, we envision a genomic design strategy by utilizing the identified QTLs to facilitate cotton breeding. Taken together, our study provides new insights into the genetic basis of phenotypic plasticity in cotton, which should be valuable for future breeding.

Plasticity in Metamorphic Traits of Rice Field Frog(Rana limnocharis) Tadpoles:The Interactive Effects of Rearing Temperature and Food Level

In organisms with complex life cycles, such as amphibians, morphological variation is strongly influenced by environmental factors （e.g. temperature） and maternal effects （e.g. diet）. Although temperature and food level exert a strong influence on larval growth, little is known about the interacting effects of these factors on age and size at metamorphosis. In this study, plasticity in growth rates, survival, larval period, and size at metamorphosis were examined in Rice field Frog （Rana limnocharis） under different combinations of rearing temperature and food level. Rearing temperature did not affect age at metamorphosis, but a significant interaction between temperature and food level revealed that of tadpoles feeding at a high food level, those reared at 32℃ had a shorter length of larval period than those reared at 29℃ or 26℃. Similarly, our results also showed high food level produced a larger growth rate and mass at metamorphosis at 32℃, but not at 29 and 26℃. Therefore, our results revealed that the effects of food level on larval growth and metamorphosis were highly dependent on developmental temperature.

Leaf phenotypic variation in natural populations of Carpinus tschonoskii in China

Carpinus tschonoskii Maxim.exhibits rich leaf phenotypic variation and various leaf shapes,but few studies show why leaf phenotypic traits have such a large variation.Basic morphological markers may provide guidance for studying plant genetic variation and species protection and utilization.To study leaf phenotypic variations and the relationship between variation characteristics and climatic and geographical factors,phenotypic traits among natural populations were investigated.Results revealed that leaf phenotypes varied significantly among and within populations.Some populations had higher phenotypic diversity,while others had lower phenotypic diversity.Among the phenotypic traits,leaf area and petiole length had the most variation.Leaf index and primary lateral veins were the most stable phenotypes,which may be important reference indexes for phenotype identification in field investigations.There was a strong consistency between leaf phenotypic traits and geographical location.Plants in high latitudes tend to have longer leaves,and plants in low temperatures tend to have longer leaves and larger leaf perimeter.In addition,plants in areas with less rainfall have longer petioles.The 13 populations of C.tschonoskii can be divided into four branches by cluster analysis,and the results show a good relationship with the geographical location of each population.Additionally,some populations geographically isolated also had unique leaf phenotypes.

Causal Analysis Between Rice Growth and Cadmium Accumulation and Transfer under Arbuscular Mycorrhizal Inoculation

Cadmium(Cd)contamination in rice has been a serious threat to human health.To investigate the effects of arbuscular mycorrhizal fungi(AMF)on the Cd translocation in rice,a controlled pot experiment was conducted.The results indicated that AMF significantly increased rice biomass,with an increase of up to 40.0%,particularly in root biomass by up to 68.4%.Notably,the number of prominent rice individuals also increased,and their plasticity was enhanced following AMF inoculation.AMF led to an increase in the net photosynthetic rate and antioxidant enzyme activity of rice.In the AMF treatment group,the Cd concentration in the rice roots was significantly higher(19.1%‒68.0%)compared with that in the control group.Conversely,the Cd concentration in the rice seeds was lower in the AMF treatment group,indicating that AMF facilitated the sequestration of Cd in rice roots and reduced Cd accumulation in the seeds.Path coefficients varied across different treatments,suggesting that AMF inoculation reduced the direct impact of soil Cd concentration on the total Cd accumulation in seeds.The translocation of Cd was consistently associated with simultaneous growth dilution and compensatory accumulation as a result of mycorrhizal effects.Our study quantitatively analyzed this process through path analysis and clarified the causal relationship between rice growth and Cd transfer under the influence of AMF.

Differences in functional traits and reproductive allocations between native and invasive plants

Because co-occurring native and invasive plants are subjected to similar environmental selection pressures,the differences in functional traits and reproductive allocation strategies between native and invasive plants may be closely related to the success of the latter.Accordingly,this study examines differences in functional traits and reproductive allocation strategies between native and invasive plants in Eastern China.Plant height,branch number,reproductive branch number,the belowground-to-aboveground biomass ratio,and the reproductive allocation coefficient of invasive plants were all notably higher than those of native species.Additionally,the specific leaf area(SLA)values of invasive plants were remarkably lower than those of native species.Plasticity indexes of SLA,maximum branch angle,and branch number of invasive plants were each notably lower than those of native species.The reproductive allocation coefficient was positively correlated with reproductive branch number and the belowground-to-aboveground biomass ratio but exhibited negative correlations with SLA and aboveground biomass.Plant height,branch number,reproductive branch number,the belowground-to-aboveground biomass ratio,and the reproductive allocation coefficient of invasive plants may strongly influence the success of their invasions.

Plant hormesis and Shelford’s tolerance law curve

Shelford’s law of tolerance is illustrated by a bell-shaped curve depicting the relationship between environmental factor/factors’intensity and its favorability for species or populations.It is a fundamental basis of ecology when considering the regularities of environment impacts on living systems,and applies in plant biology,agriculture and forestry to manage resistance to environmental limiting factors and to enhance productivity.In recent years,the concept of hormesis has been increasingly used to study the dose-response relationships in living organisms of different complexities,including plants.This requires the need for an analysis of the relationships between the hormetic dose-response model and the classical understanding of plant reactions to environments in terms of Shelford’s law of tolerance.This paper analyses various dimensions of the relationships between the hormetic model and Shelford’s tolerance law curve under the influence of natural environmental factors on plants,which are limiting for plants both in deficiency and excess.The analysis has shown that Shelford’s curve and hormetic model do not contradict but instead complement each other.The hormetic response of plants is localized in the stress zone of the Shelford’s curve when adaptive mechanisms are disabled within the ecological optimum.At the same time,in a species range,the ecological optimum is the most favorable combination of all or at least the most important environmental factors,each of which usually deviates slightly from its optimal value.Adaptive mechanisms cannot be completely disabled in the optimum,and hormesis covers optimum and stress zones.Hormesis can modify the plant tolerance range to environmental factors by preconditioning and makes limits of plant tolerance to environmental factors flexible to a certain extent.In turn,as a result of tolerance range evolution,quantitative characteristics of hormesis(width and magnitude of hormetic zone)as well as the range of stimulating doses,may significantly differ in various plant species and even populations and intra-population groups,including plants at different development stages.Using hormetic preconditioning for managing plant resistance to environmental limiting factors provides an important perspective for increasing the productivity of woody plants in forestry.

Behavioral and physiological flexibility are used by birds to manage energy and support investment in the early stages of reproduction

Interest in phenotypic flexibility has increased dramatically over the last decade, but flexibility during reproduction has received relatively little attention from avian scientists, despite its possible impact on fitness. Because most avian species maintain atrophied reproductive organs when not active, reproduction in birds requires major tissue remodeling in preparation for breeding. Females undergo rapid （days） recrudescence and regression of their reproductive organs at each breeding attempt, while males grow their organs ahead of time at a much slower rate （weeks） and may maintain them at maximal size throughout the breeding season. Reproduction is associated with significant metabolic costs. Egg production leads to a 22%-27% increase in resting metabolic rate （RMR） over non-reproductive values. This is partly due to the activity of the oviduct, an organ that may allow females to adjust reproductive investment by modulating egg size and quality. In males, gonadal recrudescence may lead to a 30% increase in RMR, but the data are inconsistent and general conclusions regarding energetic costs of reproduction in males will require more research. Recent studies on captive female zebra finches describe the impacts of these costs on daily energy budgets and highlight the strategies used by birds to maintain their investment in reproduction when energy is limited. Whenever possible, birds use behavioral flexibility as a first means of saving energy. Decreasing locomotor activity saves energy during challenges such as egg production or exposure to cold temperatures and is an efficient way to buffer variation in individual daily energy budgets. However, when behavioral flexibility is not possible, birds must rely on flexibility at the physiological level to meet energy demands. In zebra finches breeding in the cold, this results in a reduced pace of laying, likely due to down-regulation of both reproductive and non-reproductive function, allowing females to defend minimal egg size and maintain reproductive success. More research involving a range of species in captive and flee-living conditions is needed to determine how phenotypic flexibility during tissue remodeling and early reproductive investment translates to natural conditions and affects fitness [Current Zoology 56 （6）： 767-792, 2010].