Density-dependent Phenotypic Plasticity Research Articles

Comparative analysis of phenotypic plasticity sheds light on the evolution and molecular underpinnings of locust phase polyphenism

Locusts exhibit one of nature’s most spectacular examples of complex phenotypic plasticity, in which changes in density cause solitary and cryptic individuals to transform into gregarious and conspicuous locusts forming large migrating swarms. We investigated how these coordinated alternative phenotypes might have evolved by studying the Central American locust and three closely related non-swarming grasshoppers in a comparative framework. By experimentally isolating and crowding during nymphal development, we induced density-dependent phenotypic plasticity and quantified the resulting behavioural, morphological, and molecular reaction norms. All four species exhibited clear plasticity, but the individual reaction norms varied among species and showed different magnitudes. Transcriptomic responses were species-specific, but density-responsive genes were functionally similar across species. There were modules of co-expressed genes that were highly correlated with plastic reaction norms, revealing a potential molecular basis of density-dependent phenotypic plasticity. These findings collectively highlight the importance of studying multiple reaction norms from a comparative perspective.

There is no magic bullet: the importance of testing reference gene stability in RT-qPCR experiments across multiple closely related species.

Reverse Transcriptase quantitative Polymerase Chain Reaction (RT-qPCR) is the current gold standard tool for the study of gene expression. This technique is highly dependent on the validation of reference genes, which exhibit stable expression levels among experimental conditions. Often, reference genes are assumed to be stable a priori without a rigorous test of gene stability. However, such an oversight can easily lead to misinterpreting expression levels of target genes if the references genes are in fact not stable across experimental conditions. Even though most gene expression studies focus on just one species, comparative studies of gene expression among closely related species can be very informative from an evolutionary perspective. In our study, we have attempted to find stable reference genes for four closely related species of grasshoppers (Orthoptera: Acrididae) that together exhibit a spectrum of density-dependent phenotypic plasticity. Gene stability was assessed for eight reference genes in two tissues, two experimental conditions and all four species. We observed clear differences in the stability ranking of these reference genes, both between tissues and between species. Additionally, the choice of reference genes clearly influenced the results of a gene expression experiment. We offer suggestions for the use of reference genes in further studies using these four species, which should be taken as a cautionary tale for future studies involving RT-qPCR in a comparative framework.

DNA methyltransferase 3 participates in behavioral phase change in the migratory locust

DNA methylation plays important roles in the behavioral plasticity of animals. The migratory locust, Locusta migratoria, displays striking density-dependent phenotypic plasticity that can reversely transit between solitarious and gregarious phases. However, the role and the mechanism through which DNA methylation is involved in locust phase change remain unknown. Here, we investigated the expression levels of three DNA methyltransferase genes and their roles in the regulation of locust phase changes. All three Dnmt genes, namely, Dnmt1, Dnmt2 and Dnmt3 showed high expression levels in the brains of gregarious locusts. By contrast, only Dnmt3 transcript rapidly responded to population density changes, decreasing during the isolation of gregarious locusts and steadily increasing upon the crowding of solitarious locusts. Dnmt3 knockdown significantly reduced the phase-related locomotor activity, rather than the attraction index, in gregarious and crowded solitarious locusts. Transcriptome analysis showed that Dnmt3 knockdown upregulated the genes related to metabolism and transporting activity and downregulated those associated with oxidative stress response. The expression level of the phase-core transcriptional factor, hormone receptor HR3, was significantly suppressed in the brain after Dnmt3 knockdown. Moreover, there was significant overlap in the differentially expressed genes between Dnmt3 RNAi and HR3 RNAi data sets, suggesting HR3 may act as key transcriptional factor mediating Dnmt3-controlled gene expression profiles in locust brains. These findings suggest that Dnmt3 transcription is involved in locust behavioral transition, implying the possible roles of DNA methylation in phase-related phenotypic plasticity in locusts.

Revealing hidden density-dependent phenotypic plasticity in sedentary grasshoppers in the genus Schistocerca Stål (Orthoptera: Acrididae: Cyrtacanthacridinae)

Comparative quantification of reaction norms across closely related species in a clade is rare, but such a study can reveal valuable insights into understanding how reaction norms evolve along phylogeny. The grasshopper genus Schistocerca Stål (Orthoptera: Acrididae: Cyrtacanthacridinae) is an ideal group to study the evolution of density-dependent phenotypic plasticity because it includes both swarming locusts and non-swarming sedentary grasshoppers, which show varying degrees of plastic reaction norms in many traits. The swarming locusts exhibit locust phase polyphenism in which cryptically colored and solitary individuals can transform into conspicuously colored and highly gregarious individuals in response to increases in population density. The sedentary grasshoppers do not swarm in nature, and thus it has been assumed that they have little or no expression of plastic reaction norms in many traits, except for color, which has been shown to be a phylogenetically conserved trait. In this study, we have quantified density-dependent reaction norms in behavior, color, body size, and morphometric ratio in the nymphs of four sedentary species within Schistocerca by conducting explicit rearing experiments to induce potential phenotypic changes in response to isolation and crowding. In contrast to our previous assumption, we find that all four species show a certain level of density-dependent plastic reaction norms, which implies that these sedentary species have hidden reaction norms that can only be induced experimentally, some components of which must be phylogenetically conserved. Furthermore, we demonstrate that rearing density differentially affects the expression of reaction norms in different species, suggesting that different reaction norms must have followed independent evolutionary trajectories.

Density-Dependent Phenotypic Plasticity in the South American Locust, Schistocerca cancellata (Orthoptera: Acrididae)

AbstractHistorically, the South American locust, Schistocerca cancellata (Serville, 1838), has been considered the most serious agricultural pest in Argentina. An outbreak of a magnitude not recorded since 1954 started in 2015 through 2017 in northern Argentina and neighboring Paraguay and Bolivia. Schistocerca cancellata is widely considered as a true locust, with pronounced locust phase polyphenism, although the expression of its phenotypic plasticity has never been quantitatively tested under different density conditions. In this study, we explicitly quantified density-dependent reaction norms in behavior, coloration, and morphology in last instar nymphs of S. cancellata under isolated and crowded conditions. We also quantified density-dependent plasticity in adults (size) and in some life history traits. Our results showed that crowded nymphs were significantly more active and more attracted to congeners than isolated nymphs, and developed a much higher percentage of black pattern color. We also found that density had strong effects on body size and there was a sex-dependent pattern in both nymphs and adults, revealing that differences in size between males and females were less pronounced in crowded locusts. We have recorded for the isolated nymphs the presence of about 50% more hairs in the hind femora than in crowded nymphs. Finally, the mean duration of each nymphal instar and adult stage was significantly longer in isolated individuals. We have found strong resemblance with the desert locust, S. gregaria (Forskål, 1775) in several traits, and we conclude that S. cancellata exhibits an extreme form of density-dependent phenotypic plasticity in behavior, coloration, morphology, and life history traits.

Phylogeny of locusts and grasshoppers reveals complex evolution of density-dependent phenotypic plasticity

Locusts are grasshoppers that can form dense migrating swarms through an extreme form of density-dependent phenotypic plasticity, known as locust phase polyphenism. We present a comprehensive phylogeny of the genus Schistocerca, which contains both non-swarming grasshoppers and swarming locusts. We find that the desert locust, S. gregaria, which is the only Old World representative of the genus, is the earliest diverging lineage. This suggests that the common ancestor of Schistocerca must have been a swarming locust that crossed the Atlantic Ocean from Africa to America approximately 6 million years ago, giving rise to the current diversity in the New World. This also implies that density-dependent phenotypic plasticity is an ancestral trait for the genus. Through ancestral character reconstruction of reaction norms, we show that colour plasticity has been largely retained in most species in the genus, but behavioural plasticity was lost and regained at least twice. Furthermore, we show that swarming species do not form a monophyletic group and non-swarming species that are closely related to locusts often express locust-like plastic reaction norms. Thus, we conclude that individual reaction norms have followed different evolutionary trajectories, which have led to the evolutionary transition between grasshoppers and locusts - and vice versa.

Non-swarming grasshoppers exhibit density-dependent phenotypic plasticity reminiscent of swarming locusts

Locusts are well known for exhibiting an extreme form of density-dependent phenotypic plasticity known as locust phase polyphenism. At low density, locust nymphs are cryptically colored and shy, but at high density they transform into conspicuously colored and gregarious individuals. Most of what we know about locust phase polyphenism come from the study of the desert locust Schistocerca gregaria (Forskål), which is a devastating pest species affecting many countries in North Africa and the Middle East. The desert locust belongs to the grasshopper genus Schistocerca Stål, which includes mostly non-swarming, sedentary species. Recent phylogenetic studies suggest that the desert locust is the earliest branching lineage within Schistocerca, which raises a possibility that the presence of density-dependent phenotypic plasticity may be a plesiomorphic trait for the whole genus. In order to test this idea, we have quantified the effect of rearing density in terms of the resulting behavior, color, and morphology in two non-swarming Schistocerca species native to Florida. When reared in both isolated and crowded conditions, the two non-swarming species, Schistocerca americana (Drury) and Schistocerca serialis cubense (Saussure) clearly exhibited plastic reaction norms in all traits measured, which were reminiscent of the desert locust. Specifically, we found that both species were more active and more attracted to each other when reared in a crowded condition than in isolation. They were mainly bright green in color when isolated, but developed strong black patterns and conspicuous background colors when crowded. We found a strong effect of rearing density in terms of size. There were also more mechanoreceptor hairs on the outer face of the hind femora in the crowded nymphs in both species. Although both species responded similarly, there were some clear species-specific differences in terms of color and behavior. Furthermore, we compare and contrast our findings with those on the desert locust and other relevant studies. We attribute the presence of density-dependent phenotypic plasticity in the non-swarming Schistocerca species to phylogenetic conservatism, but there may be a possible role of local adaptation in further shaping the ultimate expressions of plasticity.

Density-Dependent Phase Polyphenism in Nonmodel Locusts: A Minireview

Although the specific mechanisms of locust phase transformation are wellunderstood for model locust species such as the desert locustSchistocerca gregariaand the migratory locustLocusta migratoria, the expressions of density-dependent phase polyphenism in other nonmodel locust species are not wellknown. The present paper is an attempt to review and synthesize what we know about these nonmodel locusts. Based on all available data, I find that locust phase polyphenism is expressed in many different ways in different locust species and identify a pattern that locust species often belong to large taxonomic groups which contain mostly nonswarming grasshopper species. Although locust phase polyphenism has evolved multiple times within Acrididae, I argue that its evolution should be studied from a phylogenetic perspective because I find similar density-dependent phenotypic plasticity among closely related species. Finally, I emphasize the importance of comparative analyses in understanding the evolution of locust phase and propose a phylogeny-based research framework.

To Swarm or Not to Swarm?

In desert locusts, the radical transformation from a harmless “solitarious” form to the swarm-forming “gregarious” phase is one of the most extraordinary and iconic examples of density-dependent phenotypic plasticity in nature, as well as one of the most economically devastating. Now Anstey et al. (see the Perspective by Stevenson) reveal a neurochemical mechanism linking the changed behavior of individuals to profound changes in population structure, which, in this instance, lead to swarming and mass migration. The key to the phenotypic change is the ubiquitous neurotransmitter serotonin, which is synthesized in response to multiple sensory cues that gauge locust population density, switching locusts from mutual aversion to mutual attraction—the first and essential stage in establishing swarms. M. L. Anstey, S. M. Rogers, S. R. Ott, M. Burrows, S. J. Simpson, Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts. Science 323 , 627–630 (2009). [Abstract][Full Text] P. A. Stevenson, The key to Pandora9s box. Science 323 , 594–595 (2009). [Summary][Full Text]

Genetic variation for parental effects on the propensity to gregarise in Locusta migratoria

BackgroundEnvironmental parental effects can have important ecological and evolutionary consequences, yet little is known about genetic variation among populations in the plastic responses of offspring phenotypes to parental environmental conditions. This type of variation may lead to rapid phenotypic divergence among populations and facilitate speciation. With respect to density-dependent phenotypic plasticity, locust species (Orthoptera: family Acrididae), exhibit spectacular developmental and behavioural shifts in response to population density, called phase change. Given the significance of phase change in locust outbreaks and control, its triggering processes have been widely investigated. Whereas crowding within the lifetime of both offspring and parents has emerged as a primary causal factor of phase change, less is known about intraspecific genetic variation in the expression of phase change, and in particular in response to the parental environment. We conducted a laboratory experiment that explicitly controlled for the environmental effects of parental rearing density. This design enabled us to compare the parental effects on offspring expression of phase-related traits between two naturally-occurring, genetically distinct populations of Locusta migratoria that differed in their historical patterns of high population density outbreak events.ResultsWe found that locusts from a historically outbreaking population of L. migratoria expressed parentally-inherited density-dependent phase changes to a greater degree than those from a historically non-outbreaking population.ConclusionBecause locusts from both populations were raised in a common environment during our experiment, a genetically-based process must be responsible for the observed variation in the propensity to express phase change. This result emphasizes the importance of genetic factors in the expression of phase traits and calls for further investigations on density-dependent parental effects in locust phase change. More population samples with different outbreak histories need to be analyzed to demonstrate that differences in propensity to gregarise evolve because of different outbreak histories.

Phylogenetic perspectives on the evolution of locust phase polyphenism

Locust phase polyphenism is a spectacular example of density-dependent phenotypic plasticity. It is generally interpreted as an adaptation to heterogeneous environmental conditions brought on by high population density. However, several nonlocust species are known to express phase-like traits, which is difficult to explain from an adaptive perspective alone. Here I attempt to explain this phenomenon by 1) taking a reaction norm perspective in understanding the mechanisms underlying locust phase and 2) taking a phylogenetic perspective to study how individual reaction norms of locust phase might have evolved. I argue that locust phase polyphenism is a complex syndrome resulting from interactions among different density-dependent plastic reaction norms, each of which can follow a separate evolutionary trajectory, which in turn can be reflected in a phylogeny. Using a phylogeny of Cyrtacanthacridinae (Orthoptera: Acrididae), I explore the evolution of plasticity in density-dependent color change. I demonstrate that locusts and closely related nonlocusts, express similar phenotypic plasticity due to phylogenetic conservatism. Finally, I argue that it is crucial to study the evolution of locust phase polyphenism from both adaptive and phylogenetic perspectives.