Browsing by Author "Nijhout, H Frederik"

A series of experiments and mathematical models explore the response of the bacteria E. coli to stressors. Experimentally, the effect of L-homocysteine, a non-protein amino acid, is explored, and via math models, the effect of trimethoprim, a common antibiotic, is also explored. Previous work on L-homocysteine labels it a stressor, and this assertion is refined via the presented work. A mathematical model that improves on a previous work published by Kwon et al. (2008) explores the response of E. coli to various supplementations of amino acids when exposed to trimethoprim. New methods of developing antibiotics and therapeutic drug treatments are also explored.

A major goal of biological studies is to understand how complex forms develop and evolve. Each form is the result of molecular developmental patterning, growth, and the accumulation of changes in these processes from internal and external perturbations in ancestral forms. This dissertation uses each of these lenses to investigate the complex forms in the insect family Membracidae, which arises from the pronotum. In most insects, the pronotum is a simple, domed structure just behind the head, but in membracids it has enlarged and elaborated to look like thorns, plant stipules, fungi, and ants, among other shapes. To investigate this diversity, I rely on landmark based geometric morphometrics to quantify pronotal shapes. The specimens I used included those from a laboratory colony, those collected in and around San Jose, Costa Rica, and those from the Smithsonian National Museum of Natural History collection. In Chapter 1, I examined 5th instar morphogenesis to elucidate the timing of developmental events during the transition from juvenile to adult. These findings informed Chapter 2, which revealed developmental mechanisms related to growth led to transcriptional similarity between the pronotum and wings. In Chapter 3, I identified ontogenetic changes in the patterning of membracid pronotal shape compared to that of a closely related outgroup. Finally, in Chapter 4, I used a phylogenetic framework to investigate developmental modules and the co-occurrence of pronotal shape with two life history characteristics.

Seasonal polyphenism in insects is the phenomenon whereby multiple phenotypes can arise from a single genotype depending on environmental conditions during development. Many butterflies have multiple generations per year, and environmentally induced variation in wing color pattern phenotype allows them to develop adaptations to the specific season in which the adults live. Elements of butterfly color patterns are developmentally semi-autonomous allowing for detailed developmental and evolutionary changes in the overall color pattern. This developmental flexibility of the color pattern can result in extremely diverse seasonal phenotypes in a single species. In this study, we asked the following questions: a) How do wing phenotype elements such as shape and pattern vary between seasonal forms? b) Can this variation be explained phylogenetically? c) If so, what are the various pattern development strategies used to achieve crypsis in the dry season form? To answer these questions, we used high resolution images to analyze pattern element variation of 34 seasonally polyphenic butterfly species belonging to the tribe Junoniini (Lepidoptera: Nymphalidae). We show that forewing shape and eyespot size both vary seasonally, and that the methods by which phenotype elements change in the dry seasonal forms is different in different clades and may therefore have independent and diverse evolutionary origins.

This dissertation analyzes the develop and evolution of insect phenotypes, ranging from the level of individuals to colonies. The first four chapters focus on wing patterning in the bella moth Utetheisa ornatrix. I begin by quantifying the pattern variation in the species, then attempt to identify some of the mechanisms that give rise to the observed phenotypic differences. After describing how the primitive patterns of U. ornatrix can be construed as evidence that nymphalid groundplan symmetry systems arose from the fusion of irregular spots, I present the results of a series of artificial selection experiments that highlight the species’ capacity for phenotypic evolvability. I then detail how changes in Wnt expression can lead to the evolution of bold phenotypes, and show that pterin pigments provide the primary source of coloration in U. ornatrix patterns. The final chapter focuses on the evolution of social phenotypes in insect societies. I suggest that the evolution of a physically differentiated reproductive and worker castes marks an irreversible major evolutionary transition to superorganismality. Social groups of this type are fundamentally different than monomorphic cooperative breeders, and can meaningfully be compared to metazoan organisms with differentiated germ and soma.

Organismal form emerges from the relative growth of the body and its parts. In this dissertation, I address how developmental processes produce the size relationships between body parts that shape the characteristic morphologies of animals and plants. Specifically, I study the nutrition-dependent growth of wings in butterflies and moths to determine the developmental physiological mechanisms underlying morphological scaling relationships. Additionally, I dive into how form within a tissue is established by studying the mechanism that patterns growth of butterfly wings. Using an extensive data set from rats fed on a low protein diet, I analyze how different bones respond to nutritional variation and its effect on morphological integration. Finally, I end with a theoretical chapter discussing the ways in which development can be plastic and how that shapes both genetic and phenotypic evolution. I find that developmental plasticity emerges from changes in the concentration of systemic hormones in response to environmental stimuli. Hormones, in turn, interact with every developing part. This interaction is characterized by character-specific gene regulatory networks that affect their sensitivity to hormones. Thus, developmental plasticity emerges from the interplay between the organism sensing the environment that tunes the strength of a systemic signal that moderates development, and character-specific use of molecular networks that defines the range of character states in response to signal variation.

A key question in developmental biology is how organisms attain a final size. Deviations in growth patterns can produce different/new phenotypes and these changes can play fundamental roles in ecology and evolution. The size of an organism and of its constitutive organs is determined by the growth rate and the duration of the growing period. In insects, peptide hormones such as insulin-like growth factors have been shown to be involved in determining the growth rates by coordinating metabolism, cell proliferation and cell size. In contrast, steroid hormones, such as ecdysone, are involved in determining life stage transitions, and thus the termination of the growing period. Although it is clear that insulin and steroid hormones are both involved in the regulation of growth, the ways in which these two regulators interact is yet to be determined. Furthermore, it is not clear how organs and body growth are coordinated during development to arrive to their correct proportions. In this study, using the tobacco hornworm Manduca sexta and its wings as a model system, I examine the developmental mechanisms involved in the regulation of organ growth and how developmental processes can drive morphological evolution. First, I examine how the hormonal events that take place during the termination of the body growth period affect wing disk growth. Second, by using gene expression assays and in vitro cultures, I examine the interaction between bombyxin, the Lepidopteran insulin-like growth factor, and ecdysone, the molting hormone, and their contributions to wing imaginal disk growth. Finally, by using three different size strains of M. sexta, I examine the developmental basis of the allometric relationship between the wings and the body. My results show that during the final instar of M. sexta larval development, wing imaginal disks are sensitive to the hormonal events that terminate the growth period. Furthermore, I show that the bombyxin requirement for wing disk growth is restricted to the early days of the final instar unlike the constitutive effects seen in other species. After the larva has passed a particular critical weight, bombyxin is not necessary for wing disk growth, although its absence does decrease the growth rate. In contrast, ecdysone is required for promoting the growth of wing imaginal disks primarily through its stimulation of cell proliferation. Finally, I show how selection on body size has unpredictable consequence for the response of wing size. These results demonstrate how specific allometries have a developmental basis in the cross-talk of the various signals that regulate growth itself. Therefore, direct selection on allometric relationships may not need to be strong in order to hold scaling relationships constant, at least over short evolutionary periods.

The initiation of metamorphosis causes the cessation of the larval growth period which determines the final body size of adult insects. Because larval growth is roughly exponential, differences in timing the initiation of metamorphosis can cause large differences body size. Although many of the processes involved in metamorphosis have been well characterized, little is known about how the timing of the initiation of metamorphosis is determined.

Using different strains from Tribolium castaneum, Tribolium freemani, and Manduca sexta and varied nutritional conditions, I was able to document the existence of a threshold size, which determines when the larva becomes competent to metamorphose. Threshold size, however, does not dictate the exact timing of initiation. The exact timing for the initiation of metamorphosis is determined by a pulse of the molting hormone, ecdysone, but only after threshold size has been reached. Ecdysone pulses before the larva attains threshold size only cause the larva to molt to another larval instar. These results indicate the timing of metamorphosis initiation is controlled by two factors: (1) attainment of threshold size, at which the larva becomes competent to initiate metamorphosis and (2) the timing of an ecdysone pulse after attaining threshold size.

I hypothesize the attainment of threshold size, and therefore competence to metamorphose, is mediated by the effect of changing juvenile hormone concentrations caused by the increase in size of the larva. While the larval body grows nearly exponentially, the corpora allata, which secretes juvenile hormone, grows very little if at all. The difference in relative growth causes juvenile hormone concentrations to gradually become diluted. When juvenile hormone concentrations fall below a threshold, changes in protein-protein binding occur that can cause changes in signaling networks and ultimately gene expression. These changes make the larva competent for metamorphosis.

I have demonstrated that only threshold size is consistently correlated with body size; other growth parameters such as growth rate, duration of instars, or number of instars do not consistently correlate with variation in body size. Using the black mutant strain of M. sexta I have shown that lower juvenile hormone titers correlate with lower threshold sizes. My hypothesis is consistent with the large body of literature indicating the involvement of juvenile hormone. I also hypothesize that the diversity of metamorphosis types in holometabolous insects can be explained by heterochronic shifts in the timing of threshold size and other developmental events related to metamorphosis. The heterochronic shifts affect not only the morphology of organs, but can also affect the overall phenotypic response of the larva to changes in the environment. The different phenotypic responses among species may make the more or less suited for certain types of niches.