Browsing by Subject "Manduca"

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    A Quantitative Analysis of Growth and Size Regulation in Manduca sexta: The Physiological Basis of Variation in Size and Age at Metamorphosis.
    (PLoS One, 2015) Grunert, Laura W; Clarke, Jameson W; Ahuja, Chaarushi; Eswaran, Harish; Nijhout, H Frederik
    Body size and development time are important life history traits because they are often highly correlated with fitness. Although the developmental mechanisms that control growth have been well studied, the mechanisms that control how a species-characteristic body size is achieved remain poorly understood. In insects adult body size is determined by the number of larval molts, the size increment at each molt, and the mechanism that determines during which instar larval growth will stop. Adult insects do not grow, so the size at which a larva stops growing determines adult body size. Here we develop a quantitative understanding of the kinetics of growth throughout larval life of Manduca sexta, under different conditions of nutrition and temperature, and for genetic strains with different adult body sizes. We show that the generally accepted view that the size increment at each molt is constant (Dyar's Rule) is systematically violated: there is actually a progressive increase in the size increment from instar to instar that is independent of temperature. In addition, the mass-specific growth rate declines throughout the growth phase in a temperature-dependent manner. We show that growth within an instar follows a truncated Gompertz trajectory. The critical weight, which determines when in an instar a molt will occur, and the threshold size, which determines which instar is the last, are different in genetic strains with different adult body sizes. Under nutrient and temperature stress Manduca has a variable number of larval instars and we show that this is due to the fact that more molts at smaller increments are taken before threshold size is reached. We test whether the new insight into the kinetics of growth and size determination are sufficient to explain body size and development time through a mathematical model that incorporates our quantitative findings.
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    A switch in the control of growth of the wing imaginal disks of Manduca sexta.
    (PLoS One, 2010-05-19) Tobler, Alexandra; Nijhout, H Frederik
    BACKGROUND: Insulin and ecdysone are the key extrinsic regulators of growth for the wing imaginal disks of insects. In vitro tissue culture studies have shown that these two growth regulators act synergistically: either factor alone stimulates only limited growth, but together they stimulate disks to grow at a rate identical to that observed in situ. It is generally thought that insulin signaling links growth to nutrition, and that starvation stops growth because it inhibits insulin secretion. At the end of larval life feeding stops but the disks continue to grow, so at that time disk growth has become uncoupled from nutrition. We sought to determine at exactly what point in development this uncoupling occurs. METHODOLOGY: Growth and cell proliferation in the wing imaginal disks and hemolymph carbohydrate concentrations were measured at various stages in the last larval instar under experimental conditions of starvation, ligation, rescue, and hormone treatment. PRINCIPAL FINDINGS: Here we show that in the last larval instar of M. sexta, the uncoupling of nutrition and growth occurs as the larva passes the critical weight. Before this time, starvation causes a decline in hemolymph glucose and trehalose and a cessation of wing imaginal disks growth, which can be rescued by injections of trehalose. After the critical weight the trehalose response to starvation disappears, and the expression of insulin becomes decoupled from nutrition. After the critical weight the wing disks loose their sensitivity to repression by juvenile hormone, and factors from the abdomen, but not the brain, are required to drive continued growth. CONCLUSIONS: During the last larval instar imaginal disk growth becomes decoupled from somatic growth at the time that the endocrine events of metamorphosis are initiated. These regulatory changes ensure that disk growth continues uninterrupted when the nutritive and endocrine signals undergo the drastic changes associated with metamorphosis.
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    ItemOpen Access
    Big or fast: two strategies in the developmental control of body size.
    (BMC Biol, 2015-08-04) Nijhout, H Frederik
    Adult body size is controlled by the mechanisms that stop growth when a species-characteristic size has been reached. The mechanisms by which size is sensed and by which this information is transduced to the growth regulating system are beginning to be understood in a few species of insects. Two rather different strategies for control have been discovered; one favors large body size and the other favors rapid development.
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    The Role of Threshold Size in Insect Metamorphosis and Body Size Regulation
    (2010) Preuss, Kevin Michael

    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.