Thesis Committee: Glenn Edwards, Daneil Kiehart (Biology Department), Ronen Plesser, Joshua Socolar (exofficio non-voting member)
ABSTRACT: Temperature-dependence of poiklotherm*, more specifically
Drosophila**, development has been extensively documented in the literature.
However, this research rarely focuses on individual developmental steps,
such as dorsal closure, and usually generalizes the sum of processes in
order to explain variations in the characteristics of the adult fly. The
Sharpe-Schoolfield model, a temperature-dependent model of biological processes,
is a modification of the Arrhenius equation and characterizes development
in terms of enzymatic events. It ignores any dependence on physical factors,
such as viscosity of the cytoplasm. Hutson et al. use laser microsurgery
to describe specific forces involved in dorsal closure and observe a ~20%
variability in native closure times. It has been suggested that this is,
in part, due to temperature variations in the laboratory. The three main
forces driving dorsal closure arise from the stretching of the cells in
the lateral epidermis, the purse string-like contraction of the actin cable
in the leading edge, and the contraction of the amnioserosa cells. All
of these processes are highly actin dependent. John Fuseler discovered
that microtubules depolymerized exponentially slower at reduced temperatures.
I infer from this that actin depolymerizes at a similarly slower rate,
reducing the actin-dominated forces of dorsal closure. The viscous drag
also increases at reduced temperatures, further slowing the effect of the
actin-controlled forces. For high temperatures, on one hand, metabolism
increases and viscosity decreases, on the other hand, proteins are more
likely to denature. Therefore, the rate of development peaks at a certain
intermediate temperature. Flies have a robust nature and can develop
within a significant range of temperatures (~16-29 °C). In order to
understand this temperature dependence, I examined native dorsal closure
at three different temperatures. These temperatures all lie within the
range where a fertilized egg has the capacity to develop into an adult
fly: 17°C, 24°C (room temperature), and 29°C. The rates of
change of all parameters of the general geometry of dorsal closure were
fastest for embryos at 24°C, slightly slower at 29°C, and slowest
at 17°C. These temperature shifts were accomplished relatively easily
with a stage insert whose temperature was controlled by an external heatcontrolled
water bath. The overall balance of forces at different temperatures was
also examined in order to see whether or not they had the same temperature
dependence. There must be some kind of compensating mechanism driving the
completion of dorsal closure and re-balancing the forces.
* a type of cold-blooded animal
** fruit fly
Here is the thesis in PDF: nczakon_thesis.pdf (about 20.8 MBytes)