Dissertation
Photoperiod-dependent plasticity of circadian pacemaker center in the brain of the Madeira cockroach Rhyparobia maderae
Abstract
To various degrees, insects in nature adapt to and live with two fundamental
environmental rhythms around them: (1) the daily rhythm of light and dark, and (2) the
yearly seasonal rhythm of the changing photoperiod (length of light per day). It is
hypothesized that two biological clocks evolved in organisms on earth which allow them to
harmonize successfully with the two environmental rhythms: (1) the circadian clock, which
orchestrates circadian rhythms in physiology and behavior, and (2) the photoperiodic
clock, which allows for physiological adaptations to changes in photoperiod during the
course of the year (insect photoperiodism). The circadian rhythm is endogenous and
continues in constant conditions, while photoperiodism requires specific light inputs of a
minimal duration. Output pathways from both clocks control neurosecretory cells which
regulate growth and reproduction. This dissertation focuses on the question whether
different photoperiods change the network and physiology of the circadian clock of an
originally equatorial cockroach species. It is assumed that photoperiod-dependent
plasticity of the cockroach circadian clock allows for adaptations in physiology and
behavior without the need for a separate photoperiodic clock circuit.
The Madeira cockroach Rhyparobia maderae is a well established circadian clock
model system. Lesion and transplantation studies identified the accessory medulla (aMe),
a small neuropil with about 250 neurons, as the cockroach circadian pacemaker. Among
them, the pigment-dispersing factor immunoreactive (PDF-ir) neurons anterior to the aMe
(aPDFMes) play a key role as inputs to and outputs of the circadian clock system.
The aim of my doctoral thesis was to examine whether and how different photoperiods
modify the circadian clock system. With immunocytochemical studies, three-dimensional
(3D) reconstruction, standardization and Ca2+-imaging technique, my studies revealed
that raising cockroaches in different photoperiods changed the neuronal network of the
circadian clock (Wei and Stengl, 2011). In addition, different photoperiods affected the
physiology of single, isolated circadian pacemaker neurons. This thesis provides new
evidence for the involvement of the circadian clock in insect photoperiodism. The data
suggest that the circadian pacemaker system of the Madeira cockroach has the plasticity
and potential to allow for physiological adaptations to different photoperiods. Therefore, it
may express also properties of a photoperiodic clock.
environmental rhythms around them: (1) the daily rhythm of light and dark, and (2) the
yearly seasonal rhythm of the changing photoperiod (length of light per day). It is
hypothesized that two biological clocks evolved in organisms on earth which allow them to
harmonize successfully with the two environmental rhythms: (1) the circadian clock, which
orchestrates circadian rhythms in physiology and behavior, and (2) the photoperiodic
clock, which allows for physiological adaptations to changes in photoperiod during the
course of the year (insect photoperiodism). The circadian rhythm is endogenous and
continues in constant conditions, while photoperiodism requires specific light inputs of a
minimal duration. Output pathways from both clocks control neurosecretory cells which
regulate growth and reproduction. This dissertation focuses on the question whether
different photoperiods change the network and physiology of the circadian clock of an
originally equatorial cockroach species. It is assumed that photoperiod-dependent
plasticity of the cockroach circadian clock allows for adaptations in physiology and
behavior without the need for a separate photoperiodic clock circuit.
The Madeira cockroach Rhyparobia maderae is a well established circadian clock
model system. Lesion and transplantation studies identified the accessory medulla (aMe),
a small neuropil with about 250 neurons, as the cockroach circadian pacemaker. Among
them, the pigment-dispersing factor immunoreactive (PDF-ir) neurons anterior to the aMe
(aPDFMes) play a key role as inputs to and outputs of the circadian clock system.
The aim of my doctoral thesis was to examine whether and how different photoperiods
modify the circadian clock system. With immunocytochemical studies, three-dimensional
(3D) reconstruction, standardization and Ca2+-imaging technique, my studies revealed
that raising cockroaches in different photoperiods changed the neuronal network of the
circadian clock (Wei and Stengl, 2011). In addition, different photoperiods affected the
physiology of single, isolated circadian pacemaker neurons. This thesis provides new
evidence for the involvement of the circadian clock in insect photoperiodism. The data
suggest that the circadian pacemaker system of the Madeira cockroach has the plasticity
and potential to allow for physiological adaptations to different photoperiods. Therefore, it
may express also properties of a photoperiodic clock.