Rhythms

Objectives

Topic outline

Activities

 

Periodicity

Many activities of insects—such as feeding, locomotion, mating, oviposition, eclosion, etc.—occur at regular intervals.
Periodicity can be tidal, daily, lunar, annual, multi-annual. It is governed by endogenous and exogenous components.

Rhythms

Rhythms are processes that are governed by an innate “biological clock”. They characteristically continue when external conditions are artificially kept constant. Under constant conditions, the endogenous rhythm is “free running” and shows a periodicity close to normal.

Why should an insect develop a way of maintaining an innate rhythm when the external cues are invariant?

Left. The activity record of an individual of the cockroach Leucophaea maderae in constant darkness (DD) for 13 weeks showing an abrupt change in its freerunning period(T) in the third week from 24 hours to 23 hours 48 minutes. Right. Entrainment of the activity rhythm in an individual of Leucophaea maderae by a light/dark cycle (LD 12:12) following freerun in DD. The natural period of the rhythm (T) during the first 28 days is about 23 hours 30 minutes; subsequently the rhythm is entrained to a 24hour period to match the Zeitgeber. (From Saunders, 1982.)

Resetting the activity rhythm in L. maderae by lightcycles. A. Resetting with a lightcycle (LD12:12) initially outofphase with the freerunning rhythm and with "dusk" falling during the insect's subjective day. The onset of running is gradually phaseadvanced until entrainment is achieved after about eleven transient cycles. B. Resetting with a lightcycle (LD12:l2) initially outofphase and with "dusk" falling during the insect's subjective night. The onset of running is rapidly phasedelayed and the rhythm becomes entrained. Subsequent transfer to DD (on day 20) shows that the phase of the endogenous rhythm has been shifted by the light treatment. (From Saunders, 1982.)

Minilecture:

Rhythms

Presented by D. Merritt

 

Circadian rhythms

Circadian rhythms have a periodicity of approximately 24 hours. In normal situations they are constantly entrained by environmental cues such as day length. In experimental situations of constant light (LL) or constant dark (DD), they show their inherent “free-running” rhythm of approximately 24 hours. It usually varies between individuals from 22-27 hours. Circadian rhythms are temperature-compensated. We don’t know how this is achieved.

Zeitgeber: an external time cue.

Some rhythms require a zeitgeber for a rhythm to be manifested. For example, mosquitoes kept in constant light will oviposit randomly: after a period of darkness they will oviposit rhythmically at 24 hr intervals. There is an innate ability to “count off” an approximation of 24 hours. And oviposition is gated to a certain period in the cycle.

Genetic basis of rhythmic activity

Drosophila mutants were isolated that had altered pupal eclosion rhythms. Hundreds of individual stocks were generated from individuals that had been exposed to mutagen. Progeny were also assessed for locomotory rhythms. Several alleles of a single gene were isolated. The mutation is called period (per).

--per0

has no obvious rhythm

--perl

has a longer than normal rhythm

--pers

has a shorter than normal rhythm.



Beside the daily rhythm, mating is also affected. Males sing to females by vibrating their wings. The interpulse song interval for:

--wild type males

50-65 seconds

--pers

35-45 sec

--perl

75-95 sec


There are interspecific song differences between D. melanogaster and D. simulans.
The D. simulans per gene was placed in a transposable P element and used to transform a D. melanogaster per0 recipient. Rhythmicity was restored, but the host (D. simulans) rhythmicity was manifested. Only a few amino acid substitutions are responsible.
per protein is expressed in a subset of neurons in the CNS as well as the antennae, photoreceptors, ovaries. In some tissues this can be related to function: daily turnover of photoreceptor membrane, possible turnover of olfactory membrane.


The glow-worm example

Record of the light output of a glow-worm over approximately 30 days in the laboratory under LD conditions.
Glow-worm bioluminescence is nocturnal.

Are they responding to environmental cues or are they responding to an internal clock?
Best way to distinguish between possibilities is to place in constant dark (D:D), or constant light (L:L) and observe behaviour

Glow-worm bioluminescence “drifts” when placed in constant darkness (DD)
Very characteristically not exactly 24 hours, and there is a day-to-day similar trend
The fact that it is not exactly 24 hr cycle indicates it is obeying an internally-generated rhythm
Approximately 24 hours = “circa” about, “dian” a day: circadian
They show true circadian rhythmicity
Free-running period > 24 h

Rainforest Arachnocampa flava tested in lab
1: exposed to DD to free-run
2. Exposed to LD to show L represses biolum
3. On return to DD the light has re-entrained the rhythms to match the dark phase.

All species:
light is a masking agent, i.e. it blocks bioluminescent output
Light is an entraining agent

light input graph

In caves (A. tasmaniensis) cycle but don’t free-run. Exposure to artificial light masks and re-entrains to the light cycle.

artificial light input graph

Lab experiments with weak point-sources show same synchronisation but light not strong enough to mask.

Conclusion: Synchronise to each others’ glows

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Gated rhythms

A gated rhythm is something that happens only once in individual’s life. For example, eclosion of Drosophila usually occurs around dawn. If an individual misses a “window of opportunity” on a given day, it must wait until the same time on the next day to emerge. Permits synchronicity in the population, utilisation of resources, escape from predators.

Pupal eclosion rhythms are common. In some insects, for example, drosophilids, the pupa remains light-sensitive, i.e. the eclosion rhythm can be entrained by light in DD. Others, e.g. Sarcophaga and Dacus are insensitive as pupae. Entrainment occurs in larvae.

Time of eclosion of 2 spp of silkmoth is gated at different parts of the daily cycle.

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