Developmental requirement for adult behavioural rhythmicity in Drosophila melanogaster

Abstract

Endogenous circadian clocks are a consequence of the periodic nature of the environment we live in. They allow organisms to anticipate daily environmental changes and organize a wide array of biological functions, such as daily activity, sleep and feeding. The molecular clock circuits driving rhythmic locomotor behaviour in Drosophila melanogaster consist of two interlocked negative-feedback transcription loops, with a conserved central role for the heterodimeric transcription factor CLOCK/CYCLE (CLK/CYC) and its inhibitor PERIOD (PER). Research presented in this thesis aims to describe underlying developmental requirements for clock function in adult D. melanogaster. It has been demonstrated previously that major oscillator components CLK, CYC and PER are present in larvae and some circadian modulation to behavioural is demonstrated in larvae themselves. Moreover, a light pulse administered early in development is enough to establish a phase of the locomotor rhythm of adult flies, pointing out to the connection between a function of the circadian clock during development and adulthood.

This research reveals that adult circadian behaviour does not require either a functioning clock or the expression of per during prior development (Chapter 3). However, inhibition of CLK/CYC activity during metamorphosis (post-pupal formation), either by depletion of CYC or over-expression of its inhibitor PER, irreversibly affects clock-controlled locomotor activity in adult flies (Chapter 4 and 5). Even when PER over-expression is restricted to 18 ventral lateral clock neurons (LNvs) expressing the neuropeptide PIGMENT DISPERSING FACTOR (PDF), subsequent adult circadian behaviour is disrupted. A subset of small ventral lateral neurons (s-LNvs) was implicated as particularly sensitive to CLK/CYC inhibition through constitutive PER over-expression, suggesting that these cells require CLK/CYC function during development (Chapter 5).

Circadian oscillations in the peripheral tissue of adult flies were less dependent on developmental CLK/CYC activity than locomotor behaviour (Chapter 5). Thus, the newly discovered developmental function for CLK/CYC appears to be specific to the neural clock circuits. Analysis of the daily rhythms of TIMELESS protein localisation within clock neurons revealed that molecular oscillator is severely disrupted in small ventral lateral neurons (s-LNvs). Therefore my research suggests CLK/CYC is necessary during metamorphosis to establish a proper function of the molecular oscillator in s-LNvs (Chapter 5). I hypothesized that genes downstream from CLK/CYC are involved in this process, with Pdp1? and Mef2 as the strongest candidates, however this has not been confirmed (Chapter 6). Moreover, it appears that chromatin modifications are not involved in mediating the phenotype observed as result of developmental CLK/CYC inhibition (Chapter 6).

In summary, my work presents data confirming that CLK/CYC, but not PER activity, is required in PDF-expressing ventrolateral neurons during metamorphosis for establishing adult locomotor rhythmicity.

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ORCID for Herman Wijnen: orcid.org/0000-0002-8710-5176

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