The pigment‐dispersing factor neuronal network systematically grows in developing honey bees

Abstract

The neuropeptide pigment‐dispersing factor (PDF) plays a prominent role in the circadian clock of many insects including honey bees. In the honey bee brain, PDF is expressed in about 15 clock neurons per hemisphere that lie between the central brain and the optic lobes. As in other insects, the bee PDF neurons form wide arborizations in the brain, but certain differences are evident. For example, they arborize only sparsely in the accessory medulla (AME), which serves as important communication center of the circadian clock in cockroaches and flies. Furthermore, all bee PDF neurons cluster together, which makes it impossible to distinguish individual projections. Here, we investigated the developing bee PDF network and found that the first three PDF neurons arise in the third larval instar and form a dense network of varicose fibers at the base of the developing medulla that strongly resembles the AME of hemimetabolous insects. In addition, they send faint fibers toward the lateral superior protocerebrum. In last larval instar, PDF cells with larger somata appear and send fibers toward the distal medulla and the medial protocerebrum. In the dorsal part of the medulla serpentine layer, a small PDF knot evolves from which PDF fibers extend ventrally. This knot disappears during metamorphosis and the varicose arborizations in the putative AME become fainter. Instead, a new strongly stained PDF fiber hub appears in front of the lobula. Simultaneously, the number of PDF neurons increases and the PDF neuronal network in the brain gets continuously more complex.

Pigment-Dispersing Factor is present in circadian clock neurons of pea aphids and may mediate photoperiodic signalling to insulin-producing cells

“…The expression pattern of PDF in aphid circadian clock neurons strongly resembles that of other insects, [64][65][66][67]. As true for flies, cockroaches, bugs and bees [12,65,[68][69][70], PDF is present in aphid lateral clock neurons with different soma sizes. These clock neurons send projections to the superior protocerebrum and to the contralateral brain hemisphere, more precisely to a neuropil that strongly resembles the accessory medulla (AME) of other insects.…”

“…Most interestingly, a rather sparse innervation of the optic lobes by PDF fibres was also found in honeybees [50,70]. Furthermore, many varicose endings and no fine dendritic-like fibres were found in the AME of honeybee larvae.…”

The role of temperature on the development of circadian rhythms in honey bee workers

“…At the brain level, in the fruit fly, it is known that multiple oscillators that control the timing of locomotor activity at different times of the day ( e.g ., morning and evening cells) not only need to communicate but they need to synchronize in a specific manner ( Stoleru et al, 2004 , 2005 ). Recent work in the honey bee tracking the number of Pigment Dispersing Factor (PDF) positive neurons across different ages in the worker has shown that the number of PDF+ neurons and their arborizations to different parts of the bee brain continues to change after emergence ( Beer & Helfrich-Förster, 2020 ; Beer, Härtel & Helfrich-Förster, 2022 ). It is thus possible that the establishment of connections among the multiple components of the circadian system in the honey bee brain occurs in the first 48 h after emergence.…”

Pigment-dispersing factor is present in circadian clock neurons of pea aphids and may mediate photoperiodic signalling to insulin-producing cells

“…The expression pattern of PDF in aphid circadian clock neurons strongly resembles that of other insects [ 72 – 75 ]. As true for flies, cockroaches, bugs and bees [ 18 , 73 , 76 – 78 ], PDF is present in aphid lateral clock neurons with different soma sizes. These clock neurons send projections to the superior protocerebrum and to the contralateral brain hemisphere, more precisely to a neuropil that strongly resembles the AME of other insects.…”

“…This speaks against a prominent role of the AME, or at least the PDF fibres in the AME, as a light-input pathway to the circadian clock neurons. Most interestingly, a rather sparse innervation of the optic lobes by PDF fibres was also found in honeybees [ 51 , 78 ]. Furthermore, many varicose endings and no fine dendritic-like fibres were found in the AME of honeybee larvae.…”