Hnp

666 signal from the photoreceptor; and the output pathway that connects the clock to the effector structures for the rhythms.

In cockroaches, crickets and some beetles the compound eyes are the photoreceptors, and a clock is located near the medulla regions of each optic lobe (Figure 13.5). However, in flies, moths and other beetles neither the compound eyes nor the ocelli are essential as photoreceptors for circadian rhythms. These insects may use several photoreceptors for entrainment, including groups of neurons within the central brain region. In flies the clock is located in the lateral neurons (located near the border of the optic lobe and brain) or their equivalent, while in moths the clock lies deep in the central part of the brain. Output pathways vary in their nature, depending on the rhythmic activity that is being controlled. For example, in cockroaches the clock connects with interneurons that run to the thoracic ganglia where locomotor activity is regulated. By contrast, the eclosion rhythm in moths is triggered by eclosion hormone whose release is controlled by the clock in the brain.

Though behavioral rhythms such as locomotor activity and eclosion are regulated by a central clock, it is evident that many other circadian rhythms operate independently; that is, many organs and tissues possess their own clock (Giebultowicz, 2000, 2001). This is readily shown by separating a structure from the rest of the body and observing that it retains its rhythmicity of function. To date, peripheral clocks have been reported for gonads, Malpighian tubules, endocrine glands, epidermis, and some sense organs.

Molecular studies, mainly using Drosophila mutants, have identified at least 10 genes in the central brain oscillator that are involved in circadian locomotor and eclosion rhythms. Two of these genes, period and timeless, and their protein products (PER and TIM, respectively) are responsible for the actual timing mechanism, showing circadian activity and production, respectively. A third gene, cryptochrome, codes for a photosensitive protein that modulates the action of PER and TIM, thereby resetting the clock. Other genes are on the output pathway (i.e., downstream of the clock), and their gene products induce manifestation of the circadian rhythm. Homologous genes to period have been detected in the central oscillators of moths and cockroaches, as well as in organs with peripheral clocks (Giebultowicz, 2000), indicating that there may be a common molecular basis for all circadian clocks. For further details of the molecular components of circadian clocks, see Giebultowicz (2000, 2001), Stanewsky (2002), and Zordan et al. (2003).

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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