Chapter 7
Evolutionary Aspects of Steroid Utilization in Insects
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James A. Svoboda, Mark F. Feldlaufer, and Gunter F. Weirich Insect Neurobiology and Hormone Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705
Since insects are unable to biosynthesize the steroid nucleus, they require dietary sterols for structural and physiological (hormonal) purposes. Cholesterol will satisfy this dietary need in most cases, but since phytophagous insects ingest little or no cholesterol from dietary materials, they must convert dietary C and C phytosterols to cholesterol or other sterols. Through evolutionary development, certain insects have acquired the ability to metabolize dietary sterols in unique ways and to produce and utilize a variety of ecdysteroids (molting hormones) for hormonal control of development and reproduction. Thus, insects are able to flourish in virtually every conceivable ecological niche. Certain comparative studies that illustrate these evolutionary processes will be discussed in this chapter. 28
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Steroid biochemistry research has rarely been used to demonstrate phylogenetic relationships between insects. Relatively few species belonging to only eight of the 29 or so orders of insects have been examined with respect to sterol metabolism (Figure 1). Phylogenetic relationships of families and orders have been taken from Ross, H . H . et al (7). Listed from most primitive to most advanced, these include the orders: Thysanura, Orthoptera, Hemiptera, Homoptera, Hymenoptera, Coleoptera, Lepidoptera, and Diptera (Table 1). Over the years, a number of interesting differences in sterol utilization and metabolism between species have been discovered and some of these can be related to phylogenetic relationships; the rationale for some other differences is not so obvious. These studies have made it clear that it is difficult to generalize about insect sterol utilization and metabolism. We will discuss results of our work, and the work of a number of colleagues, relevant to the topic at hand. Primitive insects date from the Carboniferous period of the Paleozoic era, or several hundred million years ago (2). Arthropods, which include the Class
This chapter not subject to U.S. copyright Published 1994 American Chemical Society
In Isopentenoids and Other Natural Products; Nes, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
In Isopentenoids and Other Natural Products; Nes, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Figure 1.
Neoptera
d
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and C
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Order includes species capable of dealkylation and species which are incapable.
Orders in which all species examined were capable of dealkylation of C
Includes species capable of producing C
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or
ecdysteroids.
° No species reported to be capable of dealkylation.
b
a
phytosterols.
Suggested family tree of the orders of insects. Orders in bold type indicate those in which sterol utilization and metabolism have been investigated.
PTERYGOTA
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