Biology Reference
In-Depth Information
vitamins. Heddi et  al. (1999) “consider symbiosis in the rice weevil a sophisti-
cated mechanism for acquiring new sets of genes.”
Symbionts may be intracellular or extracellular within the insect, and they
may inhabit specialized structures. If located within the insect, the symbiont may
inhabit the gut, the reproductive tract, the fat body and the salivary glands ( Box
4.1 ). They also may occur on the outside of the insect, sometimes in specialized
structures of the exoskeleton. Symbionts may possess metabolic capabilities that
the insect host lacks, and the insect uses these capabilities to survive on poor or
unbalanced diets while the symbiont obtains nutrition and shelter ( Douglas 1998 ).
Under such circumstances, the insect-microbe relationship is required (obligate
symbiosis). Many insects freed of their symbionts grow slowly and produce few
or no progeny; many symbionts cannot grow outside their insect host, indicat-
ing the relationship is long and intimate. Intracellular symbionts are found in the
Anoplura, Mallophaga, Isoptera, Orthoptera, Hemiptera, Coleoptera, Diptera, and
Hymenoptera. The amazing diversity of relationships and organisms involved in
these relationships with insects has raised many questions, but provided relatively
few clear-cut answers because many endosymbionts cannot be cultured outside
their hosts. Many microorganisms are contained in special structures and transmit-
ted by a highly specific method, including transovarial transmission, to progeny.
Transmission also can occur when larvae feed on contaminated eggshells or feces.
Box 4.1 Symbiotic Relationships between an Insect and Microorganisms Can Be
Complex
l
Symbiosis is a broad term that includes parasites, pathogens, and mutualistic or beneficial
interactions, although most associate symbiosis as having a positive or mutualistic relationship
between the host and the symbiont.
l
Symbionts in arthropods may be Eubacteria, fungi, yeasts, viruses, protozoa, or Archaea.
l
Insects are in contact with many types of microorganisms in their environment. Some will be found
externally, some internally. That does not make them a symbiont if they are only occasionally found
associated with the insect.
l
Symbionts may be found in the digestive tract or associated with it in specialized cells, in the fat body,
in the reproductive system, free in the hemolymph, and in the salivary glands.
l
Symbionts may be obligatory, or facultative in relation to their arthropod host. Many insects harbor
multiple symbionts, including obligatory and one or more facultative organisms.
l
Obligatory symbionts often are intracellular and may exist in specialized structures (mycetocytes,
pouches, cavities) and are transmitted in specialized ways, including transovarial transmission.
Sometimes transmission is by feeding on contaminated egg shells, or by contact in the environment.
l
Some symbionts are extracellular but may be found internally. They may not inhabit specialized
structures, although some do; often, these organisms are facultative or secondary symbionts, meaning
that the insect can survive without them.
l
Symbionts may provide nutrients, affect host range, temperature tolerance, longevity, fecundity, sex
ratio, behavior, responses to natural enemies, or other aspects of their biology.
 
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