Biology Reference
In-Depth Information
mitochondria are incapable of independent existence. In the course of evolu-
tion, some mitochondrial genes were transferred to the nuclear genome of its
host. Mitochondria are examples of symbionts that have evolved into organelles.
The study of symbiosis in arthropods has revealed diverse relationships between
arthropods and their symbionts. Molecular tools and sequencing of genomes likely
will reveal more intriguing details of these relationships. The following examples
may convince you of the critical importance of symbiosis in arthropod biology,
ecology, behavior, and evolution that we are just beginning to understand.
4.12.1 Multiple Symbionts
In addition to mitochondria, insects have intimate intra- and extracellular rela-
tionships with a diverse array of organisms, including viruses, Eubacteria, Archaea,
yeasts, fungi, and rickettsia (
Schwemmler and Gassner 1989; O'Neill et al. 1997;
Bourtzis and Miller 2003, 2006, 2009; Douglas 1992, 2010; Zchori-Fein and Bourtzis
2011
). The details of the relationships between the host and these microorganisms
often are unknown, but we are learning rapidly with the aid of molecular tools.
Insect symbionts may be beneficial to their hosts by providing nutrition, improving
tolerance to environmental perturbations, enhancing fitness and host plant suit-
ability, as well as maintaining or enhancing the insect immune system (
Oliver et al.
2010, Weiss and Aksoy 2011
).
Caldera et al. (2009)
recently noted the following:
“Considering the diversity of insects and microbes on the planet, their
mutual abundance and co-occurrence in virtually every terrestrial and fresh
water habitat and their shared ancient evolutionary histories, it is likely that
the biology of every insect species on the planet is influenced by microbial
symbionts.”
For example, the rice weevil
Sitophilus oryzae
(Rhynchophoridae) has four
intracellular genomes. These genomes are nuclear, mitochondrial, principal
endosymbiont, and
Wolbachia
(
Heddi et al. 1999, 2001
). The principal endosym-
biont is found (3
×
10
3
bacteria/cell) in specialized bacteriocytes. In total, 3
×
10
6
bacteria are found in each weevil, 10-fold more cells than there are beetle cells
(
Heddi et al. 2001
). These symbionts induce the specific differentiation of the
bacteriocytes and increase mitochondrial oxidative phosphorylation through
the supply of pantothenic acid and riboflavin. Their elimination impairs many
important physiological traits, including flight ability. This weevil supports the
“serial endosymbiotic theory”; according to this view, endosymbiosis did not
occur just once in eukaryotic evolution with the origin of a nucleus, or even
twice, when an anaerobic protist acquired a respiring bacterium to give rise
to the mitochondrion. The acquisition of genomes by eukaryotic cells “contin-
ues today in the multicellular organism” (
Heddi et al. 1999
). The rice weevil gut
symbiont allowed the weevil to colonize cereal plants by because it supplied
