Biology Reference
In-Depth Information
As for the cockroaches rampant in cities, such a safeguard using endosymbionts seems unnec-
essary, because they always gain access to nitrogen-rich leftovers. The fact that they keep an
apparently unnecessary safeguard reminds us that their ancestor used to live in the tropical rain
forests, where nitrogenous compounds were not easily available, and that many of their remote
relatives still live there. In this context, for primitive termites that feed exclusively on decayed or
rotten wood a nitrogen-recycling system, such as that found in cockroaches, would be very desirable.
In fact, though both termites and their intracellular symbionts are phylogenetic relatives of cock-
roaches and their symbionts described above, respectively, there is no evidence that the termitesÔ
symbionts mobilize uric acid. No other functions have been suggested for the intracellular symbionts
of termites, either. It is possible that many microbes with various abilities, including nitrogen
Ýxation, living in the termitesÔ gut did not necessitate the speciÝc contribution of intracellular
symbionts. There is circumstantial evidence suggesting that some gut microbes play a role in
mobilizing uric acid unaerobically without the aid of uricases (M. Ohkuma, personal communica-
tions). It is likely that the bacterium that associated with the common ancestor of cockroaches and
termites has differentiated extensively since the two hostsÔ lineages diverged.
I
NTRACELLULAR
S
YMBIONTS
OF
C
OLEOPTERA
Intracellular symbiosis of microorganisms with Coleoptera has been studied in some detail using
the weevil
Sitophilus oryzae
. The weevil, which is devoid of gut microbes, harbors intracellular
bacterial symbionts in the bacteriocyte. These bacteriocytes assemble to form the bacteriome, which
is arranged somewhat like a packsaddle around the midgut of the larvae. In adults, the bacteriomes
are located at the apexes of the mesenteric caeca and ovaries. The symbionts lie free in the cytosol
of bacteriocytes and are transmitted to the hostÔs progeny through oocytes. These symbionts are
intimately involved in the hostÔs physiology and have been suggested to provide the host with several
vitamins, such as riboÞavin, pantothenic acid, and biotin (Wicker, 1983), and to take part in amino-
acid metabolism in the host (Nardon and Grenier, 1988). It has also been suggested that these
symbionts stimulate oxidative phosphorylation in mitochondria in the host cell (Heddi et al., 1993).
The symbiont of
S. oryzae
belongs to the Enterobacteriaceae of the h-proteobacteria (Campbell
et al., 1992), and its genome size was estimated to be about 3 megabases (Mb) (Charles et al.,
1997b). It is likely that, as a result of their association with the host for a prolonged time, these
symbionts have completely lost their ability to grow outside the host cell. However, this symbiosis
is distinct from that between the aphid and
Buchnera
in that the hosts deprived of the symbionts
not only are viable but also retain fecundity. A nucleocytoplasmic incompatibility between the
symbiotic and aposymbiotic strain of
S. oryzae
has been reported (Nardon and Grenier, 1988).
I
NTRACELLULAR
S
YMBIONTS
OF
B
LOOD
S
UCKERS
It has been pointed out that regardless of their taxonomic position, those insects that are
completely dependent on mammalian blood for food throughout their life cycle harbor symbiotic
bacteria, whereas those that feed on blood during only part of the life cycle do not (Trager,
1986). Thus, Þeas, mosquitoes, sand Þies, black Þies, and house Þies, which all feed as larvae
on diets rich in microorganisms, do not have symbiotic bacteria, whereas the blood-sucking
Hemiptera and Anoplura feed only on blood and do have symbiotic bacteria. A remarkable
contrast is provided by the two closely related muscid Þies, the stable Þy
Stomoxys
, and the
tsetse Þy
Glossina
. The larvae of
Stomoxy
s are free living and feed on microorganism-rich
manure, whereas those of
Glossina
develop within the mother Þy, where they are nourished by
the milk gland, an organ used specially for this purpose. In accordance with these different
feeding habits, only
Glossina
have symbiotic bacteria, which occur in bacteriomes forming a
portion of the midgut of the Þy. These symbionts, which are designated
Wigglesworthia gloss-
inidae
, are transmitted transovarially, via the nurse cells, and probably also by means of the
