Bacteriocyte

Location

Bacteriocyte location varies depending on the insect and endosymbiont type. These cells often inhabit fat bodies inside the midgut epithelium. The proximity to the insects' digestive system facilitates the absorption of bacteriocyte-produced nutrients. However, fungal-infected bacteriocytes and some bacteria-infected bacteriocytes can sometimes populate the hemocoel, a blood-containing cavity between the organs of most arthropods.

Development

Transmission of endosymbionts

Although vertical transmission of the symbionts is pivotal, the underlying and cellular mechanisms of this process are relatively unknown. However, there are several existing hypotheses. One theory is that the microorganisms circulating in the hemolymph of the mother migrate to a posterior region of the offspring blastula containing enlarged follicle cells. Other studies suggest that symbionts are directly transferred from the maternal bacteriocyte to the follicular region of the blastula through exocytic and endocytic transport. A newer hypothesis suggests that a membranous conduit forms between the maternal bacteriocyte and blastula which acts as a bridge for symbionts. Additionally, some studies show that the recognition of stem cell niches and association with dynein, kinesin, and microtubules are crucial for transmission from the parent to the offspring germline as well as segregation to host daughter cells.

Growth

Bacteriocyte tissue grows considerably during nymphal and larval development as it organizes into two regular clusters near the gut and developing embryonic chains. As some insects grow older, such as aphids, they begin to exhibit disorganized architecture in the bacteriocyte tissue. Eventually, this trend leads to progressive dis-aggregation of the tissue caused by an increasing lack of intercellular adhesion of the cells that only increases as the insect ages. Dis-aggregation appears prominently in reproductively active as well as senescent adults. Some bacteriocyte nuclei, like those in aphids, also follow this pattern of development. They are initially round and centrally positioned but progressively become more deformed and move to the periphery of the cell.

Death

Other bacteriocytes, like those found in weevils, undergo a different form of cell death. Unlike aphids, weevils lose their bacteriocytes in adulthood. In these species, both apoptotic and autophagic mechanisms quickly eliminate bacteriomes associated with the gut. This form of cell death is more common in insects with a smaller dependence on their endosymbionts. Aphids, on the other hand, are closely evolutionarily tied to bacterial endosymbiosis resulting in a more complicated form of cell death.

Function

Nutrition

The main function of bacteriocytes is to indirectly provide nutrients to the insect through the use of symbionts. The microorganisms housed in these specialized cells produce essential nutrients for their hosts in exchange for an enclosed environment to live. The health of these endosymbionts is crucial for the host's biology as their presence changes the balance of amino acid metabolism and mitochondrial phosphorylation. Both of these processes are essential for insect flight ability and performance. Insects housing symbionts develop best when fed on a diet with a lower protein-to-carbohydrate ratio than other insects because the symbionts already add considerable amounts of amino acid and nitrogen nutrition to the host. Because of this nutritional imbalance, bacteriocytes are more prevalent in insects that utilize diets consisting of an excess of one compound while lacking some nutrients, like amino acids and proteins.

Other functions

While some endosymbionts directly provide their hosts with food, others secrete enzymes to help aid in digestion of materials the insect cannot break down itself such as wood. Additionally, some bacteriocyte endosymbionts serve an immune function are known to prime the immune system especially against trypanosomes.

Examples

Aphids

Development of aphid (Acyrthosiphon pisum) bacteriocytes has been studied with the host cells that contain the endosymbiotic bacteria, Buchnera aphidicola. Bacteriocytes of aphids have a subpopulation of the bacteriocytes that is chosen prior to the maternal transmission of the bacteria to the embryo. Even later in the aphid's life, a second population of adipose cells are selected to become bacteriocytes. Bacteriocyte development has been maintained in aphids for 80–150 million years.

Tsetse flies

The Tsetse fly's most prominent and vital endosymbiont is the bacterium, Wigglesworthia glossinidia, These bacteria are housed in the fly's bacteriocytes and produce B vitamins (B1, B6, and B9). The Tsetse fly itself lacks the ability to obtain these nutrients because of its hematophagous diet. W. glossinidia population in the maternal milk glands also help prime the immune system during the larval stage. Tsetse flies with bacteriocytes containing W. glossinidia are less susceptible to trypanosome infection later in life.

References

References

1. (2009). "Encyclopedia of Insects". Academic Press.
2. Douglas, A E. (1998). "Nutritional interactions in insect-microbial symbioses: Aphids and their symbiotic bacteria ''Buchnera''". Annual Review of Entomology.
3. (July 28, 2015). "Ultrabithorax is essential for bacteriocyte development". Proceedings of the National Academy of Sciences of the United States of America.
4. (2012). "Cellular mechanism for selective vertical transmission of an obligate insect symbiont at the bacteriocyte–embryo interface". Proc Natl Acad Sci USA.
5. (2018-02-20). "Bacteriocyte cell death in the pea aphid/Buchnera symbiotic system". Proceedings of the National Academy of Sciences.
6. Brune, Andreas. (2009). "Encyclopedia of Insects". Academic Press.
7. (2017). "Immunology of insect vectors: midgut interactions of sandflies and tsetse with kinetoplastid parasites as a paradigm for establishing infection". Advances in Insect Physiology.
8. Hoff, Mary. (2007-04-10). "When Bacteria Lose a Single DNA Base, Aphids Suffer". PLOS Biol.
9. (2003-10-13). "Developmental Origin and Evolution of Bacteriocytes in the Aphid–Buchnera Symbiosis". PLOS Biol.