Transmission of endosymbionts Bacteriocyte microorganism transfer occurs through vertical transmission from mother to offspring.
Horizontal transmission or infection does not typically appear because insects with bacteriocytes depend so significantly on their symbiotic relationships to survive. Hosts without bacteriocytes do not usually survive and reproduce into adulthood. In some cases, the bacteria and fungi are transmitted in the egg, as in
Buchnera; in others, like
Wigglesworthia, they are transmitted via a milky substance that is fed to the developing insect
embryo. Suppression of the
Ultrabithorax gene in embryos led to the disappearance of bacteriocytes in
Nysius plebius, while manipulation of the
Antennapedia gene impacted the formation of
bacteriomes but did not halt the formation of bacteriocytes altogether. 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 Bacteriocytes can undergo a controlled form of cell death distinct from
apoptosis. Elimination of bacteriocytes usually begins when the insect reaches reproductive maturity. Bacteriocyte degeneration begins with cytoplasmic hypervacuolation, meaning an excess of organelles called
vacuoles form in the cytoplasm and then progressively expand throughout the entire cell. These vacuoles, which originate from the
endoplasmic reticulum, also contain large acidic compartments are thought to aid in cellular degeneration. Hypervacuolation is a common feature in cells that undergo an
autophagic, or "self-eating" death. Bacteriocytes, however, do not experience an autophagic death based on the lack of digested cellular components in the vacuoles. Bacteriocytes do develop some organelles to break down cellular components, called
autophagosomes, but research suggests that their development is a stress response to adverse cellular conditions caused by the acidic hypervacuolation and not a contributor to cell death. This form of bacteriocyte death is also nonapoptotic, based on the irregular shape of the adult nucleus as well as a lack of chromatin condensation during degeneration and other characteristic features.
Genetic testing also reveals a significant inhibition of the apoptotic pathway. Some other cell death characteristics found in bacteriocytes include acid-induced
mitochondrial dysfunction, high levels of
reactive oxygen species, and In the late phase of cell death, the digestion of endosymbionts by
lysosomes. 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 ==