Chromosomal Ploidy Dynamics in Aphid Bacteriocytes Reveal Tight Coevolution Between Host and Symbiont

The cooperative relationship between aphids and their intracellular symbiotic bacteria is remarkably intricate. In the pea aphid Acyrthosiphon pisum, a specialized cell type known as the bacteriocyte houses the bacterial symbiont Buchnera aphidicola. These cells are not only unusually large but also play a central role in nutritional symbiosis. Because of their size and function, scientists have long suspected that the chromosomes inside these cells exist in a highly polyploid state.

The bacteriocytes within the bacteriome of the pea aphid contain Buchnera aphidicola（Image source：Nozaki T and Shigenobu S. (2022)， CC BY 4.0 ）

Polyploidy is closely associated with a cellular process known as endoreduplication, in which the genome is repeatedly replicated without subsequent cell division. This specialized cell cycle often occurs in tissues with extremely high metabolic demand. Human liver cells, for example, frequently contain four to eight sets of chromosomes. Having additional DNA templates can increase gene expression, accelerate metabolic activity, and enlarge cell volume. Similar phenomena have been observed in plants and some animals. In insect–microbe symbioses, polyploidy has been proposed as a mechanism that enables the host to supply greater nutritional resources to its symbiotic partners. However, most studies on insect bacteriocytes have focused mainly on describing their large appearance rather than quantifying their ploidy levels. A rare exception comes from research on psyllid insects, where bacteriocytes were found to be 16-ploid. For aphids, however, detailed data on ploidy distribution had remained largely unknown.

The symbiosis between the pea aphid and Buchnera aphidicola is the result of long-term coevolution. Over evolutionary time, Buchnera has lost many metabolic genes and can no longer synthesize certain amino acids independently. These nutrients must therefore be supplied by the aphid host. Conversely, aphids feeding on plant phloem sap cannot obtain some essential amino acids directly from their diet, and Buchnera compensates by producing them. In this way, each partner fills critical metabolic gaps for the other.

Bacteriocytes of the pea aphid host enormous numbers of Buchnera cells. They supply large amounts of non-essential amino acids and related precursor molecules, which the bacteria use to synthesize essential amino acids such as arginine and methionine that are required by the aphid. These host cells can reach about 100 micrometers in diameter, while their nuclei measure roughly 20 micrometers across. Because of these dimensions, researchers have long suspected that bacteriocytes possess exceptionally high ploidy levels.

Recent research has now confirmed this assumption. Regardless of whether the aphid is a viviparous parthenogenetic female, a sexual oviparous female, or a male, the nuclei of their bacteriocytes contain elevated amounts of DNA, ranging from 16C to 256C, where the C value represents multiples of the haploid genome size. The distribution of ploidy varies significantly among different aphid life forms. In viviparous parthenogenetic females, bacteriocytes are most commonly between 64C and 128C and reach the highest overall ploidy levels. Oviparous sexual females are dominated by 64C cells, while males typically show lower levels between 32C and 64C. These differences are strongly associated with the life-history strategies of each form.

Asexual females can produce more than ninety offspring during their lifetime. Their rapid reproductive rate imposes extremely high metabolic demands. Consequently, bacteriocytes must deliver large quantities of nutrients to sustain Buchnera, while the bacteria must simultaneously produce essential amino acids required by the host. Elevated ploidy in bacteriocytes likely reflects this intensified metabolic workload. In contrast, oviparous females produce far fewer eggs, and males invest relatively little metabolic energy in reproduction. In these forms, bacteriocyte activity becomes reduced, a pattern also reflected in the declining abundance of Buchnera cells.

The bacteriocyte states of viviparous females, oviparous females, and males of the pea aphid. F-actin is one of the cytoskeletal proteins（Image source：Nozaki T and Shigenobu S. (2022)， CC BY 4.0 ）

Changes in ploidy also occur in a clear and continuous pattern during aphid development. In newly born first-instar nymphs, bacteriocytes typically exhibit ploidy levels of 16C to 32C. As the insect grows, ploidy gradually increases through successive developmental stages. By adulthood the levels rise further, especially during periods of intense reproduction. Seven-day-old adults, which are at the peak of reproductive activity, display the highest ploidy levels. In viviparous females, bacteriocytes commonly reach 256C during this stage.

This synchronized increase in ploidy during peak reproductive periods strongly suggests that polyploidy serves as a mechanism to support the high productivity of the aphid–bacterial symbiotic system. In older aphids, around twenty-one days after reaching adulthood, reproduction continues but signs of cellular aging become evident. Bacteriocyte nuclei begin to develop irregular shapes, and the number of Buchnera cells declines. During this stage, ploidy levels drop slightly compared with the reproductive peak.

Observations at the cellular level reveal a consistent relationship: larger bacteriocytes contain larger nuclei, higher ploidy levels, and greater numbers of symbiotic bacteria. This pattern aligns with known physiological consequences of polyploidy. In addition, nucleolus size increases with ploidy level. The nucleolus is the site of ribosome production, and its size often reflects a cell's capacity for RNA and protein synthesis. Bacteriocytes with high ploidy likely possess enhanced ribosome production capacity and faster metabolic rates, ultimately improving the efficiency of nutrient exchange between host and symbiont.

Such finely tuned coordination between host cellular function and symbiont-dependent metabolism binds the evolutionary destinies of aphids and Buchnera tightly together. Aphids cannot properly develop or reproduce without Buchnera, while the bacteria cannot survive outside the protective environment of aphid bacteriocytes. Through this deep mutual dependence, the two organisms have followed a shared evolutionary trajectory uniquely shaped by their intimate partnership.

Author: Shui-Ye You

Reference:

Nozaki T and Shigenobu S. (2022). Ploidy dynamics in aphid host cells harboring bacterial symbionts. Scientific Reports.

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