Candidatus Tremblaya princeps

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Classification

Domain: Bacteria

Phylum: Pseudomonadota

Class: Betaproteobacteria

Genus: Candidatus Tremblaya

Species

NCBI: [1]

Candidatus Tremblaya princeps

This bacterium belongs to the Betaproteobacteria class, a diverse group known for both their ecological versatility and economic significance. Within this class, Ca. T. princeps stands out due to its specialized symbiotic lifestyle and extreme genomic reduction. It is classified as a 'Candidatus' genus, indicating it has not yet been cultured in a lab setting.

Description and Significance

Candidatus Tremblaya princeps is primarily recognized for its nested symbiotic relationship within mealybugs, particularly the citrus mealybug Planococcus citri, where it is encapsulated in specialized bacteriocytes. It is unique for housing another endosymbiont, Moranella endobia, within its own cellular structure. This multilayered endosymbiosis is a remarkable example of evolutionary adaptation, where Ca. T. princeps, despite its extremely reduced genome, plays a critical role in the nutritional pathways of its host. Its genome is one of the smallest known among prokaryotes, reflecting significant genomic streamlining possibly due to its symbiotic lifestyle.

The small size of the genome brings about significant dependencies, where essential metabolic functions such as synthesis of amino acids and nucleotides are outsourced to its nested symbiont or the mealybug host. This makes Ca. T. princeps an interesting subject for studies on cellular minimalism and symbiotic interdependence.

Genome Structure

With a circular genome of only 138,931 base pairs, Ca. Tremblaya princeps has the second smallest genome and the smallest number of functional genes present within a prokaryote. Its genome only encodes 116 protein-coding genes and 20 RNA-coding genes (same as above) and is so reduced that it no longer has a complete set of enzymes for any one pathway.

This reduction can be attributed to genome streamlining, which is a common occurrence with endosymbionts. T. princeps is an extreme example, however, due to its multi-tiered symbiosis. The incomplete pathways found within the T. princeps are complemented by the genomes of both the secondary endosymbiont and the mealybug, creating a three-way dependency.

As a reference, Mycoplasma genitalium, which is often used for minimal cell studies and widely considered the smallest free-living prokaryote. It has a genome size of 580,076 bp and about 480 proteins, remarkably making it considerably larger than T. princeps.

Cell Structure, Metabolism and Life Cycle

Candidatus Tremblaya princeps exhibits a highly specialized cellular structure adapted to its endosymbiotic lifestyle. Lacking many basic metabolic pathways, it relies heavily on both its mealybug host and the Moranella endobia for survival and replication. The cell structure is characterized by a compact design with essential genes mainly focused on the production of amino acids and ribosomal components necessary for protein synthesis. It has retained the ability to synthesize key amino acids such as valine, leucine, and isoleucine, which are critical for the host's nutrition, particularly given the nutrient-poor sap diet of mealybugs. The bacterium is an obligate endosymbiont, meaning it cannot survive outside its host's body. Its life cycle is closely tied to that of its host, with strict vertical transmission ensuring that each generation of mealybugs carries the endosymbiont. The interdependence is so critical that the bacterium and its host have co-evolved mechanisms to ensure mutual survival and replication.

Ecology and Pathogenesis

The ecological role of Candidatus Tremblaya princeps is defined through its symbiotic relationships. Its presence within the mealybug Planococcus citri is essential for the insect's survival, particularly in nutrient-poor environments where the synthesis of essential amino acids by Ca. T. princeps and Moranella endobia is vital. The nested symbiotic relationship allows for a remarkable efficiency in nutrient production and conservation, enabling mealybugs to thrive on sap diets that lack many essential nutrients. The study of Ca. T. princeps offers insights into the evolutionary dynamics of genome reduction and the ecological and evolutionary implications of such extreme symbiotic relationships. Understanding this symbiosis can also contribute to more effective biological control strategies for managing mealybug populations, which are significant pests in agriculture.

References

Arkadiy I Garber, Maria Kupper, Dominik R Laetsch, Stephanie R Weldon, Mark S Ladinsky, Pamela J Bjorkman, John P McCutcheon. "The Evolution of Interdependence in a Four-Way Mealybug Symbiosis". Genome Biology and Evolution. 2021. Volume 13, Issue 8.

Author

Page authored by _____, student of Prof. Jay Lennon at IndianaUniversity.