rosetta and the Transients
Choanoflagellates, also known as "choanos," are single-celled eukaryotes that inhabit aquatic and marine environments. They exhibit characteristics resembling sperm cells, with finger-like protrusions on their collars and a whip-like flagellum for movement and capturing bacterial prey. Their active feeding behavior plays a crucial role in the marine food web and global carbon cycle, highlighting their ecological significance. As close relatives of animals, studying choanoflagellates offers insights into the evolutionary origins of multicellular organisms, including humans.
Choanoflagellates engage in phagocytosis, consuming detritus and a wide range of microorganisms. They are found worldwide in marine, brackish, and freshwater habitats. The life cycle of choanoflagellates typically involves an ovoid unicellular stage with a single posterior flagellum, which propels the cell and generates currents to capture food particles using an actin microvilli collar. This cellular structure bears a remarkable resemblance to sponge choanocytes or "collar cells," leading to speculation about the evolutionary relationship between choanoflagellates and animals. Molecular phylogenetic evidence strongly supports choanoflagellates as the closest unicellular relatives of animals.
While some choanoflagellates, like Monosiga brevicollis, maintain unicellularity throughout their lifespan, others, such as Salpingoeca rosetta, exhibit a more complex life cycle. S. rosetta has sedentary and motile stages, including fast and slow swimmers, as well as sexual and non-sexual phases. Slow swimmers can transform into fast swimmers or become attached cells. Additionally, slow swimmers can form multicellular rosette or chain colonies through cell division, with microscopic bridges and extracellular matrices connecting the cells. These unique characteristics make bacteria-loving protists like S. rosetta intriguing model organisms for studying the evolutionary history of animals.
As aquatic unicellular eukaryotes, S. rosetta interacts with millions, if not billions, of microbes. They consume diverse microorganisms for growth, metabolism, nutrition, and development. Recent research has positioned S. rosetta as a model species for understanding the bacterial cues that regulate eukaryotic development. Microbes exert a profound influence on the overall biology of S. rosetta. For instance, rosette colony formation can be induced by specific lipids produced by the bacterium Algoriphagus machipongonensis. Another study suggests that a protein from the bacterium Vibrio fischeri plays a role in inducing mating in S. rosetta.
Unlike symbiotic relationships, the association between S. rosetta and microbes is transient and non-dependent. Transient relationships, like symbiotic ones, offer benefits to organisms. While symbiosis may have played a role in the origin of eukaryotes, transient relationships have shaped our evolutionary cousins. Both types of relationships have equally contributed to the evolution of animals. Choanoflagellates, as social organisms, encounter various living organisms that aid in their evolution and transformation into unique beings. Unlike our sophisticated language system, choanoflagellates have unique modes of communication among themselves, such as eliciting responses through recognition of specific bacterial compounds or cues. Additionally, like symbiotic relationships, transient relationships provide opportunities for organisms to exchange physical elements, including genes, leaving remnants of themselves within different organisms. The greater the diversity of transients that S. rosetta interacts with, the higher the chances of acquiring unique genes that aid in their evolution, adaptation, and increasing complexity.
Research has revealed that the genome of S. rosetta contains a repertoire of animal-like genes. Approximately 62% of the sequenced genes of S. rosetta are shared with Uropisthokont and Urchoaozoan ancestral genomes, while the remaining 38% are either choan-specific genes or unique to S. rosetta. As a voracious bacterivore, S. rosetta acquires genes from the microbes it interacts with through horizontal gene transfer (HGT), the process of gene transfer between non-directly related individuals. The "you are what you eat" gene transfer ratchet theory suggests that the evolution of protist genomes, including choanoflagellates, is driven by the acquisition of genes from external sources, including random gene fragments from food sources.
Similar to HGT studies conducted on M. brevicollis, the genome of S. rosetta contains approximately 4% of HGTs acquired from food sources and transients, such as algae and bacteria. Some of these HGTs have facilitated the digestion of diverse foods and adaptation to various ecological niches, while others have had neutral or even toxic effects on S. rosetta. These HGTs undergo natural selection, potentially filtering out genes that do not contribute to the organism's evolutionary fitness.
Just like our evolutionary cousins, we interact with numerous organisms in our environment. Although we may not be consciously aware of it, these interactions result in the exchange of genetic material, enabling us to become more complex. It is possible that choanoflagellates, like S. rosetta, possess genes or physical attributes that allow them to directly communicate with the random organisms they encounter, providing evolutionary benefits. Organisms communicate using different languages, and in the case of choanoflagellates, information received from transients can be transferred through mechanisms such as chemical and biological cue recognition or the passing of genetic information through HGT. These chance encounters with diverse transients often lead to lasting changes and relationships that weave the intricate web of life on Earth.
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