Choanoflagellates are single-celled aquatic organisms with unique features. These microscopic eukaryotes offer insights into the early stages of life’s complexity. A central question surrounding them is whether they can be considered truly multicellular. Exploring their characteristics and behaviors provides insights into the evolutionary journey from single-celled ancestors to multicellular life forms, including animals.
Understanding Choanoflagellates
Choanoflagellates are free-living, single-celled organisms found globally in aquatic environments, including marine, freshwater, and brackish habitats. They inhabit both the open water (pelagic zones) and the bottom substrates (benthic zones), existing from the Arctic to the tropics and at depths ranging from the surface down to 300 meters.
Each choanoflagellate cell possesses a distinctive morphology: a spherical or ovoid cell body, approximately 3 to 10 micrometers in diameter. A single flagellum extends from one end, surrounded by a “collar” composed of interconnected microvilli. This specialized structure is central to their feeding mechanism.
Choanoflagellates are filter feeders, using their flagellum to generate water currents. This current draws food particles, primarily bacteria and detritus, towards the cell. The collar of microvilli then acts as a sieve, trapping these particles, which are subsequently ingested by the cell.
Colonial Life and the Multicellular Threshold
While choanoflagellates are often found as independent, single cells, some species exhibit colonial behaviors, forming temporary aggregations. These colonies can arise from individual cells coming together or, more commonly, through cell division where daughter cells remain attached. For example, the species Salpingoeca rosetta can exist as a solitary cell or form spherical colonies known as rosettes, as well as chain-like structures.
The formation of such colonies in choanoflagellates is often transient, with cells capable of separating and surviving independently. This characteristic distinguishes them from truly multicellular organisms, where cells are specialized and cannot survive on their own once separated from the organism. In a true multicellular organism, cells undergo significant differentiation, developing into specialized tissues and organs that perform distinct functions.
True multicellularity also involves irreversible cell-to-cell adhesion, programmed cell death for tissue maintenance, and highly integrated physiological functions across different cell types. While some choanoflagellate colonies may show basic cell-to-cell communication and coordinated behaviors, such as electrical signaling or synchronized flagellar beating in S. rosetta, they lack the extensive cell differentiation and deep interdependence seen in animals, plants, or fungi. Their colonial forms are more akin to a group of individual cells cooperating rather than a single, cohesive organism with specialized, interdependent cellular roles.
Evolutionary Insights from Choanoflagellates
Choanoflagellates are important for understanding the evolution of multicellularity, particularly in animals. They are considered the closest living relatives to animals, occupying a key position on the phylogenetic tree. This close relationship suggests that studying choanoflagellates can provide clues about the characteristics of the last unicellular ancestor of animals.
Genetic evidence supports this evolutionary connection. Choanoflagellates possess genes homologous to those involved in cell adhesion and signaling pathways in animals. These include genes for cadherins, which are crucial for cell-to-cell adhesion in animals, and C-type lectins, involved in cell recognition and binding. They also express several types of tyrosine kinases, which are fundamental to cell signaling in multicellular organisms.
The presence of these complex genes in choanoflagellates suggests that the genetic toolkit necessary for multicellular life may have existed in single-celled organisms before the emergence of animals. These genes were likely co-opted and repurposed for new functions during the transition to complex animal life. Choanoflagellates serve as models for investigating the biological mechanisms that might have facilitated the evolutionary leap from unicellular to multicellular forms.