Symbiosis: A Creation Story
During the first 2.5 billion years of life on Earth, bacteria and archaea followed separate paths of evolution, existing as distinct domains of life [1]. However, a significant event occurred when these two domains merged, leading to the emergence of a new and complex domain known as eukaryotes [1-5]. This fusion, often referred to as "the greatest symbiosis event of all time," is considered our creation story or one of the many versions of it. Similar to other stories of union, the merging of these microorganisms was not a simple process, as duplicating this event has proven to be a significant challenge for scientists. It took billions of years to solidify this union and give rise to entirely new organisms. This evolutionary event has since served as a crucial model for understanding other fascinating symbiotic relationships on Earth.
The term "symbiosis," derived from the Greek words meaning "together" and "living," encompasses various forms of coexistence [6]. Whether it is beneficial (mutualistic), parasitic, or commensal (where one organism benefits while the other is unaffected), all these relationships fall under the umbrella of symbiosis [6-7]. A classic example of symbiosis is the partnership between the bacterium Vibrio fischeri and the Hawaiian bobtail squid, Euprymna scolopes [8-10]. This relationship provides evolutionary advantages to both organisms. It begins with the physics of the situation: the cilia (tiny hairs) in the squid's light organ create turbulent currents that propel the bacteria to the organ's surface [8-10]. Once a sufficient number of bacterial cells come into contact with the organ, a chemical reaction occurs, triggering the release of antimicrobial chemicals and enzymes. These substances repel other bacterial species and attract more V. fischeri cells [8-10]. As the bacteria colonize the surface of the organ, they penetrate and remodel the entire organ system of the squid. The mature light organ becomes a safe haven for the bacteria, while in return, the bacteria provide illumination, mimicking moonlight, which camouflages the squid and protects it from predators [8-10].
Another intriguing symbiotic story involves sexual parasitism in the anglerfish, Cryptopsaras couesi [11]. In the deep, dark depths of the ocean, female anglerfish reside in a solitary and harsh benthic environment. To adapt to this environment, females emit cues such as pheromones and bioluminescence to attract a suitable mate [11-12]. Once a male or female anglerfish finds its mate, the dwarfed male anglerfish uses specialized denticle-like structures at the tips of its jaws to bite the female. This bite serves as an attachment, causing the skin layers of the male to fuse with the female's [11-12]. Over time, the circulatory systems of both male and female physically merge, allowing the female's blood to flow through the male's body. The male subsequently loses most of its internal organs and becomes entirely dependent on the female for nutrients. Meanwhile, the female utilizes the male as a permanent sperm bank. Male anglerfishes have brief free-living phases, and once fused with the females, a significant portion of their bodies ceases to function [11-12].
The concept of symbiosis, although often romanticized, reveals the intricate and multifaceted nature of life. The examples mentioned above demonstrate that symbiosis can be a complex process, at times unsettling and even destructive. It is not always characterized by harmonious cooperation. While certain relationships may bring benefits, they can also carry inherent tension and conflict, potentially leading to the emergence of something greater than before. The interdependence of organisms may challenge the notion of individuality, raising the question of what it truly means to be an individual when our bodies serve as microcosms for others.
References
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De Bary, A (1879) Die Erscheinung der Symbiose. Strassburg: Verlag Trubner.
Webster NS (2014) Cooperation, communication, and co-evolution: grand challenges in microbial symbiosis research. Front. Microbiol. 5:164. doi: 10.3389/fmicb.2014.00164
McFall-Ngai, MJ (1998) The development of cooperative associations between animals and bacteria: establishing the détente among domains. Integr. Comp. Biol. 38, 593-608
McFall-Ngai, MJ (2014) Divining the essence of symbiosis: insights from the Squid-Vibrio Model. PLoS Biol. 12, e1001783.
McFall-Ngai, MJ & Ruby, EG (1991) Symbiont recognition and subsequent morphogenesis as early events in an animal-bacterial mutualism. Science. 254, 1491-1494
Pietsch, T. W. (1975). Precocious sexual parasitism in the deep sea ceratioid anglerfish, Cryptopsaras couesi Gill. Nature, 256(5512), 38.
PIETSCH, T. (2009). Oceanic Anglerfishes: Extraordinary Diversity in the Deep Sea. University of California Press. Retrieved from http://www.jstor.org/stable/10.1525/j.ctt1ppb32
Further Readings
Yong, E. (2016). I contain multitudes: The microbes within us and a grander view of life. Random House.
Margulis, L., & Sagan, D. (1997). Microcosmos: Four billion years of microbial evolution. Univ of California Press.