Understanding Gender Identity Through the Lens of Tetrahymena
Written on
What if you landed on a distant planet, seeking companionship, and discovered seven distinct genders? The only one you couldn't mate with was your own!
Sounds intriguing, right? Now, let's switch the scenario. Imagine you’re a single-celled organism, floating in a pond on Earth, and you encounter seven genders, again with the same mating restriction.
Fascinating, isn't it? But how does a single-celled entity discern the different sexes available for mating? This question is pivotal because every organism must identify its kin to ensure successful reproduction.
In this article, we delve into the intriguing realm of Tetrahymena, a single-celled ciliated organism that has intrigued scientists since the 1800s.
Note: Cilia are microscopic hair-like structures protruding from cells. In the featured image of Tetrahymena, these appear as small green strands resembling hair. Other organisms, including humans, also possess ciliated cells.
Today, we will explore several reasons, including its seven sexual forms, that make Tetrahymena a valuable research subject. By examining this creature, which is evolutionarily distant from us, we can uncover fundamental molecular processes related to aging, immune recognition, and more.
So, let’s dive deeper into the world of Tetrahymena cells.
A Brief History and Basic Biology of Tetrahymena
(Personal Note: In the early 1990s, I conducted my Ph.D. research under Dr. C. David Allis, a leading expert on Tetrahymena. My history with this organism runs deep!)
#### Discovery
The first observations of Tetrahymena likely occurred during the microscopic explorations of Anthonie van Leeuwenhoek in the 17th century. Historically, because various species of Tetrahymena appeared physically similar, they were grouped under one species called T. pyriformis, named for its pear-like shape.
Pyriform comes from Latin, meaning pear-shaped.
The species T. pyriformis was formally described by Ehrenberg in 1830. Since then, 22 additional species have been discovered, with T. thermophila being the most commonly used in contemporary research, complete with its own genome database.
If you’re eager to skip ahead to the more exciting aspects, feel free to jump to the Sex Cells section, but understanding the biology of Tetrahymena will enhance your comprehension of its sexual processes.
Biology and Life Cycle
#### Physical Features
The name Tetrahymena derives from two Greek words: “tetra,” meaning four, and “hymen,” referring to membrane. In human anatomy, the hymen is a membrane that partially covers the vagina and is often mistakenly associated with virginity.
Where, you might wonder, are these four membranes in our organism? They reside in its mouth, or buccal cavity, with three membranes stacked on one side and one on the other.
Since the mouth is also the site of mating, the term "hymen" is indeed fitting.
The species name thermophila indicates its preference for warmer environments (optimal at 30-35° C).
Tetrahymena typically measures 30–50 micrometers in length and 20 micrometers in width, featuring approximately 20 rows of cilia.
For context, a human hair is about 70-100 micrometers in diameter, meaning three to five **Tetrahymena* cells could fit side by side within a single hair.*
One remarkable characteristic of Tetrahymena is its dual nuclei: a micronucleus and a macronucleus. We'll discuss these further along.
Moreover, it possesses organelles for energy production, food vacuoles for nutrient storage, and structures necessary for maintaining cellular integrity.
Tetrahymena species inhabit all freshwater environments and thrive at temperatures ranging from 4° to 37° C globally, even at elevations up to 3000 meters.
In its natural habitat, T. thermophila consumes bacteria, while other species may act as parasites within invertebrates like mosquitoes and snails.
Here's a video showcasing a Tetrahymena cell feeding on bacteria, allowing you to observe its cilia in action! Enjoy!
#### Cultivating Tetrahymena in the Laboratory
One reason Tetrahymena is an ideal research organism is its straightforward and economical cultivation. By mixing a nutrient broth and adding the cells, you can grow a substantial population within a day.
Researchers can choose between a generalized medium or a precisely defined nutrient mix to study specific growth conditions or cellular processes.
Through careful sterilization of culture environments, researchers can ensure that no foreign organisms contaminate the samples, a practice known as axenic culturing, critical for reliable experimental outcomes.
#### Synchronized Cell Culture
Another advantage of using Tetrahymena is its ability to synchronize cell growth. By applying heat, most cells pause their life cycle, entering a state of suspended animation. Once conditions normalize, they resume development simultaneously, maintaining this synchronicity through several divisions.
Additionally, starving the cells can induce synchronous mating. If cultures of different mating types are combined after starvation, they can mate effectively.
Understanding these processes leads to harvesting numerous cells at the same mating stage for detailed examinations.
To appreciate the intricate mating behaviors of Tetrahymena, we must examine the roles of the micronucleus and macronucleus in greater detail.
Macs, Mics, and Mating
When cells divide, they replicate their DNA to pass onto new cells, a process that becomes particularly interesting in Tetrahymena due to its unique nuclei: the macronucleus and micronucleus.
These nuclei hold the organism's DNA. Understanding the concept of ploidy, which indicates the number of chromosome sets, is essential here.
In organisms with only two mating types, like humans, sperm and egg cells each carry haploid genomes. Upon fertilization, these genomes combine to form diploid chromosomes in the new embryo.
In Tetrahymena, a micronucleus contains five pairs of diploid chromosomes. While these replicate during asexual reproduction, they do not actively contribute to routine growth. Their role emerges during mating as germline nuclei, transitioning to haploid.
The macronucleus, on the other hand, contains multiple copies of select mic DNA segments, resulting in a ploidy of 45–50.
The mating process is intricate. When food is abundant, Tetrahymena reproduces asexually. However, under resource scarcity, it shifts to sexual reproduction by conjugating with another cell of a different mating type.
In this conjugation, the cells appear to "kiss." After this union, they undergo a series of steps to create a new hybrid cell with a combined genome.
Ultimately, the new hybrid cells cannot mate with each other, as they belong to the same mating type.
How Does the Cell Generate a New Macronucleus?
The macronucleus is formed by breaking down one of the newly created diploid micronuclei into around 200 minichromosomes in two stages. The first stage involves cutting the chromosomes into approximately 250 minichromosomes, with some being discarded later.
Next, these minichromosomes are trimmed at specific sites, removing smaller sections while reassembling the remainder. This results in the elimination of about one-third of the original genome, resulting in a macronucleus with around 45 copies.
This macronucleus is crucial for the everyday functions of the cell, distinct from the germline nuclei responsible for reproduction.
The Seven Sexual Forms
Finally, let’s explore the seven mating types of Tetrahymena. Although researchers have known about these types since the 1950s, the mechanisms behind their specification remained unclear until March 2020.
A team at UC Santa Barbara identified the mating-type genes and revealed a unique process of DNA rearrangements for sex determination. Their findings have implications for human health, particularly in tissue transplantation and cancer.
Each Tetrahymena germline micronucleus contains 6 or 7 mating-type genes, retaining the complete genome. However, after the complex rearrangement during macronucleus formation, only one mating-type gene remains, determining the organism's sex.
Thus, while we understand how mating types are specified, predicting the progeny’s mating type remains elusive, as it appears to be a random process akin to rolling a seven-sided die.
Research Significance of Tetrahymena
Why is Tetrahymena considered a valuable research organism? Recent years have seen over 3,000 publications featuring this organism, with a significant portion funded by the National Institutes of Health.
Tetrahymena possesses a comparable number of genes to humans, with many exhibiting sequence similarities related to biological functions. This makes it an effective model for exploring significant biological questions.
The diversity of research areas influenced by Tetrahymena is vast. Some key areas include mating type determination and programmed genome rearrangement, which have profound implications in genetic research.
#### Telomeres
Tetrahymena was the first organism in which the enzyme telomerase was discovered, leading to significant advancements in understanding cellular aging and chromosome protection.
Telomeres, the protective caps at the ends of chromosomes, are vital for maintaining DNA integrity. Telomerase adds these caps, crucial for preventing chromosome degradation.
As cells divide, telomeres shorten, serving as biological clocks that indicate cellular age. When they become completely depleted, the cell ceases division and may undergo programmed cell death.
This characteristic makes Tetrahymena an essential model for studying telomere dynamics.
#### Functional Genomics
Functional genomics investigates gene roles and interactions, and Tetrahymena is emerging as a key model organism for such studies. With gene functions often conserved across species, it serves as an ideal system for probing biological questions.
Understanding gene functions in Tetrahymena can lead to insights applicable to higher organisms, including humans.
#### Evolution
Tetrahymena holds a unique position on the eukaryotic evolutionary tree. It belongs to the Chromalveolates, illustrating its evolutionary significance.
In conclusion, Tetrahymena offers a wealth of insights into fundamental biological processes, including sex determination and immune response mechanisms, while presenting unique advantages for research.
If you’ve read this far, consider subscribing to my newsletter for a free eBook!
References for this article:
- How a Tiny Critter Has Seven (Yes, Seven) Sexes by Carrie Arnold (2013).
- Tetrahymena as a Unicellular Model Eukaryote: Genetic and Genomic Tools by Marisa D. Ruehle et al. (2016).
- Parallel evolution of histophagy in ciliates of the genus Tetrahymena by Michaela C Strüder-Kypke et al. (2001).
- The Biology of Tetrahymena by Alfred M. Elliott (1959).
- Tetrahymena thermophila, a unicellular eukaryote with separate germline and somatic genomes by Eduardo Orias et al. (2011).
- Whats, hows and whys of programmed DNA elimination in Tetrahymena by Tomoko Noto and Kazufumi Mochizuki (2017).
- Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila by Cervantes et al. (2013).
- Researchers discover how model organism Tetrahymena plays roulette with seven sexes by Public Library of Science (2013).
- Functional genomics: the coming of age for Tetrahymena thermophila by Aaron P. Turkewitz et al. (2002).