The Modular Animal That Lives And Dies, Then Lives Again

When we hear the word “animals,” we conceptualize discrete individual organisms that have no morphological connection to other organisms. Yet, some animal species have evolved as modular organisms that congregate to create colonies and act together as mega-organisms.

Such is the case of the marine invertebrate Botryllus schlosseri (Figure 1), which come together to create a colonial-entity of modules, where each module is a living unit. A Botryllus colony is composed of several, and up to hundreds (sometimes thousands), of individuals grouped together. The Botryllus is a tiny organism. Each mature module is typically 1 mm long, and a colony can reach the size of a few centimeters.

Figure 1: A Botryllus schlosseri colony with 23 adult modules (zooids). The modules are attached to the underneath tunic. The blood-vessels are embedded in the tunic ending in the periphery shaped as bulbs or capsules and named ampullae. The white outline shows an example of a zooid; Ampullae are marked with yellow asterisks

Colonies can be found in the photic layer of the seas (the part of the upper layer of the sea that receives sunlight), permanently attached to a solid substrate by the tunic (ergo the sub-phylum name: Tunicates), a gelatinous tissue matrix abundant with blood vessels (Figure 2). The zooids are attached to the upper side of the tunic and are physically interconnected via blood vessels. This connection between individuals in the colony promotes the synchronization of their life and death cycles.

Figure 2: A Botryllus schlosseri colony in the lab. Botryllus tunic is a jelly-like substance that enables the colony to attach itself to solid substrates in the sea (and to glass-slides in the lab, as shown here).

There are three generations of modules that simultaneously inhabit and live in a colony. The life and death of each generation is tightly connected to a continuous rhythmic cycle of asexual budding of the young generations, accompanied by maturity and aging of the old generation. The continuous cycling of Botryllus modules is called Blastogenesis, and it is temperature dependent, where at 20°C each cycle lasts a week (higher temperatures result in shorter cycles and vice versa). The Blastogenic cycle is roughly divided into four stages, A, B, C, and D, according to the developmental stage of the modules (Figure 3).

Figure 3: A Botryllus colony throughout its Blastogenic cycle (=one week), starting from stage A, up to stage D. At the beginning of the week, the zooids open their oral syphon, feed, and breed throughout the week. At stage D, the zooids are old and will go through a process of absorbance, while the younger generation, the 1st buds, complete their development, open their oral syphon, and so on.

Each module lives for about three weeks (three blastogenic cycles). For the first two weeks of life, a module is referred to as a bud and completes the development of its body and organs. At the beginning of the third week of life, the module – now called a zooid, opens its oral syphon, enabling it to feed and breed independently of the other modules in the colony. An exceptional phenomenon occurs in the colony on the last day of the third week, when the old zooids begin to shrink and disappear. The old zooids go through a 24-hour process of being absorbed into the tunic while still alive, with their hearts being the last to go.

A recent study published by Baruch Rinkevich’s lab at the Israel Oceanographic & Limnological Research (IOLR) (Ben-Hamo, O. et. al, 2018), has focused on the study of development and aging of modules throughout the Blastogenic cycle. The cycle has been examined with regard to a very interesting gene, named Mortalin. Mortalin is responsible for refolding and repair of proteins in cells. The sequence of Mortalin gene was found to be highly conserved in all tested animals which imply its necessity to cellular function. Mortalin responds to several types of stress. Previous studies showed its connection to organism development, cell proliferation, cancer, and aging.

The central dogma in biology asserts that a certain gene can be transcribed into RNA, and the RNA should be translated into proteins. The number of RNA copies transcribed imply the level of the protein’s involvement in cells. Mortalin copies number of the RNA transcripts (levels) were sampled in the entire Botryllus colony, throughout the Blastogenic cycle. The levels were found to be significantly elevated in the last stage of the cycle (stage D) when the process of absorbing the old modules occurs.

Testing also revealed that Mortalin levels were significantly higher in buds, as compared to zooids, in stage D. The mechanism of old zooid absorbance in stage D involves the controlled cell death mechanism, also known as Apoptosis. It is also noteworthy that the young and the old modules are physically connected by blood vessels, yet the buds are not affected by the death of the old zooids.

After observing Mortalin levels and manipulating them (by using a specific inhibitor drug), the scientists suggested that Mortalin may act as a guard for the young buds against the massive apoptotic death of the old zooids. In conclusion, Mortalin, an aging biomarker, was shown to play a role in the aging of modules. Moreover, Botryllus schlosseri, this esoteric animal, could serve as an unexpected promising model animal for studies in organism aging.

Figure 4: An artistic rendering of the study made by the first author Oshrat Ben-Hamo. The painting shows the Botryllus schlosseri colony and the Mortalin gene studied.

The first author, Oshrat Ben-Hamo, creates science-inspired art in her free time and has represented the discussed research here in the painting represented in Figure 4. For more of her art: https://www.instagram.com/osher.science.art/ 

These findings are described in the article entitled Coupling astogenic aging in the colonial tunicate Botryllus schlosseri with the stress protein mortalin, recently published in the journal Developmental Biology. This work was conducted by Oshrat Ben-Hamo from the Israel Oceanographic & Limnological Research and University of Haifa, Amalia Rosner, Claudette Rabinowitz, and Baruch Rinkevich from the Israel Oceanographic & Limnological Research, and Matan Oren from the Israel Oceanographic & Limnological Research and Ariel University.

Reference:

1. Ben-Hamo, O., Rosner, A., Rabinowitz, C., Oren, M., & Rinkevich, B. (2018). Coupling astogenic aging in the colonial tunicate Botryllus schlosseri with the stress protein mortalin. Developmental biology, 433(1), 33-46.