Mnemiopsis Leidyi (Warty Comb Jelly)
M.Leidyi as an Model organism
Mnemiopsis leidyi (also known as warty comb jelly or the sea walnut) is a type of ctenophore which was originally found in the western Atlantic ocean. At 10cm long and 2.5cm wide, with a distinctive biradial body, it is easy to recognize despite its small size. Another distinguishing feature is that it illuminates a blue-green color when the species is startled or threaten.
M. leidyi do not drift through the ocean passively, they move actively via comb rows. There are eight comb rows that are found on the sides of the ctenophore. Each of these rows are made up of individual comb plates which are composed of thousands of cilia. The cilia on the comb plates beat together in a rhythm and move the organism through the water. The warty comb jelly is the largest organism that moves with cilia. Also located in the comb plates are specialized photocytes which produce many colors by bioluminescence.
Warty comb jellies are not as simple as they appear. The body plan of the organism is arranged on an oral-aboral axis. The mouth(oral) is located at one end while the anal pores and apical sensory organ(aboral) are on the other end of the organism. At the oral end there are tentacles which are used to catch and feed on prey. Their prey consists of zooplankton, small crustaceans and fish eggs and larave. It also can eat smaller members of its own species. The ctenophore has a complex nervous system which is composed of the peripheral nerve net and the apical sensory organ which is found in the aboral end. This system is used to sense gravity and light in the environment. The pair of anal pores are located next to the apical sensory organ and is used to control the osmotic pressure within the organism, since it is made of 97% water.
Differences between cnidarians
There are many differences between warty comb jelly (ctenophore) and jelly fish(cnidarian).The jelly fish is radially symmetric along its oral-aboral axis while the warty comb jelly is biradial symmetric. The warty comb jelly moves by cilia movements via the comb rows. They also have defined muscle cells which move the cilia. Jelly fish do not have muscle cells and move by pumping pulsations. Cnidarians have stingers, or cnidnictes in their tentacles which inject toxins into their victims while cnetohphers have colloblast in their tentacles to stick and entangle prey not to kill the pray. Since M. leidyi do not sting, it makes it an easy meal for many birds, fish and larger jellyfish.
Why is it a good model organism
The entire genome of the M. leidyi has been sequenced, thus we know how many genes are present within the organism. Sequencing of a ctenophore genome can be helpful in determining gene content, structure and gene evolution Their embryos are crystal clear, which makes imaging early features of fertilization, the cell cycle and cell division possible. Ctenophore has complex body structures and it lives in a hostile environment that makes it prone to injury thus it is a good candidate to study its regeneration and healing process. This could be used to understand how other species including human can be helped to regenerate themselves in the future. Ctenophore are ideal systems for studying adult wound healing and regeneration in the lab or in their natural environment.
Ctenophores are “diploblasts,” because they lack a well-deﬁned mesodermal cell layer. Adult M. leidyi is very capable of regenerating missing, removed, or damaged body parts. Even if large portions are removed or damaged, it can usually repair itself in every cell type in the specific location. For instance if the comb rows are removed and placed upside down, the jelly will fix them so they are right side up. This process is called post-regeneration. In post-regeneration, embryos that are dissected during early stages are deficient in half of the animal body plan. The embryos grow into incomplete larvae and adult. Regeneration of the missing structures occurs in the larva or the adult, which is somehow able to “detect” the missing structures, even though these were never present to begin with. Post-regeneteation suggests that there is some body plan which exists even in half-animals to make a whole animal.
There are some conditions which the organism does not undergo post-regeneration when the embryo is bisected during the 4 cell stage of cleavage. Instead a stable half-animal is generated. A half-animal is one that has only half of its organs developed but is stable enough to live.
Half-animals are stable because the part of the organism that is there contains the needed positional values which are necessary for its survival. When the part is missing such information, the organism undergoes post-regeneration. The reason why this form of animal is stable is that the positional values, which are used to ‘map’ out the organism body plan, are the same on either side of the oral/aboral axis. Also if adult animals are bisected perfectly in halves, they will form stable half-animals instead of repairing itself.
How an embryo is damaged or deficient during the 4 cell stage can determine whether or not the animal exhibits post regeneration. Note that post regeneration happens only in adult animals, even if the body part is missing in the embryo design. In a normal embryo the two halves join to make a whole animal. In figure 2D, during a four cell stage, the cells in white are dissected and removed. The two adjacent cells coordinate with each other and survive as half animals. In figure 2E, the cells that are diagonal with respect to each other are dissected while the other diagonal pair is allowed to grow. Instead of being mirrored halves, each of the cells have missing parts. Therefore the animal undergoes post regeneration so that the cells can regain the information it’s missing, creating a whole animal. Only one of the cells is removed during the four cell stage in figure 2F. Since one half of the animal is complete and the other is half is incomplete, the incomplete part undergoes post-regeneration and creates a whole animal. Three of the four cells are removed in figure2G and according to the model, should form a half-animal. However, the experimenters Henry and Martidale stated this did not happen. Their reason was that the ¼ animals are too small and have an underdeveloped gut. Thus if it was able to eat, it would have developed into a half animal.
Therefore the polar coordinate model presented by Henry and Martidale demonstrates that there is some mechanism that helps an organism when undergoing regeneration to form the correct morphology along the oral/aboral axis. Only in one experiment, when three quarters of the cells are missing, the animal was unable to produce correct morphology and unable to regenerate. The authors propose that the reason why one quarter organisms cannot regenerate is due to malnourishment. Since only one quarter of a gut is produced and the small one quarter animal is difficult to feed, it does not have the nutrients to regenerate.
Mnemiopsis leidyi produces free-spawning gametes, and also enable to self-fertilize as a simultaneous hermaphrodite. After fertilization, the embryo is fully developed after only 20 hours. (GESAMP 1997)
2) Early development
Mnemiopsis leidyi experience radial cleavage after 4cell stage. The first cleavage is meridional, and second cleavage is equatorial cleavage. After the third cleavage which is unequal and oblique, the embryo is divided into four end cells (E cells) on bottom(posterior) and four middle cell on the top (anterior). The Figure is aboral view, which is view from the posterior. At the 16 cell stage, each E cell produces small e1 micromere at the aboral(posterior) and each M cell produces similar m1 micromere. At 32 cell stage, e1 micromeres divides into e11 and e12 cells, and m1 micromere divides into m11 and m12 cells. E11 and m11 cells are closest to the sagittal plane.
All muscle cells are derived from micromeres born at the oral pole of endomesodermal precursors (2M and 3E macromeres). While the development of the four quadrants is similar, diagonally opposed quadrants share more similarities than adjacent quadrants. Adult ctenophores possess two diagonally opposed endodermal anal canals that open at the base of the apical organ. These two structures are derived from the two diagonally opposed 2M/ macromeres.
Tentacular and Sagittal plane(axis) passes through the oral/aboral axis and divide the body into four quadrant. Each quadrant possesses a pairt of ctene rows(multiple plates of cilia=comb rows), as well as portion of tentacle, tentacle bulb and the apical sense organ. Apical organ is a sensory structure in ctenophores that enables the animal to sense its orientation in water. Aboral/oral axis posterior/anterior axis that goes from mouth to apical organ.
3) TGF-Beta pathway
The TGF-bsignaling pathway is a metazoan-specific intercellular signaling pathway known to be important in many developmental and cellular processes in a wide variety of animals. pathway in which certain secreted proteins had the capability of transforming cells and tissues.
Binding of TGF ligand to TGF beta receptor II initiates signaling, and the receptor I activates Receptor-Smad. Receptor-Smad and Co-smad, the complex enters the nucleus and activates the transcription of target genes. Extracellularly, the signaling pathway can be inhibited by CAN family, Noggin, Chordin, and Follistatin. Intracellularly, the pathway can be inhibited by SMURF or Smad6/7 (inhibitor Smad).
This pathway are highly conserved in metazoans, but outside the metazoan, no TGF-beta receptor or ligand has been discovered, so this pathway most likely evolved early in animal evolution. It is presumed that this signaling pathway is involved in axial patterning in cnidarians.
Ctenophores are very difficult to place in the phylogenetic tree of life. While it is easy to identify what is a ctenophore and what is not a ctenophore, there has been much biological debate on how these organism relate to other animals. The tree of life was originally based on the complexity of all living animals with the least complex being the closest to a simple common ancestor and at the ‘bottom’ of the tree. However, many scientist today insist that the tree should be based on the genes of animals and the relationships between animals is determined by the amount of genes they share with each other.
Sponges are considered the most simple organisms with a few cell types and a simple body plan and thus was placed at the bottom of the tree. Further morphological complexity, such as the evolution of true epithelial tissues, allowed the evolution of such groups as the placozoans, cnidarians, ctenophores, and subsequently the large radiation of bilaterally symmetric organisms. However, a recent molecular study has re-examined animal phylogeny by looking at many genes from a much broader range of animal diversity. This phylogenomic study unexpectedly found high statistical support for ctenophores being the earliest living animals, diverging even before sponges. The new phylogenetic position matter because it could enlighten our understanding of animal evolution.
Ctenophores have a nervous system and muscle cells, while neither have ever been found in sponges. While cnidarians have a nervous system they do not have mesodermally derived muscle cells. Ctenophores may be at the base of the animal tree of life. However, they have been evolving on their own for millions of years, so how they achieved their current complexity is a very interesting area of study.
Ecology significance of M. Leidyi
Except the developmental biology of Mnemiopsis leidyi, the species is researched as an major invasive species of the North and the Baltic Sea. In 1980s, after accidentally introduced in the Black sea, M. leidyi population density reached 400 specimens per m^3 of water. Consuming majorly eggs and larvae of pelagic fish, fish population of Black sea crashed dramatically. In same way, in 1999, the introduction of M. leidyi into Caspian Sea caused 75% of depletion of the zooplankton and an entire food chain of the lake.
Since then, the area of M. leidyi habitat is increasing, from Mediterranean basin to the northwestern Atlantic. The exact dispersal route for M. leidyi isn’t known at the present time, but numerous research papers examine the effects of M. leidyi as an invasive species on ecosystems of this region.