Category Archives: flatworms

Planarian: a living vessel built by unicellular organisms?

by Piter Kehoma Boll

Not long ago I talked about the peculiar genome of the freshwater planarian Schmidtea mediterranea and how it challenges our view of many cellular processes. Now what if I told you that planarians also challenge our view of what a multicellular organism is?

We all know that organisms may be either unicellular or multicellular, but sometimes it is hard to tell them apart, especially in what are called colonial organisms, in which clones of unicellular individuals may live together.

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Green algae of the genus Volvox are somehow at the boundary between unicellular and multicellular organisms. Although usually considered a colony of unicellular organisms, they behave somewhat like a multicellular organism. Photo by Frank Fox.*

So where lies the boundary between gathered unicellular organisms and true multicellular organisms? One of the ideas is that for a group of cells to be considered a single unit (an organism) they must have high levels of cooperation and low levels of conflict between each other and, perhaps more important than that, they have to be dependent on the association in order to survive.

As most recent evolutionary theories predict, cooperation increases with genetic similarity. As a result, multicellular organisms are (almost) always composed of cells having the exact same genetic material, i.e., they are all clones. We know, however, that during DNA replication mutations may occur, so that eventually at least some cells of an adult and many-celled organism may have become genetically distinct. This leads to a need to find a way to fix this problem by reincreasing genetic similarity, and the way most organisms found to do that is by allowing only one lineage of their cells, the germ cells (which produce the gametes) to generate the next generation. Thus, each transition from one generation to the next passes through a “zygotic bottleneck”, i.e., a new organism is always generated from a single original cell, the zygote, which assures that the genetic similarity is always brought back.

8-day-old_newt_egg

A developing new egg, with an embryo that is created from a single original cell, the zygot, assuring a higher genetic similarity of the cells in the whole organism. Photo by Stéphanie Bret.**

The zygotic bottleneck is not a rule for a lot of species though. Many animals and plants are able to reproduce asexually by budding, fission or many other ways. In such cases, the new organism usually is built from several different lineages of the parent organism. For example, some succulent plants may generate a new organism from a dettached leaf and several cells of the original leaf start to reproduce and together they build the new plant and, as each lineage may have suffered different mutations, the offspring is not necessarily composed of genetically identical cells. Nevertheless, even such organisms, which are able to reproduce asexually, still retain the ability to generate zygotes through sexual reproduction, which eventually “cleans that mess”.

But in planarians things get really strange. First of all, let’s explain some basic things about planarians. They have, as you know, a remarkable regeneration ability. This happens due to the presence of stem cells called neoblasts that fill their bodies. Those neoblasts are able to generate all cell types that make up the planarian’s body. In fact, all cells in a planarian must come from neoblasts, because, as weird as it may be, all differentiated cells in a planarian body ARE UNABLE TO UNDERGO MITOSIS! Once a neoblast differentiates into any kind of cell, it is condemned to die in a few days without ever letting descendants. All cells in a planarian body are therefore constantly replaced by new ones coming from neoblasts. The only lineage of differentiated cells that is still able to reproduce is that of the germ cells, which, as in other multicellular organisms, assure that the next generation will consist of organisms with genetically homogeneous cells.

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Girardia tigrina, a freshwater planarian. Photo by Wikimedia user Slimguy.**

Several freshwater planarians, however, have lost their ability to generate sexual organs and, as a result, germ cells. In order to reproduce, they must rely on a form of asexual reproduction, which in this case happens by transversal fission of the body and posterior regeneration of the missing parts. In these populations, the zygotic bottleneck disappeared completely and, as a result, any non-lethal mutation in the neoblasts is retained in the organism, leading to a population of genetically distinct neoblasts inside a planarian.

Therefore, considering the fact that asexual planarians are not genetically homogeneous, having several different neoblast lineages in the same body, and that the neoblasts are the only cells able to reproduce and continue the species, a recent publication by Fields and Levin (see references) suggests that asexual planarians are nothing more than a very complex environment built by neoblasts in order to survive. Considering that each neoblast is an independent cell, which only needs the environment (the planarian) to survive, but does not need other neoblasts, planarians, at least the asexual ones, do not seem to have reached completely the requirements of high internal cooperation and low internal competition to be considered multicellular organisms.

We could interpret the neoblasts as unicellular organisms that live together, cooperating to build a complex environment, the planarian body, with their own sterile descendants (the differentiated cells), as if they were a group of queen ants living among sterile castes. Kind of mind blowing, huh? But it actually makes sense.

If you want to read more about it and understand every detail in this theory, read the references below.

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Maybe you’d like:

Endosperm: the pivot of the sexual conflict in flowering plants

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References:

Fields, C; Levin, M. (2018) Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity. Evolutionary Biology: 1–11.

West, S. A.; Fisher, R. M.; Gardner, A.; Kiers, E. T. (2015) Major evolutionary transitions in individuality. PNAS 112 (33): 10112-10119. https://doi.org/10.1073/pnas.1421402112

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Friday Fellow: Green-Banded Broodsac

by Piter Kehoma Boll

Parasites are very speciose, and I often feel that I’m not giving enough space for them here, especially when I bring you a flatworm, which is likely the group with the largest number of parasite species. So let’s talk about one today at last.

The first parasitic flatworm I am introducing to you is Leucochloridium paradoxum, the green-banded broodsac. It is a member of the flatworm group Trematoda, commonly known as flukes and, as all flukes, it has a complex life cycle.

Adults of the green-banded broodsac live in the intestine of various passerine birds of North America and Europe. The eggs they lay reach the environment through the bird’s feces and are eventually ingested by land snails of the genus Succinea.

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Adult individual of Leucochloridium paradoxum (left), an infected intermediate hose, a sail of the genus Succinea (center) and the sporocysts along the snail’s internal organs (right). Images not to scale. Extracted from http://medbiol.ru/medbiol/dog/0011a975.htm

Adults of Leucochloridium paradoxum are very similar to the adults of other species of the genus Leucochloridium, the main differences being seen in the larval stages. Inside the body of the snail, the eggs hatch into the first larval stage, the miracidium, which inside the snail’s digestive system develops into the next stage, the sporocyst.

The sporocyst has the form of a long and swollen sac (the broodsac, hence the common name) that is filled with many cercariae, which are the next larval stage. The sporocyst than migrates towards the snail’s eye tentacles, invading them and turning them into a swollen, colorful and pulsating structure that resembles a caterpillar. In this stage of infection, the poor snail is most likely blind and cannot avoid light as it normally does. As a result, it becomes exposed to birds that mistake it for a juicy caterpillar, eating it eagerly.

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A poor snail of the species Succinea putris with a broodsac in its left eye stalk. There is only one terrible fate for this creature. Photo by Thomas Hahmann.*

When the snail is eaten, the sporocyst burst and the several cercariae are released. In the bird’s intestine, they develop into adults and restart the nightmarish cycle.

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References:

Rząd, I.; Hofsoe, R.; Panicz, R.; Nowakowski, J. K. (2014) Morphological and molecular characterization of adult worms of Leucochloridium paradoxum Carus, 1835 and L. perturbatum Pojmańska, 1969 (Digenea: Leucochloridiidae) from the great tit, Parus major L., 1758 and similarity with the sporocyst stages. Journal of Helminthology 88(4): 506-510. DOI: 10.1017/S0022149X13000291

Wikipedia. Leucochloridium paradoxum. Available at < https://en.wikipedia.org/wiki/Leucochloridium_paradoxum >. Access on March 8, 2018.

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You know nothing, humans! A planarian genome challenges our understanding of how life works

by Piter Kehoma Boll

We finally have a rather complete sequencing of a planarian’s genome, more precisely, of the planarian Schmidtea mediterranea, which is an important model organism for the study of stem cells and regeneration.

In case you don’t know, planarians have a remarkable ability of regeneration, so that even tiny pieces are able to regenerate into a whole organism. They are like a real-life Wolverine, but somewhat cooler! This amazing ability is possible due to the presence of a group of stem cells called neoblasts that can differentiate into any cell type found in the planarian’s body. In fact, all differentiated cell types in planarians are unable to undergo mitosis, so that neoblasts are responsible for constantly replacing cells in every tissue. But we are not here to explain the details of planarian regeneration. We are here to talk about Schmidtea mediterranea‘s genome!

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Look at its little cock eyes saying “I will destroy everything you think you know, humans!” Photo by Alejandro Sánchez Alvarado.*

A rather complete genome of S. mediterranea has been recently published and its analysis reveal some astonishing features.

First of all, 61.7% of S. mediterranea‘s genome is formed by repeated elements. Repeated elements are basically DNA strands that occur in multiple copies throught the genome of an organism. They are thought to come from the DNA of virus that was incorporated to the host’s DNA. In humans, about 46% of the genome is formed by repeated elements. Most repeated elements of S. mediterranea belong to unidentified families of retroelements, thus suggesting that they are new undescribed families. Those repeats are very long, having more than 30 thousand base pairs, which are not known to exist in other animals. The only other group of repeated elements with a similar size is found in plants and known as OGRE (Origin G-Rich Repeated Elements). The long repeat found in Schmidtea was therefore called Burro (Big, unknown repeat rivaling Ogre).

But certainly the most surprising thing about S. mediterranea‘s genome is the lack of many highly conserved genes that are found in most eukaryotes and that were thought to be essential for cells to function properly.

Schmidtea mediterranea lacks genes responsible for repairing double-stranded breaks (DSBs) in DNA, which would make them very likely to suffer a lot of mutations and sensitive to anything that induces DSBs. However, planarians are known to have an extraordinary resistance to gamma radiation that induces DSBs. Do they have other repairing mechanisms or is our current understanding about this process flawed?

gene_loss

Several “essential” genes and their presence (in green) or absence (in red) in several animals. Schmidtea mediterranea lacks them all. Image extracted from Grohme et al. (2018).**

Another important gene that was not found in S. mediterranea is the Fatty Acid Synthase (FASN) gene, which is essential for an organism to synthesize new fatty acids. Planarians therefore would have to rely on the lipids acquired from the diet. This gene is also absent in parasitic flatworms and was at first thought to be an adaptation to parasitism but since it is absent in free-living species as well, it does not seem to be the case. Could it be a synapomorphy of flatworms, i.e., a character that identifies this clade of animals?

That is not enough for little Schmidtea, though. More than that, this lovely planarian seems to lack the MAD1 and MAD2 genes, which are found in virtually all eukaryotes. Those genes are responsible for the Spindle Assembly Checkpoint (SAC), an important step during cell division that prevents the two copies of a chromosome to separate from each other before they are all connected to the spindle apparatus. This assures that the chromosomes will be evenly distributed in both daughter cells. Errors in this process lead to aneuploidy (the wrong number of chromosomes in each daughter cell), which is the cause of some genetic disorders such as the Down syndrome in humans. Planarians do not have any trouble in distributing their chromosomes properly, so what is going on? Have they developed a new way to prevent cellular chaos or, again, is our current understanding about this process flawed?

Let’s wait for the next chapters.

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Reference:

Grohme, M. A.; Schloissnig, S.; Rozanski, A.; Pippel, M.; Young, G. R.; Winkler, S.; Brandl, H.; Henry, I.; Dahl, A.; Powell, S.; Hiller, M.; Myers, E.; Rink, J. C. (2018). “The genome of Schmidtea mediterranea and the evolution of core cellular mechanisms”. Nature. doi:10.1038/nature25473

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Friday Fellow: Duckweed Chain Flatworm

by Piter Kehoma Boll

Today we have one more flatworm in our team. It is part of the most bizarre group of flatworms, the so-called Catenulida. Our fellow is called Catenula lemnae, which I adapted as the “duckweed chain flatworm”.

The duckweed chain flatworm is a very small animal, measuring about 0.1 mm in width and about two or three times this size in length. It is found worldwide in freshwater lakes and ponds and is likely a complex of species, but more detailed studies are needed to make it clear. As other catenulids, it lives close to the substract, being considered a benthic animal, and feeds on other smaller organisms, such as small invertebrates and algae. It is usually a dominant species in the community of benthic microanimals, such as microturbellarians, in places where it is found.

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A chain of several connected individuals (zooids) of Catenula lemnae. Photo by Christopher Laumer.*

The word catenula, meaning “little chain” in Latin, was given to these animals because of their peculiar way of vegetative reproduction. The organism frequently divides transversally close to the posterior end, giving rise to new organisms that are genetically identical to the original one. However, the new animals often remain connected to each other for a long time before splitting, and as this asexual reproduction continues, it eventually turn them into a chain of connected individuals (called zooids). This chain swims elegantly using its cilia as if it were a single individual.

Most recent studies mentioning the duckeed chain flatworm are simply surveys of the species composition of a certain area or broad phylogenetic studies on the catenulids or flatworms in general. Little is known about the ecology, behavior and population structuring of this species, unfortunately.

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References:

Braccini, J. A. L.; Leal-Zanchet, A. M. (2013)  Turbellarian assemblages in freshwater lagoons in southern Brazil. Invertebrate Biology132(4): 305–314. https://dx.doi.org/10.1111/ivb.12032

Marcus, E. (1945) Sôbre Catenulida brasileiros. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, série Zoologia, 10: 3–113.

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The hammerhead flatworms: once a mess, now even messier

by Piter Kehoma Boll

Few people know that land planarians exist, but when they do, they most likely know the hammerhead flatworms, which comprise the subfamily Bipaliinae.

The hammerhead flatworms, or simply hammerhead worms, have this name because their head has lateral expansions that make them resemble a hammer, a shovel or a pickaxe. Take a look:

Bipalium_vagum

The “wandering hammerhead worm”, Bipalium vagum. Notice the peculiar head. Photo by flickr user budak.*

The Chinese knew the hammerhead worms at least since the 10th century, which is understandable, since they are distributed from Japan to Madagascar, including all southern and southeast Asia, as well as Indonesia, the Philippines and other archipelagos. The western world, however, first heard of them in 1857, when William Stimpson described the first species and put them in a genus called Bipalium, from Latin bi- (two) + pala (shovel), due to the head shape. One of them was the species Bipalium fuscatum, a Japanese species that is currently considered the type species of the genus.

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Anterior region of Bipalium fuscatum, the “brownish hammerhead worm”. Photo by Wikimedia user 根川大橋.**

Two years later, in 1859, Ludwig K. Schmarda described one more species, this one from Sri Lanka, and, unaware of Stimpson’s paper, called the species Sphyrocephalus dendrophilus, erecting the new genus for it from Greek sphȳra (hammer) + kephalē (head).

Sphyrocephalus_dendrophilus

Drawings by Schmarda of Sphyrocephalus dendrophilus.

In the next year, 1860, Edward P. Wright did something similar and described some hammerhead worms from India and China, creating a new genus, Dunlopea, for them. The name was a homage to his friend A. Dunlop (whoever he was).

Dunlopea_grayia

Wright’s Drawing of Dunlopea grayia (now Diversibipalium grayi) from China.

Eventually those errors were perceived and all species were put in the genus Bipalium, along with several others described in the following years. All hammerhead worms were part of the genus Bipalium until 1896, when Ludwig von Graff decided to improve the classification and divided them into three genera:

1. Bipalium: With a head having long “ears”, a well developed head.
2. Placocephalus (“plate head”): With a more semicircular head.
3. Perocephalus (“mutilated head”): With a shorter, rudimentary head, almost as if it had been cut off.

Bipaliids

Compare the heads of typical species of Bipalium (left), Placocephalus (center) and Perocephalus (right), according to Graff.

This system, however, was soon abandoned and everything went back to be simply Bipalium and continued that way for almost a century, changing again only in 1998, when Kawakatsu and his friends started to mess with the penises of the hammerhead worms.

First, in 1998, they erected the genus Novibipalium (“new Bipalium“) for species with a reduced or absent penis papilla, and retained in Bipalium those with a “well”-developed penis papilla. It is worth noticing though that this well-developed papilla is not much bigger than a reduced papilla in Novibipalium. In both genera the actual, functional penis is formed by a set of folds in the male atrium and not by the penis papilla itself as in other land planarians that have a penis papilla.

Later, in 2001, Ogren & Sluys separated some more species of Bipalium in a new genus called Humbertium (after Aloïs Humbert, who described most species of this new genus). They were separated from Bipalium because the ovovitelloducts (the ducts that conduct the eggs and vitellocites) enter the female atrium from ahead, and not from behind as in the typical Bipalium. This separation is, in my opinion, more reasonable than the previous one.

Now we had three genera of hammerhead worms based on their internal anatomy, but several species were described without any knowledge of their sexual organs. Thus, in 2002, Kawakatsu and his friends created one more genus, Diversibipalium (the “diverse Bipalium“) to include all species whose anatomy of the sexual organs was unknown. In other words, it is a “wastebasket” genus to place them until they are better studied.

Are these three genera, Bipalium, Novibipalium and Humbertium, as now defined, natural? We still don’t know, but I bet they are not. A good way to check it would be by using molecular phylogeny, but we don’t have people working with these animals in their natural habitats, so we do not have available material for that. Another thing that can give us a hint is to look at their geographical distribution. We can assume that genetically similar species, especially of organisms with such a low dispersal ability as land planarians, all occur in the same geographical region, right? So where do we find species of each genus? Let’s see:

Bipalium: Indonesia, Japan, China, Korea, India.

Novibipalium: Japan.

Humbertium: Madagascar, Sri Lanka, Southern India, Indonesia.

Weird, right? They are completely mixed and covering a huge area of the planet, especially when we consider Humbertium. We can see a tendency, but nothing very clear.

Fortunately, some molecular analyses were published (see Mazza et al. (2016) in the references). One, which included the species Bipalium kewense, B. nobile, B. adventitium, Novibipalium venosum and Diversibipalium multilineatum placed Diversibipalium multilineatum very close to Bipalium nobile, and they are in fact very similar, so I guess that we can transfer it from Diversibipalium to Bipalium, right? Similary, Novibipalium venosum appears mixed with the species of Bipalium. I guess this is kind of messing things up one more time.

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Head of some species of Bipalium, including the ones used in the study cited above. Unfortunately, I couldn’t find a photo or drawing of Novibipalium venosum. Image by myself, Piter Kehoma Boll.**

Interestingly, among the analyzed species, the most divergent was Bipalium adventitium, whose head is “blunter” than that of the other ones. Could the head be the answer, afterall? Let’s hope that someone with the necessary resources is willing to solve this mess soon.

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See also:

Once found and then forgotten: the not-so-bright side of taxonomy.

The lack of taxonomists and its consequences on ecology.

They only care if you are cute. How charisma harms biodiversity.

The faboulous taxonomic adventure of the genus Geoplana.

Darwin’s Planaria elegans: hidden, extinct or misidentified?

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References:

Graff, L. v. (1896) Über das System und die geographische Verbreitung der Landplanarien. Verhandlungen der Deutschen Zoologischen Gesellschaft6: 61–75.

Graff, L. v. (1899) Monographie der Turbellarien. II. Tricladida Terricola (Landplanarien). Engelmann, Leipzig.

Kawakatsu, M.; Ogren, R. E.; Froehlich, E. M. (1998) The taxonomic revision of several homonyms in the genus Bipalium, family Bipaliidae (Turbellaria, Seriata, Tricladida, Terricola). The Bulletin of Fuji Women’s College Series 236: 83–93.

Kawakatsu, M.; Ogren, R. E.; Froehlich, E. M., Sasaki, G.-Y. (2002) Additions and corrections of the previous land planarians indices of the world (Turbellaria, Seriata, Tricladida, Terricola). The bulletin of Fuji Women’s University. Ser. II40: 162–177.

Mazza, G.; Menchetti, M.; Sluys, R.; Solà, E.; Riutort, M.; Tricarico, E.; Justine, J.-L.; Cavigioli, L.; Mori, E. (2016) First report of the land planarian Diversibipalium multilineatum (Makino & Shirasawa, 1983) (Platyhelminthes, Tricladida, Continenticola) in Europe. Zootaxa4067(5): 577–580.

Ogren, R. E.; Sluys, R. (2001) The genus Humbertium gen. nov., a new taxon of the land planarian family Bipaliidae (Tricladida, Terricola). Belgian Journal of Zoology131: 201–204.

Schmarda, L. K. (1859) Neue Wirbellose Thiere beobachtet und gesammelt auf einer Reise um die Erde 1853 bis 1857 1. Turbellarien, Rotatorien und Anneliden. Erste Hälfte. Wilhelm Engelmann, Leipzig.

Stimpson, W. (1857) Prodromus descriptionis animalium evertebratorum quæ in Expeditione ad Oceanum, Pacificum Septentrionalem a Republica Federata missa, Johanne Rodgers Duce, observavit er descripsit. Pars I. Turbellaria Dendrocœla. Proceedings of the Academy of Natural Sciences of Philadelphia9: 19–31.

Wright, E. P. (1860) Notes on Dunlopea. Annals and Magazine of Natural History, 3rd ser.6: 54–56.

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