by Piter Kehoma Boll
It’s time to come back to the fascinating flatworms and today I decided to talk about one of the most studied species in this group even though it was only formally described 15 years ago. Its name is Macrostomum lignano, or the Lignano’s macrostomum.
Measuring 1 to 2 mm in length, the Lignano’s macrostomum belongs to the order Macrostomida, one of the basalmost flatworm groups. Its body is elongate and transparent, there are two small eyes close to the anterior end, which has a small rostrum (“snout”). The mouth is a little behind the rostrum. The posterior end is broad, forming a tail plate with many adhesive organs arranged in a U-shape.
The Lignano’s macrostomum was first collected in marine samples in the city of Lignano on the Adriatic Sea coast in northern Italy in 1995 and soon revealed to be very suitable for laboratory cultures. The natural environment of this species includes the sand and other sediments near the shore. It avoids light and, when at rest, remains attached to the substrate by its tail plate. It feeds on small organism, especially diatoms, which it ingests using its cylindrical pharynx, similarly to how most flatworms eat.
Also like most flatworms, the Lignano’s macrostomum and other macrostomids have special stem cells called neoblasts that fill their body. All differentiated cells in the body come from neoblasts and are continuously replaced by them, since its differentiated cells cannot continue reproducing. Neoblasts also give the Lignano’s macrostomum an impressive regenerative ability like that of many other flatworms such as planarians.
Even before its formal description in 2005, the Lignano’s macrostomum had already been identified as a potentially new model organism. It can be easily cultured in laboratory in Petri dishes and fed with diatoms. Its body has about 25,000 cells, which is a number small enough to facilitate studies on development, regeneration, ageing and gene expression and that is exactly what has been done in the past decades.
The Lignano’s macrostomum is hermaphrodite. The body contains two testes and two ovaries. The male copulatory apparatus contains a stylet, a hardened penis-like copulatory organ. When two macrostomums mate, they touch their ventral surfaces in a yin yang fashion (just like the guys from last week) and exchange sperm. This behavior is easily observed in laboratory and led the Lignano’s macrostomum to become a model organism for the study of sexual selection as well. But wait! Sexual selection in a hermaphrodite organism? Yes! I discussed this topic some time ago here.
Sometimes, when two macrostomums meet, they don’t find their partner that attractive, so having their eggs fertilized by that guy is not of their interest from the female side. However, their male side is still as male as any other and they want to fertilize as many eggs as possible. As a result, if the partner is not good enough, they still want it as a female but not as a male. The other guys is not interesting in being a female only though, so copulation only occurs if both partners accept to receive each other sperm. “I let you fertilize my eggs if you let me fertilize yours.” So that’s what they do.
However, after they delivered the sperm into each other’s body, they separate and may never see each other again. So the female side evolved a strategy to select better sperm. When the “bad partner” moves away, a macrostomum that received low-quality sperm bends over itself, connects its pharynx to its female genital pore, and sucks the other guy’s sperm out before it has the chance to fertilize its eggs. A clever strategy, right? But remember: just as this guy is getting rid of the other guy’s sperm, the other guy may be doing the same with this guy’s sperm. So a strategy must evolve to prevent the female personality to discard the sperm. And that is exactly what happened! The sperm cells of the Lignano’s macrostomum have hard bristles pointing backward that, when the cells is pulled back, enter the tissues in the female copulatory apparatus and remain stuck. Trying to pull them out is just like trying to pull porcupine quills out of the skin.
Now the male side recovered the advantage that the female side would have if the bristles were not there. But this is evolution, and its effect on hermaphrodites is like having two different personalities fighting each other in the same body.
Life is not easy anywhere.
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References and further reading:
Egger B, Ladurner P, Nimeth K, Gschwentner R, Rieger R (2006) The regeneration capacity of the flatworm Macrostomum lignano—on repeated regeneration, rejuvenation, and the minimal size needed for regeneration. Development Genes and Evolution 216:565–577. doi: 10.1007/s00427-006-0069-4
Ladurner P, Schärer L, Salvenmoser W, Rieger RM (2005) A new model organism among the lower Bilateria and the use of digital microscopy in taxonomy of meiobenthic Platyhelminthes: Macrostomum lignano, n. sp. (Rhabditophora, Macrostomorpha). Journal of Zoological Systematics and Evolutionary Research 43(2):114–126. doi: 10.1111/j.1439-0469.2005.00299.x
Lengerer B, Pjeta R, Wunderer J et al. (2014) Biological adhesion of the flatworm Macrostomum lignano relies on a duo-gland system and is mediated by a cell type-specific intermediate filament protein. Frontiers in Zoology 11:12. doi: 10.1186/1742-9994-11-12
Mouton S, Willems M, Braeckman BP, Egger B, Ladurner P, Schärer L, Borgonie G (2009) The free-living flatworm Macrostomum lignano: A new model organism for ageing research. Experimental Gerontology 44(4):243–249. doi: 10.1016/j.exger.2008.11.007
Pfister D, De Mulder K, Hartenstein V et al. (2008) Flatworm stem cells and the germ line: Developmental and evolutionary implications of macvasa expression in Macrostomum lignano. Developmental Biology 319(1):146–159. doi: 10.1016/j.ydbio.2008.02.045
Pfister D, De Mulder K, Philipp I et al. (2007) The exceptional stem cell system of Macrostomum lignano: Screening for gene expression and studying cell proliferation by hydroxyurea treatment and irradiation. Frontiers in Zoology 4:9. doi: 10.1186/1742-9994-4-9
Schärer L, Joss G, Sandner P (2004). Mating behaviour of the marine turbellarian Macrostomum sp.: these worms suck, Marine Biology 145 (2) doi: 10.1007/s00227-004-1314-x
Wasik K, Gurtowski J, Zhou X et al. (2015) Genome and transcriptome of the regeneration-competent flatworm, Macrostomum lignano. PNAS 112(40):12462–12467. doi: 10.1073/pnas.1516718112
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