Phylum Platyhelminthes Part 2: Free-Living Flatworms and the Problem With Turbellaria

Summary

This tutorial explores the incredible diversity of flatworms, focusing on the clade Rhabditophora, which includes free-living species like planarians. It explains how these flatworms exhibit varied body forms and unique behaviors, such as reproductive rituals like 'penis-fencing' and defensive strategies. The spotlight is on planarians, famous for their regenerative abilities, which allow them to regrow lost body parts using pluripotent stem cells. This remarkable capacity has made them key subjects in scientific research, particularly in understanding regeneration. The tutorial concludes with a transition to studying parasitic flatworms in future sessions.

Takeaways

• 😀 Free-living flatworms were traditionally grouped in the class Turbellaria, but recent studies have shown that parasitic flatworms evolved from a subgroup within them, not alongside.
• 😀 The clade Rhabditophora includes non-parasitic flatworms that secrete mucus using rod-like structures called rhabdites, showcasing significant diversity in body shapes.
• 😀 The clade Rhabdocoela features species like Mesostoma, which are predatory, and Temnocephalida, which are parasitic or commensal, highlighting the varied lifestyles of flatworms.
• 😀 Two biodiverse clades, Polycladida and Tricladida, are particularly well-studied, with Polycladida containing some of the largest and most colorful marine flatworms.
• 😀 Marine flatworms like Pseudobiceros engage in ‘penis-fencing,’ where hermaphroditic worms compete by piercing each other’s skin to inseminate.
• 😀 In some species, like the Persian carpet flatworm, the first worm to successfully penetrate its mate becomes 'male,' while the other becomes 'female.'
• 😀 Marine flatworms employ various defense strategies, such as acquiring nematocysts from prey, producing acidic secretions, and using skeletal defenses.
• 😀 Planarians, freshwater flatworms from the clade Tricladida, are famous for their remarkable regenerative abilities, which have been a subject of study for hundreds of years.
• 😀 Planarians can reproduce both sexually and asexually, with sexual reproduction involving bilateral sperm transfer and asexual reproduction through fission.
• 😀 Planarians possess pluripotent stem cells (neoblasts) that enable them to regenerate missing body parts, even when cut into many pieces, making them a key organism in regenerative biology research.

Q & A

What is the main focus of this tutorial about Platyhelminthes?

The tutorial focuses on non-parasitic members of the clade Rhabditophora, which includes various free-living flatworms outside of Neodermata, such as those that secrete mucous using rod-like rhabdites.

How was the old classification of Platyhelminthes structured, and why is it considered paraphyletic now?

The old classification grouped free-living flatworms as class Turbellaria alongside parasitic flatworms. However, recent analyses have shown that parasitic flatworms evolved from a small subgroup within the free-living Platyhelminthes, making the old classification paraphyletic.

What is the significance of the clades Polycladida and Tricladida in flatworm biodiversity?

Polycladida and Tricladida are among the most biodiverse clades of free-living flatworms and have been the focus of much research. Polycladida includes large, colorful marine flatworms, while Tricladida includes freshwater species like planarians.

What is unique about the mating behavior of marine flatworms in the Polycladida clade?

Marine flatworms in the Polycladida clade exhibit traumatic insemination, also known as 'penis-fencing,' where hermaphroditic worms use extendable two-headed pointed stylets to pierce each other's epidermis and transfer sperm.

What role does regenerative ability play in flatworm research?

Regeneration is a key feature in flatworms, particularly planarians, and has captivated researchers for centuries. Planarians can regenerate missing body parts, including heads and tails, making them a model organism for studies in cellular regeneration and stem cell biology.

How do planarians regenerate after being severed?

Planarians regenerate missing body parts through the action of pluripotent stem cells, known as neoblasts, which have the ability to create all the various cell types needed for regeneration. These cells receive molecular signals that direct them to form the correct body part.

What is the function of the auricles in planarians?

The auricles in planarians are ear-like projections at the base of their head that contain sensory cells. They help the flatworm detect touch, chemical signals, water currents, and maintain equilibrium.

What makes planarians an important subject in biological research?

Planarians, particularly species like Dugesia, are used extensively in biological and biomedical research due to their remarkable regenerative abilities and their similarity to human cell types, tissues, and organs, which makes them a key model for studying regeneration and stem cells.

How do planarians reproduce sexually?

Planarians reproduce sexually through bilateral transfer of sperm, where both hermaphroditic individuals exchange sperm, and the fertilized eggs develop inside their bodies before being laid in capsules.

What is the historical significance of planarian regeneration studies?

Planarian regeneration was first observed in the 1700s, with notable studies like the 1898 work by Thomas Hunt Morgan, which showed that a single planarian could regenerate from 279 pieces. More recent discoveries have identified pluripotent stem cells as responsible for regeneration, making planarians a vital organism for studying regenerative biology.