Week 5: Platyhelminthes

How many species are there?

60,000 +

Characteristics of Platyhelminthes

• dorsal-ventrally flattened invertebrates

• dominated by parasitic forms but large diversity of free living groups too

• exhibit developmental plasticity, and regenerative/proliferative growth via the maintenance of somatic stem cells

• found in marine, freshwater, terrestrial - as long as it is not arid- parasitic so within other animals

What are the 2 major areas of research on flatworms?

1. Parasites: Agents of Disease

   • agents of major neglected tropical diseases

   • causes economic loss in livestock production

   • e.g tapeworm

2. Free Living: Models of Regeneration/ Stem Cells

   • e.g planaria / some planaria → whole body regeneration

   • can also regenerate brain/ cephalic structures from even a single fragment of tissue

   • lesser forms of regeneration e.g posterior regeneration = a widespread feature within the whole group

What is whole body regeneration?

entire individual created from single adult stem cell

Phylogenetic position of platyhelminthes

• historically thought to be early branching bilaterians → due to simple morphology

• molecular data shows that they are a part of Lophophotrochozoans → not primitive

Which groups are now no longer considered to be flatworms?

Acoelomorphs, Nemertodermatida, Xenoturbella → thought to be early branching bilaterians

How do platyhelminthes excrete?

Excretion via protonephridia (a.k.a. 'flame cells') → specialised cell types scattered throughout parenchyma → use flagella to create a current and remove excess liquids

Habitat

• freshwater, marine – open water, marine - interstitial, (damp) terrestrial, symbiosis, parasitism

• ^ essentially anywhere not arid → so that they don’t dry out

What is a tegument?

ciliated epithelium

protonephridia

simplified kidney system

Platyhelminthe Anatomy

• Dorsal-ventrally flattened (ish…) → some exceptions

• Hermaphroditic, rarely with separate sexes; reproduction via self- and cross-fertilisation → vast majority are hermaphrodites

• Acoelomate (i.e. no body cavity): mesoderm gives rise to parenchyma, nerve/muscle and reproductive organs

• essentially solid bodies filled with parenchymal cells

• Digestive system typically a blind gut/sac (no digestive system in tapeworms)

• most have a mouth but no anus

totipotent somatic stem cells

• ability to give rise to any and all cell types

• powerful for regeneration

Features Unique to Flatworms

• Maintenance of totipotent somatic stem cells throughout ontogeny

  • the only totipotent stem cells are sperm/eggs in most animals

• ALL new cells to support growth and homeostasis throughout ontogeny arise from the division of somatic stem cells (i.e. differentiated cells incapable of division)

  • differentiated cells divide to give rise to new cells of the same type in most animals

• Somatic stem cells comprise ~1/3 of all cells in the body

  • differentiated cells comprise 99% of cells in most animals

• Continual expression of axial patterning genes by the neural-musculature system throughout ontogeny, providing a molecular ‘road map’ for regenerating missing parts

  • axial patterning genes only expressed during embryogenesis in most animals

axial patterning genes

• common + universally shared signalling systems normally found during embryogenesis

• turn off once pattern is established

Planarians

Ubiquitous, cosmopolitan fresh-water invertebrates found in most ponds

Whole body regeneration in planarians

• Some planarian species capable of regenerating the entire animal from a single somatic stem cell

• Capable of reducing or enlarging overall body size in response to local resource availability

• Can reproduce asexually via fission as well as sexually

• The most important model organisms for understanding stem cell biology and regenerative medicine for almost 200 years (and counting)

• Can regenerate brains

‘holy grail’ of planarian biology

• how they know whether to regenerate a new head of tail – cracked in 2009: maintenance of Wnt signalling throughout ontogeny

• Experimental disruption (‘knock down’) of Wnt signalling produces 2-head or 2-tailed phenotypes

Turbellarian flatworms

• free living

Characteristics of turbellarian flatworms

• Regenerative abilities widespread (but not WBR)

• some microscopic groups found in between sand grains → freshwater

• high diversity

• Propensity for symbiosis

Neodermata

parasitic flatworms (obligate parasitism)

Major neodermatan groups

1. Trematoda (flukes)

2. Monogenea

3. Cestoda (tapeworms)

Do neodermata have a ciliated epidermis?

all flatworms have ciliated epidermis - true of neodermata larva but then they get ‘new skin’ and lose it

What are the 2 lineages of monogenea?

Monopisthocotylea → gill parasites/ blood feeders

Polyopisthocotylea → skin/ detritus feeders

Examples of flukes which cause large medical and economic impact

1. Blood flukes (schistosomiasis)

2. Human liver flukes (Clonorchis)

3. Human liver flukes (Clonorchis)

Blood flukes (schistosomiasis)

• 280,000 deaths annually (no. 1 NTD)

• schistosomiasis lives in blood system (unusual- most in gut)- separate male and females→ not very representative group

• male worm pairs with female and female lives inside groove of male together in permanent association

• microscopic

• chronic disease → damage caused is due to the way eggs migrate through the body through intestine/ bowels into environment

• tiny eggs but 100s 1000s may be shed in a day

• ultimately harms the tissues via migratory route over time

• Water-borne disease with snail vectors

Neglected tropical diseases (NTD)

• Neglected tropical diseases are a diverse group of tropical infections

• Common in low-income populations in developing regions of Africa, Asia, and the Americas

• They are caused by a variety of pathogens, such as viruses, bacteria, protozoa, and parasitic worms.

Why are blood flukes not a good example of a typical fluke?

• separate sexes and this is very uncommon

• live in blood system → most parasitic flatworms live in the gut if they are internal since that is where the nutrients are

Human liver flukes (Clonorchis)

• 30,000,000 infections

• chronic disease

• spread in flooded rice paddies → perfect environment for snails

• snails as intermediate hosts

• paddy fields → no sewage system → need a poo → if infected end up putting eggs back into that environment + perpetuating the life cycle

Cattle liver flukes (e.g. Fasciola) Economic losses

• costs of anthelmintics, drenches, labor, liver condemnation at meat inspection

• losses in production due to mortality, reduction in meat, milk and wool production

• reduction in growth rate, fertility and draught power

What animals can the Cattle liver flukes (e.g. Fasciola) infect?

• sheeps

• goats

• cattle

• buffalo

• other ruminants

Medical impact of tapeworms

• Most severe when humans play the role of intermediate host rather than final and are infected with larval tapeworms instead of adult

• tapeworms have indirect life cycles like flukes → less complicated

• undergo a complete metamorphosis from the first larval stage to the next, then gives rise to adult

• adults always in vertebrate host and generally invertebrate intermediate host e.g copepod in aquatic life cycle or beetles in terrestrial ones

• most are tiny

• chronic disease

Human Larval Cestodiases

• larval infections - huge cause of epilepsy - don’t live in gut and have a tendency to go to the CNS

• larval stage in CNS = dead end route for tapeworms

• Neurocysticercosis- Taenia solium

• Hydatid disease- Echinococcus multilocularis → cyst like structures

What is the difference between the damage caused by tapeworms and flukes in humans?

• Tapeworm → humans as intermediate host = most damage

• Flukes → adult stage = most damage to humans

How many species of Trematoda (flukes) are there?

~ 18,000

Characteristics of Trematoda

• most diverse group of flatworms

• Primarily enteric (i.e. intestinal) parasites of all vertebrate groups

• Have major medical and economic importance

• HIGHLY complex life cycles involving 3-5 hosts and multiple life stages

• First host almost always a snail; less frequently bivalves, scaphopods and annelids

• Both active and passive transmission strategies

Scientific name of the human blood fluke

Schistosoma mansoni

Difference in reproduction in tapeworms and flukes

• each segment of a tapeworm has male + female reproductive organs→ thousands of eggs

• flukes have rounds of asexual reproduction to ramp up the numbers

Features of human blood fluke

• Disease: Schistosomiasis/Bilharziasis

• Dioecious (separate sexes)

• Blood dwelling → in venule/ arteriole system of human

Schistosome life cycle

1. miracidia (free-swimming/active penetration)

2. sporocyst (intra-mulloscan)

3. cercaria (free swimming/active penetration)

4. schistosomula (juvenile worm/active penetration)

miracidia (free-swimming/active penetration)

• release eggs

• eggs release enzymes and burrow out of venule and into lumen of intestine from which point they can leave the body

• damage/ scar tissue/ pathology left- over a long time - organs damaged

sporocyst (intra-mulloscan)

• once out- eggs hatch in water and can actively move and seek out a host - snail - has to be the right species relative to Schistosome

• sporocyst = sac

• undergoes asexual reproduction inside cell

cercaria (free swimming stage /active penetration)

• 2nd free living stage

• fall down in water column and twitch tails to move them back up

• encounters next host (human) by chance → burrows into human, drops off tail, juvenile in venule and matures to adulthood

schistosomula (juvenile worm/active penetration)

• cercaria that dropped its tail

• migrates through blood system

Presence of gut/ anus in flatworms vs. tapeworms

• flatworms have a gut/ mouth , no anus

• tapeworms have NO gut (hence no mouth) and thus obtain nutrients from the gut of their hosts via absorption across the outer tegument

How many species of cestoda are there?

~ 9000 species

Features of the parasitism of Cestoda

• The majority are segmented (or ‘strobilated’)

• Increase reproductive output via repetition of the reproductive organs

• Enteric parasites of all major vertebrate groups

• Have complex life cycles usually involving an arthropod 1st intermediate host→ copepod (always exceptions)

• ALL transmission is passive via food webs

Host associations of tapeoworms

• Elasmobranchs (sharks, rays, chimeras)

  • often adult tapeworms in marine environments in sharks

• Teleosts

  • found in bony fish as adults but primarily in freshwater

• Tetrapods (amphibians, reptiles, birds and mammals)

Broad-fish tapeworm (Diphyllobothrium latum)

• bear tapeworm

• humans can be definitive host

Broad fish tapeworm life cycle

1st stage larvae = ciliated- can swim in water column

actively predated on by copepod → metamorphose in haemocoel of copepod → juvenile worm

copepod eaten by fish

(small fish eaten by bigger fish)

fish eaten by human or a bear or something

• unlike flukes- single egg in tapeworm generally gives rise to a single adult worm

• larvae develop in gut - segmented worm

How many species of Monogenea are there?

~ 3,000 species

Features of Monogenea

• External parasites of fishes (and some amphibians)

• no species that infect humans

• Direct (simple) life cycles

• Mostly give birth to live young

• hermaphrodites

• exception to 2 lineages = group that parasitises amphibians

Economic importance of Monogenea

• some economic importance, mainly to salmon farming in the UK and scandinavia

• big problem → direct transmission i.e offspring just swim through water and find another host/ fish

• can get to the point where they suffocate the gills of the fish or in any case just cause some damage

Example of parasite on fish gills e.g salmon

Gyrodactylus salaris

Example of extreme phenotypic plasticity

Diplozoon paradoxum

• 2 separate individuals have to find each other - tissues fuse into a single organism = sexual maturity