Support information for this lesson:
1. Reproduction and life cycles
All life on earth (but what is life actually?) goes through an endless sequence of ever-renewing reproduction cycles. One very important outcome of this continued process is the population growth of a certain species (but what is a species?) and overcoming the problem of ageing. Another important aspect is to provide possibilities to undergo change: organisms may undergo random changes in their genetic material (mutations) that can lead to alterations in their form and/or function; through the process of reproduction, these new properties can then be passed on to and maintained by the offspring. If these changed properties are advantageous, these individuals will be more prosperous than others within the same population and their “improved design” will prevail. This process of natural selection for inherited change is the key driver of evolution.
Single cell organisms usually reproduce via cell division. This is true for bacteria (called ‘binary fission’) or single-celled members of the kingdoms of algae (e.g. Euglena in our ponds), fungi (e.g. baker’s or brewer’s yeasts; Fig.1) or animal-like protozoans (e.g. Paramecium in our ponds). These division processes can also occur in the form of “budding“. For example, many yeast cells bud off new daughter yeast cells and accumulate scars for each such event on their surface. Once about 50 such scars have accumulated, the mother cell dies; it has undergone a true process of ageing (Fig.1). Similarly, we can take cuttings from many perennial plants, bushes or trees and grow entire plants from them. The new plant is rejuvenated and has its full life expectancy, whereas the donor plant remains advanced in its ageing process.
Budding is also found in the animal kingdom; for example polyps can grow little side polyps on their surface that either stay together (e.g. to form coral branches) or separate and become their own individuals (as observed in Hydra; Fig.1).
Fig. 1. Organismal division and budding. Bacterial reproduction usually occurs via division (referred to as binary fission): Chromosomal and plasmid DNA (red; containing all the genes) are duplicated (new copies shown in blue) and distributed evenly to both daughter cells, together with other cellular components, such as ribosomes. Bacteria have no sexual reproduction, but different bacteria (illustrated by green and red DNA) can dock to each other (conjugation) and exchange DNA through cytoplasmic bridges (pili), a process referred to as horizontal gene transfer. Baker’s yeast reproduction occurs via budding, which is an asymmetric form of cell division where the nuclear DNA duplicates and is distributed to both cells together with other cell organelles (mitochondria shown); but the “mother cell” accumulates scars from each budding event and dies (red cross) when many such scars have accumulated. Haploid yeast cells containonly one set of genes and can either be of the “a” or “a” mating type; cells of opposing mating type can complement and undergo conjugation (comparable to fertilisation) to combine both gene sets into a joint diploid set; diploid cells can amplify through further budding, or form resistant spores which can endure times of poor nutrition or drought. When conditions improve, spores revitalise into haploid yeast cells. Hydra reproduction can occur via asexual budding, or by forming male and female individuals with sperm and eggs, where fertilised eggs develop into a persistent encapsulated embryo to overcome periods of poor nutrition or drought.
Whilst organismal division and budding are “asexual” forms of reproduction and renewal, “sexual” reproduction plays an important role in most species, even single-cell organisms. For example, baker’s yeast may take on one of two gender-like states (‘a’ or ‘α‘), and two cells of opposing state can fuse to combine their genetic material (a process called ‘conjugation’); this enhances the survival strength and genetic variety of that yeast population (Fig. 1). Comparable processes of conjugation occur in other single-celled organisms, such as the ciliated Paramecium or the Malaria pathogen Plasmodium (Fig. 12). Whilst bacteria have no forms of sexual reproduction, they can often dock to each other (also called ‘conjugation’) and exchange genetic materials (Fig.1). This process of “horizontal gene transfer” poses huge problems, because bacteria can exchange genetically encoded resistances to antibiotics among each other and develop into multi-resistant bacterial strains that cannot be medically treated (e.g. the MRSA ‘superbug’).
In multicellular organisms, be it plants or animals, sexual generation cycles prevail: the process of gametogenesis (involving ‘meiosis‘) ensures that adult individuals in each generation carry sperm and/or egg cells (or their equivalents); the fertilised egg cells (‘zygotes‘) then undergo the process of development to eventually become mature individuals of the next generation (Fig.2; see details in the next section). The individuals of many animal species tend to be either male (producing sperm) or female (producing egg cells). But there are also animal species where one individual produces sperm and egg cells (referred to as ‘hermaphrodite‘). They can either carry sperm and egg cells at the same time (‘simultaneous hermaphrodites’, as found in many snails, earthworms or tapeworms; see this film about how this works), or they can change sex during their lifetime (‘sequential hermaphrodites’, as found in clown fish which are all born male and only the largest animal in a colony changes sex to become a female). Plants usually combine both sexes in one individual: either in the same flower (usually male stamen, female pistil) or by carrying distinct male versus female flowers on the same plant (e.g. male tassel flowers at the tip of a maize plant, female silks along its trunk). But there are also plants where individuals are either male or female (e.g. date palms).
Fig.2. Multicellular organisms display sexual reproduction cycles, with the adult form producing male and female gametes; following fertilisation, embryonic development occurs where all the organs and tissues form that are required for life; a juvenile stage provides further growth until maturation into the next adult generation.
Finally, also generation cycles can take the form of asexual reproduction. For example aphids (greenflies on our plants in the garden; no. 13 in Fig.6), only generate females during summer which can reproduce by avoiding the process of meiosis and male fertilisation. Instead, they give birth to individuals that are identical (clonal) to the mother, a process called ‘parthenogenesis‘. Note that these eggs develop inside the mother and individuals are born as little juveniles (as shown in no. 13 in Fig.6; referred to as life birth or viviparity). In autumn, they eventually generate also males, and sexual reproduction takes place.
As should have become clear, there is an enormous breadth of reproduction and life cycle strategies, all ensuring self-renewal and amplification of the various species, and providing opportunities to establish new genetic variants that can adapt to an ever-changing environment – and the latter may become extremely important when considering the enormous challenges of global warming and pollution to nature!
Single-celled organisms do not have to go through major developmental processes when they reproduce; they simply make sure that during the division process both daughter cells receive one full copy of the genetic information and a big enough share of required intracellular organelles and functional components (Fig.1).
This becomes very different in multicellular organisms where a single fertilised egg cell (or a sprouting bud in Hydra; Fig1) has to give rise to a huge variety of very different cells that form highly specialised tissues and organs; for example, we need cells that form…
- our skin to protect us from dehydration and infection
- muscles and bones to allow movement
- a digestive system to take in and absorb nutrition, and release undigested material as excrement
- an excretory system to remove poisonous substances from our bodies
- a respiratory system to breath
- a nervous system to coordinate our behaviours and body functions.
The same requirement of generating very distinct cell types and tissues from a single fertilised germ cell is true for plants. To achieve this, multicellular organisms first undergo embryonic development: through continued cell divisions the single egg cell becomes a multi-cellular object; during this process, the many cells gradually become different from one another and establish the distinct cell populations that can assemble into the tissues and organs required for the newly formed organism to function and live. This organism is now ready to hatch or be born, usually followed by a juvenile phase where the functional organism grows in size, to eventually undergo maturation into an adult individual which is able to reproduce and start a new life cycle (Fig.2).
Fig. 3. Amphibians: Salamander (top left), newt (top right), frog (bottom left), toad (bottom right). Image details (sources hyperlinked): top left – fire salamander (Salamandra salamandra; by Saxifraga, Edo van Uchelen); top right – smooth or common newt (Lissotriton/Triturus vulgaris; by Saxifraga, Kees Marijnissen); bottom left – common frog (Rana temporaria; by Saxifraga, Rudmer Zwerver); bottom right – common toad (Bufo bufo; by Saxifraga, Ab Baas). Question: Why are the scientific biological names of specific animals consisting of two parts? (see the answer here)
For many organisms, development is a fairly linear process, where the organs establish in the embryo and gradually mature into the adult shape and size during the juvenile stage. In contrast, other organisms undergo more dramatic changes. For example, frogs, toads and newts (belonging to the class of amphibians; Fig.3) undergo the process of metamorphosis during their juvenile development: the tadpole that hatches from the egg is equipped for under-water life (i.e. with a tail for swimming and gills for respiration), whereas the frog has legs, breathes with lungs and possesses a tongue that was not yet present in the tadpole (Fig.4). Therefore, the whole organism has be be substantially re-built during a fairly short period of the juvenile stage (see this fantastic animation).
Fig. 4. Different forms of metamorphosis. For further details see main text. Question: Consider the human life cycle; which of the above animals is the most comparable? (See the answer here).
Very similar processes of metamorphosis can be found among the insects. For example damselflies and dragonflies (Fig.5) spend their juvenile larval stages in water, using gills for respiration, whereas the adults live above water and breathe with tracheae. Tracheae are equivalent to lungs; they are tubular invaginations of the skin which branch out into fine network throughout the body, allowing air to diffuse directly to internal tissues. In contrast, lung systems are restricted to one part of the body and require blood circulation to distribute oxygen to other body parts (for more info and images see here).
Fig. 5. Damselflies (left) and dragonflies (right) belong to the insect order of Odonata and are easily distinguished by their wing posture at rest: damselflies keep them in parallel and dragonflies perpendicular to the body. Furthermore, the eyes of damselflies are separated, whereas those of dragonflies meet at the top. For further distinctions see Box 6. Image details (sources hyperlinked): left main – Ischnura heterosticta; left inset – likely same species; right main – Sympetrum flaveolum; right inset – species unknown.
Also other insects, in particular stoneflies, caddisflies and mayflies (nos. 2-4 in Fig.6) spend their larval stages in water, using gills for respiration. Of these, mayflies have developed their life cycle to an extreme, in that the larvae live for two to three years under water, and the adults emerge for only one day, unable to eat and merely destined to mate, lay eggs and die. For this life cycle to work, the process has to be highly synchronised, and all adults emerge jointly from the waters, forming huge swarms that can even be detected by radar (see YouTube movie below).
However, not all insects undergo such functional re-organisations during their development. For example, juvenile grasshoppers and crickets live in the same habitats and feed the same as the adults; accordingly, they look like mini-versions of their mature counterparts, only lacking functional wings and the ability to reproduce (Fig.4). In contrast, other insects undergo an even more dramatic metamorphosis by adding a pupal stage (details in next section) to their development. A life cycle that includes a pupal stage is referred to as ‘complete metamorphosis‘ whereas most other insects undergo ‘incomplete metamorphosis‘. See Figure 4 for examples of different insect orders and whether they display complete (red dots) or incomplete (green half circle) metamorphosis.
Fig. 6. Different insects with complete (red dot) or incomplete (green half circle) metamorphosis. Download this information as an activity sheet. Question: All insects have 6 legs; which animal class does typically have 8 legs? (See the answer here). Image details (sources hyperlinked to numbers):  cicadas (Hemiptera, comprising also aphids, thrips, bugs) – Neotibicen linnei (by Bruce Marlin);  stoneflies (Plecoptera) – Rhabdiopteryx acuminata (by SMNS/Staniczek);  mayflies (Ephemeroptera) – Rhithrogena germanica; (by Richard Bartz);  caddisflies (trichoptera) – species unknown;  lacewings (Neuroptera, comprising also antlions) – Chrysopa sp. (by Alvesgaspar);  earwigs (Dermaptera) – Forficula auricularia (by fir0002 | flagstaffotos.com.au);  ants (Hymenoptera; comprising also bees, wasps) – fire ants (by Stephen Ausmus);  cockroaches (Blattodea; comprising also termites)- Periplaneta americana (by Gary Alpert);  termites (Blattodea; comprising also cockroaches) – Coptotermes formosanus shiraki (by Scott Bauer);  beetles (Coleoptera) – Melolontha melolontha (by Darkone);  crickets (Orthoptera; comprising also grasshoppers) – Gryllus campestris (juvenile form; by Didier Descouens);  praying mantises (Mantodea) – species unknown;  aphids (Hemiptera; comprising also cicadas, thrips, bugs) – species unknown (by MedievalRich);  fleas (Siphonaptera) – species unknown;  sucking lice (Phthiraptera) – Pediculus humanus (by KostaMumcuoglu);  flies (Diptera; comprising also mosquitoes) – Musca domestica (by USDAgov)
4. Complete metamorphosis: the pupal stage
The likely best known example of insects undergoing complete metamorphosis are the butterflies, where a worm-shaped caterpillar with minute extremities (three tiny legs at the front and stumped prolegs along the abdomen) develops into a winged adult with antennae, large eyes and long legs (Fig.4; TED-ED video below). Like all other insects, the caterpillar grows by shedding its skin several times (a processed called ‘moult‘). Moults are necessary because insects have a hard cuticle layer on their outer skin surface which cannot grow in size. Therefore, a new skin is developed beneath the old skin. The new skin is laid in folds, so that it can expand into a larger-sized individual when the old skin is shed during the moult.
Insects with incomplete metamorphosis gradually develop their wings and fully matured reproduction organs during their juvenile mobile stages (referred to as nymph stages) and merely expand their wings when they emerge from their last moult. In contrast, insects with complete metamorphosis have a pupal stage in which they usually become completely paralysed, because they dissolve their old appearance and replace it by their adult body features (see details below). This said, pupae are not always completely paralysed; for example, pupae of mosquitoes and midges drift at the water surface (Fig.4; YouTube movie below) and maintain a flicking tail which allows them to translocate in response to agitation.
The presence of a pupal stage is obvious when the appearance of the juvenile and adult form of an insect are dramatically different (Fig.4). It is often coupled to extreme changes in behaviour. For example, head louse nymphs live in the same environment as the adult and also feed on human blood; accordingly, they do not enter a pupal stage. In contrast, fleas undergo complete metamorphosis: their juvenile stages look worm-like and live from any kind of organic debris in cracks and gaps of the floor, whilst the adults feed on animal or human blood.
However, in other cases it can be surprising. For example, damselflies, dragonflies, stoneflies, caddisflies and mayflies (Fig.5 and nos. 2-4 in Fig.6) all have flying adult stages and their juveniles live under water. They are often not easy to tell apart from each other (Box 6), but they do not show the same life cycle: most of them undergo incomplete metamorphosis, whereas caddisflies stick out in that they have a pupal stage. Some hints that caddisflies are holometabolous are given by the fact that their larvae, when removed from their protective cases (Box 6), look a bit caterpillar-like. Also the wings of caddisflies are hairy (hence the name trichoptera: Greek “trich” means hair and “pter” means wing) and can, in some cases, look very similar to the scaled wings of moths – and moths have of course a pupal stage (Fig.4). Read more about caddisflies in this blog post.
5. What happens in the pupal case?
To understand what actually happens during the pupal stage, let’s look at a well-studied example, the fruit fly Drosophila melanogaster (see here why and how it is being studied). Like all true flies, Drosophila spends its juvenile stage as a maggot, which lasts 4 days and involves two moults in fruit flies; thereafter it enters the pupal stage which lasts for another 4 days, after which adult flies of the new generation eclose (see animated Fig.7).
Fig. 7 Life cycle of the fruit fly Drosophila melanogaster. The middle image shows a typical vial to keep flies and their offspring in the laboratory; this one is 10 cm high, contains food at the bottom (made from corm flower) and is plugged with a cotton ball; a close-up of the vial wall with flies and maggots is shown on the right. The image on the left shows a complete 10 day generation cycle, and the yellow dot indicates the respective developmental stages of images on the right.
When comparing the maggot to the fly (Fig.8), their outer appearances have only one obvious aspect in common: both are divided up from front to end into body segments (Fig.8). However, the prominent head, eyes, wings, legs and even bristles of the adult fly, are entirely absent at the larval stage. Even inside the body, essential anatomical features are very different: the abdomen of the fly contains the huge reproduction organs (including the gonads, i.e. ovaries in females and testes in males) which are not at all prominent at the maggot stage (Fig.8C). Also the muscles underlying the body wall show very distinct patterns at both stages – and completely new sets of muscles have to be formed inside the legs and at the base of wings to power their rapid and forceful movements (Fig.8B). Also a far bigger nervous system is required (Fig.8A) for a number of reasons: (1) to innervate all the newly formed muscles, (2) to integrate information from many newly formed sensory organs (for example the large eyes, antennae and multiple sensory hairs), (3) to coordinate far more complex behaviours such as flying, walking but also social interaction such as mating (as compared to the mere crawling and feeding of the maggot). How is such a dramatic re-organisation achieved? Let us look at four examples.
Fig. 8 Comparing the outer appearances of a Drosophila maggot and adult fly. A) The maggot is worm-like and displays hardly any prominent features, except for the head skeleton visible deep inside the front end the openings of the tracheae (spiracles) at the hind end. B) Using specific light conditions, 3 thoracic and 9 abdominal segments become visible in the maggot. C) The adult fly has a head (which was hidden inside the maggot) with prominent eyes and antennae, and the thorax carries the legs and wings. Sources: A) Sekelsky lab, B) Wikimedia, C) Discover Life (by Malcolm Storey)
Skin and appendages: small precursors for the adult skin and appendages are already set aside in the embryo, in form of little groups of cells (called ‘anlagen‘), which can only be seen when using specific histological markers. During larval life, they grow to larger size so that they become easily visible in dissected maggots. But it is only during the pupal stage that they accelerate their transforming growth and development towards the adult wing. This is very similar in butterflies, only that here the onset of transforming growth occurs a bit earlier during the last larval stage – which makes sense when considering their enormous wing sizes. Notably, also the frog tongue is not yet present in the tadpole where it exists only as a tiny anlage, which does not develop into the actual tongue before the onset of metamorphosis.
Fig. 9 A) Imaginal discs and histoblasts inside the larva are set aside to form the future skin and appendages of the adult fly. Top: position of the various discs inside the maggot; middle: close-ups of the different imaginal discs; bottom: colours indicate which outer body structures are derived from which imaginal disc. B) Top and side views of wing imaginal discs (top) and wing and thoracic body parts (bottom); colours indicate corresponding regions, all explained in the wing side view. C) Corresponding regions in leg disc and leg.
In flies, those anlagen which give rise to antennae, eyes, the skin of the thorax, wings, halteres (small appendages in the position of the second wing pair), legs and genitals are called ‘imaginal discs‘, and the skin of the adult abdomen is provided by ‘histoblasts‘ (Fig.9A). For example, wing discs will form the body skin of the upper thorax and the wing (green in Fig.9A). To achieve this, the wing discs are ‘patterned‘, meaning they ‘know’ where their front/back, top/bottom, proximal/distal regions will be, and their different sections are readily assigned to their future tissue fates: reproducible regions in the wing disc give rise to body skin, others to the wing hinge, or the wing margin, or the top or bottom aspects of the wing blade (Fig.9B). Similar ‘fate maps‘ can be drawn for all other discs, such as the leg discs (Fig.9C).
The wing disc is made up the future wing tissue and a supporting cell layer (peripodial membrane) and forms a pouch or purse which is tied together at its open end. During pupal development, the disc breaks through the larval skin, opens up and continues to grow, thus spreading out along the surface, gradually replacing the larval skin which dies away in parallel; in parallel it pops out the pre-fabricated wing blade on the outer surface (see movie above by Thomas Schaffter). When the fly ecloses, the situation is comparable to the final moult in hemimetabolous insects (i.e. those without a pupal stage): the wings are fully formed but folded into compact packages; following eclosure/moult, they get inflated with body liquid to expand and then dry and stiffen up (see YouTube video by Eric Spana below). Very similar processes as described here for wing discs are used by other imaginal discs and histoblasts to develop into other appendages or areas of the adult skin.
The central nervous system (CNS): The CNS of flies is composed of a brain (like in humans) and a ventral nerve cord (equivalent to the human spinal cord; see here for more explanations). In the Drosophila embryo, many neural stem cells (‘neuroblasts‘) are formed, each dividing repeated times to give rise to a string of about 20 daughter cells that comprise nerve cells (‘neurons’) and support cells (‘glia’). They all group together to form the characteristically shaped CNS; in parallel, the neurons start to form thin, long extensions (‘neurites’): usually one cable-like ‘axon’ per nerve cell and a set of highly branched ‘dendrites‘. These neurites wire the CNS up by forming complex connective networks (comparable to an electrical circuit), and they establish the nerves that reach out to all muscles (see here for more detail). At the end of embryogenesis, a functional nervous system has been established that is able to coordinate the simple behavioural repertoire of the maggot. See here for more info about embryonic CNS development.
Fig. 10 Metamorphosis of internal organs and tissues. A) The larval CNS (central nervous system) grows an outer layer with many new nerve cells (magenta); A’) the future adult nerve cells (blue) are generated by neuronal stem cells; A”) the adult CNS is composed of embryonic and newly formed nerve cells, which functionally integrate during the pupal stage. B-B”) The larval muscles (top) are very different from the adult ones (bottom); this restructuring requires that most larval muscles dissolve in the pupa and are replaced with new ones grown from muscle stem cells. C) The maggot harbours a pair of small gonads and a genital disc (see also Fig.9). C’) in the pupa both structures grow, establish connection and differentiate to jointly form the adult reproductive organs (C”). Sources: A’) from Prokop & Technau, 1991, Development 111, 79-88; all others from Hartenstein, V. (1993) Atlas of Drosophila development, CSHL Press; for explanations of the abbreviations in the images go to Hartenstein, 1993.
In the maggot, many of the neural stem cells re-activate to form even more neurons (magenta/blue in Fig.10A/A’). These start growing axons which then halt their development half-way and pause until the pupal stage; in the pupa, these half-established neurites continue to grow and rapidly wire up, joining and expanding the networks originally formed by the embryonic nerve cells. In parallel, the larval brain (which consists of two ball-like structures that are closely associated with the ventral nerve cord; Fig.10A), changes its shape and relocates into the newly formed head capsule (Fig.10A”). In the head and the thorax region, new areas are formed which receive the enormous amount of axons coming in from sensory neurons of the eyes (optic lobes), antennae (antennal lobes) as well as wings, halteres and legs (leg neuropile).
Fig. 11. Metamorphosis of muscles during the pupal stage of Drosophila: the film first shows a view onto a late maggot with the typical segmental muscle pattern (compare Fig.10B). It then shows the rapid sub-division of the larval body into the thorax and abdomen, accompanied and followed by the loss of the larval muscles. Eventually, one sees their gradual replacement through newly growing massive flight muscles in the thorax (left half or image) and the small regular abdominal muscles (right half or image; compare Fig.10B”). Muscles are made visible with a specific marker called green fluorescent protein (GFP) which illuminates under fluorescent light and makes it possible to see structures and organs even in living animals. Source: Original film published by: Chinta, R., Tan, J. H., Wasser, M. (2012). The study of muscle remodeling in Drosophila metamorphosis using in vivo microscopy and bioimage informatics. BMC Bioinformatics 13, S14-S14; movie taken and modified from Wikimedia.
Also the musculature undergoes massive restructuring which involves dissolving most larval muscles and replacing them with new sets of adult muscles born from muscle stem cells (details in Fig. 10B and 11). The reproduction organs undergo a composite development by forming from two distinct sources (details in Fig. 10C): (1) the genital discs mentioned above (Fig.9) and (2) a pair of small gonads which readily hold the progenitors for sperm or egg cells (see here for more details).
6. Life cycles of disease-causing parasites
Understanding life cycles of animals is important for animal conservation, but can also have important implications for human health. For example, knowing that disease-spreading mosquitoes spend their juvenile life in water, can trigger initiatives to drain wet land or change attitudes of people in malaria-infested regions towards taking care and avoiding unnecessary water puddles around housing areas. Also you may want to empty rain-filled pots or buckets in your garden to lower your risk of getting stung by mosquitoes in summer!
But it is also important to understand that mosquitoes are not the primary cause of malaria, but they act as transmitting ‘vectors’ of harmful parasites (in the case of malaria the single-celled organism Plasmodium) which they pass on to humans. These harmful parasites can have complex life cycles by spending certain life stages in one host (e.g. mosquitoes) and other stages in a different host (e.g. in humans where they cause disease symptoms). There are even cases where three different hosts are involved (see Box 8).
For example, the majority of malaria cases are caused by Plasmodium falsiparum (Fig. 12). During some of its life stages, it lives and multiplies in the gut and then salivary glands of a certain mosquito species called Anopheles. In other stages it is in the liver and red blood cells of humans, where it causes the cell damage that leads to the disease symptoms. As detailed in Figure 12, its stages in the human body involve asexual reproduction and end up with the formation of male and female individuals; these are taken up again by mosquitoes, where they pair (equivalent to fertilisation) and restart the life cycle. See also film1 or film2 about malaria.
———–Fig.12 Life cycle of Plasmodium. 1) male and female gametocytes are formed in red blood cells and sucked up by the mosquitoes; 2) in the mosquito gut, the gametocytes form female macrogametes and multiply into microgamets; 3) upon conjugation of micro- and macrogametes, the resulting ookinete penetrates the midgut wall and forms an oocyst on its outside; 4) the oocyst forms many sporozoites which migrate into the salivary gland to be pumped into human blood when the mosquito stings again; 5) sporozoites enter liver cells to multiply into merozoites; 6) merozoites enter blood cells to multiply further or form new gemetocytes.
Also multi-cellular parasitic organisms can have life cycles with stages that live in more than one host. For example, the dog tapeworm Echinococccus granulosus is a flatworm that lives in its adult stage in the gut of dogs or dog-related species, lays its eggs into their stool to infect grazing animals, including farm animals. Once taken up by these animals, it forms enlarging cysts containing dormant larvae, able to survive for long periods until they are taken up by another dog that feeds on the infected meat or carcass (Fig.13).
Also humans can get infected by the dog tapeworm, which then forms cysts in almost all organs of the body, which can persist and continue to grow for decades. The only current treatment is to cut these growing cysts out, which can be difficult in some organs, such as the brain or eye. As preventive measures, don’t touch your mouth with your hand when playing outside and wash your hands first thing when you come home (see also Box 7)! Furthermore, dogs need to be vaccinated and the meat of slaughtered farm animals has to be screened by veterinarians to prevent human infection through cysts. When in less safe countries, make sure you only eat very well cooked meat and avoid raw salads.
Fig. 13 Life cycle of Echinococcus ganulosus. 1) The adult tapeworm is a hermaphrodite living in the gut of dogs where it lays its (2) eggs that leave the body with the stool. Farm animals or humans can take up these eggs. 3) In the guts of animals or humans, eggs develop into oncospheres which can penetrate the gut wall and migrate into other organs. 4) In these organs, they develop into cysts which can persist and grow for decades, increasing the chance that they will be taken up when eaten by another dog.