Lesson2: genetics or not: the diversity of the organisms
Genetic mixing and fertilization contribute to the production of an original individual. In rare cases, meiosis (crossing over) errors added to mutations may allow genetic diversification, resulting in closely related genes. But this is not the only mechanism for diversification of living beings. You can have several processes involved in this diversification. Then, the natural selection by the pressure of the ecosystem allows the emergence or not of a new species.
How can living things diversify?
I- Horizontal gene transfers
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the stable transfer of genetic material from one organism to another without reproduction or human intervention.
A- In the world of prokaryotes
This process can occur through several mechanisms, and takes place on a large scale in the world of prokaryotes (bacteria). Horizontal gene transfer is the primary mechanism for the spread of antibiotic resistance in bacteria for instance.
1- Experiments and discoveries
The discovery of horizontal gene transfer (HGT) can be traced to 1928 when Fred Griffith reported the transfer of genetic material from heat-killed virulent Streptococcus pneumoniae to an avirulent form of the bacterium by a process he described as transformation.
The experiments of Griffith and Avery, MacLeod and McCarty are closely related. Griffith developed the concept of the transforming principle. The principle was able to transform a non-pathogenic bacteria into a pathogenic strain. Changing phenotype is one of the characteristics of the hereditary material. Griffith called the « factor » that changed the phenotype the « tranforming principle ».
Avery, McCarty, and MacLeod performed a series of experiments that demonstrated the hereditary materials was DNA.
2- most common forms of horizontal gene transfer in bacteria
a) transformation :
During transformation, DNA fragments (usually about 10 genes long) are released from a dead degraded bacterium and bind to DNA binding proteins on the surface of a competent living recipient bacterium.
b) conjugation :
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus.
c) transduction :
- Step 1: A bacteriophage adsorbs to a susceptible bacterium.
- Step 2: The bacteriophage genome enters the bacterium. The genome directs the bacterium’s metabolic machinery to manufacture bacteriophage components and enzymes. Bacteriophage-coded enzymes will also breakup the bacterial chromosome.
- Step 3: Occasionally, a bacteriophage capsid mistakenly assembles around either a fragment of the donor bacterium’s chromosome or around a plasmid instead of around a phage genome.
- Step 4: The bacteriophages are released as the bacterium is lysed. Note that one bacteriophage is carrying a fragment of the donor bacterium’s DNA rather than a bacteriophage genome.
- Step 5: The bacteriophage carrying the donor bacterium’s DNA adsorbs to a recipient bacterium.
- Step 6: The bacteriophage inserts the donor bacterium’s DNA it is carrying into the recipient bacterium.
- Step 7: Recombination occurs and the donor bacterium’s DNA is exchanged for some of the recipient’s DNA
B- In the world of eukaryotes
In eukaryotes, the transfer or hybridization of the genetic code between two species is more limited due in particular to the mechanism of sexual reproduction. On the other hand, it takes place between eukaryotes and viruses or eukaryotes and bacteria.
Example of the syncytin : In humans and in other mammals, the mother uses an organ called the placenta to share useful chemicals with the foetus. The placenta has a special layer called the syncytiotrophoblast, made up of cells fused together, that acts as a protective barrier for the foetus. It emerged that the syncytiotrophoblast expresses viral genes that our ancestors picked up over 45 million years ago. One of these genes codes for a protein called syncytin, which helps the cells fuse together. Clearly, genes from viruses have shaped our evolution.
II- Homeotic genes
When we compare the development of the embryon in different species, we realize that the organism under construction passes through various stages that are very close to one species to another.
Moreover, when we compare some genes responsible of the shape of the species and its of organization plan. Those genes are called homeotic genes. We can observe that they have a strong homology of sequence. So they should come from ancestral genes. These homeotic genes form complexes which, if they are not expressed in the same order, or if they are expressed in different spaces or with a different intensity, will lead to different morphologies.
So Homeotic genes are any of a group of genes that control the pattern of body formation during early embryonic development of organisms. These genes encode proteinscalled transcription factors that direct cells to form various parts of the body. A homeotic protein can activate one gene but repress another, producing effects that are complementary and necessary for the ordered development of an organism.
III- When 2 species are linked together without genes
A- Mutualism
Mutualism is a mutually beneficial relationship in which both organisms benefit. Each individual provides an advantage to the other, enabling them to exploit each other and thereby enhance their chances of survival. An example is the anemone-clownfish mutualism, in which the clownfish gets food scraps from the anemone and uses the stinging cells of the anemone for protection. The anemone gets smothering algal cover cleaned off by the clownfish and absorbs nutrients from the clownfish’s nitrogenous waste, so both organisms benefit. Another well-known example of mutualism is the relationship between corals and zooxanthellae, a type of algae that live in corals. The coral gets extra nutrition from the algae as it photosynthesizes, and the zooxanthellae are protected by the hard coral and obtain plant nutrients from the coral (e.g., ammonia).
B- Endosymbiosis
In endosymbiosis, the host cell lacks some of the nutrients which the endosymbiont provides. As a result, the host favors endosymbiont’s growth processes within itself by producing some specialized cells. These cells affect the genetic composition of the host in order to regulate the increasing population of the endosymbionts and ensure that these genetic changes are passed onto the offspring via vertical transmission.
As the endosymbiont adapts to the host’s lifestyle, the endosymbiont changes dramatically. There is a drastic reduction in its genome size. The decrease in genome size is due to loss of protein coding genes
C- Parasitism
If one of the partners in the symbiosis discovers how to use the other effectively, it becomes a parasite. There is indeed a continuum between symbiosis and parasitism (in all English lessons, it is explained like that). The parasite exploits resources provided by another unrelated individual, the host, to the detriment of the latter. Parasitism is a long-lasting interaction with a host, unlike predation, where the interaction lasts only as long as the time of capture and digestion. However, from an evolutionary point of view, it can be said that predation is only an extreme form of parasitism.
There are parasites that slowly kill their host. This is the case of plant parasitic fungi (Mildew, Armillaries, Hoof fungus, etc…) that complete their life cycle on dead tissues.
In host parasite systems where the host survives, the duration of the interaction is quite different as it is in predation: the two organisms then live together, often one in the other, sometimes cell in cell or even genome within genome. The genetic information of each partner is expressed side by side and durably in a tiny portion of space.
Some parasites are capable of significantly modifying the physiology, morphology or behaviour of their host with the consequence of increasing their probability of transmission. This host exploitation strategy is now described in many host parasite systems phylogenetically distant. Phenotypic changes in infected hosts are generally considered an illustration of the extended phenotype concept. These phenotypic changes actually correspond to the expression of the parasite’s genes and the effect of the corresponding proteins on the host’s phenotype. According to this idea, these induced modifications are adaptive for the parasite and not for the host.
Here are a few examples :
The parasite will often manipulate the host to take care of its offspring. Thus, after the larvae of the parasitic wasp Dinocampus coccinellae emerged from the abdomen of the parasitized ladybird, then transformed into a cocoon, the ladybird will take care of it and protect the cocoon until the wasp emerges (A). The Glyptapanteles wasp lays eggs inside the caterpillar of the butterfly Thyrinteina leucocerae, which will change its behaviour after the eggs hatch and transform them into pupae: it stops feeding, becomes immobile, and protects the pupae from predators until they hatch (C). Some manipulations are even more extreme. By invading an ant in the Brazilian forest, the fungus Ophiocordyceps camponoti-rufipedis (B) manipulates its behaviour by taking control of its “brain”, leading the ant to climb to the top of a plant where conditions (light, humidity) are favourable to the fungus’ development. Once the ant is firmly attached to the stem, the fungus kills it and grows slowly: the spores produced are then easily dispersed. A somewhat similar strategy is being implemented for the snail Succinea putris infected with the parasite Leucochloridium paradoxum. The latter is housed in the snail’s antennas, which will take on the appearance and movements of a worm, becoming a prey that is all the more noticeable for birds as the snail’s behaviour is also modified because it tends to leave the protection of vegetation. The life cycle of the parasite continues in the bird whose droppings allow the spread of parasite eggs (D). Sacculin (Sacculina carcini), a small crab parasite crustacean, colonizes its host, alters its hormonal balance and prevents it from reproducing, its only function being to feed the parasite (E). After the sacculin has been fertilized, the crab will take care of the parasite’s eggs as if they were its own.
IV- Cultural transmission
Cultural transmission, also known as cultural learning, is the process and method of passing on socially learned information. Within a species, cultural transmission is greatly influenced by how adults socialize with each other and with their young. Differences in cultural transmission across species have been thought to be largely affected by external factors, such as the physical environment, that may lead an individual to interpret a traditional concept in a novel way. The environmental stimuli that contribute to this variance can include climate, migration patterns, conflict, suitability for survival, and endemic pathogens. Cultural transmission is hypothesized to be a critical process for maintaining behavioral characteristics in both humans and nonhuman animals over time, and its existence relies on innovation, imitation, and communication to create and propagate various aspects of animal behavior seen today.
Vertical transmission occurs from parents to offspring and is a function which shows that the probability that parents of specific types give rise to an offspring of their own or of another type. Vertical transmission, in this sense, is similar to genetic transmission in biological evolution as mathematical models for gene transmission account for variation. Vertical transmission also contributes strongly to the buildup of between-population variation.
Horizontal transmission is cultural transmission taking place among peers in a given population. While horizontal transmission is expected to result in faster within-group evolution due to the relationship building between peers of a population, it is expected to result in less between-group variation than the vertical transmission model would allow for.
See the different examples in your (very good) French lesson 🙂