What Are Selfish Genetic Elements and What Is Their Role in Reproductive Isolation?

Introduction

The genome of the organism refers to all the genes that work together in harmony to improve the health of an organism. But there are some genes that take control of themselves and aid their own transmission at the cost of other genes in the genome, in other ways they behave in a selfish way by keeping them first.

Such genes which project themselves overcasting other genes are called selfish genes. Most of the time this will not have any impact on the organisms but sometimes it may pose a negative impact on the organism’s health and fitness.

Reproductive isolation is the phenomenon that resulted due to various physiological or evolutionary processes that prevent individuals of two different species to mate and produce offspring.

Definition of Selfish Genetic Elements

Many names are given to selfish genetic elements like selfish genes, parasitic DNA, ultra-selfish genes, and genetic outlaws. Warren et.al defined a selfish genetic element as the segments that have characteristics that enhance their own transmission relative to the rest of an individual’s genome but neutral or detrimental to the organism as a whole. Since they enhance their own transmission there is always a genetic conflict between selfish genetic elements and other genetic elements in the genome.

Selfish genetic elements are also called Junk genes as they offer no benefit to the organism. Their transmission before other genes poses no harm to the organism most of the time but sometimes proves detrimental.

Some of the selfish genes have been shown to have different pathways by which they transmit themselves. Although much was not known about their contribution to the process of evolution now it is established that they play an important role in determining the size of the genome of an organism and the process of speciation.

Types of Selfish Genetic Elements

There are mainly three types of selfish genetic elements found in the genome of an organism although there are some other types present in minute amounts. The three main types are-

• Transposable elements.

• Segregation distorters.

• Homing endonucleases.

Transposable Elements

Transposable elements can be defined as the segments of DNA that can move from one place to another or can be copied on a chromosome. They can be found in prokaryotes and eukaryotes and make the most part of the genome. They are known to have a profound impact on the fitness of bacteria not only this they also help the organism to adapt to a new environment and in the production of distantly involved species by causing divergent evolution.

Transposable elements were discovered by Barbara McClintock. She discovered these elements before DNA double helix and genetic code were explained. for her work in this field, she was awarded the Nobel Prize for Medicine in the year 1983.

Segregation Distorters

Mendel's law of segregation states that only one out of two genes is passed on to each gamete randomly. But sometimes genotypes deviate from these assumptions in the progeny of a cross between two species, these deviations are known as segregation distortion or meiotic drive, and the set of alleles that deviate the segregation in their own favor is called segregation distorters.

Fig. Segregation of genes during meiosis

Homing Endonucleases

These are also known as meganuclease. Like any other restriction endonuclease, they cut the double-stranded DNA at specific restriction sites and help in the lateral movement of genetic elements within an organism.

This process is known as homing and the endonucleases are known as homing endonucleases. They are different from normal restriction endonucleases in the fact that, while restriction enzymes confer protection against invading DNA, homing endonucleases help in the mobility of DNA sequences.

Role of Selfish Genes in Reproductive Isolation

When two organisms of the same species mate it results in the exchange of genes between those organisms this whole process is known as gene flow. But this gene flow reduces when organisms of different species mate. When the genes are introduced to a new genetic environment of a different species it causes some harmful effects. Sex chromosome becomes functionally different between two species much faster than autosomes.

It has been found that a class of selfish genes called segregation distorters is responsible for the fast-paced reduction of genetic compatibility in such organisms.

Selfish genes act as manipulators of reproduction so that they can transmit themselves prior to their genetic counterparts.

A very good example is seen in Drosophila, where these selfish genes kill the sperms that do not carry them, so only those sperms survive which carry these selfish genes along with them.

In other words, it can be inferred that selfish genes can cause reproductive isolation if they do not experience gene flow, or they can cause convergence of species if they experience the gene flow.

Conclusion

It was Richard Dawkin who coined the term Selfish genes and gave a gene-centric theory of evolution, and according to him, “Genes are competing directly with their alleles for survival since their alleles in the gene pool are rivals for the slot on the chromosomes of future generations. Any gene that behaves in such a way as to increase its own chances of survival in the gene pool at the expense of its alleles will, by definition, tend to survive. The gene is the basic unit of selfishness.”

Selfish genes can cause two species to be completely incompatible by meiotic drive leading to reproductive isolation or can cause convergent evolution by causing leakage of genes. More studies are on the way which can explain their role in evolution in detail.