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Why Sex Chromosomes Evolve So Rapidly?

BY: Shalini Balan | Category: Genetics | Submitted: 2011-11-21 08:00:41
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Article Summary: "Details about reproductive / sex chromosome and their evolution. In animals with separate sexes, embryos assign to become male or female at an early stage. Often this key decision is made by sex determination genes on the sex chromosomes. The genes concerned in sexual development have not changed much since evolution. In contras.."


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In animals with separate sexes, embryos assign to become male or female at an early stage. Often this key decision is made by sex determination genes on the sex chromosomes. The genes concerned in sexual development have not changed much since evolution. In contrast, the sex determination genes and the sex chromosomes themselves are among the most rapidly changing features of the genome. As they are determined to be male or female are the very early stages of embryogenesis.

Theoretical study tells that sexual conflict can establish sex-determining genes and sex chromosomes. The projected mechanism extended from the theory on the origin of sex chromosomes, and it explains how sex determination can move from an ancestral sex chromosome to an autosome, a non-sex-chromosome, that then invades to become a new sex chromosome.

The mechanism suggested by these authors begins with an autosome that carries two genes with particular features.

WHAT IS AN AUTOSOME?

A human somatic cell will usually contain 23 pairs of chromosomes. Twenty-two (22) of these pairs will be autosomes, and only one of them will be a pair of sex chromosomes (the X and Y chromosomes), which determines the sex of the embryo. Therefore autosomes are sets of chromosomes other than the sex chromosomes.Hence they are also termed as a nonsex chromosome.

The hypothesis of sex determination can be well explained by the theory of natural selection. This theory explains that organisms better adapted to their environment tend to survive and produce more offspring. Genes are located on chromosomes and half of all sperm cells carry a structure called an X body or the X chromosomes. Autosome that has two genes with particular features. One of these two genes is under sexually aggressive selection. And according to natural selection an allele that is most beneficial in males will occur most often with the allele of the other gene that makes the individual male. Some versions of the gene (alleles) are more favorable in males than in females, while other alleles are more beneficial for females. The other gene influences the sex of the individual. Later this new male-making, male-benefiting (or female-making, female-benefiting) combination of genes spreads through the population, eventually replacing the old pair of sex chromosomes.

Genes with sexually antagonistic fitness shows off and mutations that influence sex determination appear to be common in nature, but how would we know if the model presented here actually caused a change in the sex-determination mechanism in a particular species?

One possible way to know is by looking at the sexually antagonistic genes on a chromosome immediately before and after that chromosome took over the role of sex determination.
This can be done by comparing closely related species with different sex chromosomes. One species would have a very young set of sex chromosomes, while the other would still use the old sex chromosomes, and might estimate the state of the chromosome right before the switch.

A latest study on fungi called Neurospora tetrasperma, shows that there are many similarities between the parts of DNA that determine sex in plants and animals and the parts of the DNA that control mating types in certain fungi. Fungi don't have sex and just called as mating types. This makes fungi as an interesting model to study the development of sex chromosomes.
In the plant and animal species there are individuals of different sexes, that is, bearers of either many tiny sex cells (males) or a few large ones (females). In the third eukaryote kingdom like the fungi kingdom, there are no sexes but rather a simpler and more primal system of different mating types. These are distinguished by different variations of a few specific genes. Hence sex determination varies from species to species .In case of human beings it is done by sex chromosomes. It is believed that this mechanism of sex determination arose in the plant and animal realm from the simpler system of mating types and that this happened several times autonomously of each other throughout the evolution . The change is believed to have happened with the inhibition of a step in the copying process in DNA, which led to two separate chromosomes. These then developed further over a long period of time. In humans, sex chromosomes are believed to have developed over the last 300 million years from a common 'proto-sex chromosome.

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