Introduction
Genetic evolution refers to change(s) in inheritable traits occurring across succeeding generation of organisms. The change (evolution) is responsible for the diversity at all levels including the molecular level e.g. genetic variations, individual organisms in the same species (intra-species variations) and among species (interspecies variations). It is believed that all organisms emanated from common ancestry about 3.7 billion years ago and then evolved over time to the present biodiversity. Therefore all organisms have some shared traits at the morphological and molecular level with the species sharing a more recent ancestry having more similar traits. The process of evolution involves speciation and extinctions resulting in present biodiversity patterns .
The theory of evolution by natural selection was first proposed by Charles Darwin who defined in three laws:
1) All organisms bear more offspring than can survive so the fittest survive.
2) Individuals have varying traits that allow them to survive and reproduce differently.
3) Because the traits are inheritable- when organisms die they are replaced by offspring that are better placed to survive and reproduce in the environment. This paper shall specifically focus on the evolution of sex genes and the corresponding traits. However to understand the evolution of sex genes it is important to review the Mendelian Laws.
Mendel's laws
Mendel’s discovery regarding genes was made while crossing flowers. He discovered that when he crossed white flower and purple flower plants, the resultant off spring was not a blend. Instead, the off spring was purple-flowered. This discovery led him to come up with the notion of heredity units, which he named “factors"; one of which had a recessive characteristic and the other one was dominant . According to Mendel, factors that would later on be known as called genes exist as pairs in ordinary cell but they undergo separation during the formation of sex cells. After separation, each member of the pair is a component of a separate sex cell. Each individual inherits one factor from each parent for each trait. The two factors may or may not have the same information.
An individual is regarded as homozygous for a given trait if the two factors contain identical information . If the information on the factors differs, the individual is regarded as heterozygous. Alleles are alternative forms of the factors. The genotype of an individual is comprised of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. Mendel’s Laws formed a basis for genecists to understand genetic makeup and by extension formed a basis for studying the evolution of sex genes. Mendel summarized his findings into two laws: the Law of Segregation and the Law of Assortment.
According to the Law of segregation, after the production of gametes by any individual, the genes undergo segregation such that each gamete receives a copy of the gene. Each gamete therefore possesses one copy of a gene (allele). The Law of Assortment states that separate genes which code for various traits are inherited by the off spring from each parent as separate entities. In eukaryotes, independent assortment specifically takes place during meiosis during metaphase I. During metaphase I, the bivalents are distributed across the metaphase plate. It is along this plate that the distribution of traits takes place. During independent assortment, all the possible outcomes of paternal and maternal outcomes of chromosomes are sorted. This process is critical in gene diversity since it results in the production of novel genes.
Evolution of sex genes
In most organisms, the absence or presence of two heteromorphic chromosomes determines the sex of the organisms. These sets of chromosomes can be XY or ZW. The theory of evolution of sex chromosomes postulates that sex chromosomes were at one time a pair of equivalent autosomes that gradually changed into sex chromosomes after the acquisition of a dominant factor for the determination of sex by one of the homologues. It is further postulated that two mechanisms were involved in the differentiation. However the two mechanisms differ in terms of the frequency of occurrence. In the first mechanism, the chromosome that contains the gene which determines disintegrates as a result of failed recombination between the two homologues.
The degeneration of the Y or W chromosome occurs through various mechanisms such as Muller’s ratchet, selective sweeps and background selection. Muller’s ratchet refers to a mechanism by which the genomes of asexual organisms acquire mutations that result in the deletion of the genome sequence irreversibly. Asexual reproduction forces genes to be inherited as indivisible blocks. According to this theory, each of the least mutated genomes in an organism carries at least one mutated gene therefore each of the subsequent generations of the organism carries a mutation. The accumulation of the mutations is known as genetic load. Selective sweeping refers to the elimination or the variation of among the nucleotide sequences of the DNA sequence that is next to a mutation. Selective sweeping occurs when mutation of DNA results in a nucleotide sequence that is fitter than the previous sequence. Given that natural selection favours the fitter sequence, the less fit sequence is eliminated gradually while the frequency of the fitter mutant sequence is enhanced with time. As the prevalence of the new mutant gene increases, the occurrence of the neutral or nearly neutral forms of the mutant gene increases, a phenomenon that is referred to as genetic hitch hiking. A strong selective sweep eventually results in a DNA sequence in which only the fitter gene together with the “swept” sequence exists.
The second mechanism involves the erosion of the Y or W chromosomes therefore setting in motion mechanisms that will favour the generation of a compensatory regulatory mechanism. The function of the compensation mechanism is to bring about the balance of aneuploidy in the heterogametic sex. This mechanism is referred to as dosage compensation.
The dosage compensation differs from one taxa to another. For instance in males, dosage compensation brings about the over expression of X chromosome. The same mechanism in worms results in the reduction of the over expressed X chromosome by half. In female mammals, the mechanism results in the inactivation of one of two X chromosomes and for males it results in the over expression of the distinct X chromosome. One of the consequences of the Y o W chromosome degeneration is a state of homizygosity in the heterogametic sex.
The X or Z chromosomes are found in the homogametic sex for two thirds of the time and for one third of the time, they are found in the heterogametic organism. This gives rise to sexual antagonism. As a result of the sexual antagonism, Y chromosomes have a greater tendency of having alleles that are beneficial to the males as compared to the W chromosome. For organisms that have UV systems as part of their se genes, the occurrence of mutations is likely to be greater since these organisms undergo haploid selection which is responsible for the selection of the genetic content of both sex chromosomes.
X and Z chromosomes possess a unique feature in that they both spend a fraction of their evolutionary phase as a part of each other. This is important for maintaining polymorphisms. The second unique feature is that homogametic sex chromosomes are hemizygous for 1/3 of their life span. Recessive alleles that have deleterious effects are strongly purified during this time. The X and Z linked genes have been known to have accelerated evolutionary effects which is known as faster X and faster Z effect respectively. These effects have been observed in mammals and birds.
During sex selection, the types of genes that are carried by the X or Z genes determine whether the genes will be “feminized or masculinised.” 2/3 of the time, the X chromosome is found in females and in males, it is always homizygous. There is therefore a greater likelihood of dominant mutations occurring which are more inclined females occurring in addition to recessive mutations of the males. For Z chromosomes, the reverse of this occurs. This process results in the feminization of the X chromosome while demasculinizing the X-linked gene. On the other the Z linked gene will be defeminised and more masculinised as a result of the same process. The gene expression patterns that have been observed in flies, mammals and birds are consistent with these hypotheses.
Another unique feature of the X and Z chromosomes of mammals, birds and Drosophila is that during meiosis, they are repressed transcriptionally in the heterogametic germ line, a process that is referred to as meiotic sex chromosome inactivation (MSCI). It is speculated that this process could linked to the “silencing” of chromatin material that is unpaired which by extension is a mechanism to prevent invasion of transposons. This theory is however defied by the fact that there is a considerable amount of X linked genes which escape the process of meiotic sex chromosome inactivation. Alternative theories on the role of MSCI have been put forward in an attempt to explain the role of the process in the evolution of sex genes. One of the hypotheses that has been put forward is that during meiosis, the X chromosome has an accumulation of female sexual antagonistic sexual alleles. These alleles are against the transcription of the gene and they are also evident in Z chromosomes where male antagonistic alleles accumulate on the Z chromosomes to hinder the transcription of the gene.
Mechanisms that lead to the evolution of sex specific genes
There are three specific mechanisms that are known to lead to the evolution of sex specific genes: gene duplication, generation of sex specific transcriptional variants and cis/trans regulatory changes. Cis or trans regulatory changes could trigger the conversion of a gene that has been widely expressed gene into a sex specific gene. There is evidence from several studies that cis regulatory changes tend to be more intraspecific while trans- regulatory changes tend to be more interspecific.
Sex transcriptional variants could be generated which may lead to the acquisition of protein isoforms which are sex specific. This does not necessarily result in the modification of the original isoform expression or function. Studies involving Drosophila suggest that between 22 and 32% of transcriptional variants are sex specific. A section of the transcriptional variants have been known to contribute to the determination of sex while others are only involved in the regulation mechanism of the transcription process. In the Drosophila flies, the sex specific variants are found in abundance in the head and in the testis while in mammals they are commonly found in the testis.
The duplication of genes can also lead to the evolution of sex specific genes. Gene duplication is widely believed to be the source of novel gene forms. Gene duplication can occur at any part of the genome with the size of the genome that is being duplicated varying from a few kilo bases to hundreds of kilo base. The duplicated gene has its origins from a single individual and in the initial stages; the survival of the duplicated gene is more often a matter of chance. The duplicated genes can be neutral in most cases therefore are subject to genetic drift.
Continuous drifting of the duplicated gene can result in a once non function gene evolving into a functional gene such as a sex specific gene.
There are two mechanisms through which duplication of genes occurs: transposition which disperses gene sequences that are related and unequal crossing over which results in the creation of gene clusters. Transposition can occur through two methods: it could be as a result of the direct movement of the parent sequences from one point to another or it could be through an RNA intermediate hence the original site is not interfered with. When the genetic material shifts, the duplication occurs in the subsequent generation after the transposition. The more common form of duplication occurs through the reverse transcription of RNA transcript into DNA after which it is inserted into the genome. This process is known as retro transposition. The retroposon cannot be larger than the RNA intermediate.
Duplication can also occur through unequal crossing over. Unequal crossing over refers to crossing over of unequal segments of genetic material. It is also known as illegitimate recombination. It can be triggered by the presence of related sequences within the genome such as highly repeated transposons. In spite the fact that the crossing over of genetic material is unequal, it is still mediated by homology present at the two non-equivalent sites. There are also non homologous unequal crossing overs which can occur in the genome although there occurrence is rarer than that of homologous crossing over. The occurrence of unequal crossing over pays no regard to functional boundaries thus the portion of genes contained in the duplicated region can vary from a few base pairs to hundreds of base pairs.
The mechanism of unequal crossing over is responsible for the recent discovery on the evolution of the Y chromosome which according to scientists has gradually been deteriorating. According to research, the Y chromosome has been evolving at a much faster rate than the X chromosome that is present in both males and females. According to scientists, if th rapid pace of the evolution of the Y chromosome is maintained then it is speculated that it would eventually disappear. It is speculated that the X and Y chromosome regularly swapped DNA but about 80 to 130 million years ago, the regular crossing over of genetic material ceased. The crossing over of materials between the two chromosomes led to the integration of the new genetic material in the respective recipient and its subsequent expression in the subsequent generations. The unequal crossing over of material has led to the unequal nature of the X and Y chromosome. The human X chromosome contains more than 1,100 genes while Y chromosome contains less than 200 genes .
Genomic location of sex biased genes
The location of the X chromosomes is dependent on the type of the cell. According to studies, it has been found that the some X linked genes seem to be over expressed in the brains of males and females. The genomic location of the X chromosome is of evolutionary significance. For instance in the Drosophila pseudoobscura lineage, ancestral Muller A(located on the XL arm) and D(located on the current XR arm) fused about 10-18 million years ago therefore leading to the formation a new X chromosome. Subsequent studies reveal that there is currently an underrepresentation of X linked genes on the chromosome indicating that evolution forces are more inclined towards the demasculinization of the X linked gene. The trend is not only limited to the old male biased genes but has also been observed that there is an emergence of male biased X genes through gene duplication. However the novel male biased genes are either translocated or lost as was the case for the older male biased genes in organisms such as Drosophila pseudoobscura.
For the Z chromosomes, the trends are not consistent and the available information scanty which is an indication of the lack of studies regarding Z chromosomes. The analysis done in chicken reveals that the male linked genes in expressed in the brain are overrepresented in the Z chromosomes. The male specific genes expressed in the chicken have been found to be randomly distributed while the female specific genes which are expressed in the ovary and the brain have been found to be relatively random in the Z chromosomes. There are other studies that have been done on other organisms that have been done on other organisms that exhibit the ZW systems. For instance, in the silk worm it has been found that genes that are specific to the testis are over represented on the Z chromosome.
Mechanisms for the evolution of reproductive proteins
The exact reasons that lead to the evolution of proteins that are involved in reproduction are not known but there are several hypotheses that are involved in the evolution of proteins:
Relaxed constraint
It is speculated that the repetitive nature of the reproductive proteins may have trigger constraint therefore commencing the process of evolution. This would result in the production of a sequence that continually changes. This theory has been applied in explaining the evolution of gametes in sea urchins. For instance, mutation may occur in one of the 22 VERL protein repeats leading to a change in the VERL protein. The mutant repeat does not interfere with fertilization because there are still 22 VERL proteins . The VERL protein in this instance displays redundancy such that its mutation is neither harmful nor beneficial. The mutant repeat within the VERL protein is passed onto subsequent generations through the conversion of genes and unequal crossing over. As a result, there is continuous pressure for lysine to adapt to the continuously evolving VERL protein. Repeated and redundant genes have also been seen in other organisms such as diatom Sig1.
Self/ non self-recognition
The molecules that are involved in self-recognition vary extremely within species. It is possible that the molecules that are involved in reproduction are also involved in self-recognition. The Euplotes mating pheromone for instance is involved in both self-recognition and non-self-recognition and based on the varying conditions might result in either sexual mating or induction of the growth of the cells .
Reinforcement
Reinforcement refers to the selection of the reproductive recognition system which occurs as a result of hybrid mating which gives rise in offspring that are less fit hence possess genomic incompatibilities. This could result in the evolution of proteins that are involved in reproduction.
Sexual conflict
The competition among the sperms to fuse with the eggs presents some problems to the egg. There is need for the egg to protect itself from poly-spermy which refers to fusion of the egg with multiple sperms, a process that could result in hampered development of the zygote. One of the methods the egg has evolved for protecting itself from poly-spermy is the adaptive evolution of the surface proteins of the egg.
Conclusion
Evolution of the sex genes and reproductive proteins is not fully understood. Further studies ought to be conducted in order to resolve the grey areas in this area of research.
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