ABO histo-blood groups were the first polymorphism (presence of two or more distinct phenotypes) discovered in humans over a century ago; scientists later discovered that this is a trait found not only in humans but a trans-species polymorphism—the same two changes in amino acids being responsible for the specificity of A or B antigens in all species that have been analyzed to this date (another two changes are present that are not directly linked to substrate specificity).
The expression of ABO genes are controlled in three different places and each place codes for a different enzyme (all belonging to a group called glycosyltransferases) that convert the precursor H substance in blood into A or B antigens. It is thought that O and B blood groups arose through mutation of the A gene. The O blood group is an absence of any antigen and is usually caused by the deletion of a nucleotide, causing a frame-shift that results in the premature termination of the polypeptide chain; the O gene does not code for a functional enzyme (this specific expression of the O gene allele, in the genotype, seems to vary across species). This results in no modification of the H substance. As such, the presence of unchanged H determinants are characteristic of individuals with group O blood.
In the rare case that a person is homozygous with the recessive gene that codes for H deficiency, their bodies cannot form the A or B as the precursor for these blood types is not present. This results in the Oh blood type where the individual has the blood type O and makes anti-H antibodies due to the lack of the antigen (in addition to anti-A, anti-B and anti-AB antibodies) and by consequence can only receive other Oh blood cells. The B gene differs by seven nucleotide substitutions with four of these substitutions coding for a different amino acid (same in every species). The AB blood type is a result of the codominance of the two genes that code for both the A and B blood type.
The discovery of these blood groups are so vital to modern medicine because of Landsteiner’s law that states that for whichever ABO antigen is not present on red blood cells, the corresponding antibody is found in plasma. This is why a lack of the H precursor antigen leads to potent anti-H antibodies. These antibodies develop naturally, even without exposure to the relevant antigen due to exposure to bacteria that carry A-like and B-like antigens in the first few months of life. This means that those with the blood type of A can only receive type O or A red blood cells.
Whether this consistent recurrence of A and B antigens is the result of a common, ancestral polymorphism that has been maintained across descendant species or whether it’s an instance of convergent evolution is a topic that has been debated for decades. There are proponents for both sides of the argument yet it seems most likely that there was an ancient, multi-allelic polymorphism that has survived through the generations.
Balancing selection has the potential to maintain multiple alleles against the expected results of genetic drift when a species becomes sufficiently reproductively isolated for an extended period of time. The expectation is for chromosomes in one species to be more closely related when compared to a chromosome of a different species. However, balancing selection preserves two or more alleles between species, the case may arise where two different sections of chromosomes may have more in common with another species that has the same allelic class than chromosomes of one’s own species that have different allelic classes. The identification of such trans-species polymorphisms can be challenged due to old last common ancestors resulting in a narrower section of similarity between species. One dominant theory in the space is that the A allele is evolutionary and (in accordance with the convergent evolution theory) the B allele involved separately over a multitude of different primates. However, the sequencing of the genes of many Old and New world primates challenge this theory instead supporting the theory of the ancestral polymorphism that has maintained this polymorphism through. This can be seen on the left. This theory suggests that the phenotype of the ABO histo-blood groups are not the expressions of them through antigens that can be found exclusively on humans. There is, however, a distinct lack in cohesion for the way in which the O allele is expressed across species.
Alleles that have been preserved by balancing selection usually have a reason for those pressures to have been places upon them and one potential reason could be their interaction with host-pathogen relationships where they often protect the host against diseases such as malaria: infected cells can no longer stick to the red blood cells. The distribution of the phylogenetic expressions of ABO blood groups can give us clues as to how the pressures of balance selection actually arose.
In apes, antigens are expressed on the surfaces of red blood cells, the inner lining of arteries and on various epithelial tissues. However, Old World Monkeys do not have antigens on their red blood cells which suggest that this expression is insignificant and not the reason for these pressures. The overall evolutionary significance or specific mechanics of the pressures of balancing selection on ABO histo-blood groups are however still largely unknown; there are only theories as of yet.
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