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Endosymbiosis | Origins of A1/A2 milk | Coevolution | Bibliography


Endosymbiosis & mitochondria | Endosymbiosis & chloroplasts

Most people are familiar with the idea that populations evolve as a result of natural selection.  One way of outlining this process is VISTA: There is variation between individuals in a population. Some of this variation can be inherited. Natural selection operates on this population, such that over time those individuals that are better adapted to the current environment are more likely to survive and reproduce, passing their genes to the next generation. The result is adaptation by that population to the current environment.

However, not all features of organisms have been shaped by natural selection.  Other signifcant processes are genetic drift and endosymbiosis.  Mitochondria and chloroplasts both originated as a result of endosymbiosis.

Endosymbiosis and mitochondria

Mitochondria have their own DNA, called mtDNA, which is separate from the nuclear DNA of the cell. This mtDNA codes for a small number of genes, including those for mitochondrial ribosomes, the tRNA molecules required for transcription, and some of the proteins involved in ATP production.  mtDNA is circular, like bacterial DNA, and shows some similarity to the genetic code of a group of purple bacteria known as proteobacteria. And like bacteria, mitochondria reproduce by splitting in two, in a process called binary fission.

All this suggests that mitochondria are descended from an ancestral purple bacterium that entered an endosymbiotic relationship with the ancestor of eukaryotic cells. A symbiotic relationship occurs when two different species live in direct contact with each other. An endosymbiotic relationship is when a smaller species actually lives inside a larger species. The endosymbiotic hypothesis suggests that mitochondria descended from a bacterium that, 1.7-2 billion years ago, somehow survived endocytosis 
by another cell, and became incorporated into the cytoplasm of the host cell.

The main advantage to the host cell was the ability of the symbiont to use oxygen in generating ATP through aerobic respiration. Before this, the host cell would have used much less efficient anaerobic processes, such as fermentation,
producing far less ATP. This increase in efficiency would have given the host cell an evolutionary advantage and would have increased the number of environments in which it could survive.

Endosymbiosis and chloroplasts

Chloroplasts show similarities in structure, chemical processes and genetic make-up to cyanobacteria.  Cyanobacteria are a distinct group of ancient bacteria that - around 3 billion years ago - evolved the ability to photosynthesise. In a similar process to the evolution of mitochondria, a cyanobacterium was engulfed by an ancestral eukaryotic cell which took advantage of the cyanobacterium’s ability to photosynthesise. Over time the cyanobacterium lost its ability to live independently and evolved into chloroplasts and the plastids present in plant cells.

Note that the evolution of aerobic photosynthesis was a significant event in the evolution of life on Earth. Aerobic photosynthesis releases oxygen as a by-product. Over time this changed the nature of the early atmosphere, from a reducing atmosphere to one relatively rich in oxygen. This change in conditions would have resulted in the extinction of many anaerobic life forms, but also made possible the evolution of organisms that respired aerobically.


Population genetics of A1 & A2 milk

The 2006 Scholarship Biology paper included a question on the genetics of A1 and A2 milk. It's worth revisiting this here, because it's a good example of patterns of inheritance and the way an allele may spread through a population. (We are not going to discuss the possible public-health implications of the two alleles.)

Beta casein is one of 3 types of casein, the most common protein in milk. In turn, beta casein comes in 2 forms: A1 & A2. These two forms are identical, with the exception of the 67th amino acid in the polypeptide chain. The A1 form has a histidine in this position, while A2 has a proline.

There are two alleles involved in the inheritance of A1 & A2 beta casein. A cow with the homozygous genotype A1A1 produces pure A1 milk, while A2A2 produces pure A2. An individual which is heterozygous for this gene, A1A2, will give milk that contains a mix of the two forms of beta casein. In other words, this is an example of incomplete dominance. Scientists feel that the A2 allele represents the original form of the beta casein gene.

In a Friesan-based dairy herd, around 30% of cows will be homozygous for the A2 allele, with 30% homozygous for A1 and 40% heterozygous.  The proportion of A2 homozygotes is higher in Jersey herds - but there is also a lot of variation between herds (Woodward, 2004).

If the A2 allele was, as scientists believe, the original form of the beta casein gene, how did the A1 allele arise and what explains its present distribution in the NZ dairy herd?

Since the A1 & A2 forms of beta casein differ by only a single amino acid, the A1 allele must have originated as a point mutation in the beta casein gene.  And this mutation must have occurred in the germ-line cells that produce gametes. or during gamete production itself, otherwise the A1 allele could not have entered the gene pool.

How would natural selection act on indivduals carriying this new allele? Because casein is expressed in the milk, then selection would work on the individual's offspring. If the A1 variant had an adverse effect on calves drinking it, those calves would be less likely to survive and reproduce - so their parent's genes would not be passed on. If it benefited the calves' health in some way, then the allele would be selected for (through enhanced survival and reproduction of the calves. A third possibility is that the mutation is neutral, in which case its frequency in the population could be affected by genetic drift, or through its being linked to another, beneficial gene. Farming practices (artificial selection) may also affect the allele's frequency. For example, mating is non-random, with much of the dairy herd inseminated artifically using semen from a relatively small number of bulls.  If some of these bulls carried the A1 allele, its frequency in the population's gene pool could increase relatively quickly.  So both natural and artifical selection may have led to evolution: a change in allele frequency in the gene pool of our dairy herds.

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Coevolution can be defined as "the interdependent evolution of two or more species [which have] an ecological relationship."  

Ruminants & microorganisms
The symbiotic relationship between ruminants and the microorganisms that inhabit their guts is an example of coevolution.  Gut symbioses are quite common among herbivorous animals, with symbiotic bacteria, fungi and protozoa providing the enzymes that their host lacks to digest its plant food. Thus ruminants (e.g. cows, sheep, deer), leaf-eating monkeys, herbivorous marsupials such as kangaroos and koalas, termites, and plant-eating lizards all have microbial symbionts in their digestive tracts (Saffo, 2001).

Both host (the ruminant) and endosymbiont community benefit from this ecological relationship. The host benefits from the microbes' ability to digest a wide variety of plant materials - including cellulose, hemicellulose, and starch - and produce volatile fatty acids and other energy-rich compounds. The microbes in turn are provided with a warm, moist, anaerobic environment - and a never-ending nutrient supply.

Cattle milk proteins & human lactase genes
Cattle were first domesticated by humans around 8,000 years ago.  A recent study examined the variation in cattle milk protein genes, lactose tolerance in modern humans, and stone-age cattle-farming sites (Beja-Pereira et al. 2003). The authors concluded that their data showed evidence of a 'gene-culture evolution between cattle and humans.'

Some human populations have the genetic makeup that allows them to digest the lactose in cows' milk - a feature that would have been selected for in populations with a heavy reliance on milk and milk products. It is found not only in Europeans (Beja-Pereira et al. 2003) but also in several distinct African populations (Gibbons, 2006).

In their study of lactose tolerance in Europeans, Beja-Pereira et al. found a high level of genetic diversity in Northern European native cattle. The distribution of this diversity was similar to the distribution of the allele that gives the ability to digetst lactose - and also with the distribution of Stone-Age cattle farms. Their explanation? 'Gene-culture' evolution between cattle & human culture. The ability to digest lactose opened up a new energy source to human populations with that ability. These populations kept larger dairy herds and actively selected for higher milk yields, changing the frequency of the milk protein genes in cattle - and this in turn would have affected the frequency of the lactase gene in humans. Gibbons notes that the same process led to the evolution of lactose tolerance in at least three different African groups in response to the domestication of dairy cattle.
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A. Beja-Pereira, G. Luikart, P.R. England, D.G. Bradley, O.C. Jann, G. Bertorelle, A.T. Chamberlain, T.P. Nunues, S. Metodiev, N. Ferrand & G. Erhardt (2003) Nature Genetics 35: 311-313

A. Gibbons (2006) There's more than one way to have your milk and drink it too. Science 314: 1672

N. Lane (2005) Power, sex, suicide: mitochondria and the meaning of life. Oxford University Press.
This is an excellent and very readable account of the evolution of mitochondria and the various roles they play in eukaryote cells.

M.B. Saffo (2001) Mutualistic symbioses in Encylopaedia of Life Sciences/www.els.net, Nature Publishing Group, Macmillan.

K. Woodward (2004) A2 milk and farmer decisions - retrieved as a pdf on 01 June 2008. This document discusses the role of selective breeding (artificial selection) in altering the gene pool of our dairy herd.

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