Tuesday, May 31, 2011

Mutations: The Good, the Bad, and the Neutral (Part 1)

Chapter two of Dobzhansky’s Genetics and the Origin of Species is a summary of lessons learned surrounding the enigmatic ‘mutation’.  What is so fascinating about this is that, first of all, he addresses a number of misconceptions that the public still have about mutations today, and, secondly, he did this without knowing the mechanisms behind mutations.  For example, today we know that point mutations can occur in DNA.  This is when one of the bases in a DNA molecule gets swapped for another one.  Thus a C (the molecule cytosine) get could replaced with a T (thymine), which could potentially affect the appearance or behaviour of the organism.  In 1937, everyone knew that biological information was stored in the chromosomes.  It was known that chromosomes were somehow composed of genes (stuffed into the chromosomes like a sausage, according to Dobzhansky), and that sometimes parts of chromosomes could break off, switch around, go missing, get doubled.  But no one knew that genes were made of DNA.  No one knew that there were four 'letters' to DNA.  No one knew that mutations could occur in the DNA.  Yet Dobzhansky could still with full confidence talk about point mutations!  He knew they had to exist, even if he could not explain how.

Today I would like to give a snapshot of the lessons Dobzhansky learned when studying mutations.

Lesson one: Mutations are necessary for diversity

Diversity is one of the central problems that evolutionary theory is trying to explain.  Diversity recognizes that the world is full of different living things.  Everything is not a clone of everything else.  Why not? 

In 1911 a man named Johannsen said that, when we study diversity, we need to distinguish between two concepts: the genotype, which is the suite of heritable molecules (what today we know to be DNA) that is partially responsible for how an organism looks, and the phenotype, which is how the organism actually looks.  The phenotype is determined both by the genotype and by environmental effects.  For example, if the environment were to rip my arm off, that would change how I look, but my children would still be born with two arms.  The genotype codes for two arms, even if the environment has given me only one.  (Today we recognize that the environment-genotype distinction is not a perfect dichotomy; phenotypic plasticity muddies the waters).

In experiments, if we try very hard we can reduce the effect of the environment to almost zero.  Any difference in phenotype between any two organisms must therefore be purely genetic.  Carefully controlled inbreeding (easily done in plants) can further reduce genetic variation to almost zero.  So a combination of inbreeding and environmental controls can remove phenotypic variation.

And yet, despite this, in carefully controlled experiments, variation reasserts itself.

Jimson weed
In 1927 Blakeslee, Morrison, and Avery performed an experiment on Jimson weed.  They discovered that sometimes Jimson weed individuals would grow that had only half the number of chromosomes of their parents.  The parents had 24 chromosomes, whereas certain offspring had only 12.  Remember that chromosomes come in pairs, one from the mom and one from the dad; these offspring had to have inherited all twelve chromosomes from one parent, meaning they had no chromosome pairs.  These 12-chromosome Jimson weeds were called haploid (hap is Greek for one, ploid = fold) because they had only one chromosome set, whereas the parents, which had both chromosome sets, were diploid (‘twofold’).  

Top: diploid parental cell, containing two copies of the same chromosome (one in yellow,
the other in blue).  In gamete formation, the DNA in the chromosomes double (the X), and the chromosome
pairs align in the middle of the cell.  This cell is still diploid.  The pair then splits, moving to opposite ends of the cell,
and the cell divides, forming two haploid cells.  Finally, the doubled chromosome splits and moves to either
end of the cell, and the cell splits again, resulting in four haploid gametes.  This process is called meiosis, and
was known during Dobzhansky's time.

A normal diploid individual produces haploid gametes (sperm or eggs in animals, pollen or ova in plants).  The reason for this is simple: if gametes were diploid, when two gametes fused, the offspring would have twice as many chromosomes as the parent.  Indeed, this sometimes happens in nature, and is called polyploidy (which will be discussed much later).  Anyways, these haploid Jimson weeds sometimes produced normal gametes, each containing 12 chromosomes.  Since this is the same number of chromosomes as the haploid plant producing the gametes, each 12-chromosome gamete must be identical.

Question: what happens if you cross the pollen of a haploid Jimson weed with the ova of the same plant?  All of the diploid offspring should be identical, since the fusing gametes all contain the same genetic information.

But this was not what Blakeslee and others found.  Out of 173 offspring, 4 were different from the others.  Mutations had occurred in some of the haploid parent’s gametes, producing new phenotypic variation.

Apparently, mutations happen quite readily in nature. 

Even in a case of full-on inbreeding with no environmental variation, phenotypic variation reappeared.

Why study mutations? 

Because mutations must be the source of all diversity on this planet.

Says Dobzhansky:

‘The genotype possesses tremendous self-regulatory powers, and can withstand unchanged the impact of most environmental agencies.  Heredity is essentially a conservative force.  Evolution is possible only because heredity is counteracted by another force opposite in effect, namely, mutation.’

But aren’t mutations bad?  How could mutations be the source of differences between species, if selection acts to remove mutants?  Check back every day this week, as I post the other lessons Dobzhansky learned about mutations.

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