Genetic diversity is a quite complex topic. I find all the questions
James Bach posed very interesting, but I would doubt that
anybody can answer them in exhausting detail without involving
speculation at some point - at least nowadays (with exception
of the first question). I will try to give you my point of view
why these questions can not be answered in a straightforward way.
> How does one measure genetic diversity, in layman's terms?
There are several ways (levels) to measure genetic diversity (GD).
The most detailed way is to look at the DNA sequence, i.e.
nucleotide diversity. Nucleotide diversity is defined
as the average proportion of nucleotide differences between all
possible pairs of sequences in the sample. Let say we
have 3 bees with a region of a DNA sequence reading:
1 2
NUCLEOTIDE # 123 456 789 012 345 678 90
bee 1: CAC TGC TAG CGA TCG TAC CT
bee 2: CAC TGC TAG TGG TCG TAG CT
bee 3: CAC TGC TAG TGA TCG TAT CT
* * *
(use e.g. Courier font to few)
These 3 sequences with 20 nucleotides (letters) each
differ at three positions (see *). There are
n(n-1)/2 pairwise comparisons, here, n=3, so there are
3 pairwise comparisons (bees 1-2, 1-3, 2-3). The number of
pairwise differences is 2 for site 10 and 12, and 3 for
site 18. So, the nucleotide diversity (nd) is
nd = (2*2 + 1*3 + 17*0)/(3*20) = 0.11667
(Maximum would be: nd = (3*20)/(3*20) = 1)
Important point:
- GD depends on the number of sequences (individuals) compared
Nowadays, there is not enough sequence information available
for the honey bee genome as well as for a really good sample
of bees.
AND:
What individuals would you count in bees? Well, one should only count
the reproductives i.e. queens and drones. But how many drones do you
count? How many of them will really mate i.e. contribute their genes
to the next generation? What about laying workers producing drones?
What about mating frequencies, mating distances, ... Which bees
belong to the same population?
Another critical point in the measurement of GD might be
whether and how you weight the different types of
DNA sequences, i.e. coding genes and non-coding sequences.
Usually there is much more variation in non-coding
sequences. And some organsims have few, and some very
many non-coding sequences.
Other levels of measuring GD include fragment length
polymorphisms of DNA sequences (detected with e.g. PCR),
allozyme electrophoresis, aso. These are not so laborious and costly,
but they more inaccurate in a that they do not detect all
differences and may overestimate others. These
polymorphisms are typically measured in the degree of
heterozygosity, which is (about) the proportion of
diploid individuals that have two different alleles.
E.g. in a survey of 104 genes in a sample including all major
human races yielded estimates of heterozygositiy of 0.06.
Note that this heavily depends on the genes investigated!
The highest average heterozygosities so far have been found in
drosophila (fruit flies) with 0.9 - 0.19.
> Dr. Sheppard determined that there are about 400 genetic lines of bees
Number depends on where you set your cut off limits for 'different'.
> How many genetic lines of honey bees do we need in the U.S to maintain or
> guarantee a desirable amount of genetic diversity?
Who knows? Depends on what our environment will ask for. It certainly
is reasonable to assume that reduced GD has negative effects in the long
term. But I never have seen a scientific study so far where it has been
claimed that e.g. below nd = 0.00001 a population dies. There are many
more factors involved. Again, we need to know a lot of other things
before we might be able to answer these GD questions.
--
Michael Haberl
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