Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 324

divide number 25 (just to give a number). So, in that assemblage of meristem
cells, there are 100,000 X 2 alleles (2 copies per cell) X 25 (the cell number)
= 5 million alleles at any point in time. Since the apical meristem cells are
continuously dividing to form a branch, suppose that 10,000 divisions of
those cells are needed to generate the next stem internode and axillary bud
with its own axillary bud apical meristem. This means that there are 5 10 (= (5
X 10 6 ) X 10,000) duplications of alleles to generate that bud lateral meristem.
Even if the mutation rate is 1 mutant allele in 10,000,000 allele divisions,
it is possible that several cells in the new lateral bud apical meristem are
carrying mutant alleles.
Now, suppose that axillary bud goes on to become 10% of the above-
ground volume of a plant, and thus produces 10% of its fl owers. Since a
meristem or developing cell with a new mutant allele must compete for
survival with surrounding cells, deleterious alleles may be eliminated,
but a low number of mutations may be benefi cial for continued stem-bud
growth and would be selected for. Large sectors of plants can harbor such
mutant alleles and may contribute those alleles to future generations. The
numbers here are both fanciful and, most likely, conservative, but they do
make the point that the chances of passing rare alleles via mutation alone to a
founding population do not necessarily approach 0. In fact, it would appear
that most plant individuals would carry numerous mutations. Such mutant
alleles could arise at any stage: source population maternal or paternal
contributor, or in the founding plant (e.g., in the developing seed to mature
individual). Note that these processes differ from animals that maintain one
continuous germ line: if a mutation occurs in one cell, it is swamped by the
thousands of wild-type germ-line cells that are continuously maintained.
In contrast, in plants, an apical meristem carrying a new mutant allele may
form a large sector of the plant, generating large numbers of fl owers that
may pass the allele along via pollen or seed (Klekowski 1988).
While mutations are rare in certain portions of genomes, rates can be
detectably elevated in other regions or when certain types of DNA are
involved. For example, tandem repetitive DNA (SSRs; e.g., microsatellites,
minisatellites) can mutate at rates detectable among offspring relative to
parents (e.g., Rogstad et al. 2003). Vaz et al. (in press) explored microsatellite
DNA allelic variation across 18 polymorphic loci in one Oryza glumaepatula
population in Brazil and found that 56% of the markers detected exhibited
frequencies of < 0.05, while Olsen et al. (2000) surveyed eight microsatellite
loci in salmon discovering an average of 23 alleles per locus. Such high
numbers of alleles at microsatellite loci are thought to arise via high
mutation rates. A number of organisms have now been studied in which
mutations via transposons arise at relatively high rates (Pritham 2009).
Mutations at microsatellites or via transposons have been found capable
of affecting functional genes. Functionally important genetic systems that