Perennials - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 234

specifi c mortality rates. However, this decline in unique alleles retained is
reversed as members of the fi rst cohorts produced by the founders begin
to increasingly contribute offspring at each new round of mating, and thus
each new cohort subsequent to age 20 has retained a greater number of
alleles than the preceding cohort. As this trend of increasing numbers of
reproducers in each successive round of matings continues, the number
of unique alleles retained by each cohort increases towards a plateau at
about 2,000 alleles in each cohort. Note that this eventual increase of unique
alleles retained in each cohort is due to both (1) increasing numbers of
individuals in each cohort and (2) continual contribution of new unique
founder alleles (from the aging founders) that have not been earlier passed
to the successive cohorts. There is a degree of difference in the number
of unique alleles retained among these trials: in the most extreme cases
(o = lowest versus P = highest), there is an approximately 4.5% difference in
the alleles retained. If there are alleles of such low frequency at 5,000 genes
(of the 25,000 in Arabidopsis , say), this amounts to a difference of 225 unique
alleles between these most extreme trials. Subdivided populations near
borders yielding more individuals dispersed and dying outside the preserve
tend to have slightly lower retention on average than when founders are
all placed in a central square. However, if additional effort or damage to
the environment is required to introduce all of the founders centrally, then
planting at preserve corners may be a preferred option since unique allele
loss is not extremely pronounced in comparison.
A comparison of the results in Figs. 15.4 versus 15.8 shows that, under
the current conditions, the unique alleles retained by the population as a
whole approach stability at about 2,000 to 2,100 alleles, while the unique
alleles retained in cohorts also achieve stability at approximately 2,000
alleles (eventually these curves should converge). Both results highlight the
importance of maintaining the founders in the population as long as possible
to ensure that their reservoir of unique alleles is preserved in the ongoing
generations. If the population comes under stress, attention to preserving
founding individuals should be given fi rst. Founding individuals should
be mapped-marked to ensure that attention can be given if needed. Such
comparative trials can be used to explore under what conditions, and at
what generations, most of the unique alleles of the founders are present in
each newly produced cohort. Analyses of this type might also be informative
as to when and how new seed can be harvested to transfer the maximum
amount of genetic diversity to new sites.
These graphs raise interesting issues with regard to genetic diversity
retention and conservation in establishing populations. It is sometimes
the case that in newly developing populations, early catastrophic events
(e.g., drought, freezing, fi re, pathogen or herbivore increases) eliminate
or greatly reduce all individuals but the latest cohort to be produced