Truly Random Mating and Selfing Rate - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 167

founders have more grid spaces between them, lowering competition for
establishment, as long as matings, even if selfi ngs, can occur (population
q). Thus, population q grows more rapidly than population c, which has
founders in closest spacing (Fig. 11.3A) . A general rule for a mating by
selfi ng is that heterozygosity in the offspring is reduced by 50% relative
to the parental generation. This can be clearly seen for population q, for
which heterozygosity drops to approximately 50% (Fig. 11.3B). In the
next generation, selfi ngs are still common but not exclusive, so although
there is a substantial drop in heterozygosity, it does not quite approach
another 50% reduction. As individuals in population q become more
common and intermixed within crossing distances of one another, observed
heterozygosity in the q population increases, although not to levels
experienced by population c after 15 generations. The pattern of increased
F values in population q relative to c ( Fig. 11.4A) refl ects these effects of
initially exclusive, and later increasingly reduced, selfi ng in population q.
As shown in earlier chapters, increased spacing of founders can also lead
to more subdivision and local inbreeding, which also probably contribute
to increased F values for trial q at generation 15. Placing self-compatible
founders with 13 grid units between them greatly improves unique allele
retention: there is an approximately 30% increase in population q (Fig. 11.4B).
These results suggest that for at least partly self-compatible species, when
preserving allelic diversity is at a premium, wide initial spacing of founders
offers an improved strategy for unique allele conservation. In the fi rst
generations, increased selfi ng and increased localized population growth
due to lower spatial competition act to preserve more unique alleles, with
later mixing restoring more heterozygosity and lower inbreeding. For many
species with some degree of self-compatibility, it is thought that deleterious
alleles that contribute to inbreeding depression may have been purged to
various degrees since the increased production of deleterious homozygotes
results in increased removal of such alleles from populations by selection
(e.g., Thornhill 1993). For species for which such purging has been largely
successful in removing deleterious alleles, and for which maintaining high
heterozygosity continuously is not otherwise recommended, introducing
founders at distances that initially prevent outcrossing may be an improved
option for preserving genetic diversity under the given trial conditions.
These last results reinforce reproductive assurance hypotheses (for example,
leaky self-incompatibility; see Harder and Barrett 2006) from a slightly
different angle: species that maintain even high levels of self-incompatibility
will self occasionally, especially when no outcrossed offspring are produced;
this not only ensures that such individuals do produce offspring, but also,
in some cases, preserves allelic diversity.