Dioecy - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 213

as much seed is produced per female compared to bisexuals (e.g., see Lloyd
1982; Heilbuth et al. 2001). This potential greater loss of unique alleles in
dioecious populations that are otherwise identical to bisexual populations
adds to the list of possible disadvantages to be overcome in the evolution
of dioecy. Ways of overcoming this disadvantage might include increasing
the number of offspring produced per female plant, and/or increase the
dispersal distance of offspring.
In the monitoring of establishing dioecious populations, special
attention may thus need to be given to unique allele loss and increased
inbreeding, even though such populations appear to be expanding at rates
equivalent to comparable bisexual populations. In such cases, it may be
necessary to manipulate offspring (e.g., increase dispersal?) and/or increase
the numbers of founders.
Since dioecious species have individuals that differ as to gamete
type and whether they produce the diploid offspring, does the pattern of
placement of males versus females make a difference to either population
growth or genetic diversity retention? In Fig. 14.3, population genetic
diversity measures are depicted for populations with slight differences
in the initial central placement of 20 founders for each. In populations g
and l, the founders are placed in a hollow square, while in populations n
and o, the founders are placed in two lines of 10 each. All of the founders
are placed with no spaces between adjacent individuals, and males and
females alternate (and are juxtaposed in the lines). Maximum dispersal
of dispersules is 10 grid units for populations g and n, and 20 units for
populations l and o.
The growth patterns of the two populations with maximum dispersal
of 10 units were almost identical, as were the two populations with 20
units, although the latter two populations grew at a more rapid pace (data
not shown). Thus, whether founders are placed in the square or two-lines
confi guration makes little difference to population growth. Comparative
genetic diversity measures of these populations through 14 generations
are depicted in Fig. 14.3. Loss of heterozygosity (graph A) and F values
(graph B) are slightly decreased when founders are placed in two lines with
males and females juxtaposed (population n) rather than in a hollow square
(population g) when maximum dispersal is 10. This difference dependent
on founder geometry disappears with more distant average dispersal
(populations l versus o). In terms of unique alleles retained, placing the
founders in a hollow square leads to the greatest loss of unique alleles
under the more limited dispersal conditions; the other populations retain
similar amounts of alleles. These results suggest that, when reintroducing
a dioecious plant species, changing the geometric pattern of founder
placement may have a small infl uence on genetic diversity retention when
maximum dispersule dispersal is relatively limited.