Agriculture Reference
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Table 10.4. Genetic diversity of Solidago altissima in fields of several ages in central
New York
Field age a (yr)
Total ramets sampled
Number ofplots
Genotypes per 0.75 m 2 plot
1
30
14
1.5
5
244
3
9.3
20
131
3
3.3
35
165
3
2.0
Notes:
a Field age for the two older fields was approximate.
Source : Maddox et al. (1989).
once highly adapted genotypes have been created they will increase in relative
abundance by vegetative propagation.
Genetic variability following colonization events
Dispersal to a new location represents a genetic bottleneck for several
reasons.First,only a sample of the genetic diversity of the parental population
is transported to the new site. Second, some variability may be lost by chance
while the population is still small (genetic drift).Third,some variation may be
lost via selection in the new habitat. Several cases of substantially depleted
genetic variation, as indicated by a reduced frequency of isozyme polymor-
phisms, have been documented following long-range dispersal of weed
species. These include Striga asiatica in the southeastern USA (Werth, Riopel &
Gillespie, 1984), and Chondrilla juncea and Emex spinosa in Australia (Burdon,
Marshall & Groves, 1980; Marshall & Weiss, 1982). In the case of E. spinosa ,
although individual populations are apparently genetically uniform or nearly
so, variability exists among populations, probably due to separate introduc-
tions from the source region (Marshall & Weiss, 1982). Thus, this species has
potential for rapid increase in variability in Australia if populations spread
into sympatry and interbreed.
In contrast to the previous examples, some outbreeding species have
retained high genetic diversity even after transoceanic colonization. Notable
examples include Echium plantagineum in Australia (Brown & Burdon, 1983),
Apera spica-venti in Canada (Warwick, Thompson & Black, 1987), and Centaurea
solstitialis in the western USA (Sun, 1997).All three species show high levels of
isozyme heterozygosity. Highly heterozygous founders together with high
rates of outcrossing in these species quickly regenerate a diversity of geno-
types following colonization. The high genetic diversity in A. spica-venti led
 
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