Biology Reference
In-Depth Information
product of initial hybrid American x Chinese lines that have been selected
for blight resistance, with successive generations of resistant progeny
then being generated by always backcrossing resistant plants to American
chestnuts. With enough rounds of such backcrossing, it is hoped that trees
with genomes composed of more than 99% American chestnut genes
that also carry the Chinese blight-resistance genes can be produced and
reintroduced. Research is also underway to identify the Chinese chestnut
blight resistance genes with the goal of introducing only those genes into
American chestnut stock via recombinant DNA technology.
If blight resistance can be conferred on American chestnut via these
or other means, such stock could then be reintroduced back to American
forests. However, certainly at fi rst, propagules of the stock will be expensive
to produce, and manual introduction will be costly. Given the above, what
are major considerations for re-establishing American chestnut populations
by the most economical means that will also preserve the greatest amount
of genetic diversity? For example, how many trees should be introduced at
any given site to best promote gene conservation? What patterns of planting
are optimal? Can subdivision of the founders improve gene retention?
NEWGARDEN was designed to examine these and other questions related
to the reintroduction of species.
Actual Data: the West Salem Site
The West Salem, Wisconsin, stand of American chestnut was planted circa
1880 at a farm site 600 km disjunct from the natural range of the species.
Owing to its isolation, the stand remained essentially free of the blight until
the 1990s, when blight appeared and began to spread rapidly. The stand
was initiated from 10 trees (all but one remains standing) and today there
are over 4,000 trees on 25 ha. This stand thus offers a unique opportunity to
examine the population genetics of re-establishment from a small number
of chestnut founders.
Such a study using highly variable DNA genetic tandem repeat markers
was conducted by Pierson et al. (2007). Trees in three separate plots were
sampled (see Pierson et al. 2007 for more details) and their genetic variation
was compared to the founding trees and among the plots. The main issues
examined that are relevant to our discussion here, and the fi ndings of the
study pertaining to each issue, are described in the following numbered
sections.
1. ISSUE: Have different originally polymorphic genetic markers been
fi xed or lost within the entire population or among the different
subpopulations because of drift occurring with such a low number
of founders? FINDING: No founding variable markers, including the
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