American Bison - Buffalo Field Campaign

5. Evaluate the overall genetic constitution of the herd by
measuring:
Unique variation (rare or private alleles) and levels of
heterozygosity in comparison to other bison herds
(Halbert 2003; Halbert et al. 2004; Wilson and Strobeck
1999);
Within-herd changes in heterozygosity and genetic
variation between generations (Halbert et al. 2004);
Current breeding structure of the herd (e.g. number of
males contributing to calf crop each year, relatedness
among calves, presence of genetic subpopulations);
Existing levels of domestic cattle introgression in both
the mitochondrial (Polziehn et al. 1995; Ward et al.
1999) and nuclear genomes (Halbert et al. 2005b).
Using the data collected from the above evaluations, informed
and sensible management plans can be implemented to best
fit the needs of the target herd. To further assist in this process,
demographic and genetic data can be used to model the
effects of various management alternatives prior to actually
implementing a definitive management plan (Gross et al. 2006;
Halbert et al. 2005a).
9.2.5 Transferring bison between herds
To maintain long-term herd health, it will be necessary in some
cases to transfer bison between herds (Table 9.2). The decision
to transfer bison between herds, however, must be made with
extreme caution with the following considerations:
1. Necessity of movement
Is there actual evidence of loss of genetic diversity or inbreeding
to necessitate the transfer? In bison and other mammalian
species, well intended but uninformed management decisions
to transfer individuals among isolated groups have resulted in
detrimental and irreversible effects, especially related to genetic
integrity and disease.
2. Domestic cattle introgression
As discussed in Chapter 4, few bison herds appear to be free
from domestic cattle introgression (Halbert 2003; Halbert et al.
2005b; Polziehn et al. 1995; Ward et al. 1999). Therefore, it is
essential to understand both the historic and genetic evidence of
domestic cattle introgression in the recipient and potential donor
herds before considering a transfer. If the two herds are related,
and especially if one is a satellite of the other, the total effect on
introgression levels due to transfer will be negligible. Care should
be taken to prevent the introduction of bison of unknown origin,
or questionable history, into conservation herds. Furthermore,
given our current levels of understanding, bison should not be
transferred into the few existing herds which appear to contain
no domestic cattle introgression, with the possible exception of
transfers between parental and satellite herds (Hedrick 2009).
90 American Bison: Status Survey and Conservation Guidelines 2010
Table 9.2 Additional factors to be evaluated when considering transfers
of bison between herds.
Number When possible, the number of imported bison
should be based on prior modelling estimates
to maximize improvements in heterozygosity
and genetic diversity while minimizing dilution
of the native bison germplasm.
Sex Importing a few new males into a herd can
have a large, positive and rapid genetic and
demographic impact. The same overall effects
can be obtained when importing females,
although the process will be somewhat slower.
In some cases, it may also be worthwhile to
consider any known genetic uniqueness of the
mitochondrial genome and Y chromosome.
For instance, prior to importing bison into the
Texas State bison herd, it was noted that this
herd contained a unique bison mitochondrial
haplotype not known to occur in other
bison herds (Ward 2000; Ward et al. 1999).
Therefore, importing males into this herd was
favoured over importing females, in part to
prevent dilution of the unique native bison
mitochondrial haplotype (Halbert et al. 2005a).
Age The most rapid infusion of germplasm will be
obtained by importing breeding-age animals.
It may be desirable to choose bison that have
already produced offspring to avoid potential
issues of sterility or offspring abnormalities.
Despite planning, genetic incompatibilities
between extant and imported bison may still
influence contributions of the imported bison
to the calf crop.
Quarantine Consider a quarantine of newly imported bison
prior to release, especially when the recipient
herd is at a high risk of extinction. This allows
for an easier adjustment of the imported bison
to their new environment, as well as early
detection and treatment/removal for latent
diseases.
Mating
regime
3. Relationship between herds
Given the observed genetic distinctions among extant bison
herds (Halbert 2003; Wilson and Strobeck 1999), dilution of
unique genetic characters (alleles) within the recipient herd
should be considered when evaluating potential donor herds
(Halbert et al. 2005a). Ideally, bison should be transferred
between satellite or related herds to reduce the loss of rare
variants.
Decide whether imported bison should have
exclusive mating privileges for one or more
years or compete with other potential breeders
for access to cows. “Exclusive” matings can
be used to increase genetic and demographic
impacts. A fully competitive mating regimen
permits extant bison to contribute to the gene
pool and provides some protection in case of
genetic incompatibility between the donor and
recipient herds.