American Bison - Buffalo Field Campaign

cross, however, the latter is more common because it is very
difficult to compel domestic cattle bulls to mate with bison
females. All F1 generation hybrids experience reduced fertility
and viability relative to either parent: F1 males are typically
sterile, but the fertility of F1 females makes introgressive
hybridisation possible (Ward 2000). Genetic studies have found
no evidence of cattle Y-chromosome introgression in bison,
which is supported by the sterility of F1 hybrid males from the
cross of cattle males with bison females, and by the behavioural
constraint preventing domestic bulls from mating with female
bison (Ward 2000).
However, a number of studies using modern molecular genetic
technologies have reported both mtDNA and nuclear DNA
introgression in plains bison from domestic cattle. The first
of these studies (Polziehn et al.1995) found cattle mtDNA
among Custer State Park plains bison. Subsequently, more
comprehensive examinations of public bison herds revealed
cattle mtDNA in seven of 21 bison conservation herds
(Ward 2000; Ward et al. 1999), suggesting that hybridisation
issues between these two species were widespread and a
significant concern to long-term bison conservation efforts.
Further investigations based on high-resolution nuclear DNA
microsatellites detected domestic cattle nuclear DNA markers in
14 of these 21 U.S. federal conservation herds (Ward 2000).
All major public bison populations in the U.S. and Canada have
now been examined using mtDNA, microsatellite markers, or a
combination of these 2 technologies. Combining evidence from
both mtDNA and nuclear microsatellite markers with information
regarding population histories provides a more complete view
of hybridisation between the two species. To date, no genetic
evidence of domestic cattle introgression has been reported in 9
of these conservation populations (plains bison unless otherwise
noted; n = sample size examined): EINP (wood bison, n = 25);
MBS (wood bison, n = 36); WBNP (wood bison, n = 23); EINP
plains bison (n = 25); GTNP (n = 39); HMSP (n = 21); SHNGP (n
= 31); Wind Cave National Park (WCNP)(n = 352); and YNP (n =
520) (Halbert et al. 2005a; 2006; Ward et al. 1999).
However, the ability to detect nuclear microsatellite DNA
introgression is highly dependent on the number of bison in each
population, the number of bison sampled from each population
and the actual amount of domestic cattle genetic material
present in the population (Halbert et al. 2005a). Considering
statistical confidence (greater than 95%) allowed by detection
limits of the technology (Halbert et al. 2006), adequate numbers
of bison have been evaluated from only two of these herds
that displayed no evidence of hybridisation (WCNP and YNP).
These two herds represent less than 1.0% of the 420,000 plains
bison in North America today (Freese et al. 2007; Chapter 7)
and both of these herds are currently providing animals for the
establishment of new satellite herds for conservations efforts
(Chapter 7). Further evaluation is urgently needed to more
accurately assess levels of domestic cattle genetics in other
public bison herds.
Hybridisation issues with domestic cattle must be considered
along with other genetic and non-genetic factors in determining
which populations are designated as ‘conservation herds’.
For example, although some public herds are known to have
low levels of domestic cattle genetics, these herds may also
represent distinct lineages that reflect historical and geographic
differences in genetic diversity (Halbert 2003; Halbert and Derr
2006; Halbert and Derr submitted). Caution is needed in long-
term conservation planning to ensure that genetic diversity that
represents historical bison geographic
differences is identified and conserved
for all important populations and
not just those thought to be free
of domestic cattle introgression.
Nevertheless, defining genetic
histories that include hybridisation is a
first step in developing a species-wide
conservation management plan. Given
that there are several substantial bison
herds that appear to be free of cattle
gene introgression, it is of paramount
importance to maintain these herds
in reproductive isolation from herds
containing hybrids.
Plate 4.2 Custer State Park plains bison
bull; a high level of cattle gene introgression
is not morphologically evident. Photo:
Cormack Gates.
American Bison: Status Survey and Conservation Guidelines 2010 23