106
populations from Southwest Asia are in striking contrast; populations from this
region had an outcrossing rate of 25 % and a selfi ng rate of 75 %. This outcome prob-
ably arose because low-frequency alleles were only d etected in individuals with het-
erozygous genotypes in several populations in Southwest Asia (Novak and Mack
1993 ); this result increased the value of H (^) obs relative to that of H (^) exp.
The estimated outcrossing rates ( t ) published by Meyer et al. ( 2013 ) are remark-
ably similar to the rates reported for six of the seven regions listed in Table 4.2.
Meyer et al. ( 2013 ) estimated outcrossing rates for four “wild” populations of B.
tectorum from the Western USA, which ranged from 0.0027 to 0.0133 (i.e., 0.27 %
to 1.33 %, with selfi ng rates that ranged from 98.67 % to 99.73 %). Similar results
( t = 0.0082, or 0.82 %) were obtained with analysis of individuals grown in a com-
mon garden (Meyer et al. 2013 ). The values of H (^) obs reported in Table 4.2 are gener-
ally consistent with observed heterozygosity values previously reported for
B. tectorum by Ramakrishnan et al. ( 2006 ), Kao et al. ( 2008 ), Leger et al. ( 2009 ),
and Scott et al. ( 2010 ). In contrast, the high levels of observed heterozygosity
reported by Ashley and Longland ( 2007 , 2009 ) in two of four populations of B.
tectorum from Northern Nevada are inconsistent with the values reported by other
investigators. For instance, using seven microsatellite DNA loci, Ashley and
Longland ( 2009 ) report an observed heterozygosity value of 0.06 for the Peavine
Mountain population. This value is approximately 35- to 54-fold larger than values
reported by Meyer et al. ( 2013 ) for the Lower Peavine (0.0017) and Upper Peavine
(0.0011) populations, even though Meyer et al. ( 2013 ) used a more polymorphic
marker system , 91–93 single nucleotide polymorphisms (SNPs). This wide discrep-
ancy among these results for populations of B. tectorum from a similar locality
(Peavine Mountain) remains unresolved.
The preponderance of data indicates that B. tectorum re production is predominantly
through selfi ng (as reported by McKone 1985 ); outcrossing rarely occurs within
populations from NA. High selfi ng rates also appear to occur in European populations
Table 4.2 Percent outcrossing and selfi ng among populations of Bromus tectorum calculated
using the Coeffi cient of Inbreeding ( F ) method (Wright 1931; Dudash and Fenster 2001). Rates of
outcrossing are low for populations from most regions, with the exception of Southwestern Asia
Region
Number
of pops. H (^) exp H (^) obs F
t (^) f , outcrossing
rate (%)
t (^) s , selfi ng
rate (%)
Central USA a 60 0.014 0.00003 0.998 0.11 99.89
Mid-continent USA b 54 0.009 0.0002 0.978 1.12 98.88
California and
American Southwest c
60 0.007 0.000017 0.998 0.12 99.88
Eastern Canada d 16 0.013 0.0003 0.977 1.17 98.83
Western Canada d 44 0.012 0.00005 0.996 0.21 99.79
Europe e 39 0.006 0.0001 0.983 0.84 99.16
SW Asia e 12 0.005 0.002 0.600 25.00 75.00
H (^) exp is expected mean heterozygosity, and H (^) obs is mean observed heterozygosity.
a Huttanus et al. ( 2011 ); b Schachner et al. ( 2008 ); c Pawlak et al. ( 2015 ); d Valliant et al. ( 2007 );
e Novak and Mack ( 1993 ) as cited in Huttanus et al. ( 2011 )
S.J. Novak and R.N. Mack