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near-average or slightly above-average seed mass are more likely to be pests (weeds)
than those with very light or very heavy seeds. Some annual species with light seeds
are widely distributed and weedy, but some with heavier seeds are also. Genea and
Bromus species with heavy seeds may take advantage of animal and human activities
for seed dissemination. Bromus diandrus has rather heavy seeds with long stiff awns
and may have been transported into new areas by livestock, in wool, and as a seed and
forage contaminate (Murrumbidgee Catchment Management Authority 2008 ).
Bromus secalinus seeds are similar to those of winter cereal crops and were often
harvested, distributed, and eaten along with wheat and other grains by neolithic
Europeans (Behre 2008 ). The highly invasive species B. tectorum and B. rubens both
have fairly light seeds and long awns. This combination may facilitate dispersal in
rangelands and other open habitats (Sales 1994 ).
6.3.5 Ploidy Level (Hypothesis 6)
We hypothesized that polyploid species are more likely to be invasive (Hypothesis
6, Table 6.2 ). Our analyses showed that polyploidy is signifi cantly correlated
with wide introduction and with weediness for crop and ruderal weeds, but the
histograms show that this pattern is much stronger in perennial species (Table 6.3 )
(Fig. 6.5 ).
Bromus species may be diploid, polyploid, or both. In a number of species, some
individuals are diploid and other individuals have varying levels of polyploidy. The
species in section Ceratochloa are all polyploid with ploidy levels ranging from 6×
to 12×. South American species are generally hexaploid. Most North American spe-
cies are octaploid and may have developed the higher chromosome counts as a
result of hybridization with species in section Bromopsis (Stebbins 1981 ; Stebbins
and Tobgy 1944 ).
Diploid species may have smaller genomes that allow them to quickly complete
mitosis and meiosis, grow rapidly, and reproduce quickly (Bennett and Smith 1972 ).
Polyploid plants may have higher levels of variation and adapt better to new and
fl uctuating environmental conditions. In addition, polyploidy may affect plant
physiology and morphology, producing slower growth rates, more robust plants,
larger fl owers and seeds, delayed or prolonged reproduction, greater stress toler-
ance, and better winter survival (Rejmánek 1996 ; Monty et al. 2010 ; te Beest et al.
2012 ). This suggests that polyploidy could be especially valuable for perennial spe-
cies. Stebbins ( 1956 ) commented that diploid species are likely to be better adapted
to their original habitats, but polyploidy allows plants to better deal with novel
environmental conditions, especially when combined with hybridization. Roy
( 1990 ) suggested that the absence of a relationship between ploidy level and inva-
siveness in annual Bromus grasses might be related to the recent development of
both polyploidization and invasions.
S.Y. Atkinson and C.S. Brown