Genetic Variation in Bromus tectorum (Poaceae): Population Differentiation in Its North American Range

Abstract

Allelic variation in seedlings from 60 North American populations of the alien annual grass Bromus tectorum was determined at 25 loci using starch gel electrophoresis. Populations were collected from four regions; east of the Rocky Mountains, Nevada and California, the Intermountain West, and British Columbia. Compared to other diploid seed plants, genetic variation within these populations of B. tectorum is low: 4.60% of loci are polymorphic per population, with an average of 1.05 alleles per locus and a mean expected heterozygosity of 0.012. Although 2,141 individuals were analyzed, no heterozygous individuals were detected, and consequently, mean observed heterozygosity is 0.000. Extensive deviations from Hardy‐Weinberg expectations were observed at every polymorphic locus due to heterozygote deficiencies. The mean genetic identity (Nei's I) between population pairs was 0.980 and indicates a high level of overall genetic similarity among populations. The among‐population component of the total gene diversity is high (GST = 0.478), indicating substantial genetic differentiation among populations. These results are consistent with previous reports for highly self‐pollinating plants of low genetic variation and substantial genetic differentiation among populations. Despite the lack of genetic variation as measured by enzyme electrophoresis, this weedy grass has become exceedingly abundant in a diverse array of arid environments throughout much of western North America, perhaps due to phenotypic plasticity.

Genetic variation in Bromus tectorum (Poaceae): comparison between native and introduced populations

“…All scored enzyme bands migrated anodally; the 15 enzymes analysed were coded by the same 25 loci detected in North American populations (Novak et at., 1991). Across all native populations 13 variable loci were detected (Table 2): Adh, Got-I, Got-3, Got-4, Idh, Lap, Mdh-2, Mdh-3, Pgi-2, Pgm-1, Pgm-2, Skdh and Tpi-1.…”

“…As part of our general assessment of the evolution of B. tectorum since its arrival in North America (Novak, 1990;Novak et at., 1991;S. J. Novak et at., unpub-lished data), we investigated the level of genetic variation of this grass in its native range.…”

“…Seeds of B. tectorum were germinated and starch gel electrophoresis was conducted with seedlings as previously described (Novak et at., 1991). The same 15 enzymes employed with North American populations were used.…”

Seed Bank Depletion: The Key to Long-Term Downy Brome ( Bromus tectorum L.) Management

“…Owing to its origins, downy brome was introduced into western North America already adapted to the local conditions followed by a time of excessive livestock grazing on land that had not evolved with such (Mack 1981). These two factors provided the means for the rapid expansion to its current range in the western U.S. Downy brome's success in the western U.S. has been due to phenotypic plasticity and not the result of genetic variation (Novak et al 1991;Novak 1994). Annual environmental variation yields substantial variation in population characteristics such as recruitment, survivorship, and fecundity (Mack and Pyke 1983).…”

“…Downy brome has perfect flowers (Harper et al 1992) and is primarily cleistogamous , usually autogamous (Hulbert 1955;McKone 1985;Novak et al 1991) but can be xenogamous (Thill et al 1984;Young et al 1987). Young and Evans (1975) and Young et al (1987) suggest that downy brome's capacity to cross breed in response to changing environmental conditions allows it to maintain plant community dominance by selecting for those individuals or ecotypes that are highly competitive under those conditions.…”