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Population Biology |
2School of Biological Sciences, Washington State University, Pullman, Washington 99164-4238 USA; 3Department of Biology, Boise State University, 1910 University Drive B, Boise, Idaho 83725 USA
Received for publication April 3, 2001. Accepted for publication November 6, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: allozyme variation • biotic invasion • Bromus tectorum • cheatgrass • genetic bottleneck • multiple introductions • Poaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Garcia et al., 1989
). In its simplest and least likely scenario, an invasion stems from the genetic sampling (sensu Lindroth, 1957
) of emigrants in a species' native range that encompasses all the species' genetic variation (Taylor and Walker, 1984
; Williams, Mack, and Black, 1995
). The organisms are then transported directly to a new range environmentally similar to their native range. These immigrants readily produce descendants that retain the genetic structure of native ancestral populations (Arthington and Mitchell, 1986
; Barrett, 1989
). Their proliferation is the direct result of preadaptation for the new range and the selection forces this preadaptation produces (Grant, 1963
).
Each step in the scenario outlined above can vary substantially. Emigrants usually stem from genetic sampling in a highly restricted area within the native range and thus may reflect only a portion of the species' genetic variation (Barrett and Richardson [1986
] and references therein). Additionally, any invasion may be the product of comingled descendants of separate introductions of immigrants; these founder populations likely differ in date of arrival, size, source area, and genetic structure (Burdon, Marshall, and Groves, 1980
; Novak, Mack, and Soltis, 1991
). Furthermore, the route to a new range may be a multistep odyssey in which the immigrants are not derived directly from the native range but from one or more other new ranges, i.e., spread occurs in a "stepping-stone" process (e.g., the spread of Lantana camara across Oceania; Thaman, 1974
). Finally, the invader's genetic variation and structure would likely reflect the permutations of genetic sampling and dispersal as well as the natural selection, genetic drift, and even mutation that have occurred along this circuitous journey as well as in the new range (Waddington, 1965
; Mayr, 1982
). As a result, any attempt to reconstruct the evolutionary history of an invader must identify and evaluate the potential for these confounding forces to have acted on the species in question.
Identifying and quantifying these confounding forces is facilitated by incorporating knowledge of the areal extent of an invader's native and introduced ranges and its introduction history (e.g., dates, locations, circumstances) and by comparing the genetic structure of populations from native and introduced ranges (Mascie-Taylor and Lasker, 1988
; Garcia et al., 1989
; Malacrida et al., 1998
). By identifying the same alleles in native and introduced ranges and correlating the geographical distribution of these alleles with introduction history, plausible scenarios can be constructed that explain the genetic consequences of the invader's introduction and subsequent spread. We have employed this approach in our ongoing reconstruction of the introduction and spread of Bromus tectorum L. in North America (Novak, Mack, and Soltis, 1991, 1993
; Novak and Mack, 1993
).
Bromus tectorum, a predominantly cleistogamous, diploid (2n = 14), annual grass, was introduced from its Eurasian native range into North America (Mack, 1981
; Novak, Mack, and Soltis, 1993
). The grass has had an exceptionally long association with humans in general and with nonirrigated crops, in particular (Hegi, 1931
). The grass's specific epiphet, tectorum, recognizes its common habitat in 18th century Europe, which was the thatched roofs of dwellings (Leopold, 1949
); it likely entered and spread through its new range as a seed contaminant (Mack, 1981
). In an era in which contamination or adulteration of crop seeds with weeds seeds was rampant (Mack, 1986
), its early arrival with pre-1800 European settlement along the North American coastline seems plausible. Yet its early collection history in eastern North America is puzzling. Wood (1863)
, whose study includes northeastern United States flora, lists B. tectoram L. (sic). But the most authoritative regional flora for the northeastern United States and adjacent Canada, Gray's Manual of Botany, does not list the grass until the sixth edition in 1889, although half a dozen other alien bromes were reported in its consecutive editions, which began in 1848. Little more heretofore had apparently been assembled about B. tectorum in pre-20th century eastern North America (Klemmedson and Smith, 1964
) that could be compared with the much more comprehensive knowledge of its entry and spread into western North America (Mack, 1981
; Yensen, 1981
).
Based on the 111 populations examined previously, genetic variation in B. tectorum appears low in both native and introduced ranges compared to other diploid seed plants (Novak, Mack, and Soltis, 1991
; Novak and Mack, 1993
). Across all introduced populations, only 28% of the loci are polymorphic compared to 52% in the native range (Novak, Mack, and Soltis, 1991
; Novak and Mack, 1993
). The level of diversity within populations is, however, higher in the introduced range on average, while genetic differentiation among both populations and regions is greater in the native range (Novak and Mack, 1993
). Significant deviations from Hardy-Weinberg equilibrium are apparent in both ranges. Novak and Mack (1993)
considered that the higher within-population component of genetic diversity in the introduced range could be the result of multiple introductions: a minimum of five separate founder events have been detected in western North America (Novak, Mack, and Soltis, 1993
).
These conclusions were drawn with little genetic information about the now widespread populations of B. tectorum east of the Mississippi River in the U.S. (Hitchcock and Chase, 1971
). Thus, the paucity of allozyme data from eastern populations has been a handicap in forming a comprehensive view of the species' genetic variation and structure in North America. Furthermore, lack of comprehensive collections from the easternmost USA hampers any resolution of whether (and to what extent) the enormous populations of B. tectorum in western North America are descended from populations that first became established along the Atlantic seaboard.
By employing both historical records and enzyme electrophoresis for populations of B. tectorum in eastern U.S., we attempted to answer the following questions: (1) Can the historical record, based on early floras, contemporaneous accounts, and most reliably, herbarium specimens, shed further light on a minimum date by when the grass had entered the eastern USA? (2) How does the level of variation compare between eastern U.S. populations and populations previously reported from elsewhere in North America and Eurasia? (3) What is the genetic structure among eastern U.S. populations compared with native Eurasian and other North American populations? (4) What genetic contribution (if any) did populations in the eastern USA make to what became the more abundant populations in the West? (5) Can source populations in Eurasia be identified for these eastern U.S. populations? With these data we attempt to establish the minimum number of introductions and infer the significance of founder effects for eastern U.S. populations of B. tectorum.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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earlier assigned populations in North America to four geographical regions: the USA east of the Rocky Mountains, Nevada-California, Intermountain West, and British Columbia. This initial analysis of B. tectorum east of the Rocky Mountains was based on only 14 widely separated populations, with only 4 of these populations (Atlantic City, New Jersey, USA; S. Philadelphia, Pennsylvania, USA; Tinicum, Pennsylvania, USA; and Kentucky, USA) actually located east of the Mississippi River (Novak, Mack, and Soltis, 1991, 1993
). Populations in the native range were assigned to two regions: Europe and Southwest Asia (Novak and Mack, 1993
). Geographical partitioning was designed to consider European settlement by humans in introduced ranges, to detect multiple introductions, and to compare variation within and among native compared with introduced ranges. Populations of B. tectorum included in this analysis were found in disturbed habitats near roadsides, railroad tracks, abandoned fields, and construction sites. The populations were located by either revisiting sites based on herbarium records or by new searches. Collecting along interstate highways was avoided because it is less likely that these populations accurately represent genotypes originally introduced in these locales. We made a deliberate attempt to collect in coastal areas because the grass may have entered at 18th- and 19th-century ports. Bromus tectorum was frequently found along or near the eastern coast, north from Virginia to southern Maine. However, the grass was not found along the coast of North Carolina, South Carolina, or Georgia; only inland populations were sampled from these states (Fig. 1).
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Enzyme electrophoresis
In the laboratory, one seed from each individual in a population was germinated on moistened filter paper in a petri dish and harvested approximately 5–7 d after germination. Starch gel electrophoresis was performed following Novak, Mack, and Soltis (1991)
. The following 15 enzymes were assayed: alcohol dehydrogenase (ADH), aldolase (ALD), glucose-6-phosphate dehydrogenase (G6PDH), glutamate dehydrogenase (GDH), glutamate oxalacetate transaminase (GOT), isocitrate dehydrogenase (IDH), leucine aminopeptidase (LAP), malate dehydrogenase (MDH), malic enzyme (ME), phosphoglucoisomerase (PGI), phosphoglucomutase (PGM), 6-phosphogluconate dehydrogenase (6PGD), shikimate dehydrogenase (SKDH), superoxide dismutase (SOD), and triosephosphate isomerase (TPI). Nomenclature for loci and alleles follows Novak, Mack, and Soltis (1991)
and Novak and Mack (1993)
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Data analysis
Electrophoresis data were analyzed using BIOSYS-1 (Swofford and Selander, 1987
) to obtain the same parameters reported in Novak, Mack, and Soltis (1991)
and Novak and Mack (1993)
. Computed estimates of the genetic variability for B. tectorum were the mean number of alleles per locus (A), the percent polymorphic loci per population (%P at the 99% criterion), and observed and expected mean heterozygosity (Hobs and Hexp, respectively). Observed heterozygosity was determined by the direct count method; expected heterozygosity was determined by the unbiased estimate method of Nei (1978)
, which adjusts for small sample sizes. The parameter, expected mean heterozygosity, is equivalent to expected genetic diversity.
Values for Wright's fixation index (F) at each polymorphic locus were analyzed using a chi-square test (Wright, 1965
). We calculated Nei's (1973, 1977)
gene diversity statistics from the variance components provided in the output of the Wright-78 analysis of BIOSYS-1. Total gene (allelic) diversity (HT) was partitioned into within- (HS) and among-population (DST) components, with the proportion of the total genetic diversity partitioned among populations calculated as GST = DST/HT. Nei's (1978)
unbiased genetic identity coefficients (I) were calculated for all population pairs. Values of I can range from 0.000 to 1.000, where a value of 1.000 indicates that a population pair is genetically identical. Two phenograms based on Nei's genetic identity values were generated using the unweighted pair-group method with arithmetic averaging (UPGMA) algorithm (Swofford and Selander, 1987
). The first phenogram was generated for 38 populations of B. tectorum from eastern USA and the second phenogram was generated for all 94 North American populations of B. tectorum analyzed to date.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Thus, our estimate of genetic variability in eastern U.S. populations of B. tectorum was based on the combined analysis of 1248 individuals from 38 populations (Fig. 1). Genetic variability of all 1248 individuals was assessed using results for the same 25 loci, as previously described for B. tectorum (Novak, Mack, and Soltis, 1991
). The mean number of individuals sampled per population was 32.8, a slightly smaller mean population size than that reported by Novak, Mack, and Soltis (1991)
. Four loci (16%) were polymorphic across all 38 eastern populations: Mdh-2, Mdh-3, Pgm-1, and Pgm-2 (Table 1). For each polymorphic population, both Mdh-2 and Mdh-3 displayed the same allele frequencies; this result was also observed for Pgm-1 and Pgm-2 (Table 1). One individual from Bryson City, North Carolina, USA, and 11 from Holland, Virginia, USA, expressed Mdh-2b and Mdh-3b (allele frequencies equal 0.033 and 0.275, respectively). Mdh-2a and Mdh-3a are fixed in all other populations. One individual from Bryson City, North Carolina, USA; four from Burnsville, North Carolina, USA; 20 from Kiptopeke, Virginia, USA; and all 35 individuals from Lexington, Kentucky, USA, displayed Pgm-1a and Pgm-2a (0.033, 0.211, 0.909, and 1.000, respectively). Pgm-1b and Pgm-2b are fixed in all other populations.
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unbiased genetic identity (I) coefficients are a measure of genetic similarity among populations. The mean value of I for 38 eastern U.S. populations of B. tectorum is 0.990, with values for individual population pairs ranging from 0.913 to 1.000 (data not shown). The lowest value of I for any pair-wise comparison (0.913) was observed for the populations from Holland, Virginia, USA, and Lexington, Kentucky, USA. Overall, these results indicate that eastern populations are very similar genetically. In fact, 33 of 38 populations (86.8%) are genetically identical (Table 1). The two UPGMA phenograms provide graphic representations of the overall genetic similarity among populations from the eastern USA and all North American populations of B. tectorum (Figs. 2 and 3). Clustering of populations from eastern USA was determined by differences in the frequency of alleles at Mdh-2, Mdh-3, Pgm-1, and Pgm-2 (Fig. 2). The three populations in the most genetically distinct cluster (Kiptopeke, Lexington, and Burnsville) all possess a high frequency of Pgm-1a and Pgm-2a (Table 1). The population from Holland, Virginia, USA, forms a separate cluster because it possesses moderately high frequencies of Mdh-2b and Mdh-3b (0.275 for both loci). The variation exhibited within the Bryson City population is so low that this population is considered genetically equivalent to the monomorphic populations (Fig. 2). Population clustering in the phenogram of all 94 North American populations of B. tectorum was determined by variation in the occurrence and frequency of alleles at several loci (Got-4, Mdh-2, Mdh-3, Pgi-2, Pgm-1, and Pgm-2) (Fig. 3). Population clusters in the phenogram exhibiting this variation are indicated by capital letters (for further explanation see the Population differentiation section of the Discussion).
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to occur in only two populations in the native range, Afghanistan-2 and Afghanistan-3. The same PGM alleles detected in Bryson City, Burnsville, Kiptopeke, and Lexington also occur in Vac, Hungary, Bratislava, Slovakia, Afghanistan-1 and Iran-1.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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. Although we cannot verify Muhlenberg's identification, he probably had the means to correctly identify the grass. A correctly labeled specimen of B. tectorum from Europe remains from his own herbarium, No. 241 (PH), the product of his receiving specimens from J. C. D. von Schreber, a German colleague (E. E. Spamer, Academy of Natural Sciences, personal communication). Presence of B. tectorum in the eastern USA for the next 70 yr is problematical. In Wood's (1863)
report of "B. tectoram L." (sic) he also contends that Torrey's (1843)
listing of B. sterilis for New York is the same species. But we are not aware whether the specimen(s) Torrey reputedly misidentified still exist(s). At least two hypotheses account for this lack of records of B. tectorum from the early 19th century. The first is that the species was present all along but simply overlooked. This hypothesis cannot be rejected, but eastern Pennsylvania, where Muhlenberg collected, was a center for plant collectors in the early 19th century. Yet neither Pursh (1814)
, a regional flora, nor Torrey (1819)
, a flora for New York City area (approximately 200 km away from Lancaster), nor Barton (1818)
, a Philadelphia flora, report the grass. Alternatively, the population(s) Muhlenberg saw were later extirpated and the species' detection later in the 19th century stems from reintroductions. Remarkable in this regard is the handwritten letter to an unknown recipient that accompanies the oldest specimen of B. tectorum in eastern North America that we have examined. The specimen was collected by Halliday Jackson in West Chester, Pennsylvania in 1859 (NY). We quote his letter.
West Chester June 13, 1859Respected Friend,The enclosed is a species of Bromus which I have found growing in this place. It seems to agree very much with B. ciliatus of Gray but not with B. purgans which I perceive is much synonymous with ciliatus and pubescens in thy work. My plant is not the B. ciliatus of Darlington's Flora (sic) Cestrica (?). The plant which he has described under this name appears to be the true B. pubescens. Dr. Darlington informs me that my plant differs from the B. ciliatus preserved in his herbarium.—If thou hast duplicate specimens of B. purgans. I should be much obliged for a specimen. The plant in question is very rare here—It appears to be a stranger and is confined to a single locality. It resembles very closely the B. tectorum of Europe, a dried specimen of which I have in my own herbarium...
Jackson was correct; the specimen is B. tectorum. Furthermore, his explicit reference to its rarity suggests (but of course does not demonstrate) that he had come across a recent introduction or even reintroduction of the grass into eastern Pennsylvania, approximately 60 km from the site of Muhlenberg's collection more than 70 yr earlier. Indirect support for the recent arrival of the grass is that Darlington's (1826)
flora for West Chester (Chester County, Pennsylvania), as referred to above by Jackson, contains no mention of B. tectorum. Whatever its pre-1859 history, B. tectorum was uncommon in the eastern U.S. Miller and Young reportedly claimed that their finding of B. tectorum at the eastern end of Long Island was for a species, "never before reported in this country" (Anonymous, 1874)
. Despite having already collected 871 species in their plant searches on Long Island, they had not detected B. tectorum until 1874 (Anonymous, 1874)
. Their contention that B. tectorum was new to the USA was in error, but their collecting diligence argues that B. tectorum had not long been established. Its rarity, if not absence, through much of the 19th century is further suggested by the aforementioned absence of any mention of the grass until 1889 within Gray's comprehensive Manual of Botany, despite the fact that half a dozen other alien bromes were reported in consecutive editions beginning in 1848. Whether the pre-1880 specimens of B. tectorum now housed at the Gray Herbarium (GH) (Table 7) were deposited long after their collection dates is not known.
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Genetic variability
Allozyme variation within the 38 eastern U.S. populations of Bromus tectorum we examined is extremely low: only four of 38 populations exhibit any variation (i.e., are polymorphic). Low levels of genetic variability have often been observed for introduced plants, especially those that possess a self-pollinating mating systems (Brown and Marshall, 1981
; Barrett and Richardson, 1986
; Barrett and Shore, 1989
; Barrett and Husband, 1990
). However, the level of genetic variation reported here is very low even when compared to other introduced self-pollinating plants (Marshall and Weiss, 1982
; Glover and Barrett, 1987
; Garcia et al., 1989
; Warwick, 1990
; Perez, Garcia, and Allard, 1991
; Neuffer, 1996
). Based on a summary of electrophoretic studies compiled by Hamrick and Godt (1990)
, the within-population variability of these 38 populations (A = 1.01, %P = 1.05%, and Hexp = 0.002) is low when compared to the mean value for all other plant species (A = 1.53, %P = 34.2%, and Hexp = 0.113); it is also low in comparison to other selfing plant species (A = 1.31, %P = 20.0%, and Hexp = 0.074).
Results from our analysis of additional eastern U.S. populations of B. tectorum are consistent with the estimate of genetic variation previously reported for eastern populations (Novak, Mack, and Soltis, 1991
). Equally low levels of genetic variation were detected within the 14 populations from east of the Rocky Mountains that were analyzed earlier (A = 1.01, %P = 1.14%, and Hexp = 0.003) and the 38 populations reported here (A = 1.01, %P = 1.05%, and Hexp = 0.002). The populations that were analyzed earlier were arrayed across the eastern two-thirds of the continent, as far west as Wyoming. In this study we sampled more populations, but across a much smaller geographical area. Yet, despite differences in the population sampling strategies employed, similar genetic variability estimates were obtained in each of these studies. For all 48 populations of B. tectorum from east of Rocky Mountains that have been analyzed to date (results of four populations are shared in the two data sets described above), A is 1.01, %P is 1.17%, and Hexp is 0.003 (data not shown).
Genetic variation within North American populations of B. tectorum exhibits strong regional differentiation. Among regions, eastern populations possess the lowest levels of genetic variation, populations from Nevada-California and the Intermountain West have intermediate values, and populations from British Columbia exhibit the highest levels (Table 3). In fact, the mean of genetic variation detected in eastern populations is the lowest of any region in either the native or introduced range of B. tectorum and most closely resembles the level of genetic variation observed in European populations (Novak and Mack, 1993
).
We did not detect any heterozygous individuals among the 1248 plants analyzed from the introduced range of B. tectorum in eastern USA (data not shown). This finding is consistent with our previous results: in an analysis of 2141 individuals collected throughout North America, we did not detect a single heterozygous individual (Novak, Mack, and Soltis, 1991
). Even in the native range of B. tectorum, heterozygous individuals have been rarely detected (Novak and Mack, 1993
). Thus, in comparison with other vascular plants, B. tectorum has an exceptionally high level of self-pollination (Schemske and Lande, 1985
). These results for B. tectorum provide empirical support for the theory suggesting that a selfing mating system is selected for in plants that experience population bottlenecks through repeated colonization of new habitats by few individuals (Stebbins, 1957
; Jain, 1976
; Lande and Schemske, 1985
; Holsinger, 1988
).
Population differentiation
The mean value of HT for the 38 eastern U.S. populations of B. tectorum (0.075) is much lower than the value reported for populations from the other three geographic regions (Table 5). However, the diversity that does occur in eastern populations is, on average, mostly partitioned among populations. The mean value of GST for all polymorphic loci is 0.560 and indicates that 56.0% of the total allelic diversity in eastern U.S. populations is distributed among populations, whereas 44.0% is distributed within populations. The distribution of allelic diversity in these 38 eastern U.S. populations is exactly the same (GST = 0.560) as the distribution previously reported by Novak, Mack, and Soltis (1991)
for 14 populations east of the Rocky Mountains (data not shown). In contrast to the populations from eastern USA, most of the genetic diversity of B. tectorum from the three western regions is partitioned within populations. In fact, the GST values for populations in the Intermountain West (0.241) and British Columbia (0.329) are one-half or almost one-half of that of eastern U.S. populations (0.560) (Table 4).
The mean genetic identity value for the 38 populations of B. tectorum analyzed in this study (I = 0.990) is consistent with the mean value previously reported for 60 North American populations (I = 0.980) (Novak, Mack, and Soltis, 1991
). These values indicate that the level of genetic similarity among all North American populations of B. tectorum is quite high. This similarity occurs because 43 of 94 (45.7%) North American populations from widely separated locations are fixed for the most common genotype (S. J. Novak and R. N. Mack, unpublished data). Moreover, the UPGMA phenogram of all North American populations of B. tectorum indicates that population clustering is not region specific. For instance, populations from Burnsville, North Carolina, USA; Kiptopeke, Virginia, USA; and Lexington, Kentucky, USA form cluster A with populations from Gunnison, Colorado, USA; Monte Creek, British Columbia, Canada; and Cache Creek, British Columbia, Canada. The population from Monte Creek, the most genetically distinct population in cluster A, was polymorphic at Pgm-1, Pgm-2, and Got-4 (Novak, 1990
), while the other populations in this cluster were monomorphic for Got-4b.
Source populations
By identifying the same geographically restricted alleles in both native and introduced populations of B. tectorum (shared alleles), we previously identified potential source populations in Eurasia and inferred the introduction dynamics of the grass into North America (Novak and Mack, 1993
; Novak, Mack, and Soltis, 1993
). Using this method, we identified several distinct populations or localities in the western Mediterranean, in central Europe, and in southwest (SW) Asia as potential sources of B. tectorum in North America. Results from the current study are generally consistent with our earlier findings. The alleles Pgm-1a and Pgm-2a are found in four populations from the native range in central Europe and SW Asia and four populations from eastern USA (Table 6). Additionally, these same alleles are also displayed in nine other North American populations: Norman, Oklahoma, USA; Gunnison, Colorado, USA; Tonasket, Washington, USA, and six populations in British Columbia (Novak, Mack, and Soltis, 1993
). Within the introduced range of B. tectorum, Mdh-2b and Mdh-3b were both detected in two populations from eastern USA (Table 6) and six populations from western North America (Novak, Mack, and Soltis, 1993
). These alleles were found in only two native range populations from SW Asia (Afghanistan-2 and Afghanistan-3), but not in Europe. Based on single-locus gene distributions, marker alleles introduced into the eastern USA appear to have been drawn either from central Europe, SW Asia, or both.
Introduction and spread of Bromus tectorum: allozyme evidence
Our initial analysis of B. tectorum from east of the Rocky Mountains (Novak, Mack, and Soltis, 1991, 1993
) was based on too few populations to assess accurately the grass' introduction and spread into the eastern USA. Results reported here now allow us to draw more reliable conclusions about the grass' regional introduction and spread. The Pgm-1a and Pgm-2a marker alleles occur in four eastern U.S. populations included in this analysis: Kiptopeke, Virginia, USA; Burnsville, North Carolina, USA; Bryson City, North Carolina, USA; and Lexington, Kentucky, USA. Also, we detected Mdh-2b and Mdh-3b in populations at Holland, Virginia, USA, and Bryson City, North Carolina, USA (Table 5). Consequently, eastern USA populations of B. tectorum appear to have arisen from a minimum of two separate introductions. Additional introductions may have taken place in the eastern USA but remain undetected because the immigrants and their descendants possess the same genotype. Marker alleles were detected in only 5 of 38 eastern U.S. populations; all other populations, including 25 populations found at localities extending from Maine to Delaware, are fixed for the most common genotype (S. J. Novak and R. N. Mack, unpublished data).
The repeated collection of B. tectorum in eastern North America after 1859 coupled with its subsequent collection almost four decades later in western North America (Mack, 1981
) suggests that the grass could have spread westward with European settlers. This collection chronology also provides no support for a west to east invasion history. Occurrence of this species in eastern and western North America may stem instead from separate introductions. Our data suggest that the introduction and spread of B. tectorum probably occurred via both routes. For instance, populations with the Pgm-1a and Pgm-2a marker alleles have been detected in 13 localities that span North America; these results suggest that this genotype may first have been introduced in the east and then spread westward. Alternatively, eastern and western North American populations with this genotype may be derived from separate introductions. Similar alternative scenarios may hold for the introduction and spread of Mdh-2b and Mdh-3b, although no populations with these alleles have been detected in the central USA (Table 3).
Other marker alleles have been found only in western North American populations and indicate that the introductions with these genotypes stemmed directly from the native range. For instance, all 38 eastern populations of B. tectorum included in this study are fixed for the allele Got-4b, whereas Got-4c occurs widely among populations of western North America. In western North America, 36 of 46 populations are polymorphic at Got-4; they possess both the b and c alleles. Six of 46 western populations are fixed for Got-4c. Similarly, populations with Got-4d and Pgi-2b alleles were detected only in western North America (Novak and Mack, 1993
; Novak, Mack, and Soltis, 1993
). Based on the allelic diversity among North American populations of B. tectorum, we conclude that eastern and western populations most probably stem in part from multiple separate introductions.
The introduction history of B. tectorum in North America emerges as a series of events and includes the possibility of its early extirpation, re-entry, and subsequent establishment in the eastern USA. Allozyme evidence reveals that there were multiple introductions, not only in western North America (Novak, Mack, and Soltis, 1991
) but also along the eastern coastline. Unlike the pattern in the west however, these immigrants in eastern North America gave rise eventually to a widely naturalized species but not an invasion (sensu Mack et al., 2000)
. Although commonly found throughout the eastern USA, cheatgrass is largely restricted to sites of frequent human disturbance (e.g., footpaths, rubble heaps, railroad beds) and coarse textured soil. Unlike its role in the arid west (Mack, 1981
), it has been unable to occupy pastoral areas, nor is it a major weed of crop fields (Muenscher, 1955
). Whether an invasion of this grass in eastern North America was averted simply through chance genetic sampling could be resolved through comparison of the performance in eastern North America of B. tectorum populations assembled from its high-latitude European range. This example with B. tectorum exemplifies an important issue in predicting the fate of introduced species in new ranges: the role of chance genetic sampling in predetermining the outcome. We need a much more comprehensive experimental resolution of this issue; it relates directly not only to our understanding of the limits of evolution (Antonovics, 1976
) but also to the efficacy of quarantine measures based on the plant species as the taxonomic level for scrutiny and control.
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FOOTNOTES |
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4 Author for reprint request (rmack{at}mail.wsu.edu
)
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LITERATURE CITED |
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