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0 Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland
Received for publication January 19, 1999. Accepted for publication July 23, 1999.
ABSTRACT
Several alpine species have outlying populations in the lowlands and lower mountains north of the Alps. These small, isolated populations are usually described as either (1) glacial relics, (2) descendants from populations living on forelands and moraines during the ice ages, or (3) populations founded by long-distance dispersal after glaciation. A floristic survey of the historic and present distributions and an allozyme investigation were performed on one of these relic species, Saxifraga aizoides. The species was historically more abundant and had more stations in more regions of northeastern Switzerland. The former population structures within regions, nowadays destroyed, were still reflected in distinct and high regional genetic diversity and variation. There was weak evidence of increased inbreeding in outlying populations, but populations did not deviate from Hardy-Weinberg equilibrium. No geographic pattern of genetic variation above the regional scale (>10 km) was found. Based on the spatial and genetic structures found, it was not possible to discriminate between the abovementioned hypotheses. Nevertheless, the study shows how a thorough evaluation of distribution and abundance data aids the interpretation of genetic data with respect to population history, biogeography, and conservation biology.
Key Words: allozymes • Alps • biogeography • conservation biology • floristic survey • glacial relics • long-distance dispersal • Saxifraga aizoides • Saxifragaceae
Past events play an important role in the evolution of species and populations. Plant biogeography and phylogeography are especially informative of how the ice ages have shaped the genetic structure of species (Soltis et al., 1997
). Partial extinctions, recurrent (re-)immigration, different migration routes, long-distance dispersal events, bottleneck situations, in situ survival in refugia, or gentic drift are crucial phenomena in this context. In Europe, most molecular studies have so far focused on arctic-alpine plants or on tree species (Comes and Kadereit, 1998
). Available results on arctic-alpine plants are contradictionary, giving either evidence in favor of glacial refugia or against them (Abbott et al., 1995
; Brochmann et al., 1996
; Gabrielsen et al., 1997
; Bauert et al., 1998
). Smouse (1998)
stressed that contradictions between different studies even on the same organism may result from inaccurate sampling strategies or inappropriate choice of genetic markers to test particular biogeographic hypotheses.
In the mountainous Prealps and the hilly adjacent lowlands north of the Alps, several subalpine or alpine plant species have isolated, outlying populations at least 50 km away from their main alpine distribution areas, e.g., Linaria alpina, Pinus montana, Saxifraga aizoides, and Thesium alpinum (Bresinsky, 1965
). These prealpine populations are usually described as "glacial relics" (Christ, 1882), although they probably do not share a common history. The origin of these extant populations was debated at length in the first few decades of this century. The main biogeographic hypotheses that were proposed to explain their origin were: (1) they have descended from populations that occurred in ice-free, glacial refugia, or on nunataks (ice-free mountain peaks within the ice shield) and survived glaciation in situ, i.e., classical glacial relics; (2) they are descendants of populations that grew on forefields and moraines of glaciers during the ice ages (extreme periglacial situation) or remnant populations along the (re-)immigration routes of a species after the end of the last glaciation (literally, nonglacial relics); (3) they originated from long-distance dispersal after glaciation (Brockmann-Jerosch and Brockmannn-Jerosch, 1926
). These hypotheses were based mainly on knowledge of the maximum extent of the alpine glaciation, the climatic conditions during the ice ages, fossil records, and evidence from the present ecology and distribution patterns of plant species (Brockmann-Jerosch and Brockmannn-Jerosch, 1926
). Since that time, the debate on the occurrence of glacial relic species in the northern lowlands and Prealps and the occurrence of refugia in these areas has continued. However, molecular techniques now provide tools to test these hypotheses, and a study by Holderegger and Schneller (1994)
, examining allozyme variation and vegetation history, indicated that very small, isolated populations of the fern Asplenium septentrionale in the Swiss Plateau are not glacial relics, but were founded by independent long-distance dispersal after glaciation.
The evaluation of a rare species' population history is not an end in itself. It is related to some fundamental questions of conservation biology (Godt, Johnson, and Hamrick, 1996
). For instance, the concept of "old rare species" states that many species are naturally rare in a specific area, occurring in small, isolated populations (Huenneke, 1991
; Oostermeijer, Berholz, and Poschlod, 1996
). It is not yet possible to assess whether these species have specific biological features (e.g., with respect to pollination biology, breeding system, or genetic load; Holderegger, 1997a
). In contrast, "new rare species" are species that were formerly much more common in a particular area, and populations only became smaller, less abundant, and more isolated because of human influence (Huenneke, 1991
). However, it is possible that "old rare species" might not always have been as rare as they presently are. To correctly understand their status, we need to establish their distribution in former times by investigating the history of their populations using evidence from population genetics as well as from biogeography, ecology, and vegetation science (Pott, 1995
).
The present study investigated what is regarded as one of the classical glacial plant relics of the Swiss Plateau, i.e., the yellow mountain saxifrage, Saxifraga aizoides. The aims of the study were (1) to determine the species' former and present distributions and population sizes in northeastern Switzerland, (2) to estimate genetic variation and structure of all its present populations in this area using allozyme electrophoresis, and, (3) by using evidence from these data, to evaluate the species' population history and status as a glacial relic.
MATERIALS AND METHODS
The species
Saxifraga aizoides L. (Saxifragaceae) is a perennial, prostrate, and iteroparous arctic-alpine plant species that forms multiramet mats. The species' small narrow leaves are semisucculent and evergreen. Each raceme usually bears less than ten partly protandrous flowers of common Saxifraga type (Webb and Gornall, 1989
) with petals that vary in color from yellow to red according to genotype (Meier and Holderegger, 1998
). The diploid Saxifraga aizoides (2n = 26) is mainly outbred, although selfing can occur, but results in reduced seed set (Meier and Holderegger, 1998
). Dispersal may occur by seeds or vegetatively via detachment of ramets, which easily root again.
In the Alps, S. aizoides is widely distributed at higher elevations, but some outlying populations also occur in the Prealps and lowlands (Bresinsky, 1965
). The latter are often considered to be endangered (Landolt, 1991
). Habitats of Saxifraga aizoides are calcareous glacier forelands and moraines, banks of mountain brooks or rivers, damp or wet rocks, gravelly steep slopes, and landslides (Kaplan, 1995
).
Former and present distribution of S. aizoides
An area in northeastern Switzerland (Fig. 1) was chosen for the study, because most Swiss outlying populations of S. aizoides are found there (Welten and Sutter, 1982
), and because it is floristically well investigated.
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Explicit hypotheses on the origin of a particular population given in the literature were noted.
Population genetics
Samples of S. aizoides for allozyme electrophoresis were taken from all populations present in the regions of Kemptnertobel, Pfannenstil, Küsnachtertobel, Üetliberg, and Sihltal, from two populations from the Tösstal, where the species is still abundant, and from one population from the region of Arth-Goldau at the periphery of the species' main alpine distribution area nearest to the outlying populations in northeastern Switzerland (11 populations from seven regions in total; Tables 1, 2). Leaves from 30 individuals per population were sampled; however, because of its small size, only 24 individuals were sampled from population UE2 (Table 2). Thus, total sample size was 324.
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and Schneller and Scheffrahn (1989)
, respectively. Buffer systems number 5 and 7 and the Tris-HCl 4% PVP grinding buffer of Soltis et al. (1983)
were used. Eight enzyme systems showed clear resolution and congruent banding patterns. Staining schedules of the following enzymes (abbreviations according to Wendel and Weeden, 1989
) followed Soltis et al. (1983)
with slight changes: Glucose-6-phosphate isomerase (GPI; buffer number 5; 7 U (units) instead of 40 U of glucose-6-phosphate dehydrogenase); isocitrate dehydrogenase (IDH; number 5; incubation at 37°C instead of room temperature); malate dehydrogenase (MDH; number 5; incubation at 37°C instead of room temperature); phosphoglucomutase (PGM; number 5; 50 mg of glucose-1-phosphate instead of 200 mg, and 7 U instead of 40 U of glucose-6-phosphate dehydrogenase); shikimate dehydrogenase (SKD; number 5; 3 mg instead of 6 mg NADP). Staining recipes for aspartate aminotransferase (AAT; number 7), aconitate hydratase (ACO; number 7), and alcohol dehydrogenase (ADH; number 7) followed Wendel and Weeden (1989)
. Genotypes of the enzyme phenotypes found were determined using the information on isozyme number and quarternary structures given in Weeden and Wendel (1989)
. Ten well-resolved, polymorphic loci, Aat1, Aat2, Aco2, Adh2, Gpi2, Idh, Mdh1, Mdh2, Pgm2, and Skd1, were used in the analyses.
The following parameters were determined: percentage of polymorphic loci (P); number of alleles per locus (A); observed heterozygosity per locus (Ho); expected heterozygosity under Hardy-Weinberg expectations (He); Wright's fixation index (F; Wright, 1965
). Significant differences between observed and expected heterozygosities were checked with paired Wilcoxon tests. Significant deviations of fixation indices from zero were tested with Wilcoxon median tests. Additionally, Nei's unbiased genetic identities I and genetic distances D between populations were calculated (Nei, 1972
). The latter's correlation to geographic distances was tested using a Spearman correlation coefficient. Pairwise genetic distances were divided into comparisons (1) within a region, (2) within a landscape, (3) within the lowlands, the Prealps, or the periphery of the Alps, and (4) among the latter ones. These pairwise comparisons were analyzed by ANCOVA with genetic distance as main effect and geographic distance as covariate. Significant differences among the above four groups were tested with Bonferroni tests. Normal distributions of genetic and geographic distance were fullfilled according to Kolmogorov-Smirnov tests. An UPGMA (unweighted pair group method using arithmetic averages) clustering was performed for genetic identities between populations.
We used the parameter GST to estimate the amount of genetic variation among populations. Additionally, gene flow Nm was calculated using a formula corrected for population number (Godt, Johnson, and Hamrick, 1996
) with GST = [1/(4Nm
+ 1)], where = (n/n - 1)2 and n is the number of populations studied.
All genetic analyses were performed using BIOSYS-1 (Swofford and Selander, 1989
) and all statitistical tests using STATGRAPHICS (STSC, 1991
), following the procedures of Sokal and Rohlf (1995)
.
RESULTS
Former and present distribution of S. aizoides
Historic distribution of S. aizoides within the study area could easily be assessed, since its former occurrence in particular regions was usually mentioned in several references or indicated by serveral herbarium specimens. Nevertheless, it was not possible to extract quantitative information on the precise number of the species' former populations or its former abundances from the literature. Hence, only qualitative results on the decline of S. aizoides in the study area are presented.
The floristic survey led to the conclusion that the species was formerly more abundant (Table 1). The historic distribution of S. aizoides consisted of at least several populations in most regions. There has been an obvious decrease in the number of populations and probably also their sizes since 1900. This was especially the case for the most outlying populations of S. aizoides in the Swiss lowlands. Stations for S. aizoides were typically reduced to one or two remnants (Kemptnertobel, Küsnachtertobel, Üetliberg, Sihltal) or even none at all (Erlenbachertobel, Bachtel, Aeugsterberg). Hence, S. aizoides is no longer present in three regions, in which it occurred in 1900 (Table 1). Extant populations were usually very small (
100 individuals) and isolated from each other (Table 2; Fig. 1). Saxifraga aizoides grew on spatially limited, naturally open habitat patches, i.e., moist cliffs, tuffs, landslides, and steep erosion slopes (Table 2). The species was still abundant in the Arth-Goldau and Tösstal regions, although, in the latter, more peripheral populations have become extinct as well. A new prealpine station of S. aizoides was recorded within the study area (PF; Table 1).
Different explanations for the sources or origins of outlying populations of S. aizoides within the study area are found in the literature (Table 1). Foundings by ramet or seed dispersal by water were usually mentioned as causes for the establishment of populations along rivers or larger brooks originating in the Prealps or lower Alps (e.g., populations in regions Kemptnertobel and Sihltal; Table 1). On the other hand, populations in small ravines or on isolated hill or mountain slopes in the lowlands were usually described as glacial relics (e.g., populations in the regions of Küsnachtertobel, Aeugsterberg, and Üetliberg; Table 1). Nevertheless, founding of populations by rare, even recent long-distance dispersal was stated as well (e.g., populations in the regions of Bachtel and Tösstal; Table 1).
Population genetics
On average, 78.2% of the loci investigated were polymorphic, with a mean of 2.7 alleles per locus (Table 3). Observed heterozygosity (mean Ho = 0.41) was not significantly different from expected heterozygosity (mean He = 0.40), either within populations or in the whole data set (Table 3). Fixation indices, F, were significantly different from zero in two small populations (KU2 and UE2) and in the whole data set (Tables 2, 3). There was no significant correlation between genetic variation within populations (measured as HS = He; Table 3) and population size (Table 2; Spearman rank correlation coefficient rs = 0.47, P = 0.14). In most populations, genetic variation was high, but ranged between 0.24 and 0.50 (Table 3).
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0.01; Table 5). Pairwise Bonferroni tests showed that only genetic distances between populations within regions (group 1) and among the lowlands, Prealps, and Alps (group 4) were significantly different from each other. This again emphasized a closer genetic relationship of populations within regions, but a lack of pattern in genetic distances at a larger geographic scale.
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DISCUSSION
Decline of S. aizoides in the Swiss lowlands and Prealps
Our floristic survey showed that S. aizoides still occurred in seven of ten regions within the study area, where it was found a century ago. Hence, general distribution maps (e.g., Welten and Sutter, 1982
) would provide little evidence that the species is endangered. However, there has been a marked decrease in the number of populations and probably also local abundances in most regions. Formerly, there were several populations in almost all regions, except in Aeugsterberg and Erlenbachertobel, which had only one station each (Table 1). In the latter two, the species is now extinct. Suitable habitats for S. aizoides in the lowlands and Prealps north of the Alps are spatially restricted, even under natural environmental conditions (Bresinsky, 1965
), not allowing the maintenance of large local populations. Seeds of S. aizoides are small (Holderegger, 1998
) and thought to be easily dispersed by wind or water, although they lack specific dispersal structures (Stöcklin and Bäumler, 1996
). Additionally, dispersal of living ramets by water is sometimes reported (Jenny-Lips, 1948
). Large spatial gaps were present between the small remnant populations of S. aizoides within regions. However, genetic analysis showed that populations within the same region were genetically closer to each other than to populations in other regions (Fig. 2). This might reflect that S. aizoides formed sets of local populations connected by gene flow.
In the prealpine species S. mutata, which often grows sympatrically with S. aizoides, a similar decline in population abundances has been reported (Holderegger, 1997b
). This is attributed to the loss of suitable habitat patches in the lowlands and Prealps because of protective river management in valleys or ravines to prevent erosion and landslides on steep slopes. In the absence of erosion, suitable habitat patches for S. mutata are no longer created, and the species is eliminated by ongoing succession on artificially stabilized sites (Holderegger, 1997b
). The cessation of intensive woodland management at the beginning of this century, which helped to keep habitat patches open, possibly enhanced the above stabilizing effect (Bonn and Poschlod, 1998
). The same factors are probably also responsible for the decline of S. aizoides in the lowlands and Prealps.
Genetic diversity and variation in remnant populations of S. aizoides
Saxifraga aizoides had high genetic diversity (A, P) and variation within (He) and among populations (GST). There was no significant correlation between population size and genetic variation within populations of S. aizoides. This lack of correlation is not unusual in plants. It is reported for many rare species (Barrett and Kohn, 1991
; Ellstrand and Elam, 1993
; but see Godt, Johnson, and Hamrick, 1996
). Nevertheless, it was surprising that the small populations of S. aizoides showed no heterozygote deficiency. Fixation indices were only significantly positive (indicating inbreeding) in two very small populations of 40 individuals (Table 3). One of these populations, UE2, had the lowest percentage of polymorphic loci, the smallest mean number of alleles per locus, and the lowest observed and expected heterozygosities. On the other end, population AG, located at the periphery of the species' main alpine distribution area, exhibited the highest percentage of polymorphic loci as well as the highest number of alleles per locus (Table 3).
How can small, isolated populations of S. aizoides maintain high genetic variation and diversity? Saxifraga aizoides is known to be self-compatible, but outcrossing leads to much higher seed set than selfing (Meier and Holderegger, 1998
). Watkinson and Powell (1993)
showed in a computer simulation on Ranunculus repens, that, in species with clonal growth, a low level of sexual compared to asexual recruitment is sufficient to maintain genetic diversity within populations. It is possible that this was also true in the small populations of S. aizoides. We did not find a significant correlation between genetic variation and population size. Nevertheless, the relatively recent decline in local abundances and the positive fixation indices found in two populations suggest that it might be only a matter of time before negative consequences of isolation and small size will become apparent in remnant populations of S. aizoides. Theoretically, inbreeding will roughly increase at a rate of 1/2N in small, panmictic populations. Most mating events will become sib-matings over time, unless significant gene flow by pollen or seeds increases local gene pools. Our estimate of gene flow was low (Nm = 0.38), but Nm values only provide rough measurements of gene flow and should be interpreted with care (Whitlock and McCauley, 1999
). Formerly, gene flow could have been higher, when the abundances of local populations of S. aizoides within regions were higher too. Gene flow between local populations would have counteracted genetic erosion (Levin, 1995
). Only direct measurements of fitness in remnant populations of S. aizoides, e.g., seed set and germination after pollination experiments in populations of different sizes, would allow the evaluation of possible negative effects caused by enhanced inbreeding in this species. Meier and Holderegger (1998)
showed that seed germination is indeed lower in population SI than in populations of S. aizoides from the main alpine distribution area.
Population history of S. aizoides
Three scenarios for the origin of the isolated populations of S. aizoides in the lowlands or Prealps north of the Alps have been proposed: glacial relics, descendants from populations growing on large moraines and open forelands after the retreat of the glaciers, or foundings after long-distance dispersal from the Alps by seeds or ramets (Brockmann-Jerosch and Brockmann-Jerosch, 1926
).
These hypotheses are not mutually exclusive. Some of the present populations may be old refugial populations, others could have been periglacial populations, and yet others perhaps originated through long-distance dispersal. Long-distance dispersal might have affected genetic diversity in refugial and periglacial populations as well as playing a role in founding new ones. Additionally, genetic drift and changing gene flow patterns may have acted upon these populations over time. These processes could have obscured any genetic patterns that were originally there. Present patterns of genetic diversity and variation do not necessarily reflect the origin and age of the studied populations of S. aizoides. Thus, it is difficult to use the present data to trace biogeographic history.
In situ glacial survival of S. aizoides would only have been possible on ice-free tops of smaller mountains in the studied area. Principally, four regions meet these assumptions, namely the regions of Üetliberg, Bachtel, Tösstal, and Arth-Goldau (Hantke, 1978–1983
). Only small and/or distinct genetic pools are likely to have survived in these refugia. They might have served as sources for the colonization of other regions. If northeastern Switzerland was not swamped by massive immigration of alpine genotypes of S. aizoides (see Brochmann et al., 1996
) and in the absence of substantial genetic exchange between regions, one would expect to find a distinct genetic clustering. Refugial source populations and subsequently colonized populations should group together. No such pattern was found (Fig. 2). However, recent events (e.g., local extinctions, bottlenecks, genetic drift) in the histories of these populations could have led to divergence within and among regions, masking any genetic pattern.
Bresinsky (1965)
stressed that after the retreat of the glaciers some species that grew on forelands and moraines or were part of the adjacent, periglacial tundra vegetation (Hantke, 1978–1983
) could have survived on small suitable habitat patches in the lowlands or Prealps. As mentioned above, stochastic events, like enhanced genetic drift because of small population size, could wipe out almost any geographic genetic structure. This scenario matches the lack of regional geographic clustering found in the present study (Fig. 2). Nevertheless, one would intuitively not expect that small remnant populations could have been able to maintain relatively high genetic variation and diversity. In view of the formerly higher local abundances and the possibly interconnected population structure of S. aizoides, the high genetic diversity and variation still found in remnant populations are perhaps not unexpected. Additionally, higher gene flow could easily explain the closer relationship of populations within the same region.
Long-distance dispersal events as origins of the outlying populations in the Swiss Plateau have been advocated by Brockmann-Jerosch and Brockmann-Jerosch (1926)
. Effective long-distance dispersal of living ramets of S. aizoides by rivers has been reported several times. For instance, a single mat of this species was found growing on the banks of the Rhine near Basel, at least 100 km away from possible source populations (Jenny-Lips, 1948
). A similar origin was suggested for populations SI, KE1, and KE2. The most probable source regions for the latter two were the Bachtel, where S. aizoides is now extinct, or the Tösstal (Table 1). They were indeed genetically close to populations from the Tösstal region (Fig. 2). Population SI should have clustered near AG, its probable source population, but this was not the case. The Swiss lowlands and Prealps are probably one of the floristically best investigated areas of Europe. All populations of S. aizoides in the study area, except the newly discovered population PF, were listed in the older literature, and herbarium sheets were available for most of them as well. It seems improbable that population PF has been formerly overlooked. If recently founded, population PF must have been colonized by several long-distance dispersal events as it contains substantial genetic diversity (Table 3). A recent colonization from the nearest populations, KU1 and KU2, seems unlikely, since, genetically, they were not particularly close to population PF (Fig. 2). Long-distance dispersal of seeds from the Alps is believed to be facilitated by the strong alpine downhill wind (Holderegger and Schneller, 1994
).
It is virtually impossible to discriminate between the different scenarios for the origin of the remnant populations of S. aizoides. For population SI, growing on the banks of a small river originating in the lower Alps, colonization through water dispersal of seeds or ramets seems to be a realistic possibility. For population PF, recent long-distance dispersal is at least possible. However, for most remnant populations of S. aizoides, it cannot be concluded whether they are refugial populations, whether they are descendants from periglacial populations, or whether they were founded by historic long-distance dispersal.
Conclusions with respect to conservation biology
Our floristic survey showed that S. aizoides has always been a rare species in Central Europe north of its main alpine distribution area, although it was clearly more abundant 100 years ago. It should be regarded as an "old rare species" there (Huenneke, 1991
), irrespective of whether its outlying populations are refugial populations, remnants from formerly larger populations at the end of the last glaciation, or whether they originated from long-distance dispersal after glaciation. "Old rare species" might not be as much threatened by negative genetic consequences of isolation and small population size as "new rare species" are. The present study showed only weak evidence of increased inbreeding in outlying populations of S. aizoides. Nevertheless, many populations have been lost during this century and the species' regional population structures were destroyed only recently. It may thus be just a matter of time before the relatively high levels of genetic diversity and variation still found in the small populations of S. azoides in the lowlands or Prealps decrease due to increased or complete isolation. Because S. aizoides only colonizes open, almost vegetation-free habitats, landscape dynamics, including natural erosion, are necessary to maintain the species' remaining, biogeographically important populations and possibly to re-establish its former regional population structures (Holderegger, 1997b
).
FOOTNOTES
1 The authors thank Andreas Keel, Elias Landolt, and Hansruedi Wildermuth for providing present stations of Saxifraga aizoides, and Richard J. Abbott, Alex Bernhard, Robert M. M. Crawford, and two anonymous reviewers for critical comments on the manuscript. A grant for laboratory consumables by the Üetliberg-Verein to JJS and RH and financial support by the Swiss National Science Foundation to RH, while writing this paper, are greatly acknowledged. 
3 Current address: School of Environmental and Evolutionary Biology, Sir Harold Mitchell Building, University of St. Andrews, St. Andrews, Fife, KY16 9TH, Scotland, UK.
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