| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Systematics and Phytogeography |
2Department of Ecology and Evolutionary Biology, and The Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045 USA; 3Jardín de Aclimatación de la Orotava, Puerto de la Cruz, Tenerife, Canary Islands, Spain
Received for publication August 25, 2005. Accepted for publication February 2, 2006.
ABSTRACT
Plants endemic to oceanic islands represent some of the most unusual and rare taxa in the world. Enzyme electrophoresis was used to assess genetic diversity within and divergence among all endemic species of a small genus of plants on the Canary Islands. Our results show that the genus Tolpis is similar to many other island groups in having generally low allozyme divergence among species, with the highest divergence found among four groups of endemics. The two rare and highly localized species T. glabrescens and T. crassiuscula are each divergent from all other species in the Canaries. Tolpis coronopifolia is also divergent at allozyme loci; this is the only endemic species that is a self-compatible annual (or weak biennial). A large, morphologically variable species complex consisting of T. laciniata and T. lagopoda together with several named and unnamed morphological variants shows low allozyme divergence among its elements. The evolution of polyploidy from diploid ancestors in situ in oceanic archipelagos is uncommon, but the tetraploid T. glabrescens is an exception. Allozyme data do not implicate any extant diploid Tolpis species as parents of the polyploid. It is possible that T. glabrescens originated early in the evolution of Tolpis in the Canary Islands and that its parents are now extinct. The nonendemic T. barbata shows no greater divergence from the Canary Island endemics than some endemics exhibit among themselves. Both changes in allele frequencies and unique alleles are responsible for genetic divergence among species of Tolpis.
Key Words: allozymes • Asteraceae • Canary Islands • genetic diversity • Tolpis
Flowering plants endemic to oceanic islands represent some of the most spectacular yet endangered products of the evolutionary process (Carlquist, 1974
). Island plants have long fascinated biologists because of their divergent morphologies, and interest in using them as systems for studying plant evolution continues to grow (Stuessy and Ono, 1998; Emerson, 2002
). Initial molecular studies of island plants used enzyme electrophoresis to assess genetic diversity within and divergence among endemic congeneric species in an archipelago (e.g., Helenurm and Ganders, 1985
; Lowrey and Crawford, 1985
; Crawford et al., 1987a
; Borgen, 1996
; Francisco-Ortega et al., 1996
, 2000
; Garnatje et al., 1998
; Kim et al., 1999
). The general, and initially somewhat surprising, picture that has emerged from these allozyme studies is one of low diversity within species and, despite sometimes striking morphological and ecological divergence among congeners, high similarity (low divergence) compared to continental species. The explanation often advanced for high similarity at allozyme loci is that recent and rapid radiation in the insular setting following establishment of continental colonizers has not allowed adequate time for divergence at the neutral or near-neutral allozyme loci (Crawford et al., 1987b
; Crawford and Stuessy, 1997
). Endemics for which both allozyme and DNA data (sequences or restriction site mutations) are available, such as Argyranthemum (Francisco-Ortega et al., 1996
, 1997
) and Dendroseris (Crawford et al., 1987a
; Sang et al., 1994
), display low divergence with all markers. The high interfertility of species in different groups of island endemics, e.g., Tetramolopium (Lowrey, 1986
) and Argyranthemum (Borgen, 1976
; Humphries, 1976
; Brochmann, 1984
), indicates there has been insufficient time for the accumulation of genetic and chromosomal differences during and subsequent to radiation and speciation.
The genus Tolpis (Asteraceae: Lactuceae) consists of approximately 12 species distributed in Africa, Europe, and the islands of Macaronesia (Jarvis, 1980
; Park et al., 2001
; Moore et al., 2002
). In addition, there appear to be several undescribed species in Macaronesia (A. Santos-Guerra, personal observations). All but two of the species of Tolpis recognized by Jarvis (1980)
are endemic to Macaronesia, with six of them endemic to the Canarian archipelago. The species of Tolpis endemic to the Canaries and other Macaronesian islands do not differ from continental relatives or from each other by the spectacular differences in growth form and other features characteristic of some endemic lineages such as the silversword alliance in Hawaii, Echium in the Canary Islands (Carlquist, 1974
) and Dendroseris in the Robinson Crusoe Islands (Crawford et al., 1998
). However, Canarian Tolpis vary in a range of characters that usually remain relatively constant in other insular lineages. There is variation in life history, with both annual (or weak biennial) and perennial life forms represented (Jarvis, 1980
). There are differences in breeding system; some species are highly self-compatible while others are self-incompatible (Jarvis, 1980
; D. Crawford, unpublished data). Two ploidal levels occur in the Canary Island Tolpis, something that is relatively uncommon within insular lineages (Stuessy and Crawford, 1998
).
The endemic species of Tolpis in the Canaries are restricted to the five more mesic western islands of El Hierro, Gran Canaria, La Gomera, La Palma, and Tenerife (Fig. 1). All of the Canarian species recognized by Jarvis (1980)
, with the exception of T. laciniata, occur on Tenerife; of the endemics, only T. laciniata and T. lagopoda are present on the other islands. The molecular phylogenetic study of Park et al. (2001)
documented that the genus as circumscribed by Jarvis (1980)
is monophyletic (according to analyses of ndhF sequence data). In a subsequent study of cpDNA restriction site data, Moore et al. (2002)
provided further documentation of the monophyly of Tolpis sensu Jarvis (1980)
and also demonstrated that the species endemic to the Canaries form a monophyletic clade with the single species from the Cape Verde Islands (T. farinulosa). The six Canarian endemic species recognized by Jarvis (1980)
are T. coronopifolia (Desf.) Biv., T. crassiuscula Svent., T. glabrescens Kämmer;, T. laciniata (Sch. Bip. ex Webb and Berthel.) Webb, T. lagopoda C. Sm. ex Buch, and T. webbii Sch. Bip. ex Webb and Berthel. Two previously described species placed in synonymy under T. laciniata by Jarvis (1980)
are T. calderae Bolle and T. proustii Pitard in Pitard and Proust. One species, T. barbata (L.) Gaetn., occurs commonly on the same five islands as the endemic species but is also widely distributed on other Macaronesian archipelagos as well as in southern Europe and northern Africa (Jarvis, 1980
). Whether T. barbata is native or introduced to the Canaries has not been resolved (Moore et al., 2002
). Tolpis is similar to many other insular endemics in having very low molecular divergence among the species, and the low variation precluded resolving relationships among species in the Canary Islands (Moore et al., 2002
; R. K. Jansen, University of Texas, personal communication). Mort et al. (2003)
provided preliminary evidence of the potential of inter-simple sequence repeat (ISSR) markers for resolving relationships among the species, but the taxon sampling was quite limited, and few inferences could be made about relationships among species; that study is being expanded to include complete taxon sampling for Macaronesia (J. K. Archibald, D. J. Crawford, A. Santos-Guerra, and M. E. Mort, unpublished data).
|
observed that T. webbii is distinct in certain parts of its range but that it appears to intergrade with T. lagopoda where their ranges overlap (Jarvis, 1980
). Tolpis laciniata, T. lagopoda, the two segregate taxa T. calderae and T. proustii, the unnamed variants, and T. webbii will be referred to collectively as the T. laciniata-T. lagopoda complex in this paper. Another question is whether allozymes provide insights into the origin of T. glabrescens, the sole tetraploid species of Tolpis in the Canary Islands (2n = 36, the other species are diploids; Jarvis, 1980
). The origin of T. glabrescens is of interest because the origin of polyploids in situ on oceanic islands is rather rare (Stuessy and Crawford, 1998
). Jarvis (1980)
used morphological and cytogenetic data to hypothesize the origin of the tetraploid species, and we wished to test his hypothesis that T. crassiuscula is one of the parents of T. glabrescens. Additionally, we wanted to see whether species occurring on any of the so-called "paleo-islands" on Tenerife are divergent from elements growing elsewhere. Tenerife has a complex geological history; the three separate mountains of Adeje, Anaga, and Teno are the oldest areas (4–12 million yr; Thirlwall et al., 2000
; Guillou et al., 2004
), and subsequent volcanic activity one million years ago connected them to produce the current topography of the island (Fig. 1; Ancochea et al., 1990
). The nonendemic species T. barbata was included in order to determine whether it is more divergent from the endemics than the endemics are from one another. MATERIALS AND METHODS
Population samples
The localities of populations from which cypselas were collected are given in Table 1 and plotted in Fig. 1. Progeny from the cypselas were grown in the greenhouses at the University of Kansas and served as the sources of enzymes. The number of progeny examined is given for each population, although not every locus was resolved for every individual. Voucher specimens collected in the field are deposited in the herbarium of the Jardín de Aclimatación de la Orotava (ORT).
|
). The two buffer systems used with 12% starch gels were: (1) electrode buffer of 0.5 mol/L Tris, 0.65 mol/L boric acid, 10 mmol/L EDTA, pH 8.0, and this was diluted 1 : 9 for the gel buffer; and (2) electrode buffer of 40 mmol/L citric acid titrated to pH 6.1 with N-(3-aminopropyl)-morpholine, with the gel buffer at a 1 : 19 dilution. For polyacrylamide discontinuous gel electrophoresis, a 6.0% running gel (0.375 mol/L Tris-HCl, pH 8.9) and a 3% spacer gel (0.06 mol/L Tris-HCl, pH 6.7) were employed together with an electrode buffer of 0.005 mol/L Tris-0.038 mol/L glycine, pH 8.3 (Davis, 1964
).
Enzymes resolved in starch gels with the Tris-EDTA-borate buffer system were glucose-6-phosphate isomerase (GPI, EC 5.3.1.9), phosphoglucomutase (PGM, EC 5.4.2.2), and triose-phosphate isomerase (TPI, EC 5.3.1.1). Enzymes run with the morpholine-citrate buffer system were isocitrate dehydrogenase (IDH, NADP form, EC 1.1.1.42), malate dehydrogenase (MDH, EC 1.1.1.17), and phosphogluconate dehydrogenase (PGD, EC 1.1.1.44). Enzymes resolved in polyacrylamide gels were aspartate aminotransferase (AAT, EC 2.6.1.1) and glutamate dehydrogenase (GDH, EC 1.4.1.2). Staining protocols followed Wendel and Weeden (1989)
.
Analyses of data
The genetic bases of the banding patterns were inferred from several sources of evidence. The known active subunit compositions of the enzymes and the expected number of loci for each enzyme (Weeden and Wendel, 1989
), together with the observed variation in banding patterns within populations, were useful for inferring loci and alleles. In some instances, segregation patterns for the progeny of individual maternal plants were examined. Finally, patterns were examined in the synthetic F1 hybrids of plants with known banding patterns.
Popgene version 1.31 (Yeh and Boyle, 1997
) was used to calculate diversity within species (Hes) and populations (Hep) (Nei, 1978
). The proportion of diversity residing among populations of each species (Gst) was determined manually by averaging the population diversities for each species, subtracting this from the total diversity for each species, and then dividing the resulting value (Dst) by the total diversity (Nei, 1978
). The percentage of polymorphic loci (PP) was determined for each species. Nei's (1978) unbiased genetic identities were calculated for all pairwise comparisons of species.
RESULTS
Genetic interpretation of banding patterns
A total of 14 loci was included in the analyses: AAT (2), GDH (1), GPI (1), IDH (2), MDH (3), PGD (2), PGM (1), and TPI (2). Additional loci were detected for AAT, GPI, IDH, and PGM but were not scored because they were not well resolved and/or they were weakly expressed (IDH, GPI) or it was not possible to interpret the patterns genetically (PGM). The zone of activity scored for IDH consists of a three-banded pattern that does not segregate in hybrids. In some material, a second zone of faintly staining activity consisting of three bands was also observed. This suggests that there is a duplication for one and possibly two IDH loci. The more anodally migrating and presumably plastid forms of GPI (Gottlieb and Weeden, 1981
; Crawford et al., 1990
) appear to consist of three bands in all plants examined, and hybrids between plants with this pattern produce the same pattern, suggesting a duplication (D. Crawford, unpublished data). Resolution was not adequate for scoring and inclusion in the data set. A slower migrating zone of activity was present for PGM and individual plants of Tolpis have from one to three bands in this zone, indicating that at least two loci are being expressed. In other Asteraceae the slower migrating PGM isozymes are from the cytosol, and duplications (two loci expressed) for cytosolic PGM are known in other groups of Asteraceae (Gottlieb, 1987
; Crawford and Whitkus, 1988
).
Unique alleles
Tolpis barbata has a unique allele for Gdh and one for Tpi-1. Both alleles are monomorphic in three populations of T. barbata, but absent from the fourth population. Tolpis coronopifolia has a unique allele for Gdh in frequencies of 0.05, 0.06, and 0.10 in three of the five populations but it was not detected in the remaining two populations. Tolpis crassiuscula has a high-frequency (0.80), unique allele for Aat-2 in the one population examined. One population of T. webbii has very low-frequency (0.08), unique alleles at both Tpi-1 and Aat-1. Tolpis glabrescens has seven unique alleles, including one allele each for Gpi-2 (0.50), Pgm-1 (0.50), Tpi-2 (1.00), Pgd-1 (0.50), Aat-1 (0.50), and two alleles for Mdh-3 (0.50, each). All populations of T. laciniata from La Gomera have a unique allele for Tpi-1 (0.94), and the two populations comprising T. sp. nov. 3 (1975, 1987, Table 1) from southern Tenerife share the same high-frequency (0.53 and 1.00) unique allele for Pgd-1.
Genetic diversity-identity statistics
As shown in Table 2, Nei's (1978) total genetic diversity (Hes) within all species in the Canary Islands varies from 0.036 in the nonendemic T. barbata to 0.212 in T. lagopoda. The self-compatible endemic T. coronopifolia has lower total diversity than any other endemic species, but it is still nearly twice as high as T. barbata (Table 2). Several species, as well as morphological variants examined from only one or several populations, have genetic diversities approaching those found in T. laciniata and T. lagopoda (cf. Tables 1 and 2). Mean diversity within populations for species where two or more populations were sampled ranges from no diversity in populations of T. barbata to 0.142 for the two populations of T. sp. nov. 1. The single populations of T. calderae and the tetraploid T. glabrescens have higher diversities than the means for other species (Table 2). There is an exceedingly wide range of Gst values among those species for which more than one population was examined (0.090–0.931), with the two self-compatible species T. coronopifolia and T. barbata having much higher values than the largely self-incompatible species (Table 2). The percentage of polymorphic loci (PP) ranges from 21.43 to 78.57, with the two self-compatible species having the lowest values (Table 2).
|
|
Level and apportionment of species diversity
The review of deJoode and Wendel (1992)
revealed a mean species-level genetic diversity of 0.064 for oceanic endemics, and Crawford et al. (2001)
reported a nearly identical mean for flowering plants endemic to the Juan Fernandez Islands. Francisco-Ortega et al. (2000)
reviewed the literature for Canary Island endemics and found a considerably higher mean genetic diversity of 0.186, but with a wide range of values (0.000–0.456). The mean species genetic diversity for Canarian endemic Tolpis taxa (considering the segregates T. calderae and T. proustii and the T. sp. nov. populations as distinct taxa) is 0.144, which is closer to the mean for other Canary Island endemics than to Juan Fernandez plants (Crawford et al., 2001
) or to oceanic plants in general (deJoode and Wendel, 1992
). Francisco-Ortega et al. (2000)
discussed possible reasons for the higher diversity in the Canaries compared to many Pacific island plants and concluded that the factors responsible remain to be elucidated.
The most striking difference in allozyme diversity among Tolpis species is the lower diversity in the two self-compatible annual (or weakly biennial) species T. barbata and T. coronopifolia, generally with less than half of the diversity found in other species, all of which are perennial (some becoming quite woody at the base), self-incompatible plants (Jarvis, 1980
; D. Crawford, unpublished data). Data available on breeding systems and allozyme diversity for Canary Island plants as a whole are not compelling, but there does not appear to be a trend toward higher diversity in outcrossing as compared to selfing species (Francisco-Ortega et al., 2000
). Crawford et al. (2001)
likewise found no significant difference in levels of diversity between selfing and outcrossing species in the Juan Fernandez Islands. While results for Tolpis differ from Canary Island and Juan Fernandez endemics in general, they are similar to the results of Weller et al. (1996)
, who found much lower variation in autogamous than in outcrossing species of Schiedea and Alsinidendron in the Hawaiian Islands.
Self-compatible, and particularly autogamous, continental species typically have a much higher proportion of their total diversity residing among populations compared to outcrossing continental species. Hamrick and Godt (1997)
summarized results for endemic species showing that selfing species have about three times the interpopulational diversity of outcrossers. By contrast, Crawford et al. (2001)
did not find significant differences between the Gst values for selfing and outcrossing species in the Juan Fernandez archipelago, although there were limited data for selfers because of the lack of detectable allozyme variation in some species. The species included in the review of Canary Island endemics by Francisco-Ortega et al. (2000)
apparently did not include selfers. Plants of the Juan Fernandez and the Canaries show mean Gst values of 0.338 and 0.280, respectively, both of which are much higher than the means of 0.179 for outcrossing endemics and 0.174 for mixed mating endemics (Hamrick and Godt, 1997
). The results for insular endemics in general suggest that factors other than breeding system are influencing the distribution of genetic diversity within species. In the present study, Gst could be calculated for five species of Tolpis and two of the T. sp. nov. designations (Table 2), and they fall into two distinct groups. The two self-compatible and ostensibly highly selfing species, T. coronopifolia and T. barbata, have >80% of their diversity among populations (Gst = 0.834 and 0.931, respectively, Table 2). The high Gst values for the self-compatible species are largely a result of populations being monomorphic for different alleles at most of the loci that are polymorphic for the species. While the very high Gst values for the nonendemic species could, under the assumption that it is introduced, be the result of multiple introductions, this is clearly not the case for the endemic T. coronopifolia. The two widespread, variable, and predominantly self-incompatible species T. laciniata and T. lagopoda have roughly half as much total diversity among their populations as the self-compatible species. However, T. laciniata and T. lagopoda still have a higher percentage of among-population diversity than the mean for other Canary Island endemics (Francisco-Ortega et al., 2000
). The more geographically restricted T. webbii has slightly lower values than the more widespread species, and the two populations of the very local T. sp. nov. 1 have even lower values. Although the two small populations of T. sp. nov. 3 occur in the same localized area in the Adeje region (Fig. 1), they are isolated in two deep separate canyons, which may account for the high differentiation between them. A wide range of Gst values has been reported for congeneric species in the Canary Islands (e.g., Kim et al., 1999
; Batista et al., 2001
), but the large differences found between self-compatible and self-incompatible species of Tolpis appear to be the first example of the correlation between breeding system and the apportionment of genetic diversity for endemic congeners in the Canary Islands.
Morphological and allozyme divergence
In many island plant groups, extensive morphological and ecological divergence may occur with little or no divergence at allozyme loci (e.g., Helenurm and Ganders, 1985
; Lowrey and Crawford, 1985
; Francisco-Ortega et al., 1996
; Garnatje et al., 1998
; Kim et al., 1999
). This situation contrasts with the common (although not universal) situation for continental plants in which morphologically distinct species display reduced identities (Gottlieb, 1977
; Crawford, 1989
, 1990
). In Canary Island Tolpis, the four clearly distinguishable species T. barbata, T. coronopifolia, T. crassiuscula, T. glabrescens, and the T. laciniata-T. lagopoda complex are divergent at allozyme loci (Table 3). Thus, in contrast to some island groups, allozyme and morphological divergence have occurred in concert during the radiation of Tolpis in the Canary Islands.
Within the T. laciniata-T. lagopoda complex, however, lack of allozyme divergence cannot be used as strong evidence against recognition of additional taxa because, as indicated earlier, ecological and morphological divergence may occur without allozyme divergence in island plants. Consider the first two segregates of T. laciniata. Plants that have been segregated from T. laciniata as T. proustii (differing in features of the leaves, capitula, and flowering axes) have slightly reduced identities with other members of the complex (Table 3) except they have a high mean identity of 0.986 with populations of T. laciniata occurring on the same island of El Hierro. In contrast, T. calderae (from the type locality on La Palma) has been segregated from T. laciniata based on several morphological characters, but it does not have consistently lower identities with other members of the T. laciniata-T. lagopoda complex (Table 3).
There are several morphological variants within the T. laciniata-T. lagopoda complex that may be worthy of taxonomic recognition (A. Santos-Guerra, personal observation). Populations of T. laciniata growing on the island of La Gomera are morphologically distinguishable (A. Santos-Guerra, personal observations), have slightly lower identities (mean of 0.903) with all other members of the complex and have a unique allele in high frequency. Further study may support taxonomic recognition for these populations. The two populations designated T. sp. nov. 3 consist of large plants with highly branched stems and extensively branched inflorescences bearing large capitula. Each population grows in a different isolated canyon in the Adeje region of southern Tenerife (Fig. 1). The two populations share a unique high frequency allele at one allozyme locus. Adeje is one of the "paleo-islands" of Tenerife and is rich in endemics (Trusty et al., 2005
). Population 1984 (T. sp. nov. 4) grows in an isolated canyon in southeastern Tenerife (Fig. 1). This population may eventually prove to be worthy of taxonomic recognition, although allozymes show no evidence of divergence from other elements in the complex. Similarly, both T. sp. nov. 1 from La Palma and T. sp. nov. 2 from Tenerife are recognizable morphologically (A. Santos-Guerra, personal observations), but neither has reduced identities with other elements of the T. laciniata-T. lagopoda complex (Table 3).
Regardless of how the T. laciniata-T. lagopoda complex is ultimately treated taxonomically, the high similarity at allozyme loci and our field observations suggest that T. laciniata and T. lagopoda as now recognized are not tenable. Separating them by "markedly leafy" flowering axis vs. "not markedly leafy" (Jarvis, 1980
) is not reliable because the characters intergrade at the intra- and interpopulational levels.
Tolpis crassiusculaand the origin of the tetraploid T. glabrescens
The morphologically distinct and rare species T. crassiuscula and T. glabrescens occur in two of the three "paleo-islands" on Tenerife. Although Bramwell and Bramwell (1974
, 1990
) considered T. crassiuscula to be present in both Teno and the third "paleo-island" of Adeje (Fig. 1), Jarvis (1980)
concluded that the species is restricted to Teno and considered the Adeje plants from the Barranco del Infierno to be more similar genetically to T. lagopoda. As mentioned, our population 1987 (T. sp. nov. 3) is from the Barranco del Infierno and population 1975 (also T. sp. nov. 3) is from another barranco in the Adeje region (Fig. 1). Our results show clear allozyme divergence between T. crassiuscula from Teno and T. sp. nov. 3.
The tetraploid T. glabrescens is an extremely rare species endemic to Anaga on the northeastern corner of Tenerife (Fig. 1), where it is known from several small populations. Jarvis (1980)
, on the basis of morphology and limited biosystematic data, suggested that T. crassiuscula could be one of the diploid parents of T. glabrescens. Keeping in mind the small sample size, one of the unexpected results of the present study is the high divergence between T. glabrescens and all endemic diploid species, including the presence of several high-frequency unique alleles in T. glabrescens. As indicated earlier, the endemic diploids and T. glabrescens occur together in a strongly supported monophyletic group (Moore et al., 2002
); therefore, the tetraploid must have originated in situ in the archipelago from diploid ancestors. This being the case, T. glabrescens should share alleles with one of the extant diploid species if it is an autotetraploid or combine alleles from each of its two parental species if it is an allotetraploid, assuming that the two species are divergent at one or more loci and that they are extant in the archipelago. Neither of these allozyme patterns is seen in T. glabrescens. Allozyme data provide no support for the Jarvis (1980)
hypothesis that T. crassiuscula is one of the parents of T. glabrescens.
It is possible that the diploid ancestor(s) of T. glabrescens is now extinct, and thus some of the alleles now seen in T. glabrescens are "orphan" alleles (Werth, 1989
). Tolpis glabrescens is restricted to a small area in the "laurisilva" forest of Anaga in volcanic rocks four million years in age (Thirlwall et al., 2000
; Guillou et al., 2004
). The habitat and substrate for T. glabrescens led Jarvis (1980
, p. 94) to suggest "that it is not a particularly recently-evolved species." Our results indicate that he may be correct.
The nonendemic Tolpis barbata
Moore et al. (2002)
demonstrated that the two continental species T. barbata and T. virgata are sister taxa and in turn are sister to a clade containing the Canary Island endemics and the single species endemic to the Cape Verde Islands. However, their results were inconclusive on whether T. barbata is native to the Canary Islands or was introduced, and the results of the present study also are not informative on this question.
The genetic identities at allozyme loci between T. barbata and the endemics are similar to identities between the endemics, although slightly lower (Table 1). Similar allozyme divergence between T. barbata and the endemic species would be expected if T. barbata were native and had evolved in the archipelago. However, a similar pattern of allozyme divergence would also be expected if, following divergence from the common ancestor of the continental clade (T. barbata and T. virgata) and the Canary Island-Cape Verde clade, allozyme divergence accumulated within and between each of the clades at similar rates, with subsequent introduction(s) of T. barbata into the Canaries and other Macaronesian archipelagos.
Processes generating allozyme divergence
Divergence at allozyme loci may be generated by one or more processes. Allele frequencies may be altered by stochastic factors such as founder events and drift in small populations, and these could be especially significant in oceanic island endemics because of their typically small populations. In addition, selfing and biparental inbreeding in small populations could alter allele frequencies. Mutation is another process generating allozyme divergence, although presumably a slower process than drift or founder events (Witter and Carr, 1988
). The lowered identities of T. glabrescens with other extant species are largely a result of species-specific alleles, regardless of their origin, and not differences in allele frequencies with other species.
The lowered identities between the annual self-compatible T. coronopifolia and the perennial largely self-incompatible diploid endemics are primarily the result of fixation of alleles in T. coronopifolia that are polymorphic and in relatively low frequencies in the other taxa. The fixation of alternative alleles may also be seen at the population level in T. coronopifolia, and this results in genetic identities ranging from 0.861 to 0.999 among populations. The origin of self-compatibility in T. coronopifolia appears to have facilitated the fixation of alleles, both within the species and within populations of the species. Weller et al. (1996)
noted that self-compatible species of Schiedea in Hawaii have lower identities than self-incompatible species. The evolution of self-compatibility in T. coronopifolia has ostensibly facilitated the rapid fixation of several floral features associated with the self-compatible breeding system, and this topic will be considered in detail in another paper (D. Crawford, unpublished data).
Tolpis crassiuscula is the self-incompatible diploid endemic that is the most divergent from other diploid endemics. Its lowered identities with other species result in a large part from the presence of a high-frequency unique allele at one locus and the fixation at a second locus of an allele that is rare in other diploid endemics. Tolpis crassiuscula occurs in several small populations on substrate dated to about six million years of age (Thirlwall et al., 2000
; Guillou et al., 2004
), and some individuals are not strictly self-incompatible, with up to 20% seed set when selfed (Jarvis, 1980
; D. Crawford, unpublished data). Drift and inbreeding in the small populations together with long-time isolation on the relatively ancient substrate of Teno could have worked in concert to promote allozyme divergence.
Weller et al. (1996)
observed that allozyme divergence among outcrossing species of Schiedea occurring in large populations in Hawaii is low regardless of how distantly related they are judged to be from other data. This is presumably because the maintenance of variability in the large, outcrossing populations retards divergence via large changes in gene frequencies, including the fixation of alternative alleles. The relative lack of divergence within the T. laciniata-T. lagopoda complex may be attributable, at least in part, to the many large populations of predominately self-incompatible plants comprising this group.
Allozyme variation and divergence in Tolpis: similarities and differences with other insular endemics
Tolpis in the Canary Islands is generally similar to other insular endemics in having low genetic diversity within species, a relatively high proportion of the diversity among populations, and low divergence among species. Tolpis differs from some other endemics in the lower genetic diversity within and higher proportion of diversity among populations of self-compatible species compared to self-incompatible species.
Divergence at allozyme loci in island plants does not invariably correspond to the taxonomy of the groups based on morphology. Notable examples include Schiedea and Alsinidendron (Weller et al., 1996
), Argyrathemum (Francisco-Ortega et al., 1996
), Lobularia (Borgen, 1996
), Cheirolophus (Garnatje et al., 1998
), Sonchus (Kim et al., 1999
), and Cistus (Batista et al., 2001
). Nonconcordance between morphology and allozyme divergence is often attributed to the sorting of ancestral allele polymorphisms by stochastic processes such as bottlenecks associated with the founding of new populations. In Tolpis, correspondence between allozyme and morphological divergence for the three species T. barbata, T. coronopifolia, and T. crassiuscula is ostensibly the result of drift and inbreeding in small populations. By contrast, the large outcrossing populations characteristic of the T. laciniata-T. lagopoda complex may retard allozyme divergence, as was suggested for Hawaiian Schiedea and Alsinidendron (Weller et al., 1996
).
Although several notable island radiations have been at the polyploid level (e.g., Bidens, Helenurm and Ganders, 1985
; Dendroseris, Crawford et al., 1998
; Robinsonia, Crawford et al., 1998
; the silversword alliance, Carr and Kyhos, 1981
, 1986
; Witter and Carr, 1988
; Barrier et al., 1999
), the origin of a polyploid species such as T. glabrescens during the radiation of a lineage in an archipelago appears to be a relatively rare occurrence (Stuessy and Crawford, 1998
). The situation for T. glabrescens is especially intriguing because allozymes offer no clues about its diploid ancestors, and the possibility that its origin predates that of the extant diploid species cannot be dismissed.
FOOTNOTES
1 This research was supported by the Department of EEB and the NHM/BRC at the University of Kansas and by Kansas NSF EPSCoR, which included a postdoctoral fellowship to J.K.A. R. Mesa provided cypselas of Tolpis glabrescens. G. Ortiz composed Fig. 1. 
4 Author for correspondence (dcrawfor{at}ku.edu
)
5 Present address: Department of Biology, Indiana University of Pennsylvania, Indiana, PA 15705 USA
LITERATURE CITED
Ancochea E. Fuster J. M. Ibarrola E.. 1990. Volcanic evolution of the island of Tenerife (Canary Islands) in light of new K-Ar data. Journal of Volcanology and Geothermal Research 44: 231-249.[CrossRef][ISI]
Barrier M. Baldwin B. G. Robichaux R. H. Purugganan M. D.. 1999. Interspecific hybrid ancestry of a plant adaptive radiation: allopolyploidy of the Hawaiian silversword alliance (Asteraceae) inferred from floral homeotic gene duplications. Molecular Biology and Evolution 16: 1105-1113.[Abstract]
Batista F. Banares A. Caujape-Castells J. Carque D. Marrero-Gomez M. Sosa P. A.. 2001. Allozyme diversity in three endemic species of Cistus from the Canary Islands: intraspecific and interspecific comparisons and implications for genetic conservation. American Journal of Botany 88: 1582-1592.
Borgen L.. 1976. Analysis of a hybrid swarm between Argyranthemum adauctum and A. filifolium in the Canary Islands. Nordic Journal of Botany 23: 121-137.
Borgen L.. 1996. Genetic differentiation in endemic Lobularia (Brassicaceae) in the Canary Islands. Nordic Journal of Botany 16: 487-502.[ISI]
Bramwell D. Bramwell Z. I.. 1974. Wildflowers of the Canary Islands Cabildo Insular de Tenerife, Santa Cruz de Tenerife, Spain.
Bramwell D. Bramwell Z. I.. 1990. Flores silvestres de las Islas Canarias Editorial Rueda, Madrid, Spain.
Brochmann C.. 1984. Hybridization and distribution of Argyranthemum coronopifolium (Asteraceae-Anthemideae) in the Canary Islands. Nordic Journal of Botany 4: 729-736.[ISI]
Carlquist S.. 1974. Island biology Columbia University Press, New York, New York, USA.
Carr G. D. Kyhos D. W.. 1981. Adaptive radiation in the Hawaiian silversword alliance (Compositae-Madiinae). I. Cytogenetics of spontaneous hybrids. Evolution 35: 543-556.[CrossRef][ISI]
Carr G. D. Kyhos D. W.. 1986. Adaptive radiation in the Hawaiian silversword alliance (Compositae-Madiinae). II. Cytogenetics of artificial and natural hybrids. Evolution 40: 959-976.[CrossRef][ISI]
Crawford D. J.. 1989. Enzyme electrophoresis and plant systematics. In D. E. Soltis, P. S. Soltis [eds.] Isozymes in plant biology 146-164 Dioscorides Press, Portland, Oregon, USA.
Crawford D. J.. 1990. Plant molecular systematics: macromolecular approaches John Wiley, New York, New York, USA.
Crawford D. J. Ruiz E. Stuessy T. F. Tepe E. Aqeveque P. Gonzalez F. Jensen R. J. Anderson G. J. Bernardello G. Baeza C. M. Swenson U. M. Silva O.. 2001. Allozyme diversity in endemic flowering plant species of the Juan Fernandez archipelago, Chile: ecological and historical factors with implications for conservation. American Journal of Botany 88: 2195-2203.
Crawford D. J. Sang T. Stuessy T. F. Kim S.-C. M. Silva O.. 1990. Dendroseris (Asteraceae: Lactuceae) and Robinsonia (Asteraceae: Senecioneae) on the Juan Fernandez Islands: similarities and differences in biology and phylogeny. In T. F. Stuessy, M. Ono [eds.] Evolution and speciation of island plants 97-119 Cambridge University Press, Cambridge, UK.
Crawford D. J. Smith E. B. Roberts M. L. Benkowski M. Hoffman M.. 1990. Phylogenetic implications of differences in phosphoglucose isomerase isozymes in North American Coreopsis (Asteraceae: Heliantheae: Coreopsidinae). American Journal of Botany 77: 54-63.[CrossRef][ISI]
Crawford D. J. Stuessy T. F.. 1997. Plant speciation on oceanic islands. In K. Iwatsuki, P. H. Raven [eds.] Evolution and diversification in land plants 249-267 Springer-Verlag, Tokyo, Japan.
Crawford D. J. Stuessy T. F. M. Silva O.. 1987a. Allozyme divergence and the evolution of Dendroseris (Compositae: Lactuceae) on the Juan Fernandez Islands. Systematic Botany 12: 435-443.
Crawford D. J. Whitkus R.. 1988. Allozyme divergence and the mode of speciation for Coreopsis gigantea and C. maritima (Compositae). Sytematic Botany 13: 256-264.[CrossRef]
Crawford D. J. Whitkus R. Stuessy T. F.. 1987b. Plant evolution and speciation on oceanic islands. In K. M. Urbanska [ed.] Differentiation patterns in higher plants 183-199 Academic Press, London, UK.
Davis B. J.. 1964. Disc electrophoresis II: methods and application to human serum proteins. Annals of the New York Academy of Science 121: 404-427.[ISI][Medline]
DeJoode D. E. Wendel J. F.. 1992. Genetic diversity and origin of the Hawaiian Islands cotton, Gossypium tomentosum. American Journal of Botany 79: 1311-1319.[CrossRef][ISI]
Emerson B. C.. 2002. Evolution on oceanic islands: molecular and phylogenetic approaches to understanding pattern and process. Molecular Ecology 11: 951-966.[CrossRef][Medline]
Francisco-Ortega J. Crawford D. J. Santos-Guerra A. Carvalho J. A.. 1996. Isozyme differentiation in the endemic genus Argyranthemum (Asteraceae: Anthemideae) in the Macaronesian Islands. Plant Systematics and Evolution 202: 137-152.[CrossRef][ISI]
Francisco-Ortega J. Crawford D. J. Santos-Guerra A. Jansen R. K.. 1997. Origin and evolution of Argyranthemum (Asteraceae: Anthemideae) in Macaronesia. In T. J. Givnish, K. J. Sytsma [eds.] Molecular evolution and adaptive radiation 407-431 Cambridge University Press, Cambridge, UK.
Francisco-Ortega J. Santos-Guerra A. Kim S.-K. Crawford D. J.. 2000. Plant genetic diversity in the Canary Islands: a conservation perspective. American Journal of Botany 87: 909-919.
Garnatje T. Susanna A. Messeguer R.. 1998. Isozyme studies in the genus Cheirolophus (Asteraceae: Cardueae-Centaureinae) in the Iberian Peninsula, North Africa and the Canary Islands. Plant Systematics and Evolution 213: 57-70.[CrossRef][ISI]
Gottlieb L. D.. 1977. Electrophoretic evidence and plant systematics. Annals of the Missouri Botanical Garden 64: 161-180.[CrossRef][ISI]
Gottlieb L. D.. 1981. Gene number in species of Astereae that have different chromosome number. Proceedings of the National Academy of Sciences, USA 78: 3726-3729.
Gottlieb L. D.. 1987. Phosphoglucomutase and isocitrate dehydrogenase gene duplications in Layia (Compositae). American Journal of Botany 74: 9-15.[CrossRef][ISI]
Gottlieb L. D. Weeden N. F.. 1981. Correlation between subcellular location and phosphoglucose isomerase variability. Evolution 35: 1019-1022.[CrossRef][ISI]
Guillou H. Carracedo J. C. Paris R. Perez-Torrado F. J.. 2004. Implications for the early shield-stage evolution of Tenerife from K/Ar ages and magnetic stratigraphy. Earth and Planetary Science Letters 222: 599-614.[CrossRef][ISI]
Hamrick J. L. Godt M. J. W.. 1997. Effects of life history traits on genetic diversity in plant species. In J. Sivertown, M. Franco, J. L. Harper [eds.] Plant life histories: ecology, phylogeny and evolution 102-118 Cambridge University Press, Cambridge, UK.
Helenurm K. Ganders F. R.. 1985. Adaptive radiation and genetic differentiation in Hawaiian Bidens. Evolution 39: 753-765.[CrossRef][ISI]
Humphries C. J.. 1976. A revision of the genus Argyranthemum Web. Ex Schultz Bip. (Compositae-Anthemideae). Bulletin of the Natural History Museum of London, Botany 5: 145-240.
Jarvis C. E.. 1980. Systematic studies in the genus Tolpis Adanson. Ph.D dissertation, University of Reading, Reading, UK.
Kim S.-C. Crawford D. J. Francisco-Ortega J. Santos-Guerra A.. 1999. Adaptive radiation and genetic differentiation in the woody Sonchus alliance (Asteraceae: Sonchinae) in the Canary Islands. Plant Systematics and Evolution 215: 101-118.[CrossRef][ISI]
Lowrey T. K.. 1986. A biosystematic revision of Hawaiian Tetramolopium (Compositae: Astereae). Allertonia 4: 203-265.
Lowrey T. K. Crawford D. J.. 1985. Allozyme divergence and evolution in Tetramolopium (Compositae: Astereae) on the Hawaiian Islands. Systematic Botany 10: 64-72.[CrossRef][ISI]
Moore M. J. Francisco-Ortega J. Santos-Guerra A. Jansen R. K.. 2002. Chloroplast DNA evidence for the roles of island colonization and extinction in Tolpis (Asteraceae: Lactuceae). American Journal of Botany 89: 518-526.
Mort M. E. Crawford D. J. Santos-Guerra A. Francisco-Ortega J. Esselman E. J. Wolfe A. D.. 2003. Relationships among the Macaronesian members of Tolpis (Asteraceae: Lactuceae) based upon analyses of inter simple sequence repeat (ISSR) markers. Taxon 52: 511-518.[CrossRef][ISI]
Nei M.. 1978. Estimation of average heterozygosity and genetic distance from a small number of individuals. Genetics 89: 583-590.
Park S.-J. Korompai E. J. Francisco-Ortega J. Santos-Guerra A. Jansen R. K.. 2001. Phylogenetic relationships of Tolpis (Asteraceae: Lactuceae) based on ndhF sequence data. Plant Systematics and Evolution 226: 23-33.[CrossRef][ISI]
Sang T. Crawford D. J. Kim S.-C. Stuessy T. F.. 1994. Radiation of the endemic genus Dendroseris (Asteraceae) on the Juan Fernandez Islands: evidence from sequences of the ITS region of nuclear ribosomal DNA. American Journal of Botany 81: 1494-1501.[CrossRef][ISI]
Stuessy T. F. Crawford D. J.. 1998. Chromosomal stasis during speciation in angiosperms of oceanic islands. In T. F. Stuessy, M. Ono [eds.] Evolution and speciation of island plants 307-324 Cambridge University Press, Cambridge, UK.
T. F. Stuessy, M. Ono [eds.]. 1998. Evolution and speciation of island plants Cambridge University Press, Cambridge, UK.
Thirlwall M. F. Singer B. S. Marriner G. F.. 2000. 39Ar-40Ar ages and geochemistry of the basaltic shield stage of Tenerife, Canary Islands, Spain. Journal of Volcanology and Geothermal Research 103: 247-297.[CrossRef][ISI]
Trusty J. L. Olmstead R. G. Santos-Guerra A. Sa-Fontinha S. Francisco-Ortega J.. 2005. Molecular phylogenetics of the Macaronesian-endemic genus Bystropogon (Lamiaceae): paleo-islands, ecological shifts and interisland colonizations. Molecular Ecology 14: 1177-1189.[CrossRef][Medline]
Weeden N. F. Wendel J. F.. 1989. Genetics of plant isozymes. In D. E. Soltis, P. S. Soltis [eds.] Isozymes in plant biology 46-72 Dioscorides Press, Portland, Oregon, USA.
Weller S. G. Sakai A. K. Straub C.. 1996. Allozyme diversity and genetic identity in Schiedea and Alsinidendron (Caryophyllaceae: Alsinoideae). Evolution 50: 23-34.
Wendel J. F. Weeden N. F.. 1989. Visualization and interpretation of plant isozymes. In D. E. Soltis, P. S. Soltis [eds.] Isozymes in plant biology 5-45 Dioscorides Press, Portland, Oregon, USA.
Werth C. R.. 1989. The use of isozyme data for inferring ancestry of polypoid pteridophytes. Biochemical Systematics and Ecology 17: 117-130.[CrossRef]
Witter M. S. Carr G. D.. 1988. Adaptive radiation and genetic differentiation in the Hawaiian silversword alliance (Compositae: Madiinae). Evolution 42: 1278-1297.[CrossRef][ISI]
Yeh F. C. Boyle T.. 1997. Population genetic analysis of c-dominant and dominant markers and quantitative traits. Belgium Journal of Botany 129: 15.
This article has been cited by other articles:
|
|
 |
|
  M. E. Mort, J. K. Archibald, C. P. Randle, N. D. Levsen, T. R. O'Leary, K. Topalov, C. M. Wiegand, and D. J. Crawford Inferring phylogeny at low taxonomic levels: utility of rapidly evolving cpDNA and nuclear ITS loci Am. J. Botany, February 1, 2007; 94(2): 173 - 183. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. K. Archibald, D. J. Crawford, A. Santos-Guerra, and M. E. Mort The utility of automated analysis of inter-simple sequence repeat (ISSR) loci for resolving relationships in the Canary Island species of Tolpis (Asteraceae) Am. J. Botany, August 1, 2006; 93(8): 1154 - 1162. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
