Am. J. Bot. Plant Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.
Agricola
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.
(American Journal of Botany. 2004;91:889-898.)
© 2004 Botanical Society of America, Inc.

Systematics

Diploid and polyploid cytotype distribution in Melampodium cinereum and M. leucanthum (Asteraceae, Heliantheae)1

Tod F. Stuessy2,4, Hanna Weiss-Schneeweiss2 and David J. Keil3

2Department of Higher Plant Systematics and Evolution, Institute of Botany, University of Vienna, Rennweg 14,A-1030 Vienna, Austria; 3Biological Sciences Department, California Polytechnic State University, San Luis Obispo, California 93407 USA

Received for publication July 17, 2003. Accepted for publication January 22, 2004.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Previous chromosomal studies within Melampodium (Asteraceae, Heliantheae) of Mexico and Central America have documented chromosome numbers n = 9, 10, 11, 12, 18, 20, 23, 25 ± 1, 27, 30, and 33. Some species also have been shown to exhibit infra- and interpopulational polyploidy. The presence of cytotype mixtures is especially pronounced in the white-rayed complex, which occurs in the southwestern United States and adjacent Mexico. This group includes M. cinereum (n = 10 and 20), M. leucanthum (n = 10 and 20), and M. argophyllum (n = 30). Cytotype distribution has been newly analyzed in 415 plants from 152 populations and added to existing data from 185 plants from 113 populations, yielding information from a total of 600 individuals from 265 populations. Within M. cinereum and M. leucanthum are parapatric distributions of cytotypes, with tetraploids centered in the eastern and diploids in the western portions of their ranges. Tetraploids are most likely of autopolyploid origin, forming recurrently, with adaptations that allow colonization and establishment in new ecological regions. Contact zones are relatively narrow and only two triploid individuals have been detected. The tetraploid cytotypes probably extended eastward into central and southern Texas to the natural barriers at the edge of the Edward's Plateau in M. leucanthum and the low sandy plains in M. cinereum. The hexaploid M. argophyllum is interpreted as a relict surviving in the low mountains of northern Mexico; it may be an allopolyploid of hybrid origin between ancestors of the evolutionary lines that eventually yielded M. cinereum and M. leucanthum.

Key Words: Asteraceae • autopolyploidy • chromosome numbers • contact zones • cytotypes • Heliantheae • Melampodium


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Although known for decades, the occurrence of polyploid races within plant species continues to be of interest to plant systematists and evolutionists. The occurrence of polyploidy among angiosperm species has been estimated to be 30% (Stebbins, 1971 ) to 80% (Goldblatt, 1980 ; Leitch and Bennett, 1997 ). Some studies (see Levin, 2002 , for review) have suggested that polyploidy has played a much more important role in plant evolution than believed previously (e.g., Stebbins, 1971 ) and that autopolyploids may originate recurrently within diploid taxa (Soltis and Soltis, 1993 , 1999 , 2000 ; Wendel, 2000 ; Ramsey and Schemske, 2002 ). The significant questions regarding polyploidy relate to mechanisms of their origin, establishment relative to diploid progenitors, and coexistence of the chromosomal races, especially in contact zones (Petit et al., 1999 ). Mechanisms of autopolyploid origin have been studied in detail (Jackson and Casey, 1982 ; Jackson and Hauber, 1982 ; Felber, 1991 ). Establishment of autopolyploids relative to diploid progenitors has also been examined (Levin, 1975 ; Felber, 1991 ; Bever and Felber, 1992 ; Rodriguez, 1996 ) with examples of distributions of infraspecific polyploid races (e.g., Kay, 1969 ; Mooring, 1980 ; Chmielewski and Semple, 1983 ; Nicholson, 1983 ; Soejima, 1992 ; Pak et al., 1995 ; Husband and Schemske, 1998 ; McArthur and Sanderson, 1999 ).

Melampodium (Asteraceae, Heliantheae) contains 39 species distributed throughout Mexico and Central America (Stuessy, 1972 ; Turner, 1988 , 1993 ). Previous studies in the genus have revealed variation in chromosome numbers from the infraspecific to the infrageneric levels (Turner and King, 1962 ; Turner and Flyr, 1966 ; Stuessy, 1971b ; Solbrig et al., 1972 ; Keil and Pinkava, 1976 ; Pinkava and Keil, 1977 ; Powell and Powell, 1977 , 1978 ; Schaack, 1982 ; Strother, 1983 ; Ward, 1983 ). At the broadest level, haploid numbers of n = 9, 10, 11, 12, 18, 20, 23, 25 ± 1, 27, 30, and 33 have led to changes in classification and interpretation of the phylogeny in the group (Stuessy, 1971b ). At the specific level, chromosomal differences between taxa have been useful for interpreting species limits, such as n = 9 in M. microcephalum vs. n = 18 and 27 in the related M. paniculatum (Stuessy, 1971b , 1972 ; Stuessy and Brunken, 1979 ). Chromosomal information, in consort with geographical data, have also been used to interpret isolating mechanisms and their roles in speciation within the genus (Sundberg and Stuessy, 1990 ).

At least four species of Melampodium possess two cytotypes: M. cinereum (n = 10 and 20), M. dicoelocarpum (n = 12 and 23), M. leucanthum (n = 10 and 20), and M. paniculatum (n = 18 and 27) (Stuessy, 1971b ). The most well-studied example of infraspecific chromosomal variation occurs within the white-rayed complex of the genus (Stuessy, 1971b ), including M. cinereum and M. leucanthum, distributed in the southwestern USA and adjacent northeastern Mexico. Viewed as one of the most advanced evolutionary lineages within the genus (Stuessy, 1979 ), the white-rayed complex is likely to have been influenced by relatively recent environmental changes, especially during the Pleistocene. We selected these two species to examine polyploid formation and their co-existence with diploids in more detail because they contain polyploid cytological races, most likely of autopolyploid origin (Stuessy, 1971a ; Sundberg and Stuessy, 1990 ). Additionally, we examined M. argophyllum, once regarded as a variety of M. leucanthum (Stuessy, 1969 ).

The purposes of this paper, therefore, are to: (1) document in detail the existence of diploid and polyploid cytological races within Melampodium cinereum and M. leucanthum; (2) characterize their contact zones; (3) discuss mechanisms leading to polyploid formation and establishment within the range of the diploids; and (4) examine the evolutionary and biogeographic implications of distributions of the chromosomal races.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Flower buds of Melampodium cinereum, M. leucanthum, and M. argophyllum were collected in the field and fixed in modified Carnoy's solution (4 chloroform : 3 absolute ethanol : 1 glacial acetic acid), transferred to 70% ethanol, and stored under refrigeration until squashed in 1% acetocarmine according to standard methods (Fukui and Nakayama, 1996 ). Chromosome numbers were documented with a camera lucida and photography with black and white film. For each individual, 5–30 cells with well-spread chromosomes were used for chromosome number determination. The numbers of individuals analyzed per population are given in Figs. 1 and 2 (showing number of individuals analyzed including previously published counts) and Table 1 (data presented here for the first time). Vouchers (Table 1) are on deposit in the herbarium of the Ohio State University (OS).



View larger version (50K):
[in this window]
[in a new window]
  		Fig. 1. Distribution in the southwestern United States and adjacent Mexico of chromosome counts in populations of Melampodium leucanthum. Numbers indicate individuals counted in each population (if more than one). Data in part from Stuessy (1971a , b ), Solbrig et al. (1972) , Keil and Pinkava (1976) , Pinkava and Keil (1977) , Powell and Powell (1977 , 1978 ), Schaack (1982) , Strother (1983) , and Ward (1983) . Stippled boundary indicates generalized distribution of the species. Inset map shows detailed populational distributions in Blanco, Hays, and Travis counties, Texas. Conversion: 1 mi = 1.6 km

 


View larger version (24K):
[in this window]
[in a new window]
  		Fig. 2. Distribution in southern Texas and northeastern Mexico of chromosome counts in populations of Melampodium cinereum (var. cinereum, var. hirtellum, and var. ramosissimum). Melampodium argophyllum shown by stars (n = 30). Numbers indicate individuals counted in each population (if more than one). Data in part from Stuessy (1971a , b )

 

View this table:
[in this window]
[in a new window]
  		Table 1. New chromosome counts (with locality and voucher) in the white-rayed complex of Melampodium. Letters after collection numbers (A, B, etc.) refer to counts from individual plants in that population. F = Fischer; S = Stuessy; SF = Stuessy & Fischer; SM = Stuessy & Meacham; SS = T. & C. Stuessy. Abbreviations of states according to Hollis and Brummitt (1992)

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
New counts for 292 plants from 105 populations of Melampodium leucanthum are reported (Table 1). These counts, together with previous reports (Stuessy, 1971a , b ; Solbrig et al., 1972 ; Keil and Pinkava, 1976 ; Pinkava and Keil, 1977 ; Powell and Powell, 1977 , 1978 ; Schaack, 1982 ; Strother, 1983 ; Ward, 1983 ), bring the total number of individuals investigated in this species to 440 plants from 188 populations (Fig. 1). In addition to sampling populations throughout the range of the taxon, we determined the occurrence of infrapopulational cytotype mixtures. The widespread cytological survey presented here for Melampodium leucanthum has continued to document the existence of both diploid (n = 10) and tetraploid (n = 20) individuals (Fig. 1). Meiotic irregularities occur commonly in tetraploids (cf. Table 1) and include presence of multivalents, univalents, bridges, laggards, and fragments. We did not, however, find any obvious geographical or ecological correlations. Tetraploids in M. leucanthum are centered in the Lampasas Plains and Edward's Plateau in the easternmost part of the range of the species. Populations with both 2x and 4x individuals have been detected, but they are not common (five populations: SM 3546, SS 3622, SW 3553, SF 2004, plus one population from Stuessy, 1971a ; see Fig. 1). One 3x individual has been encountered in Blanco County, Texas (SF 2005; Fig. 1, inset).

New chromosome counts were obtained in 119 plants from 45 populations of Melampodium cinereum. These, added to the reports in Stuessy (1971a , b ), bring the total number of counts for this species to 156 plants from 75 populations (Fig. 2). Three varieties are recognized within M. cinereum (Stuessy, 1972 ; Fig. 2). Melampodium cinereum var. cinereum is found mainly in the sandy plains of south-central Texas, variety ramosissimum in northeastern Mexico (mainly Tamaulipas) and adjacent Texas, and variety hirtellum in Coahuila, Nuevo León, and areas adjacent to the Rio Grande. Because of taxonomic complexity of the species, the cytological variation is reported independently for each variety.

Eighty-eight plants in 31 populations of M. cinereum var. cinereum have been newly analyzed (117 plants from 55 populations in total; Fig. 2). In most populations a single cytotype was recorded. Tetraploids are centered in the eastern portion of the range of var. cinereum and scattered along the Rio Grande. Diploids occur mostly in the western region, but are also intermixed with tetraploid populations to the south. No populations with cytotype mixtures or triploids have been detected.

Twenty-three plants in 12 populations of M. cinereum var. hirtellum have been newly analyzed (24 plants from 14 populations in total; Fig. 2). In 10 of the populations only diploids were recorded, all in the southern portion of the range of the taxon; three tetraploid populations were found in the northern and northeastern sectors. One population with one diploid and one triploid was found (SS 3609) in northern Nuevo León.

Eight plants in two populations of M. cinereum var. ramosissimum have been newly analyzed, and added to data in Stuessy (1970 , 1971b ), 14 individuals from six populations have been recorded (Fig. 2). In five of those populations only diploids were found; one population (SF 2018) had one tetraploid and three diploids (Fig. 2).

Melampodium argophyllum is restricted to the low mountains of northeastern Mexico (Stuessy, 1972 ) and has not previously been examined cytologically. We include it here for completeness of the cytological survey on the white-rayed complex. Results show only hexaploid (n = 30) cytotypes present in four plants from two populations of this species, suggesting that it might have been derived from diploid and tetraploid ancestors, perhaps via allopolyploidy.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Polyploids may be classified on the basis of their origin: autopolyploids arise within diploid taxa and allopolyploids are products of interspecific hybridization (Stebbins, 1971 ; Ramsey and Schemske, 1998 ). In some plant groups, particularly angiosperms, autopolyploids originate recurrently and are called neopolyploids (Soltis and Soltis, 1993 ; Ramsey and Schemske, 2002 ). New polyploid species are often adapted to new ecological conditions having a broader spectrum of tolerance (Levin, 1983 , 2002 ), thus having new evolutionary potential.

Origin of tetraploids
Based on tetravalent formation during meiosis in tetraploids of both Melampodium cinereum and M. leucanthum, Stuessy (1971a) suggested their probable autopolyploid rather than allopolyploid origin. Results of the present survey add several dozens of new records of 4x plants having irregular pairing in meiosis (Table 1) and support that hypothesis. Presence of meiotic irregularities (multivalents, bridges) does not unambiguously indicate which type of polyploidization is operating, just as bivalent formation does not automatically support allopolyploidy. Bivalent promoting mechanisms (as suggested, e.g., for Alopecurus bulbosus; Sieber and Murray, 1980 ) or small chromosome size (e.g., Arabidopsis; Weiss and Maluszynska, 2000 ) can change the expected pairing behavior of polyploids. In Melampodium, in addition to the presence of meiotic irregularities, gross morphology of chromosomal races either does not differ at all (in M. leucanthum) or is manifested by quantitative morphological features only (in M. cinereum var. cinereum; Stuessy, 1971a ). A diploid garden accession of M. leucanthum is reported to have given rise to tetraploid progeny (Turner and King, 1962 ).

The known distributions of chromosome races in Melampodium leucanthum and M. cinereum var. cinereum, with uniformly tetraploid populations in the eastern portion of these species ranges, and mixed 2x–4x populations isolated from the main range of the tetraploids, is consistent with a recent, spontaneous, and recurrent autopolyploid formation. Polyploids may arise within otherwise diploid populations either through somatic chromosome doubling or, more commonly, through unreduced gametes (Bretagnolle and Thompson, 1995 ; Ramsey and Schemske, 1998 ; Bretagnolle, 2001 ). Although no specific data bearing on these points exist for Melampodium, we suggest fusion of unreduced gametes, because it is the most common mechanism of polyploid formation (Levin, 2002 ).

Establishment, maintenance, and expansion of polyploids
The occurrence of cytotype mixtures at the infraspecific or even infrapopulational levels has been documented for plant species including mixtures of diploids and tetraploids (Dean and Chambers, 1983 ; Nicholson, 1983 ; Husband and Schemske, 1998 ) and/or higher level polyploid races (Stutz and Sanderson, 1983 ; Strother, 1989 ; Soejima, 1992 ; Weiss et al., 2002 ). The success of newly established polyploids within diploid populations depends on their fitness and ability to overcome the minority disadvantage, either by replacing diploids or spreading beyond their site(s) of origin and establishing a new populational system (Levin, 1983 ; Felber, 1991 ; Burton and Husband, 2000 ). If the two chromosomal races have different ecological preferences and tolerances, e.g., differences in germination, flowering times, or pollinators, they could coexist sympatrically (Fowler and Levin, 1984 ; Levin, 2002 ). The history of polyploid establishment in Melampodium might have followed the latter pathway. The distributional areas of 2x and 4x cytotypes in both taxa overlap only in small degree (Figs. 1, 2), being almost parapatric and suggesting differences in ecological tolerance. Once "pure" tetraploid populations have been successfully established in areas free from diploids, they might spread rapidly. This is perhaps what can be seen in Melampodium cinereum and M. leucanthum, especially in the latter where diploids occupy the majority of the species range, with tetraploids clustered in the eastern portion. A similar pattern of parapatric distribution has also been observed in Chamerion (Epilobium) latifolium (Mosquin and Small, 1971 ). The small number of populations containing both 2x and 4x cytotypes in Melampodium leucanthum and M. cinereum could be due to sampling errors. However, in one case 27 (Fig. 1) individuals were analyzed within a population (SF 2044), and no variation in chromosome number was encountered. Perhaps the polyploids, with a wider spectrum of tolerance, have adapted to ecological conditions not suitable for diploids (Levin, 1983 ). The small number of triploids suggests restriction of gene exchange, increasing reproductive isolation, and, in some cases, augmentation of morphological variation (e.g., quantitative differences in M. cinereum var. cinereum; Stuessy, 1971a ).

Contact zones and hybridization
Chromosomal races of different ploidy levels may make contact along few to several hundred kilometers. These zones may result from secondary contact between previously allopatric chromosomal races (secondary contact zones; Petit et al., 1999 ) or the expansion of newly formed polyploids from within diploid populations. Contact zones usually lie along environmental interfaces, being maintained by selection against parental types in alien environments and hybrids/new cytotypes in parental environments (Hewitt, 1988 ). It is difficult to classify unambiguously the types of contact zones within the white-rayed complex of Melampodium. It seems plausible that these zones represent primary contacts, with tetraploids being formed recurrently within diploid populations (as seen in M. leucanthum) and spreading into new ecological niches, rather than outcompeting the diploids. It is possible that secondary contact zones exist. One population of M. cinereum var. hirtellum (SS 3609) with diploid and triploid individuals has been found in Nuevo León, suggesting ongoing hybridization between diploids and tetraploids. In this particular case no tetraploids have been found (Fig. 2). The contact zone between 2x and 4x in M. leucanthum is localized in the eastern part of its range. Four mixed populations were found (SS 2004, M 3546, SS 3553, SS 3622), and one population with a triploid individual was encountered in Blanco County, Texas (SF 2005). In proximity to that population, 2x, 4x, and mixed 2x–4x populations were found (Fig. 1, inset). Population density is high with all population types represented in this region. Two other 4x populations of M. leucanthum (SS 3579, SF 2038 [=3521]) found in isolated positions in the middle of the 2x cytotype range suggest the recurrent formation of tetraploids within diploid populations rather than the presence of a contact zone. The contact zones in Melampodium cinereum var. cinereum and var. hirtellum are clearly distinguishable and lie in the center of the range of the species.

Contact zones with hybrids (Chamerion [Epilobium] angustifolium, Husband and Schemske, 1998 ; Galax urceolata, Burton and Husband, 1999 ; Ixeris chinensis, Pak et al., 1995 ; Ranunculus ficaria, Nicholson, 1983 ) and without hybrids (e.g., Turnera sidioides, Neffa and Fernández, 2001 ) have been documented. The fitness disadvantage of tetraploids in parental diploid populations may be reduced by partial viability and fertility of triploids as shown in the polyploid complex of Chamerion (Epilobium) angustifolium (Burton and Husband, 2000 ). Contact zones between cytotypes in the white-rayed species of Melampodium seem to be devoid of hybrids. The presence of only two populations with 3x (one individual in each; SS 3609, SF 2005) in all three species with more than 500 individuals analyzed suggests incompatibility of 2x and 4x plants and/or reproductive isolation, perhaps of an ecological or micro-phenotypic type. It is possible, however, that crosses may be occurring between 2x and 4x cytotypes, especially on the 4x level (i.e., via unreduced gametes in the diploid; e.g., deWet, 1980 ; Bretagnolle and Thompson, 1995 ; Ramsey and Schemske, 1998 ), which would be cytologically difficult to detect. In a polyploid Draba complex, polyploidy may help to overcome genetic depauperation caused by high rates of inbreeding at the 2x level (Brochmann, 1993 ). Whether similar mechanisms exist in Melampodium is not known. Preliminary experiments including cross- and self-pollination in a 4x population of M. leucanthum (SF 2001) indicate that some tetraploids are facultative outcrossers (T. F. Stuessy, unpublished data).

Polyploidization and speciation
Polyploidy is an important factor in plant speciation (Otto and Whitton, 2000 ; Ramsey and Schemske, 2002 ). Polyploids often undergo rapid structural and epigenetic changes that create new genetic entities (Raina et al., 1994 ; Matzke et al., 1999 ; Osborn et al., 2003 ). Species with inter- and infrapopulational cytotype mixtures are relatively common in certain groups of plants, including Asteraceae (e.g., Kay, 1969 ; Mooring, 1975 , 1980 ; Semple, 1979 ; Chmielewski and Semple, 1983 ; Dean and Chambers, 1983 ; Soejima, 1992 ; Pak et al., 1995 ). Asteraceae are attractive for systematic studies due to their recent origin, numerous species, and ease of karyological analyses. This plant family is still undergoing speciation, and polyploidy may be a mechanism that confers genetic novelty and enables processes of genome rearrangements to yield new genetic combinations. Polyploidy in certain species of Melampodium might have aided expansion of distributions of new taxa. One example (Stuessy and Brunken, 1979 ) is M. microcephalum (n = 9) and the related M. paniculatum (n = 18 and 27) in which the diploid is distributed in southern Mexico and polyploids in Central America and South America. The hexaploids of M. argophyllum may be another example of such a process. Formerly treated as a variety of M. leucanthum (Stuessy, 1969 ), it could have speciated after polyploidization involving either hybridization of tetraploids with diploids of the same or other species (possibly M. cinereum) and subsequent polyploidization or through autopolyploidization of tetraploids (e.g., fusion of unreduced 4n with 2n gametes; Weiss et al., 2002 ). This species is found in an ecologically and geographically distinct niche, in mountain ranges (1830–2440 m) distant from its closest relatives. Examples of higher polyploid formation within other species complexes have already been published, e.g., hexaploids of Dalea formosa (Fabaceae) are distributed in a large area among and beyond (to the south) diploids and tetraploids and might be the product of hybridization between 2x and 4x cytotypes, stabilized by polyploidization (Spellenberg, 1981 ).

Evolution and biogeography
A better understanding of chromosomal relationships in the white-rayed complex of Melampodium offers a chance to hypothesize on their origin and distributional relationships. Although speculative, these ideas offer a framework for future testing with molecular (and other) data. Stuessy (1971a , 1979 ) suggested that the white-rayed complex must have been derived from the yellow-rayed portion of the genus. The white-rayed complex is adapted to xeric conditions and occurs at the northernmost extreme of distribution. We suggest that an ancestral white-rayed taxon originated from the immediate ancestor of the diploid, yellow-rayed, shrubby M. americanum L. of the same taxonomic section (Stuessy, 1972 ). The modern range of M. americanum extends from Guatemala to northern Mexico (Stuessy, 1972 ).

As the Pleistocene came to a close (approximately 12 000 years ago), further divergence in the white-rayed complex might have occurred as the region became drier and warmer (Van Devender, 1980 ; Wells, 1983 ; Van Devender and Wiens, 1993 ). Drying of valleys between mountains might have correlated with the movement of some plant populations from lower to higher sites and others to more northerly regions. Melampodium cinereum could have developed more fully northeastward near the Rio Grande. Melampodium leucanthum may have evolved in isolated mountains to the west of M. cinereum in Coahuila, Mexico. Hybridization between the two early populational derivatives, followed by polyploidization, could have yielded M. argophyllum, a hexaploid, which survives today, perhaps relictually, only in isolated populations in higher mountains (above 1830 m; Stuessy, 1972 ) abutting the westernmost edge of the range of M. cinereum and the southernmost edge of M. leucanthum. The two parental diploid taxa could have continued their range extensions, with the former reaching to the Rio Grande and the latter extending northward through the Big Bend of Mexico and Texas into northern Texas and adjacent Colorado, Kansas, and Oklahoma, and westward to New Mexico and Arizona. Autopolyploidy could have occurred frequently within both developing M. cinereum and M. leucanthum. In the case of M. leucanthum, the recurring 4x cytotypes were apparently unable to develop racial status within parental diploid areas, except for those that originated on the uplifted limestone bedrock of Edwards's Plateau in central Texas, which has a strong impact on plant distributions and harbors 76 endemic vascular plant taxa (Amos and Rowell, 1988 ). Once tetraploids occurred in the region tolerated less well by diploids, they might have become established and colonized successfully. Within M. cinereum, colonizing populations might have diverged morphologically to some extent (to such a degree that they are now recognized taxonomically as varieties; Stuessy, 1972 ) and, again, autopolyploidy may have given rise to 4x populations that eventually established on the northeastern periphery of the range of the species. Quantitative morphological divergence between the cytotypes of M. cinereum var. cinereum may indicate that they differentiated earlier than those in M. leucanthum. The Edward's Plateau may have served as barrier to further northward migration of M. cinereum; no hybrids between M. cinereum and M. leucanthum have ever been reported, nor have any hybrids been found in the contact zone between the two taxa (T. F. Stuessy, personal observations).


View this table:
[in this window]
[in a new window]
  		Table 1. Continued

 

View this table:
[in this window]
[in a new window]
  		Table 1. Continued

 

   FOOTNOTES
 
1 We thank Sigma Xi Grant-In-Aid of Research (to T. F. S.) in support of some of the fieldwork, the Graduate School of The Ohio State University for major support of collecting activities for many of the cited populations, and Lara Menon for technical assistance. Costs for manuscript preparation were supported by grant number P-15225-BIO (to T. F. S.) from the Austrian National Science Foundation (FWF). Back

4 E-mail: tod.stuessy{at}univie.ac.at Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Amos B. B. C. M. Rowell Jr 1988 Floristic geography of woody and endemic plants. In B. B. Amos and F. R. Gehlbach [eds.], Edwards Plateau vegetation: plant ecological studies in central Texas, 25–42. Baylor University Press, Waco, Texas, USA

Bever J. D. F. Felber 1992 The theoretical population genetics of autopolyploidy. Oxford Surveys in Evolutionary Biology 8: 185-217

Bretagnolle F. 2001 Pollen production and spontaneous polyploidization in diploid populations of Anthoxanthum odoratum. Biological Journal of the Linnean Society 72: 241-247

Bretagnolle F. J. D. Thompson 1995 Gametes with the somatic chromosome number: mechanisms of their formation and role in the evolution of polyploid plants. New Phytologist 129: 1-22[CrossRef][ISI]

Brochmann C. 1993 Reproductive strategies of diploid and polyploid populations of arctic Draba (Brassicaceae). Plant Systematics and Evolution 185: 55-83[CrossRef][ISI]

Burton T. L. B. C. Husband 1999 Population cytotype structure in the polyploid Galax urceolata (Diapensiaceae). Heredity 82: 381-390

Burton T. L. B. C. Husband 2000 Fitness differences among diploids, tetraploids, and their triploid progeny in Chamerion angustifolium: mechanisms of inviability and implications for polyploid evolution. Evolution 54: 1182-1191[CrossRef][ISI][Medline]

Chmielewski J. G. J. C. Semple 1983 The cytogeography of Aster lanceolatus. III. Cytoecology in Southern Ontario. Canadian Journal of Botany 61: 1879-1886[ISI]

Dean M. L. K. L. Chambers 1983 Chromosome numbers and evolutionary patterns in the Aster occidentalis (Asteraceae) polyploid complex of western North America. Brittonia 35: 189-196[CrossRef][ISI]

deWet J. M. J. 1980 Origins of polyploids. In W. H. Lewis [ed.], Polyploidy—biological relevance, 3–16. Plenum, New York, New York, USA

Felber F. 1991 Establishment of a tetraploid cytotype in a diploid population: effect of relative fitness of the cytotypes. Journal of Evolutionary Biology 4: 195-207

Fowler N. L. D. A. Levin 1984 Ecological constraints on the establishment of a novel polyploid in competition with its diploid progenitor. American Naturalist 124: 703-711[CrossRef][ISI]

Fukui K. S. Nakayama [eds.] 1996 Plant chromosomes. Laboratory methods. CRC Press, Boca Raton, Florida, USA

Goldblatt P. 1980 Polyploidy in angiosperms. In W. H. Lewis [ed.], Polyploidy—biological relevance, 219–239. Plenum, New York, New York, USA

Hewitt G. M. 1988 Hybrid zones—natural laboratories for evolutionary studies. Trends in Ecology and Evolution 3: 158-167[CrossRef]

Hollis S. R. K. Brummitt 1992 World geographical scheme for recording plant distributions. Plant taxonomic database standards no. 2. International Working Group on Taxonomic Databases for Plant Sciences (TDWG), Hunt Institute for Botanical Documentation, Pittsburgh, Pennsylvania, USA

Husband B. C. D. W. Schemske 1998 Cytotype distribution at a diploid-tetraploid contact zone in Chamerion (Epilobium) angustifolium (Onagraceae). American Journal of Botany 85: 1688-1694[Abstract/Free Full Text]

Jackson R. C. J. Casey 1982 Cytogenetic analyses of autopolyploids: models and methods for triploids to octoploids. American Journal of Botany 69: 487-501[CrossRef][ISI]

Jackson R. C. D. P. Hauber 1982 Autotriploid and autotetraploid cytogenetic analyses: correction coefficients for proposed binomial models. American Journal of Botany 69: 644-646[CrossRef][ISI]

Kay Q. O. N. 1969 The origin and distribution of diploid and tetraploid Tripleurospermum inodorum (L.) Schultz Bip. Watsonia 7: 130-141

Keil D. J. D. J. Pinkava 1976 Chromosome counts and taxonomic notes for Compositae from the United States and Mexico. American Journal of Botany 63: 1393-1403[CrossRef][ISI]

Leitch I. J. M. D. Bennett 1997 Polyploidy in angiosperms. Trends in Plant Science 2: 470-476[CrossRef][ISI]

Levin D. A. 1975 Minority cytotype exclusion in local plant populations. Taxon 24: 35-43[CrossRef]

Levin D. A. 1983 Polyploidy and novelty in flowering plants. American Naturalist 122: 1-25[CrossRef][ISI]

Levin D. A. 2002 The role of chromosomal change in plant evolution. Oxford University Press, New York, New York, USA

Matzke M. A. O. Mittelsten-Scheid A. J. M. Matzke 1999 Rapid structural and epigenetic changes in polyploid and aneuploid genomes. BioEssays 21: 761-767[CrossRef][ISI][Medline]

McArthur E. D. S. C. Sanderson 1999 Cytogeography and chromosome evolution of subgenus Tridentatae of Artemisia (Asteraceae). American Journal of Botany 86: 1754-1775[Abstract/Free Full Text]

Mooring J. S. 1975 A cytogeographic study of Eriophyllum lanatum (Compositae, Helenieae). American Journal of Botany 62: 1027-1037[CrossRef][ISI]

Mooring J. S. 1980 A cytogeographic study of Chaenactis douglasii (Compositae, Helenieae). American Journal of Botany 67: 1304-1319[CrossRef][ISI]

Mosquin T. E. Small 1971 An example of parallel evolution in Epilobium (Onagraceae). Evolution 25: 678-682[CrossRef][ISI]

Neffa V. G. S. A. Fernandez 2001 Cytogeography of the South American Turnera sidoides L. complex (Turneraceae, Leiocarpae). Botanical Journal of the Linnean Society 137: 189-196[CrossRef]

Nicholson G. G. 1983 Studies on the distribution and the relationship between the chromosome races of Ranunculus ficaria L. in S.E. Yorkshire. Watsonia 14: 321-328

Osborn T. C. J. C. Pires J. A. Birchler D. L. Auger Z. J. Chen H.-S. Lee L. Comai A. Madlung R. W. Doerge V. Colot R. A. Martienssen 2003 Understanding mechanisms of novel gene expression in polyploids. Trends in Genetics 19: 141-147[CrossRef][ISI][Medline]

Otto S. P. J. Whitton 2000 Polyploid incidence and evolution. Annual Reviews of Genetics 34: 401-437[CrossRef]

Pak J.-H. N.-C. Kim K. Choi I.-S. Kim B.-B. Seo S.-D. Song 1995 Geographical distributions of diploids, triploids, and tetraploids of the Ixeris chinensis complex (Asteraceae: Lactuceae) of South Korea. Korean Journal of Plant Taxonomy 25: 221-236

Petit C. F. Bretagnolle F. Felber 1999 Evolutionary consequences of diploid-polyploid hybrid zones in wild species. Trends in Ecology and Evolution 14: 306-311

Pinkava D. J. D. J. Keil 1977 Chromosome counts of Compositae from the United States and Mexico. American Journal of Botany 64: 680-686[CrossRef][ISI]

Powell A. M. S. A. Powell 1977 Chromosome numbers of gypsophilic plant species of the Chihuahuan Desert. Sida 7: 80-90

Powell A. M. S. A. Powell 1978 Chromosome numbers in Asteraceae. Madroño 25: 160-169

Raina S. N. A. Parida K. K. Koul S. S. Salimath M. S. Bisht V. Raja 1994 Associated chromosomal DNA changes in polyploids. Genome 37: 560-564[ISI]

Ramsey J. D. W. Schemske 1998 Pathways, mechanisms, and rates of polyploid formation in flowering plants. Annual Review of Ecology and Systematics 29: 467-501

Ramsey J. D. W. Schemske 2002 Neopolyploidy in flowering plants. Annual Review of Ecology and Systematics 33: 589-639

Rodriguez D. J. 1996 A model for the establishment of polyploidy in plants. American Naturalist 147: 33-46[CrossRef][ISI]

Schaack C. G. R. Herly M. L. Rusche 1982 In Á. Löve [ed.], IOPB chromosome number reports LXXV. Taxon 31: 366-367

Semple J. C. 1979 The cytogeography of Aster lanceolatus (synonyms A. simplex and A. paniculatus) in Ontario with additional counts from populations in the United States. Canadian Journal of Botany 57: 397-402[ISI]

Sieber V. K. B. G. Murray 1980 Spontaneous polyploids in marginal populations of Alopecurus bulbosus Gouan (Poaceae). Botanical Journal of the Linnean Society 81: 293-300

Soejima A. 1992 Taxonomical study of Aster leiophyllus complex (Compositae) in Kanto District, Japan, with special reference to ploidy level. Botanical Magazine Tokyo 105: 13-28[CrossRef][ISI]

Solbrig O. T. D. W. Kyhos A. M. Powell P. H. Raven 1972 Chromosome numbers in Compositae VIII: Heliantheae. American Journal of Botany 59: 869-878[CrossRef][ISI]

Soltis D. E. P. S. Soltis 1993 Molecular data and the dynamic nature of polyploidy. Critical Reviews in Plant Sciences 12: 243-273

Soltis D. E. P. S. Soltis 1999 Polyploidy: recurrent formation and genome evolution. Trends in Ecology and Evolution 14: 348-352

Soltis P. S. D. E. Soltis 2000 The role of genetic and genomic attributes in the success of polyploids. Proceedings of the National Academy of Sciences USA 97: 7051-7057[Abstract/Free Full Text]

Spellenberg R. 1981 Polyploidy in Dalea formosa (Fabaceae) on the Chihuahuan Desert. Brittonia 33: 309-324[CrossRef][ISI]

Stebbins G. L. 1971 Chromosomal evolution in higher plants. Edward Arnold, London, UK

Strother J. L. 1983 More chromosome studies in Compositae. American Journal of Botany 70: 1217-1224[CrossRef][ISI]

Strother J. L. 1989 Chromosome numbers in Thymophylla (Compositae: Tageteae). Sida 13: 351-358

Stuessy T. F. 1969 A new variety and new combination in Melampodium (Compositae-Heliantheae). Sida 3: 348-349

Stuessy T. F. 1970 Chromosome studies in Melampodium (Compositae, Heliantheae). Madroño 20: 365-37

Stuessy T. F. 1971a Systematic relationships in the white-rayed species of Melampodium (Compositae). Brittonia 23: 177-190[CrossRef][ISI]

Stuessy T. F. 1971b Chromosome numbers and phylogeny in Melampodium (Compositae). American Journal of Botany 58: 732-736[CrossRef][ISI]

Stuessy T. F. 1972 Revision of the genus Melampodium (Compositae: Heliantheae). Rhodora 74: 1-70 161–219

Stuessy T. F. 1979 Cladistics of Melampodium (Compositae). Taxon 28: 179-195[CrossRef][ISI]

Stuessy T. F. J. N. Brunken 1979 Artificial interspecific hybridization in Melampodium section Zarabellia (Compositae). Madroño 26: 53-63

Stutz H. C. S. C. Sanderson 1983 Evolutionary studies of Atriplex: chromosome races of A. confertifolia (shadscale). American Journal of Botany 70: 1536-1547[CrossRef][ISI]

Sundberg S. D. T. F. Stuessy 1990 Isolating mechanisms and implications for modes of speciation in Heliantheae (Compositae). In T. J. Mabry and G. Wagenitz [eds.], Research advances in the Compositae, 77–97. Springer-Verlag,Vienna, Austria

Turner B. L. 1988 A new species of Melampodium (Asteraceae-Heliantheae) from Oaxaca, Mexico. Phytologia 64: 445-447

Turner B. L. 1993 A new species of Melampodium (Asteraceae, Heliantheae) from Jalisco, Mexico. Phytologia 75: 136-139

Turner B. L. D. Flyr 1966 Chromosome numbers in the Compositae. X. North American species. American Journal of Botany 53: 24-33[CrossRef][ISI]

Turner B. L. R. M. King 1962 A cytotaxonomic survey of Melampodium (Compositae, Heliantheae). American Journal of Botany 49: 263-269[CrossRef][ISI]

Van Devender T. R. 1980 Holocene plant remains from Rocky Arroyo and Last Chance Canyon, Eddy County, New Mexico. Southwestern Naturalist 25: 361-372[CrossRef]

Van Devender T. R. J. F. Wiens 1993 Holocene changes in the flora of Ragged Top, South-central Arizona. Madroño 40: 246-264

Ward D. E. 1983 Chromosome counts from New Mexico and Southern Colorado. Phytologia 54: 302-309

Weiss H. C. Dobes G. M. Schneeweiss J. Greimler 2002 Occurrence of tetraploid and hexaploid cytotypes within and between populations in Dianthus sect. Plumaria (Caryophyllaceae). New Phytologist 156: 85-94[CrossRef][ISI]

Weiss H. J. Maluszynska 2000 Chromosomal rearrangement in autotetraploid plants of Arabidopsis thaliana. Hereditas 133: 255-261[CrossRef][ISI][Medline]

Wells P. V. 1983 Paleobiogeography of montane islands in the Great Basin since the last glaciopluvial. Ecological Monographs 53: 341-382

Wendel J. F. 2000 Genome evolution in polyploids. Plant Molecular Biology 42: 225-249[CrossRef][ISI][Medline]




This article has been cited by other articles:


Home page
 Am. J. Bot.Home page
K. Halverson, S. B. Heard, J. D. Nason, and J. O. Stireman III
Origins, distribution, and local co-occurrence of polyploid cytotypes in Solidago altissima (Asteraceae)
Am. J. Botany, January 1, 2008; 95(1): 50 - 58.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Bot.Home page
J. Suda, H. Weiss-Schneeweiss, A. Tribsch, G. M. Schneeweiss, P. Travnicek, and P. Schonswetter
Complex distribution patterns of di-, tetra-, and hexaploid cytotypes in the European high mountain plant Senecio carniolicus (Asteraceae)
Am. J. Botany, August 1, 2007; 94(8): 1391 - 1401.
[Abstract] [Full Text] [PDF]

Home page
 ANN BOT (LOND)Home page
L. G. QUINTANILLA and A. ESCUDERO
Spore Fitness Components Do Not Differ Between Diploid and Allotetraploid Species of Dryopteris (Dryopteridaceae)
Ann. Bot., September 1, 2006; 98(3): 609 - 618.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.
Agricola
Right arrow Articles by Stuessy, T. F.
Right arrow Articles by Keil, D. J.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS