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Genetics and Molecular Biology |
Section of Integrative Biology, University of Texas, Austin, Texas 78713-7640 USA
Received for publication October 30, 2001. Accepted for publication April 9, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: amphiploidy • Gilia • Polemoniaceae • polyploidy rate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Different pathways to polyploidy call for different ways of expressing polyploidy rates. For our present purpose let us exclude several modes from consideration, namely, autopolyploidy, amphiploidy by somatic doubling, and amphiploidy originating in two steps by one-sided doubling. We are concerned in this paper with amphiploid formation by the union of unreduced gametes in one generation, or one-step meiotic nonreduction.
Ramsey and Schemske (1998)
point out that the rate of new amphiploid production is a function of two parameters: the frequency of the F1 hybrids in the parental two-species population and the frequency of new amphiploid individuals in the F2 generation. The frequency of F1 hybrid individuals is usually unknown and hard to measure in natural populations, as Ramsey and Schemsky (1998)
note. Furthermore, it is not applicable in experimental hybrid cultures where the foundation "population" consists of one or two plants of species A used as female parents and one or two plants of species B used as pollen parents. For this reason, I am dispensing with the first parameter mentioned above and will use only the second parameter in this paper.
To get some numerical values for the second parameter Ramsey and Schemske (1998)
make use of the fact that F2 generations that have undergone doubling often contain a mixture of even euploids and other types such as aneuploids and diploids. They surveyed numerous papers on various angiosperm genera and recorded the proportion of new even euploids to other types. For the case of diploid F1 hybrids producing allotetraploid F2 progeny, the proportion of new tetraploid individuals varies from 0.4 to 100% in the sample of examples. The mean frequency of tetraploids for a subsample of plant groups with backcrossing is 1.4%, and for a subsample of self-pollinating groups it is 64.8%. From this and with the use of a conversion factor they calculate the mean rate of doubling where selfing occurs and find it to be 0.04 tetraploid individuals per generation (Ramsey and Schemske 1998
, including Web Table 6).
In this paper I approach the same problem on the basis of old empirical data from experimental hybidizations in the genus Gilia (Polemoniaceae). The plants involved are annual herbs. The purpose of the original crossings was to explore the strength, nature, and systematic distribution of sterility barriers. Data concerning crossability of the species and breeding behavior of hybrids and hybrid progeny were duly recorded. In seven of the numerous hybrid combinations, a sterile F1 hybrid produced fertile amphiploid F2s. Analysis of the original data yields estimates of amphiploidy rates for these seven cases. One of the cases has been presented from the standpoint of polyploidy rate in a previous paper (Grant, 1952
).
Two ways of expressing the frequency of new amphiploids are adopted. The first is the proportion of the hybrid combinations that yielded any new amphiploids. This reveals the number of hybrid combinations in which spontaneous doubling did not occur. The second measure is the average number of new amphiploid F2 progeny per F1 individual. This measure can also be read as the average number of new amphiploid zygotes per parental hybrid plant per two generations (P and F1).
A subject being discussed in the current literature is recurrent polyploidy. This term is applied to cases of multiple origin of a new polyploid type from a single parental species or a single hybridizing species pair (Soltis and Soltis, 1993
, and other papers; see below). As recurrent polyploidy is common, it is related to our present subject, but it is also quite different in its focus. It will be discussed later in this paper.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The relevant data for our present purpose comprise two subsets of the more extensive original database. One is for interspecific crosses in sect. Gilia, where the parental species are mostly diploid but include tetraploids. The details are in Grant (1952
, 1954
, 1965
). The second dataset is for diploid interspecific crosses in sect. Arachnion (details in Grant and Grant, 1960
; Day, 1965
). The published data were checked against the old culture notebooks in a number of instances to resolve questions or obtain supplementary information.
Nine species of sect. Gilia were intercrossed. Four of these species were parents of interspecific hybrids that yielded spontaneous amphiploids or neopolyploids. The parental species and strains are: G. achilleaefolia, San Luis Obispo, California, USA (2n); G. millefoliata, Pt. Reyes, California, USA (2n); G. valdiviensis, Limache, Chile (2n); and G. nevinii, Santa Catalina Island, California, USA (4n).
The parental species of sect. Arachnion that were involved in amphiploid formation are all diploid. Two of these species were represented by two or more strains in the crossings; letter symbols for the strains are used for identification. The localities are all in deserts of the American southwest. The species and strains are: G. clokeyi, Deep Springs, California, USA; G. mexicana, Santa Rita Mts., Arizona, USA; G. minor, Kramer Junction, California (K); Mojave, California (M); Wickenberg, Arizona (W), USA; and G. aliquanta, Red Rock Canyon, California (R); Victorville, California (V), USA.
It may be of interest to describe how the new amphiploids are first detected. In artificial F1 hybrids in Gilia high pollen stainability is normally associated with high seed output, and low pollen stainability with sterility as to seeds. Occasionally we find an artificial F1 hybrid with an odd combination of features: high pollen sterility (1–3% good grains) and irregular meiosis on the one hand and good seed output on the other. This is the first indication of a new amphiploid. When the F2 seeds are planted the next year and the F2 plants are determined for chromosome number, the preliminary indication of amphiploid formation is invariably confirmed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 1954
, 1965
). The percentage of the 16 F1 hybrid types that produced amphiploids is 25%. Sect. Gilia consists of two natural subgroups, a group of inland continental species and a separate group of maritime species on coastlines and offshore islands. These two subgroups differ markedly in rate of amphiploid formation. Crosses of inland species inter se and of inland x maritime species produced ten F1 hybrid types, of which only one type yielded a new amphiploid, for a proportion of 1/10. By contrast, inter se crosses among the maritime species resulted in six types of F1 hybrid, which yielded three new amphiploids (3/6).
Breeding behavior of amphiploid-producing hybrids in sect. Gilia
The four hybrid combinations in sect. Gilia that yielded new amphiploids in F2 are listed in Table 1. The first cross is between a maritime species (G. millefoliata) and an interior species (G. achilleaefolia); the other crosses are between maritime species. Details regarding hybrid sterility and meiotic behavior beyond those summarized below can be found in Grant (1952
, 1954
, 1965
).
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). They represent a synthetic tetraploid with the same genomic constitution as the natural species G. clivorum.
F1 G. millefoliata x G. achilleaefolia (cross 1B)
The same cross was repeated with the same strains in a later year to test the effects of soil type. The F1 generation of 1951 contained numerous seedlings; 40 of these were planted in rich loam and grown to maturity. They were sterile as to pollen and had a mean of 6.8 bivalents per PMC. Microsporogenesis was as in cross 1A. Three of the 40 F1 plants produced a total of 14 seeds. These seeds yielded six flowering plants in F2 in 1952. The F2 plants were tetraploid with high pollen and seed fertility, as in cross 1A.
The polyploidy rate was much higher in the nutrient-starved F1 hybrids than in the loam-grown F1s. The number of tetraploid F2 individuals per single parental F1 individual was 1.00 in cross 1A and 0.15 in cross 1B (Table 1). The difference in polyploidy rate was greater than these figures show since the stunted F1 hybrids in cross 1A produced fewer flowers than the vigorous hybrid plants of cross 1B (Grant, 1952
).
F1 G. millefoliata x G. valdiviensis (cross 2)
Sixteen F1 plants (2n = 18) were grown in 1957. They were sterile as to pollen but had a high degree of chromosome pairing in meiosis (mean of 8.9 bivalents per PMC). The hybrid plants produced some giant and presumably unreduced microspores. The F2 progeny consisted of ten plants that were tetraploid (2n = 36) with good pairing (Grant, 1965
). No natural species with this constitution is known.
F1 G. millefoliata (2n) x G. nevinii (4n) (cross 3)
Sixteen F1 plants were grown in 1957 from a fraction of the abundant F1 seeds. The plants were triploid, sterile, and had low pairing with an average of 2.8 bivalents per PMC. The F1 plants had some restitution nuclei at meiosis and some dyads of microspores. They produced a total of about 1800 F2 seeds. The F2 generation consisted of 202 plants, two of which were determined for chromosome number. One of these was hexaploid (2n = 54) and the other hexaploid-aneuploid (2n = 60). They had generally good bivalent pairing at meiosis and were fertile (Grant, 1965
). This synthetic amphiploid has no known natural counterpart.
F1 G. valdiviensis (2n) x G. nevinii (4n) (cross 4)
Sixteen F1 plants were grown in 1957 from a fraction of the numerous hybrid seeds. The hybrid plants were sterile as to pollen and had reduced chromosome pairing in meiosis with an average of 5.4 bivalents per PMC. They produced numerous F2 seeds. An F2 generation of 36 plants was grown, one plant of which was counted and found to be hexaploid (2n = 54). It had good bivalent pairing and was fertile (Grant, 1965
). There is no known natural species with this constitution.
The mechanism of doubling is the union of unreduced gametes, or meiotic nonreduction. The stages in this process—restitution nuclei, dyads of microspores, and giant pollen grains—were found in microsporogenesis in F1s. Nonreduction probably occurred in the female line in the F1s as well. The new tetraploids and hexaploids appeared in the F2 seeds produced by the hybrids, which points to the functioning of unreduced gametes in both the male and female lines.
All of the F2 progeny of crosses 1–4 would be expected to be doubled on the basis of the meiotic behavior and gametic sterility of the F1s. The expectation was confirmed by chromosome counts of all F2s in cross 1A. Only a small number of F2s were counted in crosses 2, 3, and 4, but the assumption that all of these F2 progeny were doubled seems justified by other indicators.
The generation containing neopolyploids often contains some deviant cytotypes such as aneuploids and triploids, but sometimes consists entirely of even polyploids. Ramsey and Schemske (1998
, Web Table 6) surveyed numerous angiosperm genera with respect to the presence or absence of deviant cytotypes in the neopolyploid-containing genera. Only preliminary data are available on this aspect in Gilia. In sect. Gilia, the F2 progeny of diploid x diploid crosses were all even tetraploids. The F2 progeny of the diploid x tetraploid cross, G. millefoliata x G. nevinii, however, consisted of an even hexaploid (2n = 54) and a hexaploid-aneuploid (2n = 60). Irregular chromosome segregation in the triploid F1 hybrid of this combination is the probable cause of the aneuploid variation in F2. This aspect requires further study in Gilia.
The rates of amphiploid formation are given in Table 1. The rates are generally high, at least they are never low, and they are surprisingly high in the cross G. millefoliata x G. nevinii.
Proportion of hybrid combinations yielding new amphiploids in sect. Arachnion
Fifteen diploid species of sect. Arachnion were intercrossed in 94 hybrid combinations. A liberal definition of hybrid combinations is used here as in sect. Gilia. Crosses involving different strains of the same species pair, crosses of a species pair made in different years, and reciprocal crosses are all grouped as components of a single hybrid combination.
Of the 94 interspecific diploid hybrid combinations attempted, 55 failed to yield any F1 plants, while 39 combinations did yield F1 plants. In the latter group, three types of diploid F1 hybrids spontaneously produced tetraploid seeds (Grant and Grant, 1960
). The percentage of the 39 F1 types that produced spontaneous amphiploids is thus 7.7%.
Breeding behavior of amphiploid-producing hybrids in sect. Arachnion
The three diploid hybrid combinations in sect. Arachnion that yielded spontaneous tetraploids in F2 are listed in Table 2. A fourth cross is added (cross 8) in which the diploid F1 hybrid produced no tetraploids spontaneously but did produce them when treated with colchicine. The strains of the parental species are indicated by letter abbreviations in Table 2 and identified in MATERIALS AND METHODS. Additional details regarding hybrid sterility and meiotic behavior beyond those summarized below can be found in Grant and Grant (1960)
and Day (1965)
.
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, pp. 473–474; Grant, 1964
, 1981
, p. 335).
F1 G. clokeyi x G. aliquanta (cross 6)
One diploid hybrid plant of this combination was obtained. It was sterile as to pollen and had reduced pairing with an average of 4.5 bivalents per PMC. The hybrid produced two F2 seeds that developed into two F2 plants in 1960. These plants were tetraploid (2n = 36) and fertile with 62% and 80% good pollen (Grant and Grant, 1960
). No natural tetraploid with this genomic constitution is known; it is possible that one exists but has not yet been discovered.
F1 G. minor K x G. clokeyi and F1 G. clokeyi x G. minor K (cross 7)
Four hybrid plants, two from each cross, were grown in 1959. They were pollen-sterile and had reduced chromosome pairing with a mean of 4.6 bivalents per PMC. The hybrid plants produced a total of 15 F2 seeds. These yielded four F2 plants, three of which were inviable. The one viable F2 plant was a product of the G. minor 
x G. clokeyi cross. It was tetraploid and fertile with 91% good pollen (Grant and Grant, 1960
).
The new tetraploid is a synthetic form of the natural tetraploid species G. transmontana. This was confirmed by making the cross G. transmontana (4n) x G. minor-clokeyi (4n) and finding the outcross hybrid to be fertile with good chromosome pairing in meiosis (Day, 1965
).
F1 G. minor x G. aliquanta and F1 G. aliquanta x G. minor (cross 8)
Several strains of the parental species were used as noted in Table 2. The crosses were repeated in different years, and a total of 31 F1 hybrids was grown in 1960, 1961, and 1962. The F1s were sterile with an average of 4.5 bivalents or multivalents per PMC. They were given ample opportunity to produce tetraploid progeny spontaneously but did not do so (Day, 1965
).
One F1 plant of G. aliquanta x G. minor was treated with colchicine and developed several fertile branches that bore 150 plump F2 seeds. The seeds produced 61 F2 plants, which were tetraploid, fertile, and had generally good chromosome pairing. Five F2 plants were counted and had 2n = 36 (Day, 1965
).
The induced tetraploid is a synthetic form of the natural tetraploid species G. malior. This was confirmed by making the cross G. malior (4n) x G. minor-aliquanta (4n) and finding the outcross hybrid to be fertile with good chromosome pairing (Day, 1965
). Cross 8 reminds us that a hybrid combination may fail to produce amphiploids in an experimental culture but succeed in doing so in nature.
No aneuploids were found in the F2 generation of any of the crosses in sect. Arachnion. All the F2 progeny that were examined were even tetraploids (2n = 36). This finding is true of the colchicine-induced polyploidy progeny of G. aliquanta x G. minor. However, only a few F2 plants (1–5) were chromosome-counted in each cross, and thus more work on this aspect is needed.
The frequency of amphiploid formation in the set of hybrid combinations ranges from 0.0 to 2.0 new tetraploid F2 plants per single diploid F1 hybrid (Table 2). In those hybrid types that produce any new tetraploids at all, doubling is not a rare event.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, ch. 24; Ramsey and Schemske, 1998
).
The role of the breeding system in amphiploid formation
Each of the two sections involved in this study contains taxa with large flowers and a predominantly outcrossing breeding system and other taxa that are small-flowered and predominantly autogamous. All of the parental species of amphiploid-producing hybrids in sect. Arachnion are small-flowered and autogamous. In sect. Gilia, all but one of the parental species of new amphiploids are small-flowered and autogamous. The one exception is the strain of G. achilleaefolia used as a parent, but its F1 with the small-flowered G. millefoliata was automatically self-pollinating. None of the large-flowered x large-flowered crosses in either section yielded new amphiploid progeny.
The explanation for the observed pattern is not far to seek. The new amphiploids in the experimental cultures arose by the union of unreduced gametes, and the breeding system affects this process. Consider an outcrossing diploid hybrid growing in a population of the diploid parental species. The hybrid will export its few unreduced pollen grains into the population where they may or may not create a triploid plant that may or may not have any progeny. However, an autogamous breeding system in a hybrid favors the union of unreduced male and female gametes of the same plant (Grant, 1956
).
Autogamy has an advantage over outcrossing for the union of unreduced gametes in hybrids in both perennial and annual plants. But the advantage is lessened in perennials by the occurrence of multiple flowering seasons and by the possibility of somatic doubling. In annuals with their single season of flowering, the advantage of autogamy over outcrossing for doubling by union of unreduced gametes is great (Grant, 1956
).
In a theoretical study using some realistic numerical values, Ramsey and Schemske (1998)
found that the rate of formation per generation of an allotetraploid from a diploid hybrid is 34 times greater with selfing than it is with backcrossing.
We can see the effects of the breeding system in the natural polyploid species of annual gilias, which probably developed by the same pathway as the experimental polyploids. All of the natural tetraploid species of Gilia sect. Gilia are small-flowered and predominantly autogamous. All of the tetraploid species of sect. Arachnion, with one exception (G. flavocincta), are small-flowered and predominantly autogamous. The same correlation of polyploidy with autogamy occurs again in the sect. Giliandra (Grant, 1956
).
A strong association of polyploidy with autogamy in annuals is found in other groups, e.g., the Madiinae (Compositae), Mentzelia (Loasaceae), and Camissonia (Onagraceae) (Clausen, Keck, and Hiesey, 1945
; Thompson and Lewis, 1955
; Grant, 1956
; Raven, 1969
).
Other factors involved in amphiploidy rate
The degree of differentiation of the parental taxa is a factor to consider. Experimental interracial hybrids produced no polyploid progeny; neither did semifertile hybrids between semispecies. The sources of experimental polyploids were highly sterile species hybrids with reduced chromosome pairing and other aberrations of meiosis. This is as expected. Amphiploidy is built on a foundation of interspecific differences in chromosome segmental arrangement. These lead to reduced pairing, hybrid sterility, and unreduced gametes in F1; doubling then brings about good bivalent pairing and a recovery of fertility in F2.
The genotype of the parental species is implicated in polyploidy rate. All of the experimental amphiploids in sect. Gilia trace their parentage back to either G. millefoliata or G. valdiviensis. These two diploid species are morphologically very different but have related subgenomes of the same chromosomal genome (Grant, 1965
). All of the spontaneous tetraploids in sect. Arachnion have G. clokeyi as one diploid parent. Many hybrid combinations of other diploid autogamous species of sect. Arachnion failed to produce spontaneous tetraploids.
Unreduced gametes play an essential role in amphiploid formation by meiotic nonreduction. The rate of formation of unreduced gametes is thus a contributing factor. Ramsey and Schemske (1998, Web Table 1) surveyed the frequency of unreduced pollen in hybrids belonging to a series of angiosperm genera. The survey shows a wide range from 2.4% to 3.0% in diploid hybrids in Quamoclit and Manihot to large values such as 83.8% and 86.0% in diploid hybrids in Lilium and Geum, respectively.
It is interesting in this connection to compare the crosses G. minor x G. clokeyi and reciprocal with G. minor x G. aliquanta and reciprocal. The first yielded spontaneous amphiploids, the second did not despite numerous opportunities in three flowering seasons. Day (1965)
compared meiosis in PMCs in the F1 hybrids of the two crosses. Both had reduced pairing. But in F1 G. minor x G. clokeyi some cells had scattered chromosomes and restitution nuclei, and some dyads of microspores were formed. In G. minor x G. aliquanta, by contrast, a well-developed spindle existed in dividing PMCs; it segregated the paired and unpaired chromosomes to poles, and restitution nuclei were not formed. Differences between the crosses in the rate of unreduced gamete formation can explain their differences in amphiploid formation (Day, 1965
).
An environmental factor affecting meiosis and polyploidy rate is seen in the comparison of the F1 hybrids of G. millefoliata x G. achilleaefolia grown in pure sand with those grown in rich loam. Chromosome pairing was reduced and polyploidy rate was increased in the sand-grown hybrids (Grant, 1952
). To what extent direct environmental effects like this play a role in polyploid origins in nature remains an open question.
The conditions affecting amphiploidy formation vary from unfavorable to favorable. In Gilia, when the conditions are favorable, new amphiploids originate in moderate to high frequencies, so that amphiploidy rate is not a limiting factor in polyploid evolution.
Recurrent polyploidy
In a series of recent papers, Soltis and coauthors have introduced and discussed the subject of recurrent polyploidy (Novak, Soltis, and Soltis, 1991
; Soltis and Soltis, 1993
, 1995
, 1999
; Soltis and Soltis, 1991
, 2000
; Soltis, Doyle, and Soltis, 1992
; Soltis et al., 1995
; Mavrodiev and Soltis, 2001
). These papers will be cited collectively here as Soltis et al. (1991–2001)
.
Recurrent polyploidy is the multiple origin of new polyploid forms from a single parental species (in autoploids) or from a given parental species pair (in amphiploids). This subject is a product of molecular systematic approaches to polyploidy. The evidence consists of isozyme or DNA markers that indicate two or more independent origins of the derived polyploid from its ancestral stock. The term is also applied retroactively to some premolecular examples.
An example is the allotetraploid Tragopogon mirus that is a derivative of the diploid species T. dubius and T. porrifolius. Isozyme, rDNA, and other evidence indicate that T. mirus had 7–11 origins (Soltis and Soltis, 1991
, 1993
). A review of the literature reveals 41 polyploid species of bryophytes, ferns, and angiosperms that, on the basis of molecular evidence, had dual or multiple origins and five additional angiosperms in which multiple origins are a possibility (Soltis and Soltis, 1993
).
These findings support the conclusion that recurrent polyploidy is a norm rather than an exception (Soltis and Soltis, 1993
; Mavrodiev and Soltis, 2001
). The findings are a positive contribution and the conclusion is consistent with the nonmolecular evidence.
If Soltis et al. (1991–2001)
had stopped at this point, I could stop here too. However, they go on to claim that molecular systematics has discovered multiple origins of polyploidy and with it the "dynamic" nature of polyploidy. They greatly underrate the role of premolecular students of polyploidy. In the following paragraphs I will present their position and contrast it with the historical record, in order to give a more accurate picture of the state of polyploidy studies in the premolecular era.
Formerly, according to D. Soltis and P. Soltis (1993
, p. 244), "polyploidization was considered a rare process, [and] each polyploid species typically was thought to have had a single origin...." Two early workers who reported recurrent polyploidy on nonmolecular evidence, in Tragopogon (Ownbey, 1950
) and Rubus (Rozanova, 1938
), are singled out as being ahead of their times (Soltis, 1993
; Mavrodiev and Soltis, 2001
). But in general, premolecular students of polyploidy allegedly held a belief in single origins.
What early student(s) of polyploidy espoused this belief? Soltis et al. (1991–2001)
cite no examples in the literature. I cannot think of any early statement of this viewpoint. The question of single versus multiple origins was not an issue with the older workers. Their primary interests lay in other aspects: types of polyploidy, developmental pathways, ancestry of natural polyploid species, etc. They were experimentalists who lived close to their plants, followed them through successive generations, and observed what happened. If there was a doubling event, well and good; if doubling occurred more than once, that was not particularly surprising.
Multiple events of amphiploid formation from the same interspecific hybrid combination were in fact found in the early period in Madia (Clausen, Keck, and Hiesey, 1945
), Gilia (Grant, 1952
), Sanicula (Bell, 1954
), Mimulus (Mia, Mukherjee, and Vickery, 1964
), and Guttierrezia (Solbrig, 1971
), as well as in Rubus (Rozanova, 1938
) and Tragopogon (Ownbey, 1950
; Ownbey and McCollum, 1953
).
Mavrodiev and Soltis (2001)
call attention to Rozanova's (1934
, 1938
) work on the hybrid Rubus idaeus x R. caesius, in which different geographical strains of the R. caesius parent produced different amphiploid progeny. They comment that "unfortunately scientists in the West remain unaware of this work because much of it was published only in Russian." Mavrodiev and Soltis (2001)
cite the Russian version but not the English version of Rozanova's 1938
paper in their bibliography. Actually, Rozanova's work on Rubus was well known, appreciated, and widely cited in the West, and was cited inter alia by Clausen, Keck, and Hiesey (1945)
, Gustafsson (1946–1947)
, Stebbins (1950)
, Darlington (1956
, 1973
), and Gottschalk (1976)
. I read her 1938 paper and made an abstract card on it soon after I got into amphiploid origins in Gilia in 1949–1950.
Mavrodiev and Soltis (2001
, p. 470) state that "the suggestion by Ownbey and McCollum (1953)
that a polyploid species could form more than once ...received little attention at that time. This is not too surprising given that prominent students of polyploid evolution such as Stebbins (1950)
, Wagner (1970)
, and Grant (1981)
did not discuss the possibility of recurrent polyploidization." The question of multiple vs. single origins was not an issue with Stebbins (1950)
and other earlier authors. However, the subject of polyploidy rate was introduced in that period (Grant, 1952
).
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Clausen J. D. D. Keck W. M. Hiesey 1945 Experimental studies on the nature of species. II. Plant evolution through amphiploidy and autoploidy, with examples from the Madiinae. Publication 564. Carnegie Institution of Washington, Washington, D.C., USA
Darlington C. D. 1956 Chromosome botany, 1st ed. George Allen and Unwin, London, UK
Darlington C. D. 1973 Chromosome botany and the origins of cultivated plants, 3rd ed. Hafner Press, New York, New York, USA
Day A. 1965 The evolution of a pair of sibling allotetraploid species of cobwebby gilias (Polemoniaceae). Aliso 6: 25-75
Gottschalk W. 1976 Die Bedeutung der Polyploidie für die Evolution der Pflanzen. Gustav Fischer Verlag, Stuttgart, Germany
Grant V. 1952 Cytogenetics of the hybrid Gilia millefoliata x achilleaefolia. I. Variations in meiosis and polyploidy rate as affected by nutritional and genetic conditions. Chromosoma 5: 372-390[CrossRef][ISI][Medline]
Grant V. 1954 Genetic and taxonomic studies in Gilia. VI. Interspecific relationships in the leafy-stemmed gilias. Aliso 3: 35-49
Grant V. 1956 The influence of breeding habit on the outcome of natural hybridization in plants. American Naturalist 90: 319-322[CrossRef][ISI]
Grant V. 1964 Genetic and taxonomic studies in Gilia. XII. Fertility relationships of the polyploidy cobwebby gilias. Aliso 5: 479-507
Grant V. 1965 Species hybrids and spontaneous amphiploids in the Gilia laciniata group. Heredity 20: 537-550[ISI]
Grant V. 1981 Plant speciation, 2nd ed. Columbia University Press, New York, New York, USA
Grant V. A. Grant 1960 Genetic and taxonomic studies in Gilia. XI. Fertility relationships of the diploid cobwebby gilias. Aliso 4: 435-481
Gustafsson Å. 1946–1947 Apomixis in higher plants. . Lunds Universitets Årsskrift 42/43: 1-370
Mavrodiev E. V. D. E. Soltis 2001 Recurring polyploid formation: an early account from the Russian literature. Taxon 50: 469-474[CrossRef][ISI]
Mia M. M. B. B. Mukherjee R. K. Vickery 1964 Chromosome counts in the section Simiolus of the genus Mimulus (Scrophulariaceae). VI. New numbers in M. guttatus, M. tigrinus and M. glabratus. Madroño 17: 156-160
Novak S. J. D. E. Soltis P. S. Soltis 1991 Ownbey's Tragopogons: 40 years later. American Journal of Botany 78: 1586-1600[CrossRef][ISI]
Ownbey M. 1950 Natural hybridization and amphiploidy in the genus Tragopogon. American Journal of Botany 37: 487-499[CrossRef][ISI]
Ownbey M. G. McCollum 1953 Cytoplasmic inheritance and reciprocal amphidiploidy in Tragopogon. American Journal of Botany 37: 487-499[CrossRef][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
Raven P. H. 1969 A revision of the genus Camissonia (Onagraceae). Contributions from the United States National Herbarium 37: 150-396
Rozanova M. A. 1934 Modes of form genesis in the genus Rubus. Journal of Botany of URRS 19: 376-384 (in Russian)
Rozanova M. A. 1938 On polymorphic type of species origin. Comptes Rendus de l'Academie des Sciences de l'URRS 18: 677-680
Solbrig O. 1971 Polyphyletic origin of tetraploid populations of Gutierrezia sarothrae (Compositae). Madroño 21: 20-25
Soltis D. E. P. S. Soltis 1993 Molecular data and the dynamic nature of polyploidy. Critical Reviews in Plant Sciences 12: 243-273
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