Polyploidization is an important, though not the exclusive cause for speciation, i.e. the development of new species, in angiosperms. Studying all connected phenomena in detail is therefore important. Closely related pairs of species differ often in their degree of ploidy, as is the case with Nasturtium officinalis (n = 16) and Nasturtium microphyllum (n = 32) and Cardamine hirsutum (n = 38) and Cardamine flexuosa (n = 16). A third species of Cardamine (C. pratensis) has usually n = 8 chromosomes, but populations with n = 12, 14, 15, 16, 19, 20, 21 – 28, 30, 32, 37, 42, 44, 45, and approximately 48 chromosomes exist.
A comparison of the chromosomal numbers of closely related species shows quickly, whether one of them has twice as many chromosomes as the other. One of such a pair of species is thus tetraploid, the other is diploid. In a few cases, however, this method reveals the actual basic number (x). Usually, the analysis of a large number of species of a given genus is necessary in order to find their smallest possible denominator, i.e. x.
Most herbaceous angiosperms have basic numbers of 6 – 9, while the number is 11 or higher in woody angiosperm families and gymnosperms.
The degree of ploidy correlates sometimes with morphological features, geographic distribution, or ecological preferences of a species:
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Species and subspecies of the Galium pumilus group are common in South and Central Europe. All degrees of ploidy between diploidy and decaploidy exist. The diploid species Galium austriacum lives as a glacial relict in a few geographically isolated areas of the Alps. Its corresponding polyploid species and subspecies are widely distributed, and their populations merge with each other. Galium anisophyllum covers all evenly numbered degrees of ploidy between diploidy and decaploidy. Here again, the diploid races are glacial relicts, that form tessellated patterns throughout European mountainous areas. In contrast, the post-glacial octoploids are widely and evenly distributed. The areas of the different populations are clearly demarcated (F. EHRENDORFER, Botanical Institute, University of Vienna, 1949, 1964). |
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The distribution of the diploid and tetraploid races of Biscutella laevigata (family: Brassicaceae) in Europe correlates strictly with the widest expansion of the last glacial period in the Pliocene. Diploid races live in the formerly unfrozen glacial valleys. When the ice retreated, tetraploid races re-populated the Alpine ranges and spread from there to Southern Europe. (I. MANTON, 1934, 1937) |
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Diploid and tetraploid populations of Asplenium trichomans do not differ morphologically. Diploid races prefer acidic, tetraploid races basic soils. |
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Ranunculus ficaria – populations with 2x, 4x, 5x, and 6x occur in Europe. Each of them displays minor, though significant morphological variations. Everywhere, where both diploid and tetraploid races occupy the same area, the tetraploid races differ from the diploid races by a set of site-specific features, i.e. no unique features distinguish the tetraploid from the diploid. It seems instead, as if the increase in ploidy provides the plant with more flexibility in expressing its genome. It can thus react with different strategies to the variable challenges of its environment. |
These examples provide evidence for the existence of different degrees of ploidy within a single species. They do, too, illustrate, that populations with different degrees of ploidy differ in their ecological requirements or their geographical distribution. Polyploid races do usually not succeed in areas already populated by the corresponding diploid races. The do, nevertheless, have an advantage in colonizing new (disturbed) habitats, a feature called adaptive radiation. Polyploidization and adaptive radiation may therefore be the first step in the development of new species.
We showed already in the topic ‘Genetics’ that meiosis is often disturbed in polyploids and especially in autopolyploids. As a consequence, the proportion of perennial, long-lived species, as well as that of species preferentially reproducing by vegetative methods (agamospermic reproduction - apomixis) is extremely high. 30 to 75% of all angiosperms and the overwhelming majority of ferns are polyploid (G. L. STEBBINS, 1938, 1940, 1947, 1950; G. TISCHLER, 1950), while polyploidy is rare in gymnosperms.
During the mid-1940th, A. and D. LÖVE published a number of articles showing that the amount of polyploid species increases with increasing geographical latitude beginning in the province of Schleswig-Holstein, the northernmost part of Germany, and proceeding through Scandinavia to Spitsbergen. The Swedish geneticist Å. GUSTAFSON drew attention to the fact (1948), that different plant families had been analyzed in the different climatic zones. The percentage of Graminaceae, Cyperaceae, and Rosaceae does indeed increase the further North you come, and in general, only these families host a high percentage of polyploid species. Only very few species have diploid species growing in warm climates, while their polyploid races occur in cold zones. In 1932, O. HAGERUP succeeded in inducing polyploidy experimentally as a result of a cold shock, but this does nevertheless seem to be an inadequate explanation for the increase in the percentage of polyploid species in cold climates. It remains thus unanswered, why the species of certain families with a high percentage of polyploids are more successful under extreme conditions than species from families with only rarely occurring polyploidy. Some gymnosperm exceptions seem to exist, since they are typical for the boreal zone. But although they occur in abundance and are widely spread, they are represented by only very few species. Supplementary studies from A. and D. LÖVE and colleagues confirmed the original observations. In accordance, it was observed, that the number of polyploid species of mountainous areas increases with increasing altitude (A. W. JOHNSON, J. G. PACKER, 1965).
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