Results of pilot experiments indicate, that certain changes of the phenotype are caused either by the structural reorganization of individual chromosomes, as is the case in maize or Drosophila, due to either polyploidization as in cultivars, or to aneuploidy, as is the case in Datura or wheat (Triticum). Such changes of the karyotype are therefore a visible expression of the genome’s rearrangement.
In order to find rules or tendencies correlated with the systematic position of a species or a higher taxon, a comprehensive survey of the karyotypes of as large a number of species as possible was required. Considerable progress in clarifying the phylogenetic relations between species came about as a consequence of the analysis of the karyotypes, and the simultaneous study of the geographic distribution and the ecological preferences of closely related species. A general answer, however, is still far away, since the discovery of certain evolutionary strategies is no evidence for their general validity.
At the beginning of the 1960th, the chromosome numbers of more than 17,000 angiosperms were known. V. GRANT (1963, Rancho Santa Ana Botanical Garden) compiled and presented them in a frequency distribution histogram. The lowest number (n = 2) belonged to the Compositae Haplopappus gracilis, while a species of Kalanchoe has a rather high value of n = 250. Ferns do also have extremely high values. An increase in the number of chromosomes is usually associated with a reduction in chromosome size. The comparison of the karyotypes of closely related species revealed some conspicuous, though not universally applicable patterns of distribution.
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he numbers or frequently multiples of a basic number (x), such as 2x, 3x, 4x, ...., nx. We will focus on this topic in a section of its own, because of the eminent importance such ploidy series have in connection with the speciation problem. |
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Related species do often have the same number of chromosomes. Occasionally, even all species of a genus (Pinus, n = 12; M. C. FERGUSON, 1904, for example) or of a whole family (e.g.Thymeleaceae, n = 9; E. STRASBURGER, 1910) have the same number of chromosomes. Identical numbers of chromosomes do not necessarily mean identical shapes of comparable chromosomes. Examples of different shapes at roughly the same chromosomal size occur in species of the genus Lilium. Related species are often characterized by differently sized chromosomes. |
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Homologies of single chromosomes or chromosomal segments occur also in related species. The differences in the karyotypes of Narcissus bulbodicum and Narcissus cantabricus, sympatric species from the south-west Mediterranean, for example, seem to be the result of two inversions and three translocations. . |
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The different chromosomal numbers and sizes of legumes and North American species belonging to the genus Crepis were caused by diminution of chromosomes, a loss of DNA, loss of whole chromosomes, and translocations. A detailed analysis of their karyotypes revealed, that certain chromosomal sections from different species could be homologous, and that changes of the chromosomal patterns, especially reductions of chromosomal sizes, correlate with an increased specialization of the respective species. The most primitive, i.e. less specialized, species has n = 6 chromosomes. Both annual and perennial forms exist. Species with n = 3 chromosomes are predominantly annuals. Even the seemingly immutable rule of the constant chromosome number per species is not unbreakable. |
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As said above, intraspecific races differing with respect to their degree of ploidy exist. |
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The chromosomal numbers of the species of a few families are hard to determine, because of the accidental presence of the so-called B-chromosomes, that occur in variable amounts. B-chromosomes are especially frequent in grasses, Liliaceae, and Compositae. They are usually heterochromatic and do not affect the phenotype. In general, they are much smaller than the normal chromosomes, also called A-chromosomes. An inexperienced cytologist may nevertheless not always be able to distinguish them. |
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All species of a few genera, like Carex, Luzula, Scirpus, Eleocharis, as well as some species of other genera, like Poa alpina have variable chromosomal numbers. One Carex-species, for example, has chromosomal values ranging from n = 36 to n = 66, though no differences in the phenotype occur. One cause for this variability is the existence of diffuse, polycentric, or multiple centromers. On one hand, the genomes of these species seem to be well balanced, the loss of chromosomes is compensated for by the remaining chromosomes. On the other hand, chromosomes present in high numbers are smaller than in cells with lower numbers. |
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Environmental parameters may add to the selection of variable chromosomal patterns, thereby contributing to the development of local races. I. FUKUDA and R. B. CHANELL (1975) studied the karyotypes of distinct North- American populations of white trillium (Trillium ovatum). Undivided populations of the coastal region with its extensive coniferous forest zone and its almost homogeneous climate display only minor differences in the structure of a particular chromosome. In contrast, the karyotypes of the isolated, fairly small populations of the Idaho and Montana Rocky Mountains vary considerably. These regions are variable both in climate and geology. The colonization with Trillium occurred after the last glaciation. The variation of the chromosomal patterns correlates with a morphological variation of the isolated populations. |
In summary, the distribution of chromosome sizes shows several trends (G.L. STEBBINS, 1971).
Gymnosperms (Cycadales, Gingko, Coniferales) have on average larger chromosomes than most other plants. They are surpassed by a few angiosperm genera and families (Paeonia, Lilium, Trillium, Tradescantia, Krameriaceae, Loranthaceae).
Wooden angiosperms have without exception small and usually hardly distinguishable chromosomes. Between related species and genera exist often no detectable differences.
Related species and genera of herbaceous angiosperms display considerable differences in the sizes of their chromosomes. The shapes of the chromosomes do provide no clue for the phylogenetic position of the family, nor do they allow predictions of the chromosome size of any species.
In spore-producing pteridophytes, smaller chromosomes are associated with heterosporous families or genera (Selaginella, Isoetis, Marsiliaceae, Salviniaceae), while larger chromosomes are typical for homosporous groups.
Chromosome sizes of significant difference have two, partly complementary causes:
Species with larger chromosomes have more active genes than species with smaller chromosomes.
Species with large chromosomes contain large amounts of non-coding DNA, also called repetitive DNA or heterochromatin.
These two alternatives are discussed in the topic ‘Changes at the Molecular Level’.
Apart from analyzing the chromosomal numbers of distinct species, interpretations of their variability in larger phylogenic groups of plants are feasible. As a result, the chromosomal numbers of herbaceous angiosperms have been shown to vary by the factor 100, while this factor is 14 in woody angiosperms and just 2 in conifers. The chromosomal numbers of cycads do not vary at all. An increase in the morphological variability correlates usually with an increase of the chromosomal variability. The next chapter shows, that herbaceous angiosperms use a wide range of reproductive strategies. The sizes of their populations are far smaller than that of woody species. Optimized genotypes can easily be accumulated, the speed of evolution is thus increased (D. A. LEVIN, A. C. WILSON, 1976).
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