The formation of new species by crossing of different species will be explained with a few out of the very many well-known examples.
Phleum pratense, the Timothy grass, Cat's tail, is a common species in damp meadows. It has 2n = 42 chromosomes. J. W. GREGOR, F. W. SANSONE, 1930 and H. NORDENSKIÖLD, 1937, succeeded in producing this already naturally occurring species anew. They crossed Phleum nodosum (2n = 14) with Phleum alpinum (2n = 28). The hybrid proved to be fully fertile and corresponded in all its properties to Phleum pratense. Irrespective of this, NORDENSKIÖLD produced Phleum pratense by autopolyploidy of Phleum nodosum (1949). It seems that this species is equipped with a simple, Phleum alpinum with a double and Phleum pratense with a threefold genome. What seemed to be an allotetraploid hybrid (Phleum nodosum x Phleum alpinum) was thus shown to be an autopolyploid one.
Around 1870, a new species of
grass turned up at the salt marches near the coast of the
English Channel: Spartina
townsendii . It was taller than the indigenous Spartina
alternifolia. Another relative, Spartina stricta,
inhabits the North-American east coast. It was brought in to
Europe and began to occupy the sites of Spartina alternifolia
- and exactly there was also the new species found. It was now
suspected that Spartina townsendii was a hybrid of the two
original species. The fact that Spartina townsendii has 2n
= 126 chromosomes, Spartina alternifolia has 2n = 70 and
Spartina stricta has 2n = 56 chromosomes makes this
suggestion seem likely (C. L. HUSKINS, 1931).
It has long since been presumed, that the cultivated variety of tobacco (Nicotiana tabacum) with its 2n= 48 chromosomes is an allotetraploid hybrid of Nicotiana tomentosiformis (2n = 24) and Nicotiana sylvestris (2n = 24). The assumption was hardened by the analysis of the respective crossings since the cross with other wild varieties resulted in combinations that bore only faint resemblances to Nicotiana tabacum. A final proof was found several years ago when it could be shown with molecular methods that chloroplast genes used by Nicotiana tabacum resemble those of Nicotiana sylvestris completely (see theme 42; CHEN et al., 1976). This finding showed at the same time that the hybrid had been generated by the cross of Nicotiana sylvestris (female) x Nicotiana tomentosiformis (male).
In 1928, G. D. KARPETSCHENKO (Institute of Applied Botany, Detskoje Selo near St. Petersburg) produced a new species: Raphanobrassica (2n = 36) by crossing Raphanus sativus (2n = 18; radish) and Brassica oleraceae (2n = 18; cabbage). It seemed at first as if the hybrid was sterile but after numerous experiments, a fertile specimen could be found. Fertility was preceded by the doubling of the chromosomal set. Accordingly, we have the following combinations: 9+9= 18 (sterile) and 18 x 2 = 36 (fertile).
In the genera Brassica, species have developed under
natural conditions by the crossing of different species, too. The
analysis of the karyotype of several species regarded as related
led to the conclusion that always two of them have common genome
parts. The results can be depicted in a U-scheme
that gives a clear representation of the species' relations.
The Japanese cultivator H. KIHARA found out during the twenties that the genome of wheat (Triticum aestivum) consists of several partial genomes of which at least one tallies with the genome of Emmer (Triticum dicoccum), a primitive cultivated variety. During the 1940s, the genesis of wheat largely was clarified by R. SEARS and H. KIHARA. There exists a whole range of different cultured wheat varieties of which some are diploid, others tetraploid and some, like Triticum aestivum are hexaploid (n = 21). The genome of this last species consists of three parts, A, B and D. A corresponds to Triticum monococcum (n = 7), B could not reliably be assigned to any wild variety but combinations of A and B were found in a number of cultured varieties, like, for example Triticum dicoccoides, Triticum dicoccum, Triticum turgidum, Triticum persicum, Triticum polonicum and Triticum durum. D stems from Triticum tauschii (= Aegilops squarrosa; n = 7). Between A and B exist certain similarities though they are outweighed by the differences since only bivalents -and never quadrivalents- are found in Triticum aestivum. In other words: A-chromosomes pair only with themselves, never with B-chromosomes. The same is true for B- and D-chromosomes. In the following picture are the assumed relations of descent illustrated.
These examples point out the implications allopolyploidy has in the formation of new species. We will talk more about the formation of new species in the section about evolution. The following picture (for explanation) shows the different possibilities to increase the number of chromosomes by allopolyploidy.
|
|