Cytogenetical Studies in the Genus Phaseolus Linn

III. Evolution in the genus Phaseolus

抄録

The detailed cytogenetical studies have been undertaken in fifteen species of genus Phaseolus, Linn. for the first time to determine the chromosomal basis of speciation and evolution.
Cytogenetical investigations in 15 species of Phaseolus wild and cultivated so far studied revealed the somatic chromosome number to be 2n=22 (except wild allotetraploid Phaseolus species 2n=44). The somatic chromosomes are small, their size range from 0.85μ to 3.0μ. The longest being 3.0μ in P. vulgaris and shortest in 0.85μ in P. ricciardianus. Though the chromosome number of all the species is same they differ from each other in certain karyotypic details. This is of great significance in the origin of new taxonomic units.
Inversion heterozygosity has been observed in five species. The number of such bridges are either one or two. The role of both pericentric and paracentric inversions in the evolution and origin of species has been quite significant.
The univalent chromosomes occur in six species. The average of univalents per PMC range from 1.68 (P. bracteatus) to 0.96 (P. trilobus). The role of genetical factors, precocious separation, inversion heterozygosity, desynapsis and environmental factors such as change in temperature are to a great extent responsible for the appearance of univalents in different species of Phaseolus.
Pollen sterility appears to be fairly low among various taxa of Phaseolus. The percentage of pollen sterility ranges from 18% (P. semierectus) to 3% (P. acutifolius and P. aconitifolius, etc.). Inversion heterozygosity and presence of univalents have been emphasized as the main causes of pollen sterility.
Evolutionary trend in the genus Phaseolus has been studied. Polyploidy has not played any significant role in evolution. Structural chromosomal changes as pericentric and paracentric inversions, gene mutations, hybridization and weak development of isolation mechanism have dominated the evolutionary history in the genus phaseolus.. Diminution in the size of chromatin material with advancing evolution has been a special feature. Species with symmetrical karyotypes having large chromosomes with median primary constrictions are primitive while those with asymmetrical karyotypes having smaller chromosomes with subterminal primary constriction are regarded as advanced.
Phaseolus vulgaris and P. aureus with large somatic chromosomes and longest total chromatin length has been considered primitive respectively in climbing and erect habit series of Phaseolus species. While P. bracteatus and P. ricciardianus with short chromosomes and shortest total chromatin length are considered as advanced in climbing and erect habit series respectively. Presumably climbing habit Phaseolus species are ancient and erect ones are modern.
It is thus concluded that Phaseolus vulgaris which is under cultivation in Mexico for the past 7000 years is primitive and it has given rise to other Phaseolus species including erect ones.