Cytological studies in a tetraploid potato (Solanum polytrichon Rydb.).

Abstract

The haploid number of chromosomes in S. polytrichon Rydb. is found to be 24 (n=24) and is therefore tetraploid, since the basic number of chromosomes in potato is n=12. Frequencies of chiasmata at diplotene and diakinesis are analysed. The chiasma frequency per bivalent at diplotene and diakinesis varied from 1.62 to 1.22 respectively.The incidence of quadrivalents in the tetraploid species investigated is high 44.33%. This gives indication that they are autotetraploid and therefore shows a high degree of affinity between four sets of chromosomes.The percentage of univalents is 21.62. Mechanical interference with pairing owing to presence of four sets of homologous chromosomes is likely to cause more failure of pairing in tetraploids. It is also probably that genetic dissimilarity of individual chromosomes is also responsible for certain amount of failure of pairing amongst pairable chromosome mates in Solanum. The univalents behave in dif-ferent ways; they either pass to the poles or lag in between separating groups of chromosomes or divide. The division and elimination of univalents cause unequal distribution of chromosomes on second metaphase plates. Univalents and fragments which failed to be included in the daughter nuclei formed micronuclei (44.06%).The frequency of chromatid bridge is 3.94. There is no relationship between the chromatid bridge and deformity of pollen in S. polytrichon. The abortion of pollen grains in this species is presumably due to gene mutation.The strong homology between four genomes indicates that it is probably of autopolyploid origin. It can therefore be suggested that one of the main lines of evolution is the tetraploid S. polytrichon Rydb. has been the occurrence of polyploidy.