Nature of chromosome pairing in allopolyploids of Arachis and their stability.

Abstract

Studies of chromosome pairing in interspecific hybrids and induced allopolyploids of Arachis elucidated the genomic structure of A. hypogaea and other tetraploid species. The diploid species (2n=20) probably originated from 10 chromosome (2n) ancestors as yet unknown. In view of the paucity of evidence from studies on diploid hybrids and their amphidiploids, the nature of genomic differentiation of the individual diploid species is not well understood at present. The chromosomes of the haploid complement of the diploid species have homeologous counterparts in the A and B genomes of the tetraploid species and a set of 5 chromosomes in one of the genomes of the latter appears to exhibit a high level of differentiation. In the allohexaploids which comprise the genomes of the tetraploid and diploid species, pairing seems to be enhanced by genetic mechanisms even to give maximum expression to the residual affinity. It is probable that the genotype of the diploid species brings in the accentuation of pairing due to a breakdown of diploidising factor present in the polyploids of Arachis.Triploid and tetraploid hybrids and the allohexaploid manifested multivalent chromosome pairing and aberrant meiotic behaviour. The realisation of balanced euploid derivatives in different proportions signified the organisation of gametes with three different chromosome numbers as a result of directed genomic segregation in the meiosis of the progenitor polyploids. Euselectivity of gametes in ferfertilization was also evidenced from the analysis of progenies resulting from backcrosses of A. hypogaea with the three synthetic polyploids. Auxiliary mechanisms arising from genomic combinations seemed to be effective in determining the breeding behaviour of the experimental polyploids. The deviations in the strength of expression of the genic mechanisms also appeared to depend upon the differences in the genomic composition of the interspecific hybrids and induced polyploids.