Karyotype stability and genome-specific nucleolar dominance in peanut, its wild 4× ancestor, and a synthetic AABB polyploid

Abstract

Allopolyploidy is a significant evolutionary process involved in the origin of many crops, including peanut (Arachis hypogaea L.). The process usually results in a series of chromosome, genomic and epigenetic rearrangements in the derived polyploids. Here, we examined the chromosomal consequences undergone by AABB tetraploids of Arachis after the genome merger. For that objective, different chromosome markers and DNA contents were compared among peanut, its wild tetraploid ancestor, and the diploid genome donors A. duranensis Krapov. & W.C. Gregory (AA, female) and A. ipaënsis Krapov. & W.C. Gregory (BB, male). The analysis also included an artificially synthesized allotetraploid using A. ipaënsis as a female [(A. ipaënsis × A. duranensis)4×]. The karyotypes in the natural (originated ~10,000 yr ago) and newly synthesized allopolyploids have largely maintained the patterns of heterochromatin and ribosomal RNA loci detected in the diploid progenitors. Intergenomic translocations were not evident using genome in situ hybridization, and the DNA contents of the allotetraploids corresponded to the expected sum of those observed in their parental species. The analysis of ribosomal DNA loci and their association with nucleolar organizing regions revealed a rapid establishment of nucleolar dominance in favor of the A genome. The large macrostructural stability of karyotype observed here after polyploidization has not been frequently cited for polyploid crop plants. This stability is significant for peanut breeding, since it suggests that effective introgression of wild useful alleles into cultivated peanut may potentially occur in most of the extension of the A and B chromosome complements.