Pennisetum squamulatum: Is the Predominant Cytotype Hexaploid or Octaploid?

Apomixis is a mode of asexual reproduction where maternal clones are produced through seeds. Consequently, genetic segregation is prevented in hybrid progenies. Pennisetum squamulatum has been used to transfer apomixis into the related sexual species Pennisetum glaucum by the introgression of an apospory-specific genomic region (ASGR)-carrier chromosome. Crosses between P. glaucum and P. squamulatum or Pennisetum purpureum have been relatively easy to make even though P. squamulatum has been reported to have a different basic chromosome number than the other 2 species (9 vs. 7) and to be hexaploid (2n = 6x = 54). Our extensive examination of one accession had shown a chromosome number of 2n = 56. In order to determine if there was a variation among accessions, we counted the number of chromosomes in 5 accessions of P. squamulatum using centromeric and 18S-5.8S-26S rDNA probes as molecular cytological markers. Our results showed that P. squamulatum is most likely octaploid with a basic chromosome number of 7 (2n = 8x = 56) and may belong to the secondary gene pool of Pennisetum. Moreover, a morphologically similar ASGR-carrier chromosome that confers apomixis was observed in all accessions.