Ploidy and genome composition of Musa germplasm at the International Institute of Tropical Agriculture (IITA) - 2006



Musa spp (bananas and plantains) constitute a hybrid-polyploid complex and are classified according
to different genome compositions such as AA, BB, AB, AAA, AAB, ABB, AAAA, ABBB, AAAB and
AABB. Knowledge of ploidy and exact genome compositions of the parental material is essential for
Musa breeding. This study determined the ploidy levels and genome composition of the Musa
germplasm collection, constituting over 300 accessions, at the International Institute of Tropical
Agriculture in Nigeria and Uganda. Flow cytometric analysis of nuclear DNA content was used to
estimate ploidy levels, while genome composition was ascertained with RAPD markers that are specific
for the A and B genomes of Musa. It was determined that at least 8% of the plants in the germplasm
collection were miss-classified in terms of ploidy and/or genome composition. The cultivars 'Pisang
awak', 'Foulah 4' and 'Nzizi', previously classified as triploids, were found to be tetraploids by flow
cytometry and conventional root tip chromosome counts. Similarly, cultivars that were previously
classified as diploids including ‘Too’, and ‘Toowoolee’ were found to be triploids in our analysis. Ploidy
and genome classification in Musa was generally determined from morphological characteristics. While
our study showed that such a system is not always reliable, it was interesting to find that none of the
plantains in the germplasm collection were miss-classified with regards to both ploidy and genome
composition.


Genetic diversity and population structure of Musa accessions in ex situ conservation - 2013

Background
Banana cultivars are mostly derived from hybridization between wild diploid subspecies of Musa acuminata (A genome) and M. balbisiana (B genome), and they exhibit various levels of ploidy and genomic constitution. The Embrapa ex situ Musa collection contains over 220 accessions, of which only a few have been genetically characterized. Knowledge regarding the genetic relationships and diversity between modern cultivars and wild relatives would assist in conservation and breeding strategies. Our objectives were to determine the genomic constitution based on Internal Transcribed Spacer (ITS) regions polymorphism and the ploidy of all accessions by flow cytometry and to investigate the population structure of the collection using Simple Sequence Repeat (SSR) loci as co-dominant markers based on Structure software, not previously performed in Musa.

Results
From the 221 accessions analyzed by flow cytometry, the correct ploidy was confirmed or established for 212 (95.9%), whereas digestion of the ITS region confirmed the genomic constitution of 209 (94.6%). Neighbor-joining clustering analysis derived from SSR binary data allowed the detection of two major groups, essentially distinguished by the presence or absence of the B genome, while subgroups were formed according to the genomic composition and commercial classification. The co-dominant nature of SSR was explored to analyze the structure of the population based on a Bayesian approach, detecting 21 subpopulations. Most of the subpopulations were in agreement with the clustering analysis.

Conclusions
The data generated by flow cytometry, ITS and SSR supported the hypothesis about the occurrence of homeologue recombination between A and B genomes, leading to discrepancies in the number of sets or portions from each parental genome. These phenomenons have been largely disregarded in the evolution of banana, as the “single-step domestication” hypothesis had long predominated. These findings will have an impact in future breeding approaches. Structure analysis enabled the efficient detection of ancestry of recently developed tetraploid hybrids by breeding programs, and for some triploids. However, for the main commercial subgroups, Structure appeared to be less efficient to detect the ancestry in diploid groups, possibly due to sampling restrictions. The possibility of inferring the membership among accessions to correct the effects of genetic structure opens possibilities for its use in marker-assisted selection by association mapping.










Your Gateway to U.S. Federal Science

African Journals Online

Review: Domestication, Genomics and the Future for Banana

Ploidy and genome composition of Musa germplasm

Genetic diversity and population structure of Musa accessions in ex situ conservation

Can an individual subgroup contain cultivars that have differing

ploidy levels and or genome compositions?