Domestication, genomics and the future for banana

Abstract

Background: Cultivated bananas and plantains are giant herbaceous plants within the genus Musa. They are both sterile and parthenocarpic so the fruit develops without seed. The cultivated hybrids and species are mostly triploid (2n = 3x = 33; a few are diploid or tetraploid), and most have been propagated from mutants found in the wild. With a production of 100 million tons annually, banana is a staple food across the Asian, African and American tropics, with the 15 % that is exported being important to many economies.

Scope: There are well over a thousand domesticated Musa cultivars and their genetic diversity is high, indicating multiple origins from different wild hybrids between two principle ancestral species. However, the difficulty of genetics and sterility of the crop has meant that the development of new varieties through hybridization, mutation or transformation was not very successful in the 20th century. Knowledge of structural and functional genomics and genes, reproductive physiology, cytogenetics, and comparative genomics with rice, Arabidopsis and other model species has increased our understanding of Musa and its diversity enormously.

Conclusions: There are major challenges to banana production from virulent diseases, abiotic stresses and new demands for sustainability, quality, transport and yield. Within the genepool of cultivars and wild species there are genetic resistances to many stresses. Genomic approaches are now rapidly advancing in Musa and have the prospect of helping enable banana to maintain and increase its importance as a staple food and cash crop through integration of genetical, evolutionary and structural data, allowing targeted breeding, transformation and efficient use of Musa biodiversity in the future.

Fig. 1.

The diversity of banana and plantains on sale in a shop in south India (Varkala, Kerala State) with various genome compositions. Cultivars are indicated by letters above bunches: a, cultivar ‘Red’ (AAA genome constitution), a prized sweet dessert banana cultivar. Differences between bunches are mostly from water and nitrogen conditions in the field, and not genetic. b, ‘Palayam Codan’ (AAB). c, ‘Njalipoovan’ AB (unripe and ripe, green and yellow) sweet dessert banana with small fingers, thin skin and delicate flavour but poor keeping quality and the fruits fall off bunches. d, ‘Robusta’ (‘Cavendish’ group, AAA); ‘Cavendish’ cultivars ripen without changing to yellow (green ripe) when above 22 °C. e, ‘Nendran’ (AAB), used for cooking and for making chips. f, ‘Peyan’ (ABB) used as a vegetable for curries and for cooked snacks. g, ‘Poovan’ (AAB). (A light tube has been edited out of the picture in the top left.)

Fig. 2.

A, A banana plant with ripening fruit bunch. A sucker is growing from the base of the stem which will form a replacement plant after the fruit is harvested and the mother plant cut down. B, The dessert banana ‘Gros Michel’ (AAA, 2n=3x=33) killed by Panama disease or Fusarium wilt. ‘Gros Michel’ was the major export banana before spread of the disease led to its replacement by the variety ‘Cavendish’ which accounts for nearly all the export trade in banana.

Fig. 3.

In situ hybridization to banana chromosomes (2n = 3x = 33) stained blue (A, D) with the fluorochrome DAPI. (A–C) hybridization of 5S (labelled green, B) and 45S (red, C) rDNA to chromosomes from the ABB cooking banana ‘Fougamou’ showing three major 45S rDNA loci (one chromosome in each genome carries locus), while multiple chromosomes have 5S loci. (D–F) A metaphase from the ABB (2n = 3x = 33) cultivar ‘Bluggoe’ labelled with total genomic DNA from the diploid genome donor Musa acuminata (A genome; red in E). The DNA hybridizes predominantly to the centromeres of A genome origin chromosome and identifies these 11 chromosomes, shown in the drawing in F (Osuji et al., 1997, 1998). Scale bar in (D) = 5 µm.

Fig. 4.

A dot-plot comparing about 100 000 bp (x and y axes) of the genomic DNA sequence in homoeologous BAC clones from the A and B genome. Horizontal: Genbank accession AC186955 from Musa acuminata; vertical AC186754 from M. balbisiana (submitted to database by Chris Town et al., TIGR, 2006; dot-plot made using program by Sonnhammer and Durbin, 1995). Similar DNA sequences are indicated by lines made up of dots, while gaps indicate regions of different sequence or insertions and deletions.