Musa as a model for genomics
The Musa genome offers new perspectives in the monocotyledon, with its relatively small haploid genome of 500 to 600 Mb (only 25% larger than rice) divided among 11 chromosomes. Because of its relatively small size, the Musa genome is highly tractable to complete functional and sequence analysis and extensive characterization of the genes will be possible using realistic high-throughput technologies.
Musa offers an interesting model for genetic studies, as it is one of the few plant species with bi-parental cytoplasmic inheritance: paternal inheritance of mitochondria and maternal inheritance of chloroplasts.
In the centre of origin and diversity in Southeast Asia there are many sterile clones that have been genomically static or fixed for thousands of years of vegetative propagation in the same environment. There are also partially fertile and highly fertile wild diploid equivalents that have been actively evolving for the same period in the same environment, ranking Musa among one of the most perfect models to study plant evolution at a genomic level. Moreover, they have co-evolved for the same period and in the same environment with most of the Musa pathogens. Both of these co-evolutions reinforce the position of Musa as a perfect model to study plant and pathogen evolutions at a genomic level.
About 3000 years ago, a few plantain varieties were introduced from the centre of origin into Africa where they underwent secondary diversification exclusively through mutations, in the absence of most of their pathogens. This provides also a valuable tool for the study evolution in the presence and absence of pathogens. As a vegetatively propagated crop, it offers a good model to study the role of somaclonal variation and phenomena such as "imprinting".
The variability in the different ploidy levels in Musa offers a very special opportunity to gain insight to the greater-than-additive gains in crop productivity that often accompany polyploidy. Even the most well studied polyploids like cotton and sugarcane, contain only one type of polyploidy within the taxon, whereas Musa includes a number of autopolyploids (AAA, AAAA, AAAAAA), and different types of allopolyploid (allotriploids AAB, ABB and allotetraploid AABB, AAAB) in addition to the diploid M. acuminata (AA) and M. balbisiana (BB) and AB hybrids. Bananas and plantains are attractive models to study the role of hybridisation and polyploidy in the evolution of cultivated crops, and to analyse the interaction of parental genomes at chromosomal and sequence level.
The diverse arrays of polyploid and diploid genomes in Musa create opportunities not only to study the relationship of polyploid formation to phenotype, but also the causes and consequences of polyploidy for genome organization.
The combination of parthenocarpy and sterility that has led to the typical edible banana is of basic interest in a wide range of other fruit crops, and is especially unusual in that there are relatively few parthenocarpic monocots.
The small size of the Musa genome and the different ploidy levels of the existing crops and species within Musa as well as the combination of parthenocarpy with sterility and vegetative propagation, place the banana among one of the few opportunities to study the specific gene expression in different chromosomal environments.
Being a monocotyledon but taxonomically very distantly related to rice, Musa is an ideal candidate for studying synteny between distantly related species.
Musa was the first species where a pararetrovirus was shown to be integrated in the plant genome with the capacity to give rise to episomal banana streak badnavirus. Understanding the mechanism behind this phenomenon may lead to important applications, such as gene targeting.