Banana’s Ancestors are a Mystery

It is believed that humans domesticated bananas for the first time 7,000 years ago on the island of New Guinea. However, the history of banana domestication is complicated, and the distinction between species and subspecies is often unclear.

A recently published study reveals that this history is significantly more complicated than previously imagined[1]. The findings show that the genomes of the current domesticated varieties include remnants from three extra, as of yet unidentified, ancestors.

“We show that most of today’s diploid cultivated bananas that descend from the wild banana Musa acuminata are hybrids between different subspecies. At least three extra wild ‘mystery ancestors’ must have contributed to this mixed genome thousands of years ago, but haven’t been identified yet,” said Dr. Julie Sardos, the study’s lead author.

Domesticated bananas (except for Fei bananas in the Pacific) are believed to have descended from a group of four ancestors, which were either subspecies of the wild banana Musa acuminata or different but closely related species. Before being domesticated, Musa acuminata existed in Australasia and seems to have developed on the northern borderlands between India and Myanmar about 10 million years ago. Another complication is that domesticated varieties may contain two (‘diploid’), three (‘triploid’), or even four (‘tetraploid’) copies of every chromosome, and many are derived from the wild species Musa balbisiana.

Recent smaller-scale studies suggested that other ancestors linked to Musa acuminata may have been involved in the domestication, suggesting that even this highly complicated scenario may not be the whole story. The latest findings not only validate this to be the case but also demonstrate for the first time that these gene pools are common in domesticated banana genomes.

The authors sequenced the DNA in 226 extracts leaf extracts. Among these samples, 68 belonged to nine wild subspecies of Musa acuminata, 154 to diploid domesticated varieties descended from Musa acuminata, and four more distantly related wild species and hybrids as comparisons. Many had previously been gathered in dedicated ‘banana collecting missions’ to Indonesia, the island of New Guinea, and Bougainville, an island which is part of Papua New Guinea.

The researchers first measured the levels of relatedness between cultivars and wild bananas and made “family trees” based on the diversity at 39,031 Single Nucleotide Polymorphisms (SNPs). They used a subset of these – evenly spread across the genome, with each pair demarcating a block of approximately 100,000 “DNA letters” – to statistically analyze the ancestry of each block. For the first time, they detected traces of three further ancestors in the genome of all domesticated samples, for which no matches are yet known from the wild.

The mystery ancestors might be long since extinct. “But our personal conviction is that they are still living somewhere in the wild, either poorly described by science or not described at all, in which case they are probably threatened,” said Sardos.

Genetic comparisons show that the first of these mystery ancestors must have come from the region between the Gulf of Thailand and west of the South China Sea. The second is from the region between north Borneo and the Philippines. The third, from the island of New Guinea.

[1] Sardos et al: Hybridization, missing wild ancestors and the domestication of cultivated diploid bananas in Frontiers in Plant Science - 2022. See here.