Study Traces Banana Chromosome Evolution From 17 to 9–11

Scientists have reconstructed the ancestral chromosome arrangement of the banana family, concluding that its common ancestor likely had a haploid chromosome number of 17 before successive rearrangements produced the 11, 10 and 9 chromosome complements found in living Musaceae lineages.

The research was led by scientists at the South China Botanical Garden of the Chinese Academy of Sciences, working with Sichuan University and other institutions. The findings were published in Current Biology, according to an August 21 announcement from the South China Botanical Garden.

The researchers did not recover chromosomes or DNA directly from an ancient banana ancestor. Instead, they inferred the ancestral karyotype, meaning its chromosome organization, by comparing chromosome structures across living members of the banana family.

How scientists reconstructed the chromosome changes

A key part of the work was a telomere-to-telomere, gap-free genome assembly of Musa exotica, an ornamental banana relative from an early-diverging lineage. Researchers combined that genome with other available Musaceae genomes for comparative analysis.

The resulting reconstruction pointed to an ancestral haploid chromosome number of n = 17. The researchers concluded that the chromosome complements found in living lineages at n = 11, n = 10 and n = 9 did not arise independently but developed progressively through repeated structural changes.

Those changes included reciprocal chromosome translocations, in which chromosome segments are exchanged; end-to-end joining; and nested chromosome fusions, in which one chromosome becomes incorporated into another.

The reconstructed pattern was also highly consistent with evolutionary relationships previously inferred from DNA sequence data. That gives researchers an additional source of evidence, based on chromosome structure, for examining relationships among major Musaceae lineages.

Chromosome changes may be linked to bract coloration

The study also examined whether chromosome restructuring could be connected with differences in bract color. Bracts are specialized leaves associated with flowers or inflorescences and can be brightly colored in ornamental banana relatives.

Regions associated with chromosome-rearrangement breakpoints were enriched in genes involved in anthocyanin production and regulation. These included structural genes such as CHS and F3H, which encode enzymes involved in the anthocyanin biosynthesis pathway, as well as regulatory transcription factors including MYB and bHLH.

Transcriptomic comparisons of differently colored bracts also showed coordinated activity among several anthocyanin-related genes, including CHS, CHI, F3′5′H and ANS. According to the researchers, differences in their expression were associated primarily with transcriptional regulation rather than simple changes in the number of gene copies.

The findings suggest that chromosome restructuring may have changed genomic environments and regulatory networks in ways that contributed to bract-color diversification. They do not establish that chromosome rearrangements alone caused the different colors seen across the banana family.

Why the ancestral karyotype matters

The reconstruction gives scientists a reference for comparing chromosome structures among Musaceae species and tracing how genetic material was reorganized as different lineages evolved.

The researchers say it could also help scientists investigate genetic information retained by wild banana relatives and assess its potential value. That is relevant because many cultivated banana varieties are vegetatively propagated and have relatively narrow genetic diversity, while wild Musaceae contain additional genetic variation.

The study also provides a framework for investigating how genome structural changes relate to visible traits such as bract coloration in ornamental bananas.

It does not immediately produce a new banana variety, disease-resistant cultivar or farming treatment. Any practical crop improvement would require further work to identify useful traits, establish their genetic effects and determine whether they can be incorporated effectively into cultivated bananas.

An earlier version of the research was released as a bioRxiv preprint in May. The Current Biology study now provides a chromosome-level evolutionary framework that researchers can use to investigate genome restructuring and trait diversification across the banana family.