Young Hass has a mixed background
New study sheds light on the avocado's family history.
The Hass avocado, the world’s most popular, is a 61:39 mix of Mexican and Guatemalan varieties, researchers say.
They know, because they sequenced its genome.
In fact, the US-Mexican team sequenced a variety of avocados (Persea Americana) from Mexico, Guatemala and the West Indies, which are each home to genetically distinct, native cultivars of the fruit.
"These varieties are genetic resources for avocado's future. We needed to sequence the avocado genome to make the species accessible to modern genomic-assisted breeding efforts," says Luis Herrera-Estrella from Texas Tech University in the US.
The work also involved researchers from University at Buffalo, US, and Mexico’s National Laboratory of Genomics for Biodiversity. The findings are published in the journal Proceedings of the National Academy of Sciences.
The avocado belongs to a relatively small group of plants called magnoliids, which diverged from other flowering plant species about 150 million years ago.
If this is right, the researchers say, it would not mean that avocados themselves are older than eudicots and monocots, but that avocados belong to a hereditary line that split off from other flowering plants before the eudicots and monocots did.
"One of the things that we did in the paper was try to solve the issue of what is the relationship of avocados to other major flowering plants, and this turned out to be a tough question," says Buffalo's Victor Albert.
"Because magnoliids diverged from other major flowering plant groups so rapidly and so early on, at a time when other major groups were also diverging, the whole thing is totally damn mysterious.
“We made contributions toward finding an answer by comparing the avocado genome to the genomes of other plant species, but we did not arrive at a firm conclusion."
The new paper suggests that the avocado experienced two ancient polyploidy events, in which the organism's entire genome got copied.
Many of the duplicated genes were eventually deleted, but some went on to develop new and useful functions, and these genes are still found in the avocado today. Among them, genes involved in regulating DNA transcription, a process critical to regulating other genes, are over-represented.
The research also finds that avocados have leveraged tandem duplicates – the product of isolated events in which an individual gene gets replicated by mistake during reproduction – for purposes that may include manufacturing chemicals to ward off fungal attacks.
"In the avocado, we see a common story: two methods of gene duplication resulting in very different functional results over deep time," says Albert.
The scientists' new map of the Hass avocado genome reveals large chunks of contiguous DNA from both its Mexican and its Guatemalan parents, reflecting its recent origin.
Hass were first planted in the 1920s and because commercial growers typically cultivate avocados by grafting branches of existing trees onto new rootstocks, the researchers say today's Hass avocados are genetically the same as the first ones.