A multinational team of researchers has sequenced the genomes of Mexican, Guatemalan and West Indian avocados, and the most commercially popular hybrid cultivar, Hass, shedding light on the ancient origins of this buttery fruit and laying the groundwork for future improvements to farming.
The avocado (Persea americana) is a commercially important tree fruit species in the Lauraceae family, otherwise known for the spices cinnamon, bay leaves, and sassafras. It is a vital crop for Mexico, from which almost 50% of all avocado exports originate, valued at about $2.5 billion US dollars.
Although the avocado has an ancient cultivation history in Mexico and Central to South America, its extreme worldwide popularity as an oily, nutty-flavored fruit with highly beneficial nutritional properties dates mainly from the early 20th century.
Cultivated avocados occur in three landraces with possibly independent cultivation origins that reflect their current distribution: the Mexican, Guatemalan, and West Indian varieties.
The principal industrial avocado cultivar is known as Hass, after the grower who first patented it in 1935.
“Avocado is a crop of enormous importance globally, but particularly to Mexico,” said Texas Tech University’s Professor Luis Herrera-Estrella, co-lead author of the study.
“Although most people will have only tasted Hass or a couple of other types, there are a huge number of great avocado varieties in the species’ Mexican center of diversity, but few people will have tried them unless they travel south of the U.S. border.”
“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.”
The team successfully sequenced and analyzed the nuclear genomes of Mexican and Hass variety avocados, and also resequenced the genomes of Guatemalan and West Indian varieties.
“We study the genomic past of avocado to design the future of this strategic crop for Mexico,” Professor Herrera-Estrella said.
“The long life cycle of avocado makes breeding programs difficult, so genomic tools will make it possible to create faster and more effective breeding programs for the improvement of this increasingly popular fruit.”
The avocado belongs to a relatively small group of plants called magnoliids, which diverged from other flowering plant species about 150 million years ago.
The new research supports — but does not prove — the hypothesis that magnoliids, as a group, predate the two dominant lineages of flowering plants alive today, the eudicots and monocots.
“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,” said co-lead author Professor Victor Albert, from the University at Buffalo and Singapore’s Nanyang Technological University.
“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.”
Having addressed some ancient mysteries of the avocado, the study also moves forward in time to explore a modern chapter in the story of this beloved fruit: how humans have altered the species’ DNA.
Because commercial growers typically cultivate avocados by grafting branches of existing trees onto new rootstocks, today’s Hass avocados are genetically the same as the first Hass avocado planted in the 1920s.
These modern-day Hass avocados are grown on Hass branches grafted onto various rootstock that are well adapted for particular geographic regions.
While the Hass avocado was long thought to be a hybrid, the details of its provenance — 61% Mexican, 39% Guatemalan — were not previously known.
The Hass avocado genome reveals huge chunks of contiguous DNA from each parental type, reflecting the cultivar’s recent origin.
“Immediately after hybridization, you get these giant blocks of DNA from the parent plants,” Professor Herrera-Estrella explained.
“These blocks break up over many generations as you have more reproductive events that scramble the chromosomes. But we don’t see this scrambling in the Hass avocado. On chromosome 4, one whole arm appears to be Guatemalan, while the other is Mexican. We see big chunks of DNA in the Hass avocado that reflect its heritage.”
The study was published in the Proceedings of the National Academy of Sciences.
Martha Rendón-Anaya et al. The avocado genome informs deep angiosperm phylogeny, highlights introgressive hybridization, and reveals pathogen-influenced gene space adaptation. PNAS, published online August 6, 2019; doi: 10.1073/pnas.1822129116