“Seedless cherimoya, the next banana?” read the headlines of a story that has circulated on the Web since Monday, referring to the discovery of a gene for seedlessness in a fruit related to cherimoya by Charles Gasser, professor of plant biology at UC Davis, and three Spanish colleagues. The prospect sounds inviting, because the cherimoya is supremely delicious, but contains numerous hard black seeds (whence its name, which means “cold seeds” in Quechua, the Inca language).
Seedless cherimoyas have been around for decades, but have not become common, either in home gardens or commercial orchards, because the trees are not very productive; the seeds are necessary for normal fruit development, so seedless fruits are small and misshapen. Researchers in Japan have also figured out how to make standard varieties of cherimoya seedless by controlling pollination and applying natural plant growth hormones called gibberellins, but this approach has not proved commercially practical.
Gasser’s group does not yet have a seedless cherimoya, but a seedless form of a more tropical relative, sugar apple. This was found in Thailand and brought to Spain, said Gasser on a recent visit to his office. Seedless sugar apples are common in backyards, but previous varieties have been malformed, with mediocre flavor. This one, however, is full size, with good flavor, said Gasser, whose article appeared online Monday in the journal Proceedings of the National Academy of Sciences; the seed coats appear to develop just far enough to induce the fruit to develop normally, but no seeds develop.
His colleagues in Spain, where cherimoyas are grown commercially, are working to breed a seedless cherimoya by crossing their seedless sugar apple with a normal cherimoya. The trait for seedlessness is recessive, so it will take several generations, but Gasser is confident that a seedless cherimoya-like hybrid will result. (This is all through conventional breeding, not genetic modification.)
Most interestingly, Gasser has identified the same mutant gene for seedlessness in Arabidopsis, a species of cress that is the standard model organism used for studying plant biology. Because sugar apples and cherimoyas are in the magnolia family, which is among the most primitive of flowering plants, “that means that we can probably control this gene to induce seedlessness in basically all flowering plants,” Gasser said.
Cherimoyas, which are cultivated on about 300 acres in California, will likely become more popular if seedless varieties become available. But they probably won’t ever be as abundant as bananas and pineapples, two other fruits that lost their seeds during domestication. Cherimoyas require labor-intensive hand-pollination and are strongly climacteric, passing quickly from firm to ripe to squishy and brown, so they’ll probably remain an expensive delicacy best suited for local sale.
-- David Karp
Photo: Thai seedless sugar apple, related to cherimoyas, are being used by a team of Spanish researchers to hybridize a seedless cherimoya-like fruit. Credit: Emilio Guirado