Think pumpkins are for Halloween only? Think again. For many people around
the world, pumpkins are source of nutrition. Now, scientists at Boyce
Thompson Institute (BTI) in the US and the National Engineering Research
Center for Vegetables in Beijing, China have sequenced the genomes of two
important pumpkin species, Cucurbita maxima and Cucurbita moschata to
understand their desirable traits.

The researchers sequenced the two different pumpkin species to better
understand their contrasting desirable traits: C. moschata is known for its
resistance to disease and other stresses, such as extreme temperatures,
while C. maxima is better known for its fruit quality and nutrition. The
hybrid of these two species called Shintosa has even greater stress
tolerance than C. moschata, and is often used as a rootstock for other
cucurbit crops.

The sequencing project revealed an interesting evolutionary history for
Cucurbita species. By comparing the Cucurbita genome sequences to those of
other cucurbits, the researchers discovered that the pumpkin genome is
actually a combination of two ancient genomes, making it a paleotetraploid.
Although the pumpkin is considered a diploid today, the genome sequence
analysis revealed that between 3-20 million years ago, two different
ancestral species combined their genomes to create an allotetraploid - a new
species with four (tetra-) copies of each chromosome, from two different
(allo-) species.

Typically, when an allotetraploid is formed, the genome experiences
downsizing and gene loss, eventually transforming the new species back into
a diploid, or one of the contributing genomes will dominate over the others
to retain more genes, a phenomenon observed in maize and cotton. This did
not happen for pumpkins. The ancient Cucurbita allotetraploid lost its
duplicated genes randomly from both of the contributing diploids.
Furthermore, the ancestral chromosome remained largely intact, leaving the
modern pumpkin with two subgenomes representing the ancient species that
contributed to the paleotetraploid.

[btiscience.org]