The project was no small feat. According to national program leader Peter Bretting, who has programmatic leadership responsibilities for all ARS germplasm collections, the set of genetic data they generated was one of the largest ever compiled of its kind.
“Now,?says research leader and plant pathologist Richard F. Lee, “we’ve got a relatively small core subset that represents much of the diversity in the Citrus genome. This makes it easier to search for specific genes related to pest and pathogen resistance and to high flavonoid and nutrient levels.?/p>
For their work, the researchers used 25 molecular markers, 13 of which they created, to help them track genetic similarities between accessions. The markers -- not unlike those used in forensics for determining genetic identity and parentage -- helped the researchers identify specimens of unknown origin and differentiate between closely related species.
Of the roughly 900 total accessions, about 400 of unknown, but probable, sexual origin were studied. The others were either known to be crosses from breeding programs or mutations from a parent tree -- so they wouldn’t differ enough from other accessions to be surveyed.
“About 50 of the 400 accessions represent more than 90 percent of the collection’s true diversity,?says Krueger. “That’s just 13 percent of the collection.?/p>
That may sound surprising, but it reinforces a long-held theory among citrus experts that there are just a few naturally occurring forms of citrus in the world and that the rest are hybrids of these ancestral forms. The research also supports the generally held concept that most familiar citrus