Zooming in on the lettuce genome: species relationships in Lactuca s.l., inferred from chromosomal and molecular characters

Lactuca sativa (cultivated lettuce) is the world's most important leafy salad vegetable. Apart from L. sativa , the genus Lactuca contains ca. 75 wild species, potentially useful to improve, for example, taste, texture, and disease resistance of cultivated lettuce. The wild species L. serriola (Prickly Lettuce), L. saligna (Least Lettuce), and L. virosa (Great Lettuce) are commonly used for lettuce improvement.

In preliminary experiments, we established that there is a close connection between evolutionary distances of wild species relative to cultivated lettuce, and their position in the lettuce gene pool (i.e., the possibility to hybridize them with cultivated lettuce). In the present thesis, we established evolutionary relationships among L. sativa and 22 wild species in order to predict this position.

We determined that L. sativa , L. serriola , L. dregeana , and L. altaica are closely related, and can be regarded as conspecific. L. aculeata is closely related to them, but is a distinct species. L. serriola , L. dregeana , L. altaica, and L. aculeata occupy the primary gene pool of cultivated lettuce. They can be easily hybridized with cultivated lettuce, and thus are readily accessible gene sources for lettuce improvement. L. saligna and L. virosa are less closely related to L. sativa , and occupy the secondary gene pool (i.e. hybridization with L. sativa is possible, but difficult). All primary and secondary gene-pool species can be classified in Lactuca sect. Lactuca subsect. Lactuca . We found that all tertiary gene-pool species (hybridization with L. sativa only possible with radical techniques) can be classified in the remaining sections of the genus Lactuca (sections Phaenixopus , Mulgedium, and Lactucopsis ). These sections are the most promising sources of wild species for future improvement of cultivated lettuce. In the experiments, the tertiary gene-pool species were represented by L. viminea , L. tatarica , L. sibirica , and L. quercina . Surprisingly, the species classified in Lactuca sect. Lactuca subsect. Cyanicae are not evolutionary close to cultivated lettuce. They are not part of the lettuce gene pool, and should be excluded from Lactuca .

To determine the evolutionary relationships among L. sativa and its wild relatives, we examined the genomes of the species at various levels, which provided additional information on genome evolution. We established, that in general the genome sizes in the group increased during evolution, while the ratio of AT/GC nucleotides decreased. Genome complexity for species with 2C DNA amounts below 8.5 pg was similar, but species with 2C DNA amounts exceeding 8.5 pg had more complex and less similar genomes. The species from the primary gene pool share a common ancestor, but the genomes of L. sativa / L. serriola , L. saligna , and L. virosa , evolved in different directions.

The present thesis demonstrates that with the proper combination of techniques, a plant systematic study can provide both practically applicable results and fundamental evolutionary insights, thus bridging the gap between fundamental and applied research.