Combining genome size and pollen morphology data to study species relationships in the genus Daucus (Apiaceae)

Background: The genus Daucus (Apiaceae) comprises about 40 wild species and the cultivated carrot, a crop of great economic and nutritional importance. The rich genetic diversity of wild Daucus species makes them a valuable gene pool for carrot improvement breeding programs. Therefore, it is essential to have good knowledge of the genome structure and relationships among wild Daucus species. To broaden such knowledge, in this research, the nuclear DNA content for 14 Daucus accessions and four closely related species was estimated by flow cytometry and their pollen morphology was analyzed by light and scanning electron microscopy (SEM).

Results: The flow cytometric analysis showed a 3.2-fold variation in the mean 2C values among Daucus taxa, ranging from 0.999 (D. carota subsp. sativus) to 3.228 pg (D. littoralis). Among the outgroup species, the mean 2C values were 1.775-2.882 pg. The pollen grains of Daucus were tricolporate, mainly prolate or perprolate (rarely) in shape, and mainly medium or small (rarely) in size (21.19-40.38 µm), whereas the outgroup species had tricolporate, perprolate-shaped, and medium-sized (26.01-49.86 µm) pollen grains. In the studied taxa, SEM analysis revealed that exine ornamentation was striate, rugulate, perforate, or the ornamentation pattern was mixed. At the time of shedding, all pollen grains were three-celled, as evidenced by DAPI staining. We also found high positive correlations between the length of the polar axis (P) and the length of the equatorial diameter (E) of pollen grains, as well as between P and P/E. However, when comparing cytogenetic information with palynological data, no significant correlations were observed.

Conclusions: This study complements the information on the nuclear DNA content in Daucus and provides comprehensive knowledge of the pollen morphology of its taxa. These findings may be important in elucidating the taxonomic relationships among Daucus species and can help in the correct identification of gene bank accessions. In a broader view, they could also be meaningful for the interpretation of evolutionary trends in the genus.