Experimental evidence for contemporary production of a triploid agamospermous hybrid between European and Japanese dandelions through backcrossing

Background/Question/Methods

Asexual reproduction through non-fertilized seeds is called agamospermy. Agamospermous taxa often show high genetic variability. It have been inferred that this high genetic variability is derived from hybridization between an agamospermous species and its sexual relatives. To our knowledge, however, there has been no direct evidence of this hypothetical hybridization process in natural populations. Agamospermous dandelions of hybrid origin between a native sexual diploid species (Taraxacum platycarpum) and an alien agamospermous triploid (T. officinale) are now widely distributed throughout mainland Japan. These hybrid dandelions are known to be genetically variable. We hypothesized that this variability is maintained by repeated ongoing hybridization, based on the fact that triploid dandelions not only produce seeds agamospermously, but also produce some functional pollen grains that can sire seeds of sexual dandelions. To test this hypothesis, first, we examined whether heads of Japanese diploid dandelions produce new hybrid seeds after fertilization with pollen from triploid agamosperms under field conditions. Second, we determined genotypes at three microsatellite loci for 38 backcrossed polyploid hybrids obtained from artificial backcrossing of F1 hybrids as pollen parents with diploid T. platycarpum; ploidy and hybridity were confirmed by flow cytometry and ITS genotyping, respectively.

Results/Conclusions

First, one of the 430 tested plants grown from sexual dandelion seeds was a triploid and had morphological and molecular characteristics, which are consistent with a hybrid origin; a bagging experiment showed that it can produce some germinable seeds by agamospermy. The plant formed the hybrid seed were surrounded in the field by many individuals having recurved involucral bracts, which were hybrids (96.5%) or T. officinale (3.5 %). These results suggested that the new hybrid is derived from a backcross of a hybrid to the native sexual species. Second, backcrossed hybrids showed high phenotypic variability probably as a result of segregation of various alleles inherited from both parents. In conclusion, these findings supported that high genetic variability found among triploid hybrids in the field is the result of backcrossing of hybrids with T. platycarpum and subsequent genetic segregation.