Morphological Evolution in Layia (Compositae): Character Recombination in Hybrids Between L. discoidea and L. glandulosa

Abstract

Layia glandulosa and L. discoidea are self-incompatible annual plants, native to Cali- fornia, which are completely interfertile and appear to be related as progenitor and recent derivative. They are conspicuously different in floral morphology. Layia glandulosa has sunflower-like heads with showy female ray florets associated with clasping involucral bracts, while L. discoidea lacks both ray florets and clasping bracts, a difference governed primarily by two genes. F2 and backcross progenies were examined in order to determine whether the quantitative traits distinguishing the species were correlated with each other or with the major gene differences governing head type. A number of significant pairwise correlations were found between the quantitative characters. Radiate and discoid F2 plants differed in head size, disk floret corolla length, and disk floret pappus length but did not consistently differ in vegetative traits. In backcross progenies, radiate and discoid plants differed slightly in pappus length but not in other traits. The associations of quantitative characters with head type accounted for only a fraction of their variance in the hybrid progenies. Thus, the evolution of discoid heads in L. discoidea does not appear to have required the coordinated evolution of other traits by which it differs from L. glandulosa.