Cytogenetic evidence supporting the merger of heterotheca and chrysopsis (Compositae)

Conclusion

The average pollen stainability of 21 % in the F₁ hybrid, although markedly reduced from that of the parental plants, is reasonably high. The essentially regular meiosis in the F₁ hybrids indicates the existence of sufficient chromosomal homology between the parental genomes for chiasmata formation with few evident gross structural differences. Lack of crossability in itself hardly constitutes valid evidence against genetic relationships since crossability may be blocked by the action of relatively few genes. On the other hand, the ability to produce viable hybrids with good chromosome pairing, which is presumably indicative of extensive chromosomal homologies between the parental genomes, seems to constitute convincing proof of close relationship.

Both morphological and cytological evidence seems to support Shinners’ merger ofChrysopsis andHeterotheca. This would appear to be another case where pappus characters have been unduly emphasized in the separation of genera in Compositae. The species ofHeterotheca s. str. form a recognizably distinct, obviously monophyletic, close-knit group that seems best treated as one of five sections under a broadened concept of the genus. It appears to represent a segregate taxon derived from, and probably even more closely related to the mid-western and western members of theChrysopsis villosa complex than the latter group is to either sect.Pityopsis orChrysopsis of the southeastern United States. But the congeneric status of all sections withinChrysopsis, with the possible exception of sect.Pityopsis, is attested to by the presence of at least some morphological forms which tend to bridge the gaps between them.