Karyotypes of Bensoniella, Conimitella, Lithophragma, and Mitella, and Relationships in Saxifrageae (Saxifragaceae)

With the exception of Mitella breweri and M. trifida, all North American species of Mitella, Conimitella williamsii, and species of Lithophragma possess essentially identical chromosome morphology. The high degree of karyotypic similarity among genera is further indicated by normal bivalent formation and high pollen stainability in intergeneric hybrids involving Conimitella wil- liamsii and Mitella stauropetala. Conimitella, Lithophragma, and most species of Mitella possess karyotypes that are indistinguishable from those of species of Heuchera, Tolmiea, and Tiarella. Chromosomal data, as well as evidence from morphology, intergeneric hybridization, flavonoid chemistry, and the host preferences of Puccinia rusts, suggest that Conimitella, Lithophragma, Mitella, Heuchera, Tiarella, and Tolmiea form the core of a natural group of genera having x = 7. The monotypic Bensoniella, as well as Elmera and Tellima, differ in chromosome morphology from species of these six genera. Other lines of evidence, including morphology and flavonoid chemistry, indicate, however, that all of these genera are closely allied. A second natural group of genera having x = 7 is composed of Bolandra, Boykinia, Suksdorfia, and Sullivantia. Studies of species throughout Saxifrageae indicate that chromosome morphology is highly conserved in the tribe. Chromosomal data, in conjunction with insights from enzyme electrophoresis, suggest that this group radiated rapidly in North America with little accompanying chromosomal or genetic divergence.