Botanical Gazette 131(3): 245-259 (Sept 1970)
Genetic and Transplant Studies on Contrasting Species and Ecological Races of the Achillea millefolium Complex
W. M. Hiesey, M. A. Nobs

Genetic relationships between species and contrasting ecological races of the same species of the Achillea millefolium complex have been investigated at Stanford and at altitudinal field stations in California. Diploid species (n = 9), including A. asplenifolia, A. setacea, and A. tomentosa, all native to central Europe, are separated by strong sterility barriers and are easily distinguished morphologically. Tetraploid (n = 18) forms of A. millefolium from North America include a multitude of polymorphic ecological races that are highly interfertile among themselves. Second-generation progeny from such crosses segregate widely. Hexaploids (n = 27), also native to North America, have evolved equally diverse ecological races which, when intercrossed, also yield fertile hybrids and highly segregating F2 progeny. Tetraploids cross with hexaploids to yield pentaploids (2n = 45-48) that vary in fertility, but mostly produce viable seed. The responses of parental, F1, and segregating F2 progeny of a cross between contrasting latitudinal forms of hexaploid A. millefolium were studied when grown as vegetatively-propagated clones at Stanford (elevation 30 m), Mather (1,400 m) and Timberline (3,050 m) over a 5-year period. The northern form, originally from Kiska Island, becomes less than 10 cm tall when grown at Stanford in contrast with a giant southern form from the San Joaquin Valley of California which grows up to 180 cm. The Kiska parent fails to survive for more than 1 or 2 years in gardens at Stanford, in contrast with the southern parent that thrives indefinitely in this environment. The segregating F2 progeny differ widely in their capacity to survive at Stanford. At Mather, neither of the parents can survive, but the F1 and most of the F2 progeny are remarkably vigorous. At Timberline, where the Kiska parent survives with some difficulty and the San Joaquin parent is completely eliminated, the F1 hybrid displays heterosis over both parents, but only few F2 individuals survive over an extended period. These results are compared with data from similar selection experiments with contrasting ecological races of Potentilla and Mimulus. Achillea, like Potentilla and Mimulus, shows evidence of genetic coherence of characters that distinguish contrasting ecological races.