Amphitropical Species Pairs in Microseris and Agoseris (Compositae: Cichorieae)

Abstract

In the flora of temperate South America are two species of Compositae, tribe Cichorieae, subtribe Micro-seridinae, that are disjunct from the North American centers of distribution of their respective genera. Microseris pygmaea, an autogamous annual of central Chile, is morphologically similar to several species of like habit, found in the climatically comparable Californian region. A hybrid obtained between M. pygmaea and the closely related North American species, M. bigelovii, showed normal meiotic behavior and was almost fully fertile. In an F2 generation of 55 plants, pheno-typic segregation was more in the direction of M. bigelovii, and two individuals resembled that species with only minor differences. There was a complex pattern of inheritance for the number of pappus scales on the fruit, a characteristic of major taxonomic importance in distinguishing the parent species. A reduction in seed fertility of certain hybrid plants was ascribed to segregation of a small, nonreciprocal chromosome translocation. It was concluded that the two species are adequately distinct taxonomically but are genetically closely related and interfertile. Microseris probably reached South America by longdistance dispersal, the most likely time for migration being the middle or late Pliocene Epoch. In the second example, Agoseris coronopifolia is a variable species, distributed over the southern tip of the continent, which can be differentiated from the North American Agoseris heterophylla only with difficulty. Both species are composed primarily of annual or perennial plants with autogamous reproduction. Adaptations for out-crossing are present in some races of A. heterophylla but not in A. coronopifolia. Preliminary crossing experiments suggest that genetic barriers exist between the species. The general pattern of phytogeographical and evolutionary relationships in Agoseris and Microseris is like that in certain other families of similar distribution.