Cytogeography of the South American Species of Epilobium (Onagraceae)

Abstract

Greenhouse hybrids involving ten of the 12 native species of Epilobium in South America and two from elsewhere were analyzed for meiotic chromosome behavior and hybrid fertility. Two chromosome end arrangements-AA and BB-characterize nine of the ten South American species studied; the one strain of E. puberulum used here differs from BB by one translocation. One strain of E. hirtigerum is shown to have a novel arrangement that differs from two other South American strains of this species, both of which possess the BB arrangement. Biogeographic interpretations suggest that the species with AA chromosomes were derived from North American ancestors and those with BB chromosomes originated from Australasian ancestors. Solomon (1982) recognizes 12 native species of Epilobium in South America, ten of which are endemic or nearly so. This contrasts sharply with two earlier treatments of the South American species by Haussknecht (1884) and Samuelsson (1923, 1930) in which 18 species and 34 species and ten varieties were recognized, respectively. All South American species belong to sect. Epilobium, which consists of about 180 species found in moist, open habitats at relatively high latitudes and elevations throughout the world (Raven 1976). The species of sect. Epilobium that have been examined cytologically have a gametic chromosome number of n = 18, with the chromosomes morphologically indistinguishable from one another. The discovery of reciprocal translocations within sect. Epilobium by Mosquin (1968) and subsequent elaboration of this phenomenon by Seavey and Raven (1977a, 1977b, 1977c, 1978) and Hair et al. (1977) provide a fairly detailed knowledge of the number and geographic distribution of these translocations in sect. Epilobium. These chromosome analyses have proven to be very useful for elucidating the interrelationships of the species in the section. Three common chromosomal arrangements characterize the species of sect. Epilobium and have been given the arbitrary designations of AA, BB, and CC (Seavey and Raven 1977b); the original arrangement, from which the other two are believed to be derived, is BB (Seavey and Raven 1978). Of the 12 native species considered here, six have been used previously in a preliminary study of artificial hybridization and chromosome homologies (Seavey and Raven 1977a): two were found to possess chromosomes homologous with those of the North American E. ciliatum subsp. watsonii, designated the AA arrangement, and three were found to have chromosomes homologous with those of the European E. obscurum, the BB arrangement. The AA arrangement differs from the BB arrangement by one reciprocal translocation. One strain of E. hirtigerum was found to have neither of these standard arrangements-it differed from AA by two non-overlapping translocations and from BB by one translocation. We describe the chromosomal homologies of four additional species, extend the geographic range of species used previously, analyze the homologies of additional strains of E. hirtigerum, and interpret the phylogeny and cytogeography of the native South American species of Epilobium in the context of a modern taxonomic treatment of these species (Solomon 1982). MATERIALS AND METHODS Plants were grown from seed in experimental greenhouses at the Missouri Botanical Garden and at Lewis & Clark College. Hybrids were produced by pollination following emascula-