Abstract
To elucidate the ancestry of the allopolyploids <i>E. stevenii</i> and <i>E. boöphthona</i>, I examined eleven isozyme loci and 24 morphological characters from 28 populations representing five related <i>Euphorbia</i> species from Australia. According to an analysis of genetic and morphological data, three diploid species differentiated recently, but two independent polyploid species are estimated to have differentiated a relatively long time ago. Fixed heterozygosity for most isozymes in <i>E. stevenii</i> and <i>E. boöphthona</i> strongly suggests that these two species are allopolyploids rather than autopolyploids. The isozyme profiles of <i>E. stevenii</i> indicate that it is an allopolyploid that evolved from interspecific hybridization between the diploid <i>E. tannensis</i> and unidentified or extinct tetraploid species. In addition, isozyme patterns strongly suggest that <i>E. stevenii</i> was one of the ancestors of <i>E. boöphthona</i>. However, <i>E. boöphthona</i> showed a large number of fixed alleles that were not detected in any other Australian <i>Eremophyton</i> species. The most likely hypothesis for the origin of <i>E. boöphthona</i> is that it was formed by hybridization and chromosomal doubling between an extinct diploid species and the hexaploid <i>E. stevenii</i>.
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