A Consideration of the Species Relationships in Subtribe Brassicinae (Cruciferae) in View of Cluster Analysis of Morphological Characters

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The taxonomic relationships of 53 species (122 strains) in subtribe Brassicinae were studied by using Euclidian distances calculated from 53 metric characters. Through cluster analysis applied to the species and subspecies in the genus Brassica, six large clusters could be defined. By the same method, Diplotaxis was grouped into four clusters, Erucastrum into two, and Sinapis into two. These clusters coincided in general with the taxonomic sections of Schulz (1936), but some exceptions were noted. Most of the exceptional species were those placed in critical positions by cytogenetical observations. It was found that cluster grouping reflected the cytogenetical relations of species fairly well. Relationships among clusters and genera are represented by a Euclidian distance matrix, and the evolutionary trends of those taxa are discussed. Clusters B-3 (in Brassica), D-1 (in Diplotaxis) and E-1 (in Erucastrum) are closely related to each other, forming a complex. This complex is closely related to Sinapidendron. It is postulated that this complex and Sinapidendron might have retained some characters in common with their putative ancestor. Species in the other clusters are considered to be derived from that hypothetical ancestor in the course of evolutionary divergence.
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