Molecular tests of the proposed diploid hybrid origin of Gilia achilleifolia (polemoniaceae)

Gilia achilleifolia is a putative diploid hybrid species. Hybrid origin was hypothesized based on traditional biosystematic evidence (i.e., morphological, cytological, and crossability data), which may be insufficient to establish genealogical history. Here, phylogenetic analysis of sequence data from the internal transcribed spacer (ITS) regions is used to examine the relationship between the putative hybrid species and its proposed parents. Isozyme variation is assayed to test for genetic additivity in the putative hybrid taxon and morphological data are analyzed cladistically to evaluate the characters that led to the original hypothesis of hybrid origin. The ITS-based gene tree placed G. achilleifolia in two divergent clades, each sister to one of the putative parental lineages. Little isozyme additivity was observed and G. achilleifolia possessed six unique alleles among 42 alleles observed. However, ITS and isozyme trees differed in their placement of the two lineages of G. achilleifolia; both lineages are closer to a third putative parent in the isozyme tree. Also, G. achilleifolia is intermediate or polymorphic for all nine morphological characteristics differentiating the parental species. Sorting of ancestral polymorphisms cannot easily account for expression patterns of seven of these characters. In our view, these results fail to distinguish between alternative hypotheses of ancient hybrid origin and divergent evolution, belying the difficulty of detecting ancient hybrids.