Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

James F. Smith, Ph.D.


Evidence to support the origins of a putative hybrid species with certainty must be determined using several lines of evidence: the presence of genetic additivity of parental marker alleles in a putative hybrid species, along with ecological or niche separation. Novel or transgressive morphological traits obtained through chromosomal rearrangements during hybridization may facilitate niche separation of the hybrid species from progenitor habitats. These evolutionary processes together enforce reproductive isolation and promote an independent evolutionary trajectory in hybrid species. By studying these evolutionary processes in putative hybrid species, researchers may identify hybrid species with confidence.

We employed multiple lines of evidence to examine a putative hybrid origin in the rare endemic Castilleja christii, which is known from only one population on 80 hectares at the summit of Mt. Harrison, Cassia Co., Idaho. We utilized granule-bound starch synthase II (waxy) to initially address hybridization between Castilleja christii and widespread species C. miniata and/or C. linariifolia in an area of sympatry. We aligned cloned sequences from all three Castilleja species and scored all direct sequenced individuals based on this alignment for the presence, absence, or a combination of species-specific indels and/or substitutions. Interestingly, all 230 direct-sequenced Castilleja christii individuals had no unique alleles, and contained both C. miniata and C. linariifolia sequences within their genomes, indicating that C. christii is likely of hybrid origin. Morphologically, ANOVA and discriminant functions analyses tested among all three Castilleja species for 33 morphological characters revealed that C. christii shared traits with both parents while also displaying characters that were unique and transgressive. Ecological data were collected to address whether phenology, spatial, and/or ecological differences provide barriers to hybridization between the three sympatric Castilleja species at the summit of Mt. Harrison. Pollen mother cells were collected from all three Castilleja species at the summit to address cytological differences and the potential of polyploidy to act as a barrier to hybridization. All three taxa were found to be diploids (2N = 24). All three Castilleja species associated with different plant communities, were spatially distinct, and were found growing on different aspects of the summit. Based on these lines of evidence, we conclude that Castilleja christii is a stabilized homoploid hybrid derivative of C. linariifolia and C. miniata and is likely following an independent evolutionary trajectory from its progenitors.

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