Homoploid hybrid speciation in a rare endemic Castilleja from Idaho (Castilleja christii,Orobanchaceae)

• Premise of the study: Hybridization is an important evolutionary force in the history of angiosperms; however, there are few examples of stabilized species derived through homoploid hybrid speciation. Homoploid hybrid species are generally detected via the presence of genetic additivity of parental markers, novel ecological and spatial distinctions, and novel morphological traits, all of which may aid in the successful establishment of hybrid species from parental types. Speciation and diversification within the genus Castilleja (Orobanchaceae) has been attributed to high levels of hybridization and polyploidy, though currently there are no examples of homoploid hybrid speciation within the genus. We employed multiple lines of evidence to examine a putative hybrid origin in C. christii, a rare endemic, known only from 80 hectares at the summit of Mt. Harrison (Cassia Co., Idaho). • Methods: We used granule-bound starch synthase II (waxy) sequences and 26 morphological characters to address hybridization between C. christii and widespread congeners C. miniata and/or C. linariifolia in an area of sympatry. Chromosomes of C. christii were also counted for the first time. • Key results: All 230 direct-sequenced C. christii individuals had the additive genomes of both C. miniata and C. linariifolia. Castilleja christii shares traits with both parents but also has floral characters that are unique and transgressive. Cytological counts indicated that all three taxa are diploid. • Conclusions: We conclude that C. christii is a stabilized homoploid hybrid derivative of C. linariifolia and C. miniata and is likely following an independent evolutionary trajectory from its progenitors.