Conservation of rare, endemic braya species (Brassicaceae): Breeding system variation, potential hybridization and human disturbance

Abstract

The closely related, endangered Braya longii and threatened Braya fernaldii (Brassicaceae) are endemic to the Limestone Barrens on the Northern Peninsula of Newfoundland, Canada. These species were not known to co-occur naturally, but anthropogenic disturbance has mediated contact. Future preservation strategies must consider population variability, breeding system and possible hybridization in the context of human use. Breeding system and hybridization were assessed using floral measurements and hand-pollination experiments under field conditions. Morphological analysis revealed the presence of two well-defined groups as well as population-level variation in floral characters. PCA confirmed intraspecific variation in both species, mediated by origin of disturbance. Natural populations of B. longii are differentiated due to very limited seed dispersal, whereas non-differentiated anthropogenically disturbed populations reflect the homogenising effect of aggregate movement among sites. Although both species are primarily autogamous, floral characters indicate B. longii has great potential for outcrossing compared with B. fernaldii. Outcrossed hand-pollinations resulted in significantly lower fruit set for B. fernaldii (<18%) than for B. longii (64%). Hybridization potential was suggested by the presence of intermediate forms in wild populations as well as fruit set from hand-pollinations between the two species. These results indicate that conservation strategies should focus on preserving populations throughout the entire range of each species in order to capture genetic variation among populations. Human activities that promote the population homogenisation, such as movement of quarried gravels among sites, should be minimized. Reintroductions should be undertaken with caution to prevent possible hybridization.