Conservation conflicts may develop on restoration sites with multiple species recovery objectives. For example, the recovery of Taylor’s checkerspot butterfly (Euphydryas editha taylori) and golden paintbrush (Castilleja levisecta), two federally listed species, have come into conflict on several remnant prairies in southwest Washington. This conflict arises from the routine planting of Harsh paintbrush (C. hispida), which is a favored pre-diapause larval host plant for Taylor’s checkerspot. Recently, biologists have confirmed Castilleja specimens of mixed morphology on recovery sites, prompting fears that genetic swamping could threaten golden paintbrush. The primary objective of this research is to assess the introgressive threat to golden paintbrush by determining whether hybrids are fertile and capable of producing viable offspring. Because populations of harsh paintbrush can be diploid (2n = 12), tetraploid (4n = 24), or hexaploid (6n = 36), and golden paintbrush is only diploid (2n = 12), this project explores mixed ploidy crosses between the two species. We conducted a series of controlled crosses between golden paintbrush and the three ploidy levels of harsh paintbrush, and measured the resulting fruit set, seed set, and seed germination.
Results/Conclusions
Hybrid crosses between diploid golden paintbrush and all ploidy levels of harsh paintbrush produced equal fruit set (p=0.295), but the type of cross significantly affected seed set (p<0.0001) and seed germination (p=0.002). Seed set from crosses between golden paintbrush and 2n harsh paintbrush averaged 54% (± 6%), while golden paintbrush crosses with 4n and 6n harsh paintbrush yielded 1.2% (±0.7%) and 0.2% (±0.1%) seed set. Also, seed germination from golden paintbrush crossed with diploid harsh paintbrush averaged 73% (± 11%), while germination of seeds from golden paintbrush crossed with tetraploid harsh paintbrush averaged 15% (±13%). Hybrid crosses between the diploid types appear to pose a potential threat to golden paintbrush through introgression and genetic swamping. However, crosses with polyploid harsh paintbrush have lower risk of successful hybridization. Therefore, the conservation conflict between golden paintbrush and Taylor’s checkerspot may be reduced by planting only polyploid races of harsh paintbrush at sites where both rare species are managed.