- Conservation conflicts may develop on restoration sites with multiple species recovery objectives. For example, on Pacific Northwest prairies, the co-planting of the diploid cytotype of the common native wildflower Castilleja hispida with the endangered wildflower C. levisecta has resulted in putative Castilleja hybrids on restoration sites, prompting fears that genetic swamping could threaten C. levisecta. Because C. hispida is a larval host for the endangered Taylor’s checkerspot butterfly (Euphydryas editha taylori), this situation puts the recovery of both of these species at risk. However, hybrid fertility in this system is unknown.
To assess hybrid fertility and introgression in Castilleja hybrids, we conducted a series of controlled reciprocal crosses between C. levisecta and C. hispida, and backcrosses between F1 hybrids and their progenitors. We measured the resulting fruit set, seed set, and seed germination to look at post pollination barriers to reproduction. Because populations of C. hispida can be diploid (2n = 2x = 24), tetraploid (2n = 4x = 48), or hexaploid (2n = 6x = 72), and C. levisecta is only diploid (2n = 2x = 24), this project explores mixed ploidy crosses between the two species.
Reproductive isolation from C. levisecta was between 32 – 61% in crosses between C. levisecta and diploid C. hispida, 89 – 100% in C. levisecta and tetraploid C. hispida crosses, and 98 – 99% in C. levisecta and hexaploid C. hispida crosses. Reproductive isolation from C. levisecta was between -22 – 0% in backcrosses between C. levisecta and diploid F1 hybrids, suggesting higher diploid hybrid fitness. Reproductive isolation from C. levisecta was between 99 – 100% in backcrosses between C. levisecta and triploid F1 hybrids, suggesting interploidy crosses act as a barrier to gene flow.
In addition, in order to prevent introgression, the eradication of putative hybrids is critically important to the conservation of the C. levisecta genome, but hybrids can appear extremely morphologically close to either parental species, or highly distinct. Because ploidy differences between two interspecific mating partners can be a potent isolation mechanism in plants, land managers might choose to co-plant a polyploid C. hispida cytotype with C. levisecta at recovery sites in order to mitigate hybridization between these two species, but little morphological data exists on C. hispida cytotypes. To better identify hybrids and C. hispida polyploids, we looked at whether measurable differences are detectable in 15 bract, calyx, and floral characteristics of C. levisecta, three cytotypes of C. hispida, and their diploid F1 hybrids, and if so, whether those differences are distinct enough for field technicians to distinguish them.
Using multivariate analyses and univariate ANOVA, we found groups of traits that distinguished between hybrids and their progenitors, and C. hispida cytotypes, that could prove useful for field biologists.