Format: ORAL

Rafael Urbina-Casanova1,2, Maria Giro1,2, Jeremy Foster1,2, Bryce Richardson3, Scott Jensen3, Jeremie B. Fant1,2 Andrea T. Kramer1,2.

1 Plant Biology and Conservation, Northwestern University, Chicago, IL, USA. 2 Negaunee Institute for Plant Conservation Science and Action, Chicago Botanic Garden, Glencoe, IL, USA. 3 Rocky Mountain Research Station, USDA Forest Service, USA.

Outbreeding depression is a main concern when mixing multiple source populations in restoration, as it can reduce seed quantity and quality in seed production fields and restoration sites. The main causes for outbreeding depression are unnoticed taxonomic issues and fixed chromosomal differences, but these aspects are rarely assessed when designing and implementing a restoration project. Penstemon pachyphyllus was identified as a restoration priority in the Great Basin, where the Great Basin Restoration Initiative took a regional admixture provenancing approach, combining as many as 18 source populations in pooled seed production fields, to generate large quantities of genetically diverse seeds for wide-scale restoration across this region. However, outbreeding depression was previously reported for this species when mixing populations from two different ecoregions. Because this species extends beyond the Great Basin, we assessed genome size variation across the whole species range to study potential causes of this pattern of outbreeding depression. As we found a geographically structured pattern of genome size variation, with southern populations showing a smaller genome size, we designed a controlled crosses experiment to test if genome size difference is the main driver of outbreeding depression. Moreover, we took a phylogenomic approach to assess species delimitation on our target species including all closely related taxa (i.e., the Coerulei-Gentianoides clade). We found that genome size difference is the main driver of outbreeding depression in this species, with crosses between populations with different genome sizes showing a significantly lower seed production in terms of number and weight, than crosses between populations in different geographic regions but with the same genome size. Species delimitation analyses are consistent with this incipient reproductive barrier.