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How Are Rare Species Maintained?: Reproductive Barriers Between Layia jonesii, A Rare Serpentine Endemic, And L. platyglossa Natalie L. Rossington Reproductive barriers are vital to generating new species as well as maintaining distinct species. Investigating reproductive barriers between closely related plant taxa helps us to understand how these barriers are maintained, particularly between rare and widespread relatives. Layia jonesii, a rare San Luis Obispo County serpentine endemic, and L. platyglossa, a common coastal species, co-occur on serpentine derived hillsides and are interfertile. At these locations, L. jonesii is isolated to dry soils near serpentine rock outcrops and L. platyglossa is located on slightly deeper grassland soils surrounding the rock outcrops. On hillsides where they co-occur, I observe two morphologically distinct species, therefore the two species must be maintaining reproductive barriers, yet mechanisms that maintain this isolation are unknown. I studied this system to investigate possible mechanisms contributing to the maintenance of reproductive barriers. I hypothesize prezygotic reproductive isolation in this system is due to (1) habitat isolation due to local adaptation to differential edaphic environments on the hillside, (2) flowering time differences, and (3) reduced seed set resulting from hybrid crosses. To investigate the local adaptation of L. jonesii and L. platyglossa, I reciprocally transplanted both species into the center of each species’ distribution. I also conducted a competition experiment to determine if L. jonesii is sensitive to resource competition beyond its natural distribution. To investigate flowering time differences, I tracked flowering time of both wild and reciprocally transplanted populations. I also performed controlled crosses to determine if heterospecific, or hybrid crosses, result in lowered seed set than conspecific crosses. The reciprocal transplants showed L. platyglossa is locally adapted to the grassland habitat. Local adaptation likely prevents L. playtyglossa from dispersing into the rock outcrop habitat. Results of the competition experiment revealed L. jonesii is sensitive to competition and this may contribute to its constrained distribution to shallow soils. Local adaptation and competition likely contribute to habitat isolation between the two species. I also documented stark differences in flowering time between the species which contributes to reproductive isolation by reducing pollen flow. Hybrid crosses also resulted in lowered seed set than conspecific crosses. These results suggest prezygotic barriers to reproduction likely maintain the majority of isolation between the two species. These results provide insight into mechanisms that maintain reproductive barriers between closely related taxa existing in similar habitats. The results also contribute to our understanding of how rare plants preserve genetic integrity near common and interfertile relatives.

How are rare species maintained?: Reproductive barriers between Layia jonesii, a rare serpentine endemic, and L. platyglossa