Benefits of outcrossing and their implications for genetic management of an endangered species with mixed-mating system

Mixing gene pools in translocations can create populations with higher genetic diversity and evolutionary potential compared to single sources. However, the benefits of outcrossing on genetic rescue may depend on species' life history, notably mating systems. We assessed outcomes on growth and survival of different kinds of cross in a translocated population of the highly endangered mixed-mating species Banksia brownii, established from multiple sources. We genetically assigned plants grown from field-pollinated seed as of selfed, within-source or between-source (referring to the original source populations) cross types, and estimated proportion of heterozygous loci for each plant. We modeled the associations among genetic predictors (cross type, level of genetic diversity, and ancestry) and fitness proxies measured in glasshouse and field. At the translocation site, selfing rates were approximately 53-70% and crossing among sources was rare, both likely owing to different patterns of flowering among plants. Offspring of selfed plants were consistently smaller in volume in the glasshouse than were offspring from within-source crosses. There was no evidence from the first 3 years that survival in the field was influenced by any predictor tested, but the smaller volume of selfed plants in the glasshouse and correlation of glasshouse size at 5 months with the size of the same plants at 3 years in the field suggest that more time may be needed to observe any effects of outcrossing on survival and fitness. Our results highlight the importance of monitoring mating systems, genetic diversity, and fitness data for evaluating translocation success and informing adaptive management.

Automated summary

Mixing gene pools in translocations can lead to populations with higher genetic diversity and potential for evolution. In a translocated population of the endangered mixed-mating species Banksia brownii, offspring from self-pollination were consistently smaller than offspring from within-source crosses. No evidence was found for the influence of predictors on survival, suggesting that more time may be needed to observe the effects of outcrossing on fitness. Monitoring mating systems, genetic diversity, and fitness data is important for evaluating translocation success and guiding adaptive management.