Geographical parthenogenesis in tetraploid brambles: Do competition and reproductive output in the secondary contact zone matter?

Abstract

Apomicts often show geographic distributions different from their sexual relatives, a phenomenon known as geographic parthenogenesis. Traits that have been suggested to influence the likelihood that apomicts and sexuals co-occur include those responsible for resource acquisition and reproductive traits. Here we test the contribution of these traits to geographic parthenogenesis, previously observed in a unique agamic system (Rubus ser. Glandulosi) free from the potential confounding effects of different ploidy. We conducted two competition experiments with different source materials (stem tips, root cuttings) of tetraploid apomicts and sexuals of Rubus ser. Glandulosi from their secondary contact zone to investigate their resource acquisition efficiency and reproductive characteristics. Both apomicts and sexuals were grown in monocultures and in mixtures of both reproductive groups, either with or without an additional competitor (grass Elymus repens). We found no consistent differences in survival, vegetative traits, biomass production or reproductive traits between sexuals and apomicts across treatments in either experiment. Thus, our data show similar competitiveness and fitness of apomicts and sexuals. Competitive exclusion, although potentially delayed by neutral dynamics, supports the strong parapatric distribution of sexuals and apomicts in the previously observed contact zone. However, the position of the contact zone and its shifts through time are largely determined by selection-independent factors like priority or stochastic effects and neutral population genetic processes.