Newly discovered differences of reproductive traits between island and mainland plants support Baker’s law

Abstract

Abstract Background: Reproduction in angiosperms involves either one or two parents, through which selfed/cloned or outcrossed progeny is formed, respectively. Uniparental reproduction is advantageous when lack of mates and/or pollinators limits outcrossing opportunities. Baker’s law predicts that the capacity for uniparental reproduction should be enriched in habitats colonized via long-distance dispersal, such as volcanic islands. To test Baker’s law, we quantified variation of reproductive traits at multiple hierarchical levels and compared seed set after selfing and crossing experiments in both island and mainland populations of Limonium lobatum, a widespread species that Baker described as self-incompatible based on observations of stigma-pollen dimorphism in their flowers. In species with the type of pollen-stigma dimorphism that Baker detected in L. lobatum, pollen of one floral morph typically cannot fertilize ovules of the same floral morph. Results: We discovered new variation and combinations of pollen-stigma traits never described before and determined that plants with such novel combinations were more common in island than mainland populations. We also documented, for the first time, a lack of correspondence between specific pollen-stigma combinations and pollen compatibility. Furthermore, the results of manual pollination experiments established that selfed seed-set was higher in island than mainland plants, while outcrossed seed-set was lower in island than mainland plants. Overall, more than 80% of all plants were self-compatible, while less than 20% were partially or entirely self-incompatible. Conclusions: Contrary to previous descriptions by Baker, L. lobatumis a species that includes both self-compatible and self-incompatible plants characterized by both known and previously undescribed combinations of pollen-stigma traits. This type of variation was previously unknown in Plumbaginaceae and the lack of correspondence between pollen-stigma combinations was never described outside the highly variable species complex of Armeria maritima. Furthermore, island populations of L. lobatum harbor more plants with the newly discovered pollen-stigma combinations than mainland populations and are enriched in their capability for uniparental reproduction, corroborating Baker’s law. Our study establishes a link between variability of reproductive traits and capability of uniparental reproduction on islands, connecting research on reproductive and island biology.