Difference in reproductive traits of two sympatric species of Stigmaphyllon (Malpighiaceae) throughout their distribution range: Are they context-dependent?

Abstract

Populations of widely distributed plants may experience varying climatic or ecological conditions, leading to uneven evolution of reproductive traits. However, studies on reproductive biology usually focus on just one, or rarely two, sampling areas. In this study, we performed a geographical comparative analysis of the flowering pattern, pollination biology and breeding system of Stigmaphyllon bonariense and S. jatrophifolium (Malpighiaceae) throughout their natural distribution in Argentina, classifying them as ‘northern’ or ‘southern’ according to their geographic location. For this, we selected 4 populations for S. bonariense and 3 for S. jatrophifolium to characterized the flowering peak at both the individual (IFP) and population (PFP) levels, the floral display per inflorescence, the richness of Malpighiaceae species and their pollinators, and the stigmatic and body pollen loads of the pollinators. In addition, we performed manipulative experiments to determine the breeding system. Our results showed that the duration of flowering phenology and the floral display vary between northern and southern populations of both species. These traits include flowering duration and the number of flowers at anthesis per inflorescence (floral display). In addition, the pollination context exhibited latitudinal differences in species richness, phenology, body size of oil-collecting bees, and visitation frequency. Besides, the number of co-flowering Malpighiaceae species varied among populations. However, no considerable differences were observed among populations in terms of stigmatic and body pollen load in pollinators of both species. The breeding system was invariable and consistent in both species with self-incompatibility (SI). These findings suggest that certain reproductive traits of both Stigmaphyllon species are context-dependent and would allow the species maximise reproductive success in each population. Therefore, our results demonstrate that large-scale geographical studies (i.e., several populations with different climatic conditions) and the analysis of various phenological traits are necessary to better understand the reproductive biology of plant species.