Primula secundiflora is a distylous [long-style and short-anther phenotype (L-morph); short-style and long-anther phenotype (S-morph)], self- and intra-morph incompatible perennial herb. Reproductive success of this species relies on insect pollen vectors. Previous studies have implied that nectar robbers may influence the trait–fitness relationships and mediate the direct and/or indirect effects of floral traits on fitness. 

To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences experimentally quantified the phenotypic selection on floral traits through female fitness. They also estimated the casual effects of nectar robbing with different nectar robbing intensities on trait-fitness relationships in both the L- and S-morph flowers among P. secundiflora populations. 

They found that more flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in five natural populations of P. secundiflora.  

Nectar robbers mediated indirect effect, whose strength increased with increasing nectar robbing intensity. It implied that nectar robbing was one of the agents that influence the number of flowers-fitness relationships in P. secundiflora. 

In contrast, the indirect effect of corolla tube width on female fitness was only observed in the L-morph flowers and only in a few populations. 

In addition, the average effect was stronger in the S-morph flowers than in the L-morph flowers. In S-morph flowers, the positive effects of nectar robbers on female fitness are significant in populations with low nectar robbing intensity. However, the effects are not significant in populations with high nectar robbing intensity. 

The study entitled "Nectar robbers influence the trait–fitness relationship of Primula secundiflora" was published online in Plant Biology.