Primula tibetica is an inconspicuous herb distributed in the Himalayas where lacks pollinators. The species may undergo severe pollen limitation in the field. It is an ideal model system to see whether pollen limitation selects on the traits that increase the floral display in a case in which a small, inconspicuous plant undergoes obligate out-crossing and faces difficulty in evolving self-fertilization.

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study to see whether the small distylous herb increases its female fitness by increasing its floral display. They asked what the incompatibility feature of Primula tibetica was and how pollinator-mediated selection on Primula tibetica occurred.

In the selection analysis, the researchers included the date at which flowering began and four morphological traits (stalk height, flower number, corolla width, and flower tube length). They considered the female fitness as the dependent variable.

The researchers conducted hand-pollination experiment, including four treatments: hand self-fertilization, autonomous self-fertilization, hand intra-morph pollination, and hand inter-morph pollination.

During the 2016 flowering season, they found significantly positive selection gradients for traits involved in the floral display (corolla width, flower number, and stalk height) in the focal population. P. tibetica underwent severe pollen limitation during the 2016 flowering season.

Primula tibetica was demonstrated to be highly self- and intra-morph incompatible and was unable to autonomously self-fertilize. The results indicated that P. tibetica highly relied on pollinators for sexual reproduction. The traits involved in the floral display of P. tibetica were positively selected by pollinators.

The researchers thus reasonably concluded that P. tibetica was under selective pressure to increase its floral display. Enlarging the flower display may be vital for the inconspicuous distylous plant.

The study entitled “Self- and intra-morph incompatibility and selection analysis of an inconspicuous distylous herb growing on the Tibetan plateau (Primula tibetica)” has been published in Ecology and Evolution.

(a) Flowering plants of the long-styled morph of Primula tibetica,

(b) a flower of Primula tibetica being visited by a tachinid fly,

and (c) a flower of Primula tibetica being visited by a syrphid fly

(Images by JIANG Xianfeng)