Ecology and genetics of transitions from outcrossing to selfing in Primula

The evolutionary breakdown of the outcrossing floral polymorphism heterostyly is a classic example in plants of a mating system-transition. In primroses (Primula), comparative evidence indicates that the pathway from outcrossing to selfing has occurred on numerous occasions, but little is known about the ecological mechanisms driving this transition or its population genetic consequences. Shuai et al. demonstrate that in the sub-alpine P. oreodoxa, a species endemic to south western China, increasing elevation is associated with a deterioration in pollinator service, increased inbreeding and reduced genetic diversity in populations.

Fourteen populations throughout the range of P. oreodoxa were sampled, and morph frequencies and floral characteristics were recorded. Polymorphism at microsatellite loci and chloroplast DNA (cpDNA) variation were used to quantify population genetic structure and genetic relationships among populations. Controlled pollinations and studies of pollen tube growth and fertility were conducted to determine the compatibility status of populations and their facility for autonomous self-pollination. Finally, visitation rates of long- and short-tongued pollinators to distylous and homostylous populations at different elevations were compared to determine if increased elevation was associated with deterioration in pollinator service.

n contrast to most heterostylous species, both distylous and homostylous morphs of P. oreodoxa are highly self-compatible, but only homostyles have the facility for autonomous self-pollination. Homostyles set significantly more fruit and seeds following open pollination than the distylous morphs. Visitation by long-tongued pollinators was significantly lower in homostylous populations, and overall rates of insect visitation decreased with elevation. Genetic diversity was significantly lower in homostylous populations, with evidence of increased inbreeding at higher elevation. Patterns of cpDNA variation were consistent with multiple transitions from distyly to homostyly and limited gene flow among populations.

These changes result from the spread of self-pollinating variants (homostyles) within populations. The results of this study support the hypothesis that the multiple loss of floral polymorphism in distylous P. oreodoxa is associated with unsatisfactory pollinator service, with homostyles benefiting from reproductive assurance as a result of autonomous self-pollination.

This paper is part of the Special Issue on Morphology and Adaptation. It is FREE access for a limited period to the end of January 2018. It will then be free access from November 2018.