A roadmap for research in octoploid strawberry

Abstract

The cultivated strawberry (Fragaria × ananassa) is an allo-octoploid species, originating nearly 300 years ago from wild progenitors from the Americas. Since that time the strawberry has become the most widely cultivated fruit crop in the world, universally appealing due to its sensory qualities and health benefits. The recent publication of the first high-quality chromosome-scale octoploid strawberry genome (cv. Camarosa) is enabling rapid advances in genetics, stimulating scientific debate and provoking new research questions. In this forward-looking review we propose avenues of research toward new biological insights and applications to agriculture. Among these are the origins of the genome, characterization of genetic variants, and big data approaches to breeding. Key areas of research in molecular biology will include the control of flowering, fruit development, fruit quality, and plant-pathogen interactions. In order to realize this potential as a global community, investments in genome resources must be continually augmented.

Keywords: Agricultural genetics; Genome evolution; Plant breeding.

Fig. 1. Genome-wide prediction (GWP) can reduce the strawberry breeding cycle from a minimum of 3 to 2 years.

In this example, a 2016 replicated trial of advanced selections (training population) was phenotyped and genotyped and a model generated to predict parental performance (genomic estimated breeding values) for seedlings from the same year (test population) for which phenotypic data has not yet been collected. Some of the untested seedlings with high predicted performance for predicted traits of interest were used in 2017 crosses, one year before they would be used as parents without genome-wide prediction. Combining GWP for complex traits with marker-assisted seedling selection (MASS) for traits controlled by one or few genes results in a comprehensive strategy for genetic improvement

Fig. 2. A proposed model of the regulation of flowering and runnering in strawberry.

Arrows indicate activation and bars indicate repression

Fig. 3. Fruit set and early fruit development set the stage for a symphony of color, flavor, and sweetness that accompanies ripening.

Aspects of hormone homeostasis, transport, and signaling resulting in fruit set and early fruit development in diploid or octoploid strawberry have been implied from multiple transcriptome studies. Hormone metabolism genes are indicated in red, hormone transporters in blue, and hormone signaling components are indicated in purple. However, many question remain to be resolved (dashed lines) regarding pathway components for the two major hormones, auxin and gibberellin. These components need to be known before we can ask questions about how the genes involved in the process are regulated. Over 50 years have passed since auxin was identified as being required for receptacle enlargement, yet a fundamental question remains: what is it about fertilization that turns on the production of auxin? What form(s) of the hormones are transported from the ovule to stimulate growth of the receptacle, and what are the transporters involved? Which of the auxin and gibberellin responsive genes are critical to fruit enlargement?

Fig. 4

A roadmap for future research in octoploid strawberry that begins with high-quality genome resources and leads to genetic improvement, through both basic and applied avenues