Kim S. Lewers

Multiple Models for Rosaceae Genomics

The plant family Rosaceae consists of over 100 genera and 3,000 species that include many important fruit, nut, ornamental, and wood crops. Members of this family provide high-value nutritional foods and contribute desirable aesthetic and industrial products. Most rosaceous crops have been enhanced by human intervention through sexual hybridization, asexual propagation, and genetic improvement since ancient times, 4,000 to 5,000 B.C. Modern breeding programs have contributed to the selection and release of numerous cultivars having significant economic impact on the U.S. and world markets. In recent years, the Rosaceae community, both in the United States and internationally, has benefited from newfound organization and collaboration that have hastened progress in developing genetic and genomic resources for representative crops such as apple (Malus spp.), peach (Prunus spp.), and strawberry (Fragaria spp.). These resources, including expressed sequence tags, bacterial artificial chromosome libraries, physical and genetic maps, and molecular markers, combined with genetic transformation protocols and bioinformatics tools, have rendered various rosaceous crops highly amenable to comparative and functional genomics studies. This report serves as a synopsis of the resources and initiatives of the Rosaceae community, recent developments in Rosaceae genomics, and plans to apply newly accumulated knowledge and resources toward breeding and crop improvement.

Genetic diversity in wild and cultivated black raspberry (Rubus occidentalis L.) evaluated by simple sequence repeat markers

Breeding progress in black raspberry (Rubus occidentalis L.) has been limited by a lack of genetic diversity in elite germplasm. Black raspberry cultivars have been noted for showing very few phenotypic differences and seedlings from crosses between cultivars for a lack of segregation for important traits. Despite these challenges, little molecular work has been done to explore genetic diversity and relationships in wild and cultivated black raspberry germplasm. Microsatellite, or simple sequence repeat (SSR), markers are highly polymorphic codominant markers useful for studying genetic diversity, population genetics, genetic fingerprinting and other applications. We examined genetic diversity in 148 wild and cultivated black raspberry accessions using 21 polymorphic SSR markers. Black raspberry cultivars clustered tightly and showed higher than expected heterozygosity while that of wild accessions was low. Relationships between wild black raspberry accessions were poorly resolved and regional clusters were mostly absent from our analysis. Our results indicated that wild black raspberry germplasm is a relatively untapped resource available for future breeding.

Identification of QTL associated with flower and runner production in octoploid strawberry (Fragaria×ananassa)

BACKGROUND: Understanding the genetics of flowering in the strawberry (Fragaria × ananassa) will aid in the development of breeding strategies. OBJECTIVE: To search for quantitative trait loci (QTL) associated with remontancy and weeks of flowering in the strawberry. METHODS: Previously collected phenotypic data from two non-remontant ‘Honeoye’ × remontant ‘Tribute’ strawberry populations and simple sequence repeats (SSR) markers were used to search for QTL associated with repeat flowering, weeks of flowering and runner production, as well as the ability to produce flowers and runners at 17, 20 and 23 ◦ C. RESULTS: As was discovered in other studies, we found a major QTL that regulated remontancy and weeks of flowering on homeologous linkage group IV of ‘Tribute’. This QTL also had a negative effect on runner production and a positive influence on flower production under high temperatures. A number of additional QTL were discovered that significantly (LOD >3.0) influenced flower and runner production. CONCLUSIONS: Remontancy/non-remontancy is controlled by a major gene/locus and several minor modifying ones.

Low Tunnels as a Strawberry Breeding Tool and Season-Extending Production System

ABSTRACT Strawberries (Fragaria ×ananassa Duchesne ex Rozier) are economically valuable and popular with consumers who want to buy them all year round. However, a typical harvest season in most U.S. states is limited to only 3 to 4 weeks. Repeat-fruiting cultivars grown in these states could help meet that demand, especially if bred to be adapted to those states. To develop new repeat-fruiting cultivars that meet modern standards when grown in the U.S. Mid-Atlantic and similar regions, a production system utilizing low tunnels over raised white-plastic-covered beds was developed as a breeding tool to enable meaningful selection. The system also showed promise as a commercial production system. Fruit production using repeat-fruiting strawberries was compared with and without low tunnels. Season length was extended to 9 months. Total yield and marketable yield under low tunnels was 188% greater and 313% greater, respectively, compared with open beds. With no fumigation or fungicides, losses to fruit rots under low tunnels was 12% less than in open beds. The cost of materials to construct low tunnels was determined to be recoverable from the increased yield. Management strategies and applications of the findings from this study towards breeding repeat-fruiting strawberry cultivars are discussed.

Evaluating Strawberry Breeding Selections for Field and Postharvest Fruit Decay

Fruit from the annual replicated yield assessments for the USDA-ARS strawberry (Fragaria x ananassa Duchesne ex Rozier) breeding program at Beltsville, MD in 2010 were evaluated for postharvest decay development after storage at 5°C. A statistically significant correlation between percentage decay of fruit in the field and percentage decay of fruit from post-harvest evaluation was observed when data were analyzed on a genotypic mean basis (r = 0.37) or a field plot basis (r = 0.25) across all harvests. Analysis of the same data on a plot by harvest combination basis resulted in a statistically significant correlation for only one harvest date. While significant, the level of correlation on a genotypic mean basis is not strong enough to dismiss the need for post-harvest evaluation. The percentage postharvest decay increased over harvests, while the percentage decay at harvest, in the field, did not. Weather data from 2010 indicated that field conditions just a few days before harvest can affect percentage decay at harvest differently than percentage decay in postharvest storage; rain events were correlated with increased percentage decay in the field but not postharvest decay, while dry air was correlated with decreased percentage decay postharvest but not decay in the field. These findings suggest that, in some environments, conditions after flowering can have a more significant role in the fruit decay than previously has been reported.