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Title: Genetic mapping with octoploid strawberry

item Lewers, Kimberly

Submitted to: North American Strawberry Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/31/2007
Publication Date: 4/2/2008
Citation: Lewers, K.S., Ashman, T., Hancock, J., Main, D. 2008. Genetic mapping with octoploid strawberry. North American Strawberry Conference Proceedings. 83-86.

Interpretive Summary: Breeding of strawberries is slow in part because, without advanced technology, seedlings derived from breeders’ crosses must be grown to maturity for evaluation of many horticultural traits such as fruit quality and yield. This of course takes years to accomplish, and requires large investment in land, greenhouses, staff and equipment. The breeding process would be greatly accelerated, and would be much more efficient, if a breeder could test a small seedling and know with confidence what traits that seedling will have if grown to maturity. A DNA based method, called “marker assisted selection” is available to accomplish this, but requires DNA “markers” that can be used to identify the seedlings the breeder should select. This makes the selection step easy; developing the markers is not as easy. We are developing a set of such markers for use in strawberries. So far we have identified 80 potential markers. Our results will greatly accelerate the breeding of strawberries for desirable horticultural traits, including production of more than one crop per year and ease of harvest. Our results are of immediate interest to strawberry breeders and in the near future to the strawberry producers and consumers worldwide.

Technical Abstract: In 2004, the USDA-CSREES call for proposals for the National Research Initiative (NRI) Program 52.1, Plant Genetics, focused on crops within the plant family Rosaceae. Two of the funded projects focus on molecular marker mapping of flowering traits of strawberry: sex determination and remontancy. Both traits affect fruit yield qualitatively and quantitatively and are important to the strawberry industry. The population developed for mapping sex-determination was generated from to two F. virginiana clones derived from wild populations in Pennsylvania. One parent is female, and the other is a hermaphrodite; both are heterozygous for the recessive allele conferring female-sterility or “maleness”. The accepted hypothesis is that sex-determination is controlled at one locus with three alleles (F>h>m) with F for female, h for hermaphrodite, and m for male. Developing markers surrounding this locus would allow strawberry breeders to introgress wild germplasm more efficiently. The other mapping population was developed from two F. ×ananassa cultivars. Multiple loci are expected to affect the trait, remontancy, because multiple physiological traits are thought to contribute. Developing markers near these loci will allow breeders to retain the remontancy trait in breeding populations as they introgress non-remontant germplasm. Both mapping populations have been planted and evaluated in the field at Beltsville, Md., and both were observed to be segregating for the remontancy trait. Approximately 300 Simple Sequence Repeat (SSR) markers were identified specifically for these projects. Around 200 primer pairs have been used in parental screens which include Rubus L. (blackberry and raspberry) and Prunus persica (L.) Batsch (peach) DNA. Twenty strawberry primers amplified a product in the polymerase chain reaction (PCR) with blackberry DNA, twenty-four with raspberry, and two with peach DNA. These primer pairs may be useful in comparing genomes of species within Rosaceae. Around 90% amplify a product in strawberry DNA, and around 73% of those detect polymorphisms between mapping parents. Primers used with mapping populations amplified between one and eight products, averaging around four products per primer pair; it was difficult to determine if any primer pairs amplified more than eight products. To be used for generating maps, a product should be either: 1) present in only one parent and segregating in a 1:1 ratio among the progeny; or 2) present in both parents and segregating in a 3:1 (present : absent) ratio among the progeny. In the F. virginiana population, more of the PCR products segregate in a 1:1 ratio (47%) compared with the 3:1 ratio (16%). The reverse was true for the F. ×ananassa population in which 15% of the products segregated in a 1:1 ration while 26% segregated in a 3:1 ratio. So far, none of the SSRs have been linked to the phenotypes of sex-determination or remontancy; this includes SCAR2, a marker linked to remontancy in F. vesca, a diploid progenitor species of F. virginiana and F. ×ananassa. However, two SSRs previously linked in F. vesca, also are linked in F. virginiana.