Frogsleap Farm

Frogsleap Farm

Friday, November 23, 2018

Introgression of traits from commercial tomato hybrids into heirloom types


Heirloom type tomatoes are the result of generations of selection for flavor.  However they typically lack other elements of fruit quality, such as tolerance to cracking/splitting and improved shelf life. Some heirlooms have low-level tolerance to some disease complexes, but lack the resistance to specific tomato pathogens that can greatly improve plant health and productivity.

On the other hand, many new commercial hybrids have multiple pest resistance to key disease and nematode pests, and improved tolerance to splitting, cracking and cat-facing.  Some commercial hybrids also demonstrate significant improvement in shelf life when compared to heirloom types.  However, strong selection pressure for fruit quality, fruit yield, plant health and traits that make the fruit “transportation friendly” has led to commercial hybrids that typically lack the excellent flavor of the best heirloom types.

The breeding challenge is to efficiently and effectively combine the great flavor of the best heirloom types with the improved plant health, fruit quality and productivity of newer commercial hybrids.  Harry Klee and colleagues at the University of Florida have attempted this by crossing a disease resistant commercial parent line to flavorful heirloom types to create novel hybrids (e.g. Garden Gem and Garden Treasure).   A step in the right direction perhaps, but it is a quick fix that likely falls short of what could be achieved in a breeding program designed to preserve or enhance flavor and stack dominant pest resistant genes on both sides of the hybrid pedigree. Introgression of traits associated with plant health (e.g . pest resistance) and fruit quality (e.g. tolerance to cracking) from newer commercial hybrids into selected heirloom types provides an opportunity to capture the “best of both worlds” in multiple parent lines that can be used to produce novel hybrids with a combination of high flavor, high yield, high fruit quality and multiple pest resistance/improved plant health – a trait combination that does not exist in the market today.

University tomato researchers have done an outstanding job of identifying sources of resistance to key tomato pests in tomato wild relatives, and then finding a molecular marker that co-segregates with the desired gene for resistance to the specific disease or nematode.  Selection for the associated genetic marker (i.e. marker assisted selection or “MAS”) allows tomato breeders to efficiently identify plants that carry the desired pest resistance gene (MAS reference).  Combining MAS with phenotypic selection in breeding populations derived from crosses between heirloom types and commercial hybrids allows the design of a breeding program that, over several generations of selection, stacks multiple pest resistance genes into stable inbred lines which have also been selected for high flavor and fruit quality.

2017 MAS nursery and trials

After 8 generations of MAS genotypic selection and two cycles of field phenotypic selection for flavor, fruit quality and fruit yield, we have created our first generation of multiple pest resistant experimental hybrids.  These were evaluated in open field and greenhouse trials in 2018.  We were able to document the multiple pest resistance (up to 10 pest resistance genes) of the hybrids and found individual test cross hybrids with heterosis for both fruit yield and fruit flavor.  

The best of these new hybrids tasted as good as or better than our best heirloom-derived lines, with generally improved fruit quality characteristics.  The first generation hybrids are cherry to saladette size – larger fruited types are a year or two behind.  The initial crosses were designed to generate breeding lines segregating for fruit color and stripes, so a diversity in fruit color, shape and size will be available in these new hybrid products.

In parallel we have been working on improving shelf life by selection per se, and by incorporating mutant alleles of RIN or NOR - key genes regulating fruit ripening in tomatoes.  Extended shelf life (XSL reference) will be a key feature of several of our new Cream of the Crop TomatoTM multiple pest resistant hybrids.  

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