Dr. Amy Iezzoni, Michigan State University
The S-locus is selected as one of RosBREED’s “Jewels in the Genome," because knowledge of S-locus variants is leading to more efficient breeding of self-fertile varieties of cherry, almond, apricot and plum.
Self-fertility is the ability to set fruit following self-pollination. Self-fertility contributes to higher more consistent yields in many Prunus species, because pollen from self-incompatible almond, sweet cherry, tart cherry, plum, and apricot cultivars is unable to grow the length of its own pistil. In self-fertile selections, pollen borne in a flower is able to successfully grow down the style, resulting in self-fertilization and fruit set. Self-fertile varieties have the advantage of not requiring orchard space for “pollinator” varieties. Additionally, self-fertile varieties characteristically have higher yields than self-incompatible varieties, as compatible (“self”) pollen is always available at the perfect time.
For many Prunus species; such as almond, sweet cherry, tart cherry, apricot, and plum; we know the two major genes that control self-fertility! These two genes reside at the S-locus on stone fruit chromosome 6. One gene is expressed in the style and encodes a cytotoxic ribonuclease, called the S-RNase. The second gene, expressed in the pollen, encodes an F-box protein, named SFB,that interacts with the S-RNase in an allele specific manner. Most of the self-fertile alleles described to date in these Prunus species disrupt either the S-RNase function in the style or the SFB function in the pollen, leading to self-fertility (Fig.1).
Figure 1: Incompatible versus compatible pollination: If the S-allele in the pollen matches the S-allele in the style, pollen growth is arrested in the style. When all pollen is incompatible, fertilization does not occur and the fruit does not develop. However, when the S-allele in the pollen has a mutation that causes it to be self-fertile (Sf), pollen grows the length of the style, fertilization is successful, and fruit development follows. Photo credit: Michigan State University.
Knowledge of genetic changes in S-RNase and SFB that lead to self-fertility has resulted in the development of simple diagnostic tests to screen progeny for self-fertility early in a breeding program. For example, the S-allele in the self-fertile sweet cherry varieties, ‘Lapins’ and ‘Sweetheart’, is called S4' (Boškovic et al. 2000). This allele has a four base pair deletion in the S4 SFB gene that results in the production of a shortened non-functional SFB protein. Pollen containing non-functional SFB protein is self-fertile and compatible on styles of all other cherry varieties. The DNA diagnostic test for self-fertility simply involves differentiating between S4 and S4', which can be accomplished by resolving the four base pair difference. Identification of self-fertile variants enables breeders to select parental combinations that maximize their chances of obtaining self-fertile offspring.
Development of this page was supported in part by the USDA's National Institute of Food and Agriculture (NIFA). Project title: RosBREED: Enabling marker-assisted breeding in Rosaceae is provided by the Specialty Crops Research Initiative Competitive Grant 2009-51181-05808. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s)and do not necessarily reflect the view of the United States Department of Agriculture.