Population Structure Analysis of Advanced European Pear Breeding Selections

Pears are an economically important and nutritionally valuable fruit in the U.S and throughout the world. There are a few major cultivars grown in the U.S. that were developed in Europe over two centuries ago. Continued pear breeding in the United Kingdom, Spain, Italy, and France has resulted in the development of several advanced breeding selections. These selections include potential rootstocks with desirable properties, such as dwarfing, precocity, and disease resistance. In order to meet the needs of the U.S. pear industry, a pear rootstock breeding program is currently being established. The advanced breeding selections from Europe can serve as elite parental material for the breeding program. However, prior to their utilization as parental material, there is a need to determine the inherent genetic diversity in the advanced selections. We conducted a population structure analysis of advanced European pear breeding selections using target region amplified polymorphism (TRAP) markers as a means of diversity analysis.  Six primer combinations produced 86 polymorphic loci in a population of 200 advanced pear genotypes. We scored the polymorphisms using the software programs STRUCTURE, CLUMPP, and DISTRUCT. The custom software program seeDNA™ was used to generate visual representations of the diversity between and within the subpopulations. Interestingly, we observed that the advanced selections from the UK and Italy represent only three subpopulations, indicating a rather narrow genetic diversity among these selections. The lack of sufficient diversity between the tested samples strongly indicates that there is a need for the use of diverse germplasm for pear breeding in the U.S.  The results of this analysis will guide the selection and utilization of the most diverse parental material from the sample set tested.