The team led by Prof. Zhang Shaoling, head of the Center of Pear Engineering & Technology Research, College of Horticulture, Nanjing Agricultural University (NAU), recently published a research paper titled “Diversification and Independent Domestication of Asian and European Pears” (IF5year=13.554) in Genome Biology. The paper has NAU as the primary institution, Prof. Zhang as the corresponding author, Prof. Jun Wu as the first author, and Associate Prof. Shutian Tao as the co-first author. The study not only reveals the origin and path of dissemination of the pear, but also discovers the independent domestication of Asian and European pears.
“The Pear (Pyrus) is a globally grown fruit and the earliest cultivation of pears can be traced back to more than 3,000 years ago. The big family of cultivated pear species can be divided into two major groups, the Asian or ‘Oriental’ pear and the European or ‘Occidental’ pear. The Asian pear species, widely grown across China, Japan and South Korea, bear round-shaped fruits with crisp flesh, high sugar content, low acid content, minimal aroma and mild flavor, which usually ripen for harvest while still on the tree. Major cultivars of this group include Chinese white pear, sand pear, Ussurian (or Harbin) pear and Xinjiang pear. The European pear, predominantly grown in Europe and North America, bears typical calabash-shaped fruits with soft and smooth flesh, a sweet and sour taste as well as strong aroma and flavor, which, like kiwifruit, usually will not become ripe until after a post-harvest ripening process. This group has only one cultivar and shares the common name of Occidental pear,” said Prof. Zhang.
A pear tree is a typical self-incompatible species, which means that one pear tree usually cannot bear fruit after being pollinated by another pear tree of the same species. This reproductive characteristic resulted in a very high degree of interspecific hybridization and extensive gene flow and genetic recombination among pear groups. Thus, the study of the genetic background and relationships of pears remained a challenge and little was known of divergence and genetic connections among different pear species until recently. In this study, the research team collected a total of 113 typical accessions from germplasm material across 26 countries and conducted a population-level analysis of genetic variation of pears based on the resequencing of pear genomes that were not published until recently. As a result, the evolutionary history of the pear is traced back millions of years and a complete and detailed “family tree” is depicted for the big family of pears.
According to Prof. Wu, the study uncovered the myth about the origin, dissemination, divergence and independent domestication of Asian and European pears in which the pear, after originating in southwest China and then being disseminated throughout central Asia, had eventually spread to western Asia, and then on to Europe. It was estimated that Asian and European pears diverged between 6.6 and 3.3 million years ago, far prior to any possible human intervention. This divergence in domestication has contributed to wide differences observed between Asian and European pear populations. The study identified candidate genes in selective sweep regions in Asian and European pears related to such key traits as fruit size, sugar, acidity, stone cell and aroma. Of these genes, sugar-related genes outnumber any other type of genes, indicating a preference for sweetness during domestication.
Interestingly, the study also finds that an “intermarriage” or hybridization must have occurred between Asian and European pears over 2,000 years ago, giving birth to the interspecific hybrid species – the Xinjiang species (as represented by Korla pear). Judging from the timing of such hybridization, it is highly likely that trade in goods and cultural exchanges along the Silk Road may have contributed to the development of this unique species of cultivated pear.
The study also finds evidence for rapid evolution and balanced selection for S-RNase genes that have contributed to the maintenance of self-incompatibility, which facilitates outcrossing and drives pear genome diversity across the Eurasian continent.
This paper studied an unprecedented amount of global genomic resources for wild and cultivated pears. Alongside its identification of candidate genes in selective sweep regions and colocalized quantitative trait loci (QTLs), the paper will significantly contribute to efforts for genetic improvement and molecular breeding of pears.
Key collaborators of this study include the Shijiazhuang Fruit Tree Research Institute of Hebei Academy of Agriculture and Forestry Sciences, BGI, the University of Illinois at Urbana-Champaign and Cornell University.
The Center of Pear Engineering & Technology Research at NAU, headed by Prof. Zhang, is mainly specialized in pear germplasm resources and genetic breeding, pear self-incompatibility, genome and functional genomics of pear, and the formation and regulation of pear fruit quality. It has published a number of research papers in journals such as Genome Research, Plant Cell, Plant Journal and New Phytologist, including one that is among the ESI Highly Cited Papers and has received enormous attention and recognition from peers from home and abroad.