BackgroundPear (Pyrus) is a globally grown fruit, with thousands of cultivars in five domesticated species and dozens of wild species. However, little is known about the evolutionary history of these pear species and what has contributed to the distinct phenotypic traits between Asian pears and European pears.ResultsWe report the genome resequencing of 113 pear accessions from worldwide collections, representing both cultivated and wild pear species. Based on 18,302,883 identified SNPs, we conduct phylogenetics, population structure, gene flow, and selective sweep analyses. Furthermore, we propose a model for the divergence, dissemination, and independent domestication of Asian and European pears in which pear, after originating in southwest China and then being disseminated throughout central Asia, has eventually spread to western Asia, and then on to Europe. We find evidence for rapid evolution and balancing selection for S-RNase genes that have contributed to the maintenance of self-incompatibility, thus promoting outcrossing and accounting for pear genome diversity across the Eurasian continent. In addition, separate selective sweep signatures between Asian pears and European pears, combined with co-localized QTLs and differentially expressed genes, underline distinct phenotypic fruit traits, including flesh texture, sugar, acidity, aroma, and stone cells.ConclusionsThis study provides further clarification of the evolutionary history of pear along with independent domestication of Asian and European pears. Furthermore, it provides substantive and valuable genomic resources that will significantly advance pear improvement and molecular breeding efforts.Electronic supplementary materialThe online version of this article (10.1186/s13059-018-1452-y) contains supplementary material, which is available to authorized users.

Ethylene response factor (ERF) has been widely studied in regulating fruit ripening in tomato, apple, banana and kiwifruit, but little is known in pear. In this study 1-methylcyclopropene (1-MCP) treatment, an inhibitor of ethylene perception, was conducted at approximately 30 days before harvest to delay fruit ripening in a climacteric white pear cultivar Yali. Transcriptome libraries were constructed and sequenced in pre-ripening, ripening, and 1-MCP treated fruits. Data analysis showed that 73 candidate genes related to fruit ripening were induced by 1-MCP, among which two were positively related, namely 1-aminocyclopropane-1-carboxyla oxidase and an ERF gene (designated as ACO54 and ERF24). Transient transformations in pear fruit revealed that over-expression of ACO54 enhance transcription level of ERF24 and most ripening-related genes. Meanwhile, over-expression of ERF24 raises expression level of ACO54 and partially ripening-related genes. Moreover, dual-luciferase and yeast-one-hybrid assays unravel an interaction between ERF24 and the ACO54 promoter. Therefore, the ERF24 could directly regulate ACO54 expression by binding to its promoter. These results suggested that the first identified ERF24 is involved in regulating fruit ripening in Chinese white pear.

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Protein disulfide isomerase (PDI) is an important enzyme for protein folding in endoplasmic reticulum. The PDI gene family has been systematically studied in Arabidopsis, barley, rice, and bread wheat; however, little is known about this gene family and their roles during fruit development and ripening in fruit trees. In this study, 63 PDI genes were isolated from 8 fruit trees. Phylogenetic trees showed that these genes were clustered into six different groups, designated as A to F. In the groups, the PDI genes had significant differences in gene structure and conserved regions. The chromosome location of each PDI gene was determined in complete genome-assembly fruit trees and the synteny of chromosome segments was detected among peach, pear, and strawberry. Expression profiles of PDI genes in peach, pear, and strawberry showed that nearly all genes in group D and E were more highly expressed in developmental and ripening fruit tissues than those in other groups, while all genes in group A and B presented the lowest levels of expression in fruits of each stage. Moreover, qRT-PCR analyses revealed that these expressed genes were stable expressed in pear and peach fruits, as well as the reported reference genes. Eventually, PbPDI.F1 presented the highest expression stability in pear fruit while PpPDI.F displayed stronger stability than other genes in peach fruit. Thus, these two genes, which were clustered in group F, are good reference genes for gene expression studies during fruit development and ripening.