Abstract
BackgroundPears (Pyrus spp.) are one of the most important fruit crops in temperate regions. Japanese pear breeding has been carried out for over 100 years, working to release new cultivars that have good fruit quality and other desirable traits. Local cultivar ‘Nijisseiki’ and its relatives, which have excellent fruit texture, have been repeatedly used as parents in the breeding program. This strategy has led to inbreeding within recent cultivars and selections. To avoid inbreeding depression, we need to clarify the degree of inbreeding among crossbred cultivars and to introgress genetic resources that are genetically different from modern cultivars and selections. The objective of the present study was to clarify the genetic relatedness between modern Japanese pear cultivars and diverse Asian pear genetic resources.ResultsWe genotyped 207 diverse accessions by using 19 simple sequence repeat (SSR) markers. The heterozygosity and allelic richness of modern cultivars was obviously decreased compared with that of wild individuals, Chinese pear cultivars, and local cultivars. In analyses using Structure software, the 207 accessions were classified into four clusters (K = 4): one consisting primarily of wild individuals, one of Chinese pear cultivars, one of local cultivars from outside the Kanto region, and one containing both local cultivars from the Kanto region and crossbred cultivars. The results of principal coordinate analysis (PCoA) were similar to those from the Structure analysis. Wild individuals and Chinese pears appeared to be distinct from other groups, and crossbred cultivars became closer to ‘Nijisseiki’ as the year of release became more recent.ConclusionsBoth Structure and PCoA results suggest that the modern Japanese pear cultivars are genetically close to local cultivars that originated in the Kanto region, and that the genotypes of the modern cultivars were markedly biased toward ‘Nijisseiki’. Introgression of germplasm from Chinese pear and wild individuals that are genetically different from modern cultivars seems to be key to broadening the genetic diversity of Japanese pear. The information obtained in this study will be useful for pear breeders and other fruit breeders who have observed inbreeding depression.Electronic supplementary materialThe online version of this article (doi:10.1186/s12863-016-0380-7) contains supplementary material, which is available to authorized users.
Highlights
Pears (Pyrus spp.) are one of the most important fruit crops in temperate regions
Iwate Prefecture (IWA) Wild individuals from Iwate, BRE P. bretschneideri cultivar, USS P. ussuriensis cultivar, KAN Local cultivar from Kanto region, NSJ Local cultivar from near the sea of Japan, WJ Local cultivar from western Japan, CFH Cultivar released in the first half of the 20th century, CLH Cultivar released in the latter half of the 20th century or breeding line developed during that time, MDC Modern cultivar or breeding line developed during that time cultivars generally considered to be P. bretschneideri (BRE) and P. ussuriensis (USS); and Japanese pear local cultivars that originated in the Kanto region of Japan (KAN), near the Sea of Japan (NSJ), and in western Japan (WJ)
The modern cultivars seemed to be genetically close to local cultivars that originated in the Kanto region, especially ‘Nijisseiki’ (Figs. 2 and 3)
Summary
Pears (Pyrus spp.) are one of the most important fruit crops in temperate regions. Japanese pear breeding has been carried out for over 100 years, working to release new cultivars that have good fruit quality and other desirable traits. Local cultivar ‘Nijisseiki’ and its relatives, which have excellent fruit texture, have been repeatedly used as parents in the breeding program. This strategy has led to inbreeding within recent cultivars and selections. Selection of local cultivars from wild populations during domestication has increased productivity but narrowed genetic diversity [10, 11]. An organized breeding program is generally started with limited genetic resources that have been already domesticated, such as local cultivars. The genetic diversity of modern cultivars has decreased dramatically over time
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