Dear Colleagues,

Germplasm repositories primarily provide the raw material for plant breeding and crop improvement programs. Hundreds of living germplasm collections of fruit tree crops across the world have played an incredible role in fruit tree breeding and enabled many newly improved cultivars available to the global fruit industry. Germplasm repositories for fruit trees are uniquely constructed as clonal living collections preserved in orchards, vineyards and plantations or nurseries, etc. They include diverse valuable resources such as current commercial cultivars, traditional cultivars, landraces, breeding materials, elite selections and wild relatives. In addition to the preservation and maintenance of well-documented and managed living collections, living fruit tree collections are not only used by fruit breeders; other biologists may have different interests and possibly different requirements. While fruit breeders focus on the commercial traits of immediate perceived value, other biologists may be more interested in studies to better understand the properties and behavior of the plant, as well as its domestication history or evolutionary phylogeny, especially at the genomic level.

One of the prioritized tasks in a germplasm repository of fruit tress is to characterize the genetic diversity and composition of accessions maintained in the repository to capture a genetic and morphological diversity that is as broad as possible; to facilitate the characterization of intra- and interspecific variation; and to understand phylogenetic relationships among all resources including cultivars, varieties, subspecies and species—particularly those between wild relatives. The quantity and quality of data documentation are of crucial importance for any germplasm repository management. Although data documentation has changed over time with initial morphological evaluation, genomic data have been increasingly obtained from a wide range of fruit tree germplasm collections. Genotyping accessions has become routine protocol in fruit tree germplasm collections to verify pedigrees and track a trait of interest in breeding. Nevertheless, genetic information obtained using SSR, SNP, AmpSeq, etc. provides a valuable resource for breeding designs of fruit trees for both traditional breeding and molecular breeding programs—that is, parentage choice.

The nature of fruit trees as mostly perennial plants also helps other biologists to study and understand many aspects about basic plant biology, such as growth, development, reproduction, biotic and abiotic stress tolerance, metabolite changes, etc. because germplasm repositories of fruit trees are maintained in the same location across multiple years, which offers an important opportunity to study inter-annual variation under common garden conditions. One approach, for example, is to study phenotypic plasticity in response to climatic changes and the adaptive evolution of long-lived plants.

Ultimately, living fruit tree collections are of particular use to fruit tree breeding and genetic improvement for new cultivar development—for example, many new kiwifruit cultivars recently released to the global kiwifruit industry rely heavily on the Chinese National Kiwifruit Germplasm Repository at Wuhan Botanical Garden, Chinese Academy of Sciences. Recently, molecular approaches such as GWAS, QTLs, and other marker-assisted selection techniques have increasingly been used as tools for selecting potential parents from germplasm collections for use in many fruit tree breeding programs to accelerate the breeding process. It is evident that fruit tree improvement is greatly enhanced by standardized and statistically robust procedures for discovering quantitative trait loci (QTLs) in germplasm relevant to breeding programs and helps in the validation of important breeding parents (IBPs) by estimating average allelic representation in wild relatives, also demonstrating importance of construction of core germplasm sets in living fruit tree germplasm collections.

Given this context, in this Special Issue articles (original research papers, perspectives, hypotheses, opinions, reviews, modeling approaches, and methods) that focus on germplasm repository management, evaluation and data documentation, genetics and novel breeding approaches, molecular methods for genetic improvement, gene discovery of traits, and new fruit tree domestication are most welcome.

Prof. Dr. Hongwen Huang
Guest Editor

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• germplasm repository management
• germplasm evaluation and dada documentation
• gene discovery of important traits
• germplasm core sets at genomic level
• population genomics
• integration of traditional and genomic breeding
• climatic changes and adaptive evolution
• germplasm innovation
• new fruit tree domestication
• germplasm networking