Date palm (Phoenix dactylifera L.) is one of the oldest fruit crops grown in the arid regions of the Arabian Peninsula, North Africa, and the Middle East. The most probable area of origin of the date palm was in or near what is now the country of Iraq, but date cultivation spread to many countries starting in ancient times. Dates are a major food source and income source for local populations in the Middle East and North Africa, and play significant roles in the economy, society, and environment in these areas. In addition to serving directly as a food source, dates are packed and processed in a number of ways, and other parts of the tree are used for various purposes. The date palm is a diploid, perennial, dioecious, and monocotyledonous plant adapted to arid environments. It has unique biological and developmental characteristics that necessitate special propagation, culture, and management techniques. Thousands of date palm cultivars and selections exist in different date-growing countries. Different genetic marker systems have been used to study genetic relationships among date palm cultivars. The long life cycle, long period of juvenility, and dioecism of date palms make breeding challenging. Worldwide date production has grown from 1,809,091 t in 1962 to 6,924,975 t in 2005. Worldwide date production will continue to grow, especially in the Middle East, despite current and future challenges.

Domestication of the apple was mainly driven by interspecific hybridization. In the present study, we report the haplotype-resolved genomes of the cultivated apple (Malus domestica cv. Gala) and its two major wild progenitors, M. sieversii and M. sylvestris. Substantial variations are identified between the two haplotypes of each genome. Inference of genome ancestry identifies ~23% of the Gala genome as of hybrid origin. Deep sequencing of 91 accessions identifies selective sweeps in cultivated apples that originated from either of the two progenitors and are associated with important domestication traits. Construction and analyses of apple pan-genomes uncover thousands of new genes, with hundreds of them being selected from one of the progenitors and largely fixed in cultivated apples, revealing that introgression of new genes/alleles is a hallmark of apple domestication through hybridization. Finally, transcriptome profiles of Gala fruits at 13 developmental stages unravel ~19% of genes displaying allele-specific expression, including many associated with fruit quality.

Apple (Malus X. domestica Borkh.) is one of the world's most valuable fruit crops. Its large size and long juvenile phase make it a particularly promising candidate for marker-assisted selection (MAS). However, advances in MAS in apple have been limited by a lack of phenotype and genotype data from sufficiently large samples. To establish genotype-phenotype relationships and advance MAS in apple, we extracted over 24,000 phenotype scores from the USDA-Germplasm Resources Information Network (GRIN) database and linked them with over 8000 single nucleotide polymorphisms (SNPs) from 689 apple accessions from the USDA apple germplasm collection clonally preserved in Geneva, NY. We find significant genetic differentiation between Old World and New World cultivars and demonstrate that the genetic structure of the domesticated apple also reflects the time required for ripening. A genome-wide association study (GWAS) of 36 phenotypes confirms the association between fruit color and the MYB1 locus, and we also report a novel association between the transcription factor, NAC18.1, and harvest date and fruit firmness. We demonstrate that harvest time and fruit size can be predicted with relatively high accuracies (r > 0.46) using genomic prediction. Rapid decay of linkage disequilibrium (LD) in apples means millions of SNPs may be required for well-powered GWAS. However, rapid LD decay also promises to enable extremely high resolution mapping of causal variants, which holds great potential for advancing MAS.

MicroRNAs (miRNAs) are a class of non-protein-coding small RNAs. Considering the conservation of many miRNA genes in diVerent plant genomes, the identiWcation of miRNAs from non-model organisms is both practicable and instrumental in addressing miRNA-guided gene regulation. Citrus is an important staple fruit tree, and publicly available expressed sequence tag (EST) database for citrus are increasing. However, until now, little has been known about miRNA in citrus. In this study, 27 known miRNAs from Arabidopsis were searched against citrus EST databases for miRNA precursors, of which 13 searched precursor sequences could form fold-back structures similar with those of Arabidopsis. The ubiquitous expression of those 13 citrus microRNAs and other 13 potential citrus miRNAs could be detected in citrus leaf, young shoot, Xower, fruit and root by northern blotting, and some of them showed diVerential expression in diVerent tissues. Based on the fact that miRNAs exhibit perfect or nearly perfect complementarity with their target sequences, a total of 41 potential targets were identiWed for 15 citrus miRNAs. The majority of the targets are transcription factors that play important roles in citrus development, including leaf, shoot, and root development. Additionally, some other target genes appear to play roles in diverse physiological processes. Four target genes have been experimentally veriWed by detection of the miRNA-mediated mRNA cleavage in Poncirus trifoliate. Overall, this study in the identiWcation and characterization of miRNAs in citrus can initiate further study on citrus miRNA regulation mechanisms, and it can help us to know more about the important roles of miRNAs in citrus.

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The apple (Malus domestica) is one of the world’s most commercially important perennial crops and its improvement has been the focus of human effort for thousands of years. Here, we genetically characterise over 1000 apple accessions from the United States Department of Agriculture (USDA) germplasm collection using over 30,000 single-nucleotide polymorphisms (SNPs). We confirm the close genetic relationship between modern apple cultivars and their primary progenitor species, Malus sieversii from Central Asia, and find that cider apples derive more of their ancestry from the European crabapple, Malus sylvestris, than do dessert apples. We determine that most of the USDA collection is a large complex pedigree: over half of the collection is interconnected by a series of first-degree relationships. In addition, 15% of the accessions have a first-degree relationship with one of the top 8 cultivars produced in the USA. With the exception of ‘Honeycrisp’, the top 8 cultivars are interconnected to each other via pedigree relationships. The cultivars ‘Golden Delicious’ and ‘Red Delicious’ were found to have over 60 first-degree relatives, consistent with their repeated use by apple breeders. We detected a signature of intense selection for red skin and provide evidence that breeders also selected for increased firmness. Our results suggest that Americans are eating apples largely from a single family tree and that the apple’s future improvement will benefit from increased exploitation of its tremendous natural genetic diversity.

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Sib selection for mating competence among isolates of Magnaporthe grisea that are pathogenic on rice has led to the selection of isolates where the mating type alleles are the only restriction on mating competence. Isolates are able to mate with all sibling isolates of opposite mating type and produce asci with ascospores. Isolates can function as male, female, or hermaphrodite in mating depending on the isolates with which they are mated. Mating competence is controlled by the genes in both mated isolates. Key words: mating competence, sib selection, rice blast, genetics.