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.
ChihCheng T. Chao and Robert R. Krueger
Robert R. Krueger and Mikeal L. Roose
New potential citrus germplasm accessions may be received as seed rather than budwood, thereby reducing phytosanitary risks. However, trueness-to-type may be an issue with seed materials because many varieties produce both apomictic (nucellar) and sexual (zygotic) embryos and most citrus is fairly heterozygous. To identify nucellar seedlings of polyembryonic types and to retain these as representing the type, we screened 1340 seedlings from 88 seed sources for markers amplified with two inter-simple sequence repeat (ISSR) primers. Sixteen seed sources produced no seedlings classified as being of nucellar origin. Among the remaining seed sources, seedlings classed as nucellar were identified for potential addition to the collection. In 37 accessions, both nucellar and zygotic seedlings were detected, and in some cases both types were retained. Inclusion of established accessions of the same cultivar group in the analysis allowed an initial assessment of similarity to existing accessions. This technique improved the efficiency of acquiring new germplasm of polyembryonic types by seed. The method identifies those seed sources that produce few or no nucellar seedlings, but it is not useful for determining which seedlings of monoembryonic types should be retained in collections.
Deqiu Fang, Robert R. Krueger, and Mikeal L. Roose
ISSR markers were analyzed to study phylogenetic relationships among 46 Citrus L. accessions representing 35 species. A dendrogram based on the unweighted pair-group method, arithmetic average cluster analysis was constructed using a similarity matrix derived from 642 polymorphic ISSR fragments generated by 10 primers. These 46 accessions could be classified into five major groups: 1) C. indica Tan.; 2) C. maxima (Burm.) Merrill; 3) lemon [C. limon (L.) Burm.] or lime [C. aurantifolia (Christm.) Swingle] type accessions; 4) C. halimii B. C. Stone; and 5) sour orange (C. aurantium L.), mandarins and their hybrids. Group 5 was further divided into three subgroups. Although some previous work had grouped it with mandarins, C. indica appeared to be a distinct genotype or species that was not close to mandarins. C. tachibana Tan. grouped closely to mandarins. C. vulgaris Risso was not related to sour orange but was similar to accessions usually classified in the lime or lemon group. Sour orange and its hybrids, C. nippokoreana Tan., C. hanayu Hort. ex Shirai, C. sudachi Hort. ex Shirai, and C. yuko Hort. ex Tan. had close phylogenetic relationships with mandarins. Although the mandarin accessions studied were divergent in morphology, the genetic distances among them were relatively small. Relationships among these Citrus accessions revealed by ISSR markers were generally in agreement with previous taxonomic classifications.