The genes involved in F1 seedling abnormal development and lethality in inter-gene pool crosses have been designated as Dl1 (MesoAmerican=MA) and Dl2 (Andean=A) (Shii et al., 1980, J. Hered. 71:218–222). The different degrees of leaf crippling (C) in segregating populations of crosses was due to the interaction between the Dl1 or Dl2 loci, growing environment, and the lcr allele (Singh and Molina, 1996, J. Hered., In press). The objective was to identify RAPD markers linked to the genes for crippling (lcr) and seedling lethality (Dl) using the bulked segregation analysis procedure for F2 of MA × A crosses. Crosses were made between C lines, FB 10413-24-2, WA 7807-305, and TY 5578-220 and normal (N) parents and tester stocks for Dl1 and Dl2 genes. The F2 FB 10413-24-2 × Carioca segregated 13 N:3C. F3 families segregated 3N:1C. RAPD marker OPB-10 was linked to Lcr at 31.2 cM. F3 families segregated 1N:3C. RAPD marker OPO16 was linked to Dl1 at 27 cM. The F2 WA-7807-305 × Rio Tibagi segregated 3N:1C. RAPD marker OPS-03 was linked to Lcr at 32.6 cM.
Martha Dávila, Dermot Coyne, Shree Singh, and Guenhwa Jung
Majid R. Foolad
In tomato, Lycopersi conesculentum Mill., currently there are >285 known morphological, physiological and disease resistance markers, 36 isozymes, and >1000 RFLPs, which have been mapped onto the 12 tomato chromosomes. In addition, currently there are >162,000 ESTs, of which ∼3.2% have been mapped. Several tomato genetic maps have been developed, mainly based on interspecific crosses between the cultivated tomato and its related wild species. The markers and maps have been used to locate and tag genes or QTLs for disease resistance and other horticultural characteristics. Such information can be used for various purposes, including marker-assisted selection (MAS) and map-based cloning of desirable genes or QTLs. Many seed companies have adopted using MAS for manipulating genes for a few simple morphological characteristics and several vertical disease resistance traits in tomato. However, MAS is not yet a routine procedure in seed companies for manipulating QTLs although it has been tried for a few complex disease resistance and fruit quality characteristics. In comparison, the use of MAS is less common in public tomato breeding programs, although attempts have been made to transfer QTLs for resistances to a few complex diseases. The potential benefits of marker deployment to plant breeding are undisputed, in particular for pyramiding disease resistance genes. It is expected that in the near future MAS will be routine in many breeding programs, taking advantage of high-resolution markers such as SNPs. For quantitative traits, QTLs must be sought for components of genetic variation before they are applicable to marker-assisted breeding. However, MAS will not be a “silver bullet” solution to every breeding problem or for every crop species.
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.
Margaret R. Pooler, Louise G.H. Riedel, S.E. Bentz, and A.M. Townsend
Controlled pollinations were made between five hemlock (Tsuga) species from eastern North America and Asia, resulting in over 5700 germinating seedlings. A subset of putative hybrid seedlings from each cross was tested for authenticity by various DNA marker systems. The most reliable and useful system for verifying hybrids was amplified fragment-length polymorphism (AFLP) markers. Hybridizations between the eastern North American species, T. canadensis [L.] Carriere and T. caroliniana Engelm., and the Asian species, T. chinensis (Franch.) E. Pritz., were used as a model to test the inheritance, reliability, and ease of use of these markers. Using AFLP markers, we were able to verify 58 hybrids between T. caroliniana and T. chinensis, one hybrid between T. caroliniana and T. canadensis, but could find no definitive hybrids between T. canadensis and T. chinensis. Results using other marker systems, including RAPD, SCAR, ITS, and SSR, are also presented.
Darlene M. Lawson, Minou Hemmat, and Norman F. Weeden
Five morphological and developmental traits (branching habit, vegetative budbreak, reproductive budbreak, bloom time, and root suckering) were analyzed in a family obtained from the apple (Malus domestica Borkh) cross `Rome Beauty' × `White Angel'. The phenotypic variation in these traits was compared with a selected set of marker loci covering the known genome of each of the parents to locate genes with major effects on the traits. The contrasting branching habits of the two parents appeared to be controlled by at least two loci. One of these, Tb, governed the presence or absence of lateral branches, particularly on the lower half of shoots. The locus was heterozygous in `White Angel' and was mapped to a 5 CM interval on linkage group 6. At least one other locus conditioning spur-type branching appeared to be segregating, but the locus or loci could not be linked to segregating markers. The timing of initial vegetative growth was tightly associated with the chromosomal region in which the Tb gene is located and maybe a pleiotropic effect of this gene. Time of reproductive budbreak correlated with segregation at the isozyme marker, Prx-c, on linkage group 5. Variation in time of bloom and later stages in flower development appeared to be controlled by different genes not linked to Prx-c. The tendency to produce root suckers cosegregated with a marker on `White Angel' linkage group 1, suggesting control by a single locus, Rs. Data from a `Rome Beauty' x `Robusta 5' family provided additional information on the inheritance of these traits.
Zhanao Deng, Jinguo Hu, Fahrettin Goktepe, Brady A. Vick, and Brent K. Harbaugh
Cultivated caladiums are valued for their bright colorful leaves and are widely used in containers and landscapes. More than 1500 named cultivars have been introduced during the past 150 years, yet currently only about 100 cultivars are in commercial propagation in Florida. Caladium tubers produced in Florida account for 95% of the world supplies. Loss of caladium germplasm or genetic diversity has been a concern to future improvement of this plant. In addition, the relationship among the available cultivars, particularly those of close resemblance, has been lacking. This study was conducted to assess the genetic variability and relationship in commercial cultivars and species accessions. Fifty-seven major cultivars and 15 caladium species accessions were analyzed using the target region amplification polymorphism marker technique. This marker system does not involve DNA restriction or adaptor linking, but shares the same high throughput and reliability with the amplified fragment length polymorphism system (AFLP). Eight primer combinations amplified 379 scorable DNA fragments among the caladium samples. A high level of polymorphism was detected among the species accessions as well as among cultivars. These markers allowed differentiation of all the cultivars tested, including those hardly distinguishable morphologically. Clustering analysis based on these DNA fingerprints separated the cultivars into five clusters and Caladium lindenii far from other caladium species. The availability of this information will be very valuable for identifying and maintaining the core germplasm resources and will aid in selecting breeding parents for further improvement.
Hong Y. Yang, Schuyler S. Korban, Jutta Krüger, and Hanna Schmidt
Almost 200 random sequence decamer primers were used to screen a pair of bulked samples of apple (Malus ×domestica Borkh.) DNA and that of the donor parent Malus floribunda Sieb. clone 821 for molecular markers linked to the Vf gene conferring resistance to apple scab [Venturia inaequalis (Cke.) Wint.]. Identified was a single primer that generated a polymerase chain-reaction (PCR) fragment, OPAR4/1400, from the donor parent M. floribunda clone 821 and the scab-resistant selections/cultivars bulk, but not from the scab-susceptible recurrent-parent bulk. Cosegregation analysis using a segregating apple progeny and polymorphism analysis of individual scab-resistant selections/cultivars confirmed that this marker was linked to the scab-resistance gene Vf OPAR4/1400 was then cloned and sequenced. Sequence-specific primers of 25 oligonucleotides based on the marker were developed and used to screen further M. floribunda clone 821, scab-susceptible apple cultivars, scab-resistant apple cultivars, and scab-resistant Purdue, Rutgers, and Univ. of Illinois apple breeding program selections. The sequence-specific primers identified polymorphisms of OPAR4/1400 based on the presence or absence of a single band. This molecular marker is at a distance of about 3.6 cM from the Vf gene.
Mohammed A.M. Aly, Robert G. Fjellstrom, Gale H. McGranahan, and Dan E. Parfitt
Somatic embryos derived from walnut (Juglans regia L.) ovule tissues were evaluated to determine whether they were of zygotic or maternal origin. Molecular markers were used to permit evaluation at an early stage, before whole plant development. Somatic embryos developed from potentially apomictic `Sunland' and `Cisco' ovule tissue isolated from bagged putatively unpollinated flowers. Phosphoglucomutase (PGM) isozyme analysis showed that all of these embryos, except one from each cultivar, carry the same zymotype as the maternal tissue. However, restriction fragment length polymorphism (RPLP) analysis combined with isozyme evaluation demonstrated that the tested embryos originated from zygotic rather than maternal tissues. This study demonstrates the application of molecular marker analyses, particularly RFLPs, evaluation of somatic embryo origin.
Thomas Horejsi and Jack Staub
106 POSTER SESSION (Abstr. 335–343) Breeding and Genetics–Vegetables II (Molecular Markers and Physiological Genetics)
Roberto F. Vieira, Peter Goldsbrough, and James E. Simon
Molecular markers were used to assess genetic diversity in basil (Ocimum L. spp., Lamiaceae). Using randomly amplified polymorphic DNA (RAPD) analysis, 11 primers generated 98 polymorphic bands, ranging from 300 to 2,000 base pairs, that discriminated among 37 accessions across nine Ocimum spp. Means of genetic similarities within Ocimum spp. showed that the domesticated species, O. minimum L. (0.887), O. basilicum L. (0.769), and O. ×citriodorum Vis. (0.711) had highest similarity indices within species, while the nondomesticated, O. americanum L. (0.580), O. gratissimum L. (0.408), and O. kilimandscharicum Guerke (0.559) showed the lowest similarity. RAPD results indicated that O. minimum should not be considered a distinct species but rather a variety of O. basilicum. Consistent clusters among all but one of the O. ×citriodorum spp., all containing citral as the major constituent, were identified using bootstrap analysis. RAPD analysis was useful in discriminating among Ocimum spp., although within species resolution will require a higher number of polymorphic bands.