Differences in structural gene expression are responsible for a wide range of responses from human cancer to patterned flowers. Gene silencing is one of the ways in which gene expression is controlled. We have developed a model system to study gene silencing using a gene silencing mutation in Petunia ×hybrida (Star mutation) and the ability of certain viruses to reverse the silencing mutation. This model system was used to characterize how the Star flower color pattern was controlled.
Genetically characterized isozyme loci are useful for taxonomic studies. In an initial study a few Ananas genotypes were used to determine which enzyme systems would give well-resolved banding patterns on starch gels. The enzyme-staining systems that resulted in well-resolved banding patterns were used to survey more Ananas genotypes to identify and characterize isozyme polymorphism. Genetic studies were performed using seedling populations to determine the basis of variability observed among genotypes. Two peroxidase loci and three phosphoglucomutase loci were identified and characterized. Information from these studies, was used to formulate a system by which species and plant introductions could be identified and distinguished.
Strawberry flowering habit can be classified as either day-neutral (DN) or short-day (SD), depending on whether plants are insensitive or sensitive to photoperiod, respectively. Short-day (SD) cultivars produce mature fruit for just a few weeks in early summer. New floral initiation does not commence until triggered by the combination of short daylength and low temperature in the fall. Day-neutral (DN) cultivars do not require particular daylength conditions to initiate flowering, and so continue to produce flowers and mature fruit into late summer and early fall. We are using a map-based approach to characterize the genetic determinants of flowering habit in strawberry at both the diploid and octoploid levels. A recessive gene conferring DN flowering habit has been identified, and its position determined with respect to molecular markers on the Fragaria vesca genetic linkage map. We are using the technique of bulked segregant analysis (BSA) in an effort to find random amplified polymorphic DNA (RAPD) markers linked to a putative dominant gene conferring the DN habit in the octoploid, cultivated strawberry, F. ánanassa.
The polymorphism in nine enzyme systems in apple (Malus domestica Borkh.) was analyzed using horizontal starch gel electrophoresis. The systems studied included aspartate aminotransferase, diaphorase, glucosephosphate isomerase, isocitrate dehydrogenase, phosphoglucomutase, and triosephosphate isomerase. The products of at least 27 loci could be distinguished in these systems, 19 of which displayed polymorphism. Joint segregation analysis in populations derived from crosses between highly heterozygous cultivars revealed four multilocus linkage groups: Aat-c–Idh-1, Dia-2–Mdh-4, Gpi-c2-Aat–p, and (Dia-5, Pgm-p1)–(Mdh-2, Tpi-c2). Although several of the populations investigated had been prescreened for resistance to apple scab, cedar-apple rust, or fire blight, no correlation could be established between the inheritance of an allozyme and a resistant phenotype. The high frequency of duplicate loci encountered is in accordance with the postulated tetraploid nature of the genome.
In husk tomato, Physalis ixocarpa Brot., a wide range of self-incompatibility phenotypes was observed upon self-pollination. Self-incompatibility seems to be a polygenic trait of low heritability, affected by inbreeding depression and by environment. Organ coloration and leaf serration also were found to be polygenic traits. The chromosomes of this species are characterized by conspicuous heterochromatic knobs which would facilitate identification of each pair. Attempts to hybridize P. ixocarpa with P. floridiana L. and P. peruviana L. were unsuccessful. Propagation by cuttings is readily attainable in P. ixocarpa and might be useful for growing and intercrossing field selections in isolation plots.
F2 and backcross segregation for heat tolerant × heat sensitive crosses in Chinese cabbage (Brassica campestris L. Group Pekinensis) indicated that heat tolerance was controlled by a single recessive gene.
In a study of chilling requirement in Malus, broad-sense heritability estimates for the length of vegetative bud dormancy in 43 clones growing under simulated subtropical winter conditions were 0.76 ± 0.04 in 1986 and 0.81 ± 0.04 in 1987. Narrow-sense heritability estimates were 0.66 ± 0.13 in 1986 and 0.69 ± 0.13 in 1987. Seedlings with low chilling requirements (CR) were not observed in crosses where both parents had high bud-chilling requirements. `Koningszuur' did not transmit its long CR to its seedlings. Open-pollinated (OP) seedling populations from the Malus × domestics Borkh. cultivars Anna, Dorsett Golden, Ein Shemer, Khashabi, Winter Banana, and Zabaoani, and the species and interspecific hybrids M. baccata L. DE#98, M. brevipes Rehd., M. ×robusta (Carr.) Rehd. DE#485, M. × robusta No. 5 (`R5'), M. rockii, M. turesi Rehd. PI 34143, and `Rosedale' had at least 5% of their descendants in the lower CR classes. In all but one instance, 50% or more of `Anna' descendants had low CR. Many of these seedlings were within a few classes of the extreme low CR. It is postulated that the low-CR character present in `Anna' is controlled by at least one major dominant gene and that minor genes interact to modulate its effects. Very low-CR cultivars have a shallow bud dormancy. This highly heritable component for low bud CR is related to a failure to develop a deep dormancy state, rather than to acceleration of the termination of the dormancy process.
In growth chambers at 26°±2°C (day) and 21°±2° (night), a 15 hour photoperiod delayed flowering on ‘Great Northern (GN) Nebraska #1 sel. 27’ by 47 days and in Plant Introduction (PI) 207262 by 48 days over a 10 hour photoperiod. The cultivars ‘GN 1140’ and ‘GN Nebraska #1’ and a near-isogenic determinate ‘GN Nebraska #1’ flowered at the same time under both photoperiods. Both near-isogenic lines, determinate and the early flowering ‘GN Nebraska #1 sel. 27’ flowered earlier than ‘GN Nebraska #1 sel. 27’ in all field experiments. The near-isogenic determinate ‘GN Nebraska #1’ flowered earlier than the ‘GN Nebraska #1 cultivar in 2 out of 3 field experiments. The delay in flowering of ‘GN Nebraska #1 sel. 27’ was much greater at higher night temperatures at Lincoln than under lower temperatures at Scottsbluff when the photoperiod was the same. Delayed flowering of PI 207262 under long photoperiod was controlled by a single recessive gene.