Richard E. Durham

Evidence of gene introgression in apple using RAPD markers

SummaryA genomic remnant of Malus floribunda clone 821 introgressed into the cultivated apple M. x domestica Borkh. was identified using randomly amplified polymorphic DNA (RAPD) markers obtained by the polymerase chain reaction (PCR). Using a set of 59 oligonucleotide decamer primers, polymorphic DNA markers were identified among three pooled DNA samples. Based on the presence or absence of bands among bulked apple scab-resistant selections and cultivars, bulked scab-susceptible cultivars, and a M. floribunda clone 821 sample, one primer, A 15, identified amplified fragments in the scab-resistant bulked sample that was also unique to the M. floribunda clone 821. The unique band from M. floribunda clone 821 was amplified in four out of 17 scab-resistant selections/cultivars. This RAPD, designated OA15900, identifies an introgressed fragment that has as yet no known function.

Somatic Embryogenesis in Legumes

Legumes are members of the third largest family (Leguminosae) of flowering plants, and are globally distributed, with as many as 19,000 species. Several species have the ability to undergo a symbiotic association with nitrogen-fixing bacteria of the genera Rhizobium or Bradyrhizobium. Consequently, legumes tend to have high protein contents, and are an essential source of food, feed, and forage. They are also used as ornamentals, or valued for green manure, timber, gums, and other compounds.

Effects of explant size, pretreatment, and light intensity on shoot regeneration from in vitro-grown apple leaves

The effects of explant size, pretreatment, and light intensity on shoot regeneration of apple (Malus × domestica Borkh.) were investigated using the rootstock genotype Mailing 7a. Young leaves were harvested from in vitro proliferating shoot cultures, sliced into sections or minced into small pieces, dipped in either sterile distilled water or liquid regeneration medium, and placed onto a medium consisting of half-strength Murashige and Skoog salts, Staba vitamins, 20 μ m thidiazuron, and 2.7 μ M tα -naphthaleneacetic acid. During the first four weeks of culture, explants were exposed to various light treatments: continuous darkness, two weeks of darkness followed by two weeks of low-light (15 μ mol m-2 s-1), continuous low-light, two weeks of low-light followed by two weeks of high-light (100 μ mol m-2 s-1), or continuous high-light. The regeneration frequency observed increased with decreasing light intensity and light duration. Mincing leaves and dipping explants in a liquid regeneration medium prior to culture also resulted in an increase in the number of shoots produced per leaf but did not significantly influence the regeneration frequency. The highest regeneration frequency (92%) was obtained when explants were sectioned, dipped in a liquid regeneration medium, and cultured in darkness for 4 weeks. However, the highest number of shoots (59.8 per leaf) was observed on minced explants cultured under the same conditions. A significantly higher regeneration frequency was obtained for five apple genotypes tested when leaves were minced rather than sectioned.