High frequency somatic embryogenesis in peanut (Arachis hypogaea L.) using mature, dry seed.

Peanut (Arachis hypogaea L.) somatic embryos were produced from the embryo axes of mature, dry seeds of cultivar GK-7. Percent embryogenic explants ranged from 88-100% using 10-40 mg/1 of 2,4-D in the induction medium. Neither 2,4-D concentration nor photoperiod during the induction period had a large effect on percent embryogenesis, mean number of embryos per explant, or embryo morphology. However, embryos obtained from cultures grown in the dark were easier to remove from the explant than those under a 16-h photoperiod. Somatic embryos developed on the epicotyl portion of the embryo axis, primarily on the young, expanding leaves. A survey of 14 genotypes indicated that genotype had a large influence on embryogenic capacity, with all genotypes being embryogenic to some extent. The ability to recover somatic embryos from axes of harvested, stored seeds represents significant advantages for the establishment of peanut embryogenic cultures, including the use of simple sterilization procedures and a constant source of explant tissue.

Linkage of restriction fragment length polymorphisms and isozymes in Citrus.

Genetic linkage analysis was performed using two segregating populations of citrus. One population arose from an intergeneric backcross of Citrus grandis (L.) Osb. cv 'Thong Dee' and Poncirus trifoliata (L.) Raf. cv 'Pomeroy', using the former as the recurrent (female) parent. The other population came from an interspecific backcross of C. reticulata Blanco cv 'Clementine' and C. x paradisi Macf. cv 'Duncan', using the former as the recurrent (male) parent. A total of 11 isozyme and 58 restriction fragment length polymorphisms were found to segregate in a monogenic fashion in one or both populations. Linkage analysis revealed that 62 of the loci examined mapped to 11 linkage groups, while 7 loci segregated independently from all other markers. Gene order was highly conserved between the maps generated from the two divergent segregating populations. Possible applications of the use of such maps in tree fruit breeding are discussed.

Repetitive somatic embryogenesis from peanut cultures in liquid medium.

A regeneration system based on repetitive somatic embryogenesis was developed for peanut (Arachis hypogaea L.). Embryogenic suspension cultures were initiated using individual somatic embryos induced from immature cotyledons cultured on a modified Murashige and Skoog medium containing 40 mg/l 2,4-D for 30 days. After transfer to a modified MS liquid medium, the somatic embryos produced masses of secondary and tertiary embryos which continued to proliferate following manual separation and subculture of the embryogenic clumps. The cultures exhibited exponential growth, and have been maintained for over one year without apparent loss of embryogenic potential. Further embryo development, germination, and conversion were achieved by placing embryo clumps onto hormone-free, solid medium. The inclusion of a desiccation period during embryo development enhanced conversion four-fold. Plants have been established in soil and appear to be phenotypically normal.

The use of leaf GPI and IDH isozymes to examine the origin of polyembryony in cultured ovules of seedless grape.

Isozyme analysis of young leaf tissue was performed on progeny produced from cultured ovules resulting from crosses of seedless × seedless grapes. Two polymorphic loci, Idh and Gpi-c, were found to segregate in a simple Mendelian fashion in the populations examined. Analyses of 70 seedlings indicated that most seedlings produced by this method were zygotic. Isozyme data of 11 plants from 5 polyembryonic ovules, along with data previously reported, suggest that several mechanisms may be operative in grape that give rise to polyembryony: 1) fertilization and development of more than one cell in the embryo sac, 2) adventive embryogenesis from the zygote, or 3) embryogenic development of gametic cells in addition to the zygote in the embryo sac.