Rapid Generation Cycling and Inbreeding Depression in Chrysanthemums

Abstract

Advanced inbred lines that are nearly homozygous for favorable alleles could serve as parents for creating uniform, commercially acceptable F1 hybrid seed garden and greenhouse chrysanthemums, Dendranthema grandiflora Tzvelv. (Chrysanthemum morifolium Ramat.). Since this species is highly self incompatible, it is difficult to inbreed via self pollination. Consequently, there is a paucity of information on inbred line development and the effects of inbreeding depression in this hexaploid (2n = 6x = 54), perennial species. Objectives of this research were to: (1) use rapid generation cycling techniques (laboratory seed ripening and embryo rescue) to reduce the total generation time; (2) produce as many inbred generations in one year as possible, and (3) analyze the inbred generations for reproductive characteristics and inbreeding depression. Embryogenesis occurred at a faster rate using laboratory seed ripening when compared with in situ methods. ER of heart-stage embryos significantly improved seed set, percent germination, and survivorship in comparison to in situ treatments. Sixty-six inbred, noninbred, hybrid and species parents were self pollinated to generate four inbred generations in 1.33 years using RGC techniques. Inbred populations were evaluated for inbreeding depression (IBD) at three life cycle stages (seed set, seed germination, and survivorship). IBD was greatest primarily in noninbred cultivars and hybrids during the period in which the zygotic genome became activated (heart-stage to germination). Most F1 and F2 hybrid-derived inbreds expressed less or equal amounts of IBD in comparison to advanced inbreds. These recombinant inbreds have some control system(s) operating that negates the deleterious effects of inbreeding. Linear regression coefficients for germination aand survivorship were negative, indicating a reduction in progeny performance as homozygosity increased. IBD in chrysanthemums could be explained by a genetic model based on dominance and epistasis. Lethal equivalent values ranged from 2B = $-$4.5 to 10.8 (germination) and 2B = $-$3.6 to 6.2 (survivorship). Negative lethal equivalents occurred for all inbred chrysanthemum families and were not due to fixed heterozygosity in polyploid species, as they occurred in lethal equivalents calculated for fitness traits in published work with tetraploid Cyclamen and diploid Borago, Gilia, Phlox, Zea, and Helianthus.