are largely physiological, but can be long-lasting. Breeding and development Yields of older, established nucellar budlines have often been higher than those of their parent lines. James W. Cameron, Professor of Horticulture But most importantly, many viruses and viruslike and Geneticist, Emeritus, and agents are eliminated at embryo formation. This Robert K. Soost, Professor of Genetics, has led to widespread propagation of nucellar lines Department of Botany and Plant Sciences and has contributed to the improvement of citrus plantings in many countries during the last two decades. hen Howard B. Frost began citrus breeding at the University of California Citrus Ex- W periment Station in 1914, knowledge of Inheritance of nucellar embryony crossing relationships in the genus was limited to One problem associated with citrus breeding- earlier studies in Florida by the U.S. Department as with many tree fruits-is the scarcity of char- of Agriculture. Frost began a wide series of crosses acters controlled by one or a few genes. Most char- among edible types, but the numbers of hybrids acters are inherited quantitatively, involving many first obtained were limited by nucellar embryony. genes. In addition, genes are apparently seldom This is the phenomenon by which somatic cells of homozygous (that is, existing in two identical the nucellus (tissue in the ovule but outside the copies, or alleles) within a clone. Thus, fruits embryo sac) develop into embryos. Since these showing a wide range of size, shape, color, and nucellar embryos develop asexually, with no male flavor may occur in a single progeny from two cells contributing to their formation, they are usu- parents. ally genetically identical with the seed parent. Because of the importance of nucellor embry- Nucellar embryony has been a major obstacle to ony, U.C. researchers began to study its pattern of the systematic production of F1 hybrids and self- inheritance. By 1959, data indicated that its inher- pollinated progenies. Many cultivars produce al- itance was rather simple within common Citrus most entirely nucellar embryos, and there may be species. Crosses between strictly sexual parents several in a single seed. Others form both nucellar regularly yielded hybrids that were themselves and zygotic (sexual) embryos. Studies have identi- sexually reproducing; crosses between sexual and fied additional cultivars that produce only sexual nucellar parents usually produced hybrids of both progeny, and these have been largely used in more types. Two parents that could reproduce by nucel- recent crosses. lar embryony sometimes produced offspring that The occurrence of nucellar embryony enabled were completely sexual. Tetraploid hybrids (hav- Dr. Frost to compare carefully the tree and fruit ing four sets of chromosomes) were also examined, characters of recent nucellar seedling budlines and they showed the same kind of segregation pat- with those of their older parental budlines. The terns. Nucellar embryony thus appears to be dom- nucellar budlines were usually more vigorous, inant over sexual embryony, and very few genes faster growing, more thorny, and slower to fruit may be involved. than parental ones. These juvenile characteristics In crosses between Citrus and its near relative, Poncimcs, the inheritance is somewhat more com- plex. Many F, hybrids were sexually reproducing, even when both parents could reproduce by nucel- lar embryony. Yet backcrosses of these sexual hy- brids to sexually reproducing Citrus in experiment station tests still resulted in progeny that were essentially all sexual, adding to the evidence that strictly sexual plants may be homozygous for this character. Self-incompatibility in citrus Hybridization in citrus can also be complicated by self- and cross-incompatibility, which occurs in a number of cultivars. Self-incompatible forms set Triploid orange (with 3 little or no seed, and often very little fruit, after sets of chromosomes) self-pollination. Climatic factors, however, some- with few seeds (center) times favor fruit set even in the absence of seeds; is from cross of this condition is called parthenocarpy. tetraploid Ruby orange Self-incompatible cultivars may produce many (left)by diploid King fruits and seeds after cross-pollination by particu- tangor (right). lar pollen parents, indicating cross-compatibility. Right: CES-developed Reactions of this sort occur in a number of plant seedless Chandler pum- €amiliesand are considered to be caused by one or melo was grown in isola- more series of self-incompatibility alleles. Studies tion; seedy fruit at left reported by Dr. Soost in 1965 indicated that such received other citrus alleles are present in several citrus taxa, including pollen. 5ome pummelos and their hybrids, certain grape- Fruit hybrids such as Minneola and Orlando, and CR-4 the Clementine mandarin. In such cases, cultivars from two or three basic histogenic layers, but a that can serve as pollinators are often needed to nucellar embryo stems originally from a single seed produce good crops of fruit. parent cell. The evidence indicates that the pink grapefruits carry genetic factors for color in only The use of polyploidy one or two of their histogenic layers. Thus nucellar Most common citrus is diploid, with two sets of embryos (believed to arise from layer 11) can either chromosomes, but tetraploids often occur spon- lack a color factor or, conversely, can carry it in all taneously among seedling populations. Dr. Frost their tissues. was one of the first to identify and collect these forms, among a series of citrus cultivars. Although Chemical factors as markers not themselves commercially useful, tetraploids Citrus taxonomy has long been controversial, have been important in breeding few-seeded tri- and the origin and relationships of many forms are ploids (with three sets of chromosomes). Fruits uncertain. Intensive studies of a range of chemical with few seeds are desirable for eating out of and physical characters have been directed toward Oroblanco triploid grape hand, but most diploid hybrids are seedy. Triploids solving these problems. Under the leadership of fruit hybrid, developed at can be produced by crossing diploids with tetra- R.W. Scora, U.C. scientists have investigated ter- U.C. in 1980, is nearly ploids, and by 1938 Frost had obtained limited penes of leaf and rind oils, leaf hydrocarbons, pro- seedless and earlier numbers of such hybrids. Recently, large popula- tein and isozyme patterns, oxidase browning, virus ripening than the Marsh tions have been produced and are under study. resistance, and momhological characters. The grapefruit. When a partly sexual tetraploid is used as seed overall results suggest that there are four primary Below: Immature fruit of parent and a diploid as pollen parent, triploids are species, namely, the subtropical Citrus reticulata variegated orange. usually obtained, but the reciprocal cross results in and C. medica, the tropical C. grandis and the yellow rind sectors have few triploids and numerous hybrid tetraploids. recently discovered c. halimii. The latter is some- normal growth; green U.C., Riverside, studies showed that the what intermediate between the genera citrus and sectors are &Dressed. chromosomes occasionally doubled in the diploid Fortunella. Other forms that have often been clas- seed parent, followed by selective survival of the sified as species can, in fact, be identified as mem- resulting tetraploid embryos. An unfavorable bers of complexes involving asexual reproduction. balance between chromosome numbers in the In studies of closely related biotypes, relationships young embryo and endosperm (the nutritive tissue) were found between diploid and tetraploid forms, is responsible for this unusual behavior. However, among identical twin individuals, and among F, such new tetraploid plants, some of which are hybrids of Citrus and Pcm.cirus. strictly sexual, have already been used as addi- U.C. researchers, in cooperation with the Uni- tional seed parents to produce more triploids. versity of Kansas, have recently identified enzyme The triploid embryos that arise from pollination characters that are simply inherited and useful for of tetraploid seed parents by diploids also have a both breeding and taxonomy. The browning reac- certain developmental disadvantage, since they tion in young shoot tissue results from oxidation of sometimes tend to mature earlier and produce phenolic substrates. Presence of the substrate is smaller seeds. They can usually be germinated sat- under single-gene control, which often permits the isfactorily by use of in vitro techniques. separation of nucellar from zygotic seedling plants. Another reaction causes coagulation of shoot Mutations and chimeras tissue in some forms. Coagulation is recessive to Somatic mutations occur frequently in citrus; non-coagulation so that, again, separation of dif- they are often unfavorable, but occasionally highly ferent genetic types is often possible. Analyses of valuable. In Japan, mutations in the satsuma man- several other leaf enzymes, also inherited in darin have resulted in closely related clones differ- single-gene fashion, have been especially reward- ing in characteristics such as color and ripening ing. The presence or absence of as many as eight time. At Riverside, Dr. Frost found that similar of these markers, taken together, can permit early mutations had produced satsuma seedling lines and rapid separation of nucellar from zygotic that varied in ripening time, productivity, and