Summary Crossing barriers occur frequently when intra- or interspecific crosses are attempted. These barriers are the result of incompatibility and incongruity. Sexual barriers preventing interspecific hybridization have been distinguished into pre- and postfertilization barriers. The nature of the barrier governs the method to be used to overcome the specific barrier. A range of techniques, such as bud pollination, stump pollination, use of mentor pollen, and grafting of the style, have been applied successfully to overcome prefertilization barriers. In vitro methods in the form of ovary, ovary-slice, ovule, and embryo culture are being used to overcome postfertilization barriers that cause endosperm failure and embryo abortion. An integrated method of in vitro pollination and fertilization followed by embryo rescue has been applied in many crosses. Vital hybrid plants may display lack of flowering or male and female sterility, resulting in failure of sexual reproduction. If sterility is caused by a lack of chromosome pairing during meiosis, fertility may be restored by polyploidization, enabling pairing of homologous chromosomes in the allopolyploid hybrid. Integration of these techniques into the breeding programs would enable the breeder to introgress genes across the species barriers. Introduction The phenomena underlying crossing barriers are incompatibility and incongruity. Incompatibility operates in intraspecific crosses and is the result of the activity of S-alleles. Incongruity occurs in interspecific crosses as a result of lack of genetic information in one partner necessary to complete pre- and postpollination processes in the other (Hogenboom 1973). This chapter focuses on different methods used to overcome incongruity. Interspecific and intergeneric crosses are made to introduce new genetic variation into cultivated plants.In breeding ornamental crops, interspecific hybridization is the most important source of genetic variation.

Methods for overcoming interspecific crossing barriers.

In vitro corm formation and field evaluation of corm-derived plants of Gladiolus

The pathogenicity and vegetative compatibility of mainly Dutch isolates ofFusarium oxysporum collected from diseased gladioli and other Iridaceae were investigated. Based on their pathogenicity to two differential gladiolus cultivars, the isolates could tentatively be divided into two races. All self-compatible isolates ofFusarium oxysporum f.sp.gladioli belonged to one of three distinct vegetative compatibility groups, VCG 0340, 0341 or 0342, and were incompatible with isolates that were not pathogenic to gladiolus. Isolates of one of the two races were restricted to one VCG while isolates of the other race were present in all three VCGs.

Physiological races and vegetative compatibility groups within Fusarium oxysporum f.sp. gladioli

Some of the economically important horticultural crops have two common features. First, they are of interspecific hybrid origin and, second, they are mostly polyploids. The histories of the origin of such ornamentals as Rosa, Narcissus, Iris, Crocus and Chrysanthemum, among others, have been well documented (Stern, 1946; Wylie, 1952; Darlington, 1976; Brighton et al., 1980; Brandham, 1987) and they can be traced back to 18th and 19th centuries or even earlier. Initially, most of these crops were diploids, the polyploid forms originated spontaneously from interspecific hybrids in the breeders’ nurseries. In those early days, the knowledge regarding the status of the species, the genomes and polyploidy was nonexistent; nevertheless the horticultural breeders were successful in creating considerable genetic variation. These efforts were subsequently responsible for the selection of thousands of horticultural varieties in some of those crops. For example, in Narcissus, more than 25,000 cultivars have been recorded (Throckmarton, 1980). In Lilium, which has probably followed a similar path of origin as other horticultural crops, more than 7000 cultivars have been registered since 1960 alone (Leslie, 1982). The existence of thousands of cultivars in Tulipa is well documented.

Interspecific Hybridization and Introgression

Chromosome counts onCostus lucanusianus in Nigeria were carried out. Diploid (2n=18) and triploid (2n=27) chromosome numbers were found. The diploid cytotype is reported for the first time for this taxon.

Chromosome numbers of costus lucanusianus (costaceae) in nigeria

Based on x=15, the species in the genus depict a range of ploidy from diploid (2n=30) to dodecaploid (2n=180), whereas all the garden coultivars studied are tetraploid (2n=60). The two diploid taxa G. tristis L. and Acidanthera bicolor Hochst. show regular meiosis with 15 bivalents and near normal pollen stainability. Among tetraploid (2n=60) taxa, G. italicus Mill. is characterised by regular occurrence of 30 bivalents leading to normal anaphase segregation and 91.6 per cent pollen stainability. The garden cultivars, however, show at metaphase I predominant bivalent formation with some quadrivalents, trivalents and univalents. While normal anaphase I segregation (30:30) was observed in most of the cells, unequal segregation also resulted due to trivalents and univalents. Pollen stainability ranges from 49.8 to 98.6 per cent. The pentaploid taxa comprising of six cultivated types of G. psittacinus hybr. also exhibited predominant bivalent formation along with pentavalents, quadrivalents, trivalents and univalents. Anaphase I is irregular due to unequal disjunction of trivalents and univalents. Pollen stainability ranges from 65.07 to 94.2 per cent. G. communis (2n=176) is characterized by trivalents, bivalents and univalents. Bivalent formation is unexpectedly high with a mean of 84.75 per cell and full complement of 88 bivalents was observed in 20 per cent of the cells studied. Pollen stainability is 87.16 per cent.A correlation of ploidy with general geographical distribution shows that the species native to Cape Winter-Rainfall region are diploid and those occurring in Summer-Rainfall region and Tropical Africa range from diploid to hexaploid; while Mediterranean, European and West Asian species are all polyploid, ranging from tetraploid to dodecaploid.

https://www.jstage.jst.go.jp/article/cytologia1929/50/2/50_2_213/_pdf

Cytogenetics of Garden Gladiolus

The present studies are based on three species of the genus Costus viz., C. speciosus Sm., C. malortieanus H. Wendel and C. megalobractea K. Schum. C. spe-ciosus contains intraspecific chromosomal races namely diploid (2n=18), triploid (2n=27) and tetraploid (2n=36). This taxon has been studied in cytogeographic angle, and it is clear that the materials originating from different geographic regions of India have different ploidy levels. Intraspecific triploid hybrids (2n=27) were obtained by crossing natural tertaploid and diploid C. speciosus. The chromosome number recorded for the species C. megalobractea (2n=36) in the present studies is the first report on the taxon. The detailed karyotype analyses were made in all these taxa, while the meiotic analyses were carried out on the diploid and tetraploid cytotypes of C. speciosus. On the basis of the present cytological data, it can be concluded that polyploidy and structural changes of the chromosomes have played and important role in the evolution of the genus Costus.

T. N. Khoshoo

Papers

Cytogenetics of Garden Gladiolus

Cytological studies in the genus Costus L.