Development of new cultivars via anther culture

Crop species Reference Asparagus officinalis Pelletier et al. (1972) Brassica napus Wenzel et al. (1977) Brassica oleracea Kameya and Hinata (1970) Capsicum annuum Wang et al. (1973) Hordeum vulgare Clapham (1973) Nicotiana tabacum Nitsch and Nitsch (1969) Oryza sativa Niizeki and Oono (1968) Secale cereale Thomas et al. (1975) Since the technique of anther culture was first reported (Guha and Maheshwari, 1966), its promise as a means of generating haploid plants that might be instrumental in developing new cultivars of major crop species has been impatiently awaited. By placing immature anthers containing microspores at the late uninucleate/early binucleate stage of development onto a synthetic medium, development can be modified such that some microspores proceed along an embryogenic, or androgenetic, pathway. By conversion of such embryos into plants, or regeneration of microspore-derived callus, haploid plants can be obtained. Subsequently, the chromosome number of such haploids can be doubled to yield homozygous inbred lines. At first, the technique was limited to only a few species; however, with refinements in tissue culture and selection of responsive genotypes, androgenesis has been observed in a wide array of crops representing diverse taxonomic groups (Bajaj, 1990). Given the length of time generally required to develop new cultivars, we can only now begin to expect release of cultivars that owe their origin to anther culture (Table 1). The citations in this table are not necessarily the first anther culture reports for each crop, but the first reports where haploid green plants were produced. Some species used in early anther culture work are not included in the list because of the difficulties that subsequent researchers encountered when applying the techniques to obtain haploid plants (e.g., tomato). Using anther culture for plant improvement depends on the breeding system of a crop. For self-pollinated crops, such as wheat (Triticum aestivum L.), rice (Oryza sativa L.), or tomato (Lycopersicon esculentum Mill.), anther culture can be most effectively applied to an F1 hybrid between two breeding lines with complementary characteristics of agricultural significance to fix homozygosity in a single generation, thus avoiding the time required for several generations of inbreeding. Anther-derived doubled haploids could be released directly as cultivars once their field performance has been sufficiently documented. Two possibilities exist for applying anther culture to cross-pollinated crops, depending on whether homozygous lines and hybrid cultivars are already available. If they are available, e.g., maize (Zea mays L.), then anther culture can be used to develop additional inbred lines, as in self-pollinated crops. The difference is that the superior inbred lines developed by anther culture would not be grown directly as cultivars, but would be used to generate hybrid cultivars. For crops where completely homozygous inbred lines are either not already available or difficult to develop, e.g., Brassica or Asparagus, anther culture offers a new means to obtain them. Survival as a haploid provides an immediate and efficient selection against deleterious and lethal genes. Because only a single allele is present at each locus, expression of such deleterious recessive alleles is guaranteed with the result that embryos or plants bearing them will perish through the “monoploid sieve” (Wenzel et al., 1979). Also, derivation of haploids does not depend on male and female fertility, as does inbreeding. Of course, once doubled haploids have been obtained, fertility is generally a prerequisite to their use in further breeding. Finally, for highly heterozygous, clonally propagated crops such as potato (Solanum tuberosum L.), heterogeneity among seedlings derived from heterozygous parents is generally too great for most agronomic characters for releasing and producing hybrid cultivars on a massive scale. Anther culture provides the potential to develop homozygous inbreds for F1 hybrid production without sacrificing uniformity, thereby converting propagation from vegetative to hybrid seed. Work has progressed on all of the above applications of anther culture toward cultivar development. In some instances, a general reduction in vigor has been associated with anther-derived doubled haploids (Baenziger et al., 1989; Marburger and Jauhar, 1989), even