Haploid induction in onion via gynogenesis
Onion (Allium cepa L.) is a biennial monocotyledoneus plant belonging to the Alliaceae family. Within the Allium species onion is the most economically important with a total world acreage of 2,7 million ha dry onions and production of over 46 million ton in 2001 (FAO, 2001).The demand for specific varieties adapted to local agroclimatic conditions is very high since onion is a photoperiodically sensitive plant and forms bulbs only after specific environmental conditions, which vary among genotypes. According to the breeding method, two types of onion varieties are cultivated: open pollinated and hybrid varieties. Hybrid varieties have been produced for over 50 years from elite inbred lines. Expected benefits from hybrid cultivars are: higher yield (expressed heterosis), improved uniformity and for the seed producers — protection of plant material. The characteristic for onion inbred lines is their relatively high heterozygosity resulting from limited (two to three) cycles of self-pollination. Doubled haploids provide an alternative strategy that offers, for the first time in onion breeding, complete homozygosity and phenotypic uniformity. Another advantage is a substantial reduction in the time required to produce inbred lines, considering that onion is a biennial plant that requires up to 10 years to obtain nearly homozygous inbred lines by conventional breeding. Haploid plants can be obtained from male or female gametic cells. However, as reviewed by Keller and Korzun (1996), large anther culture experiments in onion have failed. Flowering is induced by environmental factors causing the apex to cease production of leaf primordia and initiate an inflorescence. A single onion inflorescence (umbel) might consist of up to 2,000 or more flowers and some genotypes form more than one (even more than a dozen) flower stalks. Within the umbel, flowers open in successive order so that the blooming time of each umbel is over 10 days. Each flower consists of a single superior pistil with three locules having two ovules in each carpel. The nectaries are between the carpels of the ovary and the three inner stamens. According to Klein and Korzonek (1999), flower development can be divided into seven stages related to the umbel size and developmental stages of anthers and ovules. In variety ‘Kutnowska’ during first 3 stages, the umbel (smaller than 2 cm) is enveloped by a spathe. The beginning of meiosis in anthers starts in stage IV, with the subsequent meiotic divisions in stage V. Ovule development starts later than meiosis in the anthers, when flower buds are 3.5–4.0 mm long (predominantly in stage VI). Flowers in stage VI are actually used in majority of gynogenic haploid production procedures.
KeywordsInbred Line Induction Medium Donor Plant Chromosome Doubling Haploid Plant
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