The production of doubled haploid plants is desirable as an alternative to sexual inbreeding of longer-generation crops. Onion (Allium cepa L.) is a biennial plant and amenable to the production of gynogenic haploids. Although a strong population effect has been observed for gynogenic haploid production, there is no report describing the genetic basis of greater haploid production in onion. We evaluated over years the frequency of haploid production among onion inbreds and identified lines showing significantly (P < 0.01) greater production of haploids. The onion inbreds, B0223B and B2923B, produced the highest mean frequencies of haploids so far reported. Hybrid families from crosses of B2923B with inbreds having relatively low haploid production showed significantly higher haploid production than the low-producing parent and significantly lower haploid production than B2923B. Plants from B0223B and B2923B with established rates of haploid production were testcrossed and/or self-pollinated. The F1 family from B1717A-1 × B2923B-3 showed rates of haploid production slightly greater than the low parent (B1717A-1) and significantly less than the high parent (B2923B-3). Self-pollination of plants from B2923B showing relatively high rates of haploid production generated S1 progenies also producing relatively high frequencies of haploids. Selfed progenies from plant B2923B-6 showed a high mean rate of haploid production (56.8% ± 14.5%) and, more importantly, the highest level of haploid production (82.2%) reported for any single onion plant. These results indicate that relatively high haploid production, at least for B2923B, was quantitatively inherited with dominance towards low production. We suggest S1 family selection as an effective method to increase gynogenic haploid production of onion populations.
Borut Bohanec, Marijana Jakse and Michael J. Havey
Borut Bohanec, Marijana Jakše, Predrag Šesek and Michael J. Havey
Bulbous leek-like plants are a poorly defined group usually assigned to the Allium ampeloprasum complex. Studies were initiated to determine the origin of an unusual bulbous accession received in Shanxi province in China, where it was used in diet as garlic but propagated by seeds, and to genetically compare this accession with morphologically similar plants from Europe. Genetic analyses included karyotypes and genomic in situ hybridization, pollination to leek, genome size determination and nuclear rDNA and plastid DNA polymorphisms. Results revealed that this agriculturally interesting accession from China is a so far unknown variant within tetraploid A. ampeloprasum cultivated taxa. We also observed that great-headed garlic did not share derived states in the chloroplast with leek, revealing that this cultivated plant does not possess the cytoplasm of leek or garlic, while its 1C genome size was 17% bigger than those of studied leek and bulbous-leek accessions.
Marijana Jakše, Pablo Hirschegger, Borut Bohanec and Michael J. Havey
Although haploid induction has been used in onions (Allium cepa L.) for over 20 years, several obstacles limit its use in plant breeding programs. To address these limitations, we evaluated the responsiveness of doubled haploid (DH) lines and their selfed progenies, an alternative protocol for chromosome doubling using somatic regeneration of haploid lines, and pollen viability of DH lines. Twenty-one DH lines were self pollinated and tested for haploid induction in the second generation. Among the DH lines, 18 lines showed an average of 20% decrease in gynogenic responsiveness compared with the original lines, while three lines registered an average increase of 5.7%. Using a two-step induction/regeneration procedure, 8,589 somatic regenerants were obtained from 16,170 flower buds from haploid plants, and shoot culture was established. A more laborious procedure using extraction of ovaries in the regeneration stage was found equal to flower bud culture. Chromosome doubling via somatic regeneration was found to be 83% and 100% efficient when the source material was haploid or mixoploid, respectively. Based on the results achieved in this and previous studies, an alternative protocol for chromosome doubling of gynogenic haploids is proposed.