Hybrid Breeding System Research Articles

Identification of monogenic dominant male sterility and its suppressor gene from an induced mutation using a broccoli (Brassica oleracea var. italica) microspore culture

Male sterility (MS) has been studied intensively in Brassica crops to ensure high purity and the protection of F1 hybrids. In the preceding trial, broccoli microspores were treated with a mutagen, n-nitroso-n-methyl urethane (NMU), during the cultural process to produce MS. Many MS plants were derived from the 0.01 μM and 0.1 μM treatments just before and at the fourth day of culture, respectively. In this study, most MS plants inherited sterility from a single recessive gene. Only one plant, 9-4C-106, which originated from the 0.1 μM treatment on the fourth day, was controlled by a single dominant gene. Another dominant mutant gene suppressing the expression of this dominant MS gene was discovered in a plant treated with 0.01 μM NMU. A hybrid breeding system using this MS structure is discussed.

Expression of active Streptomyces phage phiC31 integrase in transgenic wheat plants

Site-specific recombination systems are becoming an important tool for the genetic modification of crop plants. Here we report the functional expression of the Streptomyces phage-derived phiC31 recombinase (integrase) in wheat. T-DNA constructs containing a phiC31 integrase transgene were stably transformed into wheat plants via particle gun bombardment. A plant-virus-based assay system was used to monitor the site-specific recombination activity of the recombinant integrase protein in vivo. We established several independent doubled haploid (DH) inbred lines that constitutively express an active integrase enzyme without any apparent detrimental effects on plant growth and development. The potential of phiC31 integrase expression in crop plants related to transgene control technologies or hybrid breeding systems is discussed.

Secondary traits in parental inbreds and hybrids under stress and non-stress environments in tropical maize

Secondary traits other than grain yield (GY) have been successfully used to enhance the rate of genetic improvement for maize ( Zea mays L.) populations under abiotic stresses. With increasing trend towards the development of hybrids in the tropics there is a need to understand the genetics of these traits in inbred–hybrid breeding systems. The objectives of this study were to estimate the general combining abilities for secondary traits and their relationship with GY in a group of tropical white inbred lines and their hybrids under stress and non-stress environments. Traits were measured in inbred lines and their hybrid combinations across stress and non-stress subtropical and tropical environments in México. Hybrids showed earlier flowering, taller plants, more ears per plant (EPP), higher shelling percentage, slower leaf senescence, and higher leaf chlorophyll content than inbreds under all environments. La Posta Sequı́a (LP)-derived inbreds in general had desirable GCA for traits such as anthesis–silking interval (ASI), EPP, and leaf senescence but were more susceptible to foliar diseases. Conventionally bred maize lines had better standability, stay green, and resistance to diseases. Higher GY in lines and hybrids was associated with shorter ASI, earlier flowering, increased plant and ear height, increased EPP, increased shelling percentage, delayed senescence, and greater leaf chlorophyll concentrations. Under stress vs. non-stress conditions, we observed a higher variability for ASI and EPP, a higher inbred–hybrid correlation, and significant correlations between these traits and GY. Inbreds from LP appear to have high frequency of favorable dominant alleles for ASI and EPP.

Molecular diversity of male sterility inducing and male-fertile cytoplasms in the genus Helianthus.

The organisation of mtDNA was investigated for 28 sources of cytoplasmic male sterility (CMS) and a fertile line (normal cytoplasm) of Helianthus annuus by Southern hybridisation. In addition to nine known mitochondrial genes ( atp6, atp9, cob, coxI, coxII, coxIII, 18S, 5S and nd5) three probes for the open reading frames in the rearranged area of PET1, orfH522, orfH708 and orfH873, were used. Genetic similarities of the investigat-ed cytoplasms varied between 0.3 and 1. Cluster analyses using the UPGMA method allowed the distinction of ten mitochondrial (mt) types between the 29 investigated cytoplasms. Most mitochondrial types comprise two or more CMS sources, which could not be further separated, like the PET1-like CMS sources (with the exception of ANO1 and PRR1), or ANN1/ANN2/ANN3, ANN4/ ANN5, ARG3/RIG1, BOL1/EXI1/PEF1/PEP1 and GIG1/ PET2. ANL1, ANL2 and the fertile cytoplasms are also regarded as one mitochondrial type. Unique banding patterns were only observed for ANT1 ( atp6), MAX1 ( atp6, orfH522 and orfH708) and PRR1 ( coxII). However, four of the mitochondrial types showed unique hybridisation signals: ANN4/ANN5 had characteristic bands for atp6 and orfH708, PEF1/PEP1/EXI1/BOL1 for atp6and coxII, and PET2/GIG1 for atp9. The PET1-like cytoplasms all shared the same patterns for orfH522, orfH708and cob (except ANO1). It could be demonstrated that CMS sources, like, e.g., PET2 and PEF1, are different from PET1 in mtDNA organisation and the CMS mechanism. Therefore, these CMS sources represent interesting candidates for the development of new hybrid breeding systems based on new CMS mechanisms.

CMS‐D8 Restoration in Cotton Is Conditioned by One Dominant Gene

Cytoplasmic male sterile (CMS‐D8) and fertile restorer lines (D8R) of cotton (Gossypium hirsutum L.) (AD1) were developed by transferring the cytoplasm and nuclear gene(s) from G. trilobum (DC.) Skovst. (D8) into the cotton nuclear background. Understanding the genetics of fertility restoration in this CMS system is essential for its use in a hybrid breeding system. The objective of this investigation was to determine the mode of inheritance of D8R restoration to CMS‐D8. The experimental approach involved a series of crossing schemes involving nuclear permutations among the AD1 and D8 alloplasms. Eighteen normal genotypes did not restore fertility to the CMS‐D8 (A line) and could be used as maintainer (B) lines. D8R crossed as female with B lines yielded F1 and F2 populations with all fertile plants. F1 pollen also produced all fertile progeny in crosses on A lines. Thus, the D8 restorer functions at the gametophytic level. When heterozygous restored plants were pollinated with B lines, or when reciprocal F1's with normal cytoplasm were crossed as male with the A line, the progeny ratio was one fertile to one sterile. A 3:1 ratio was obtained when restored F1 plants with D8 cytoplasm were pollinated by their reciprocal F1's with normal cytoplasm. Accordingly, restoration of CMS‐D8 by the D8R restorer is conditioned by a single dominant gene (Rf2). The genotypes for A, B, and D8R lines in the CMS‐D8 system are designated S (rf2rf2), N (rf2rf2), and S (Rf2Rf2), respectively. This gametophytic restoration system is potentially useful for utilizing F2 heterosis in cotton.

Nectar quality in open-pollinated, pol CMS hybrid, and dominant SI hybrid oilseed summer rape

Nectar sugar composition and temporal patterns of nectar sugar production were examined in oilseed summer rape (Brassica napus L. ssp. oleifera (Metzg.)) from six open-pollinated, eight pol cytoplasmic male-sterile (CMS) hybrid and seven dominant self-incompatible (SI) hybrid cultivars at three field plot sites in Manitoba. The total sugar content of nectar samples was measured by the Dreywood anthrone reaction for total carbohydrate, and simple sugar composition was determined using an enzymatic bioanalysis for D-glucose, D-fructose and sucrose. Hybrid and open-pollinated cultivar flowers had similar sugar content. Mean total sugar content per flower also did not vary among hybrid breeding systems when compared within individual weeks of the bloom period or within daily sampling periods. However, for all cultivars, total nectar sugar content per flower was lower during the 08:00 and 11:00 h sampling periods and increased to maximum levels during the 14:00 and 16:00 h sampling periods. Significant differences in nectar sugar content were also found in relation to the bloom phenology of the cultivars. Cultivars produced the greatest amount of sugar per flower during the first 2 wk of the bloom period, then sugar production decreased in the third and fourth weeks. Nectar sugar ratios from all cultivars averaged approximately 1:1 glucose:fructose. Nectar glucose content among cultivars was similar but, among breeding systems, CMS cultivars tended to have lower amounts of glucose than SI or open-pollinated cultivars. Selecting for higher total sugar content may produce nectars more attractive to foraging honeybees (Apis mellifera L.), thereby ensuring adequate pollination of hybrid parental lines and F1 hybrid plants. Selecting for lower nectar glucose will produce honeys with more desirable granulation characteristics. Overall, the production and quality of nectar sugar in oilseed rape hybrids are similar to those of open-pollinated cultivars, and are not likely to adversely affect the pollinating activities of honeybees or their potential for honey production. Key words: oilseed summer rape, Brassica napus, honeybees, Apis mellifera, nectar, simple sugars

The potential use of gametophytic factors in the production of F1 hybrid varieties

Attention is drawn to the potential of gametophytic (Ga) factors which act only in pollen, or preferably in both pollen and egg cells, in the production of F1 hybrids in a wide range of crop plants when used in conjunction with linked chromosomal male sterility genes. The genetic stocks required to develop practical hybrid production systems are outlined along with the potential advantages and disadvantages of the use of gametophytic factors in hybrid breeding systems.