The genomes of Arachis hypogaea 2. The implications in interspecific breeding

Summary The existence of structural differentiation between genomes in section Arachis of the genus Arachis has important implications in the utilization of diploid wild species in this section as a germplasm resource. Maximum expression of desirable characters may not be achieved unless tetrasomic dose levels can be achieved. Possible breeding strategies discussed include natural and induced gene exchange between genomes and chromosome substitution which could be brought about by manipulation of ploidy level and where appropriate the use of ionizing radiation. Such strategies could be tested in the improvement of resistance to the Cercospora leafspots.Paper number 5561 of the Journal Series of the North Carolina Agricultural Experimental Station, Raleigh, NC 27650.     

Cytology and Fertility of Hybrids and Amphiploids between Aegilops caudata L. ×Triticum turgidum (L.) Thell

Interspecific hybrids and amphiploids between Aegilops caudata L. (2n = 2x = 14, CC) and Triticum turgidum (L.) Thell. conv. durum Desf M. K. (2n = 4x = 28, AABB) were produced. Such hybrids can be used to introduce desirable traits such as disease resistance into cultivated durum wheats. One of the durum parents was a ph I mutation of the cv. ‘Cappelli’ used for testing the possibility of direct introduction of alien variation into cultivated species. The amphiploids were obtained both through colchicine chromosome doubling and as natural non-reductional mciosis products. In both hybrids and amphiploids, meiotic pairing and fertility were studied. Hybrids showed varying degrees of pairing and, in addition to the one involving the ph 1 mutant, one high pairing hybrid was found (Ae. caudata× cv. ‘Capinera’). Cytological examination of microsporogenesis in amphiploids revealed a high frequency of bivalent formation. Fertility proved to be a very variable character since some of the amphiploids were almost completely sterile. The use of amphiploids in breeding programmes is discussed in relation to meiotic and fertility data.     

Crossability and cytological studies in Solanum macrocarpon and Solanum linnaeanum (Solanaceae).

Summary Crossing experiments involving the various groups of Solanum macrocarpon L. complex, including wild (dasyphyllum), semi-wild and cultivars produced fully fertile F1 and F2 hybrids. This confirmed earlier findings that these groups belong to the same biological species. S. macrocarpon complex and S. linnaeanum Hepper ex Jaeger are isolated by reproductive barriers. F1 and F2 hybrids between various groups of S. macrocarpon showed heterosis. F2 superior hybrids were isolated as candidates for the future breeding programmes. Most wild-type traits like prickliness, hairiness were dominant to those possessed by cultivars. Relatively resistant hybrids to pest attack were noted. S. macrocarpon complex and S. linnaeanum are diploids (2n=24). The overall nature of the karyotype suggests that the two are related. The F1 hybrids between the cultivars of S. macrocarpon and the wild group (S. dasyphyllum) showed normal meiosis. A lack of isolating barriers confirmed that S. macrocarpon complex constitute a single biological species.     

Introduction of a self-compatible gene of Lolium temulentum L. to perennial ryegrass (Lolium perenne L.) for the purpose of the production of inbred lines of perennial ryegrass

Interspecific hybrids between self-compatible species, Lolium temulentum L. and self-incompatible species, L. perenne L. were obtained using embryo rescue. Two cycles of backcrossing of interspecific hybrids with L. perenne were carried out. A 1: 1 segregation ratio of self comatibility and incompatibility was observed in backcross generations. These segregation data confirmed that self-compatibility of L. temulentum was controlled by a single gene. It suggested that the self-compatible gene of L. temulentum could be introduced to self-incompatible plants of L. perenne through interspecific hybridization. It appears that utilization of a self-compatible gene of L. temulentum would be useful for production of inbred lines of L. perenne. The possibility of breeding procedures of perennial ryegrass using a self-compatible gene was discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of sethoxydim resistance in foxtail millet, Setaria italica (L.) Beauv.

Interspecific hybridization between foxtail millet cultivars (Setaria italica) and a green foxtail (S. viridis) resistant to the herbicide sethoxydim were undertaken to breed foxtail millet for improved herbicide resistance. Parents, reciprocal F₁ hybrids, F₂ selfed derived populations and BC₁ backcross progeny were produced and analysed for mortality and fresh weight over a range of dosages. All resistant progeny were 700 times more resistant than susceptible cultivars and was symptom free under current field dosages. Segregations of resistant and susceptible progeny (3:1 in F₂ and 1:1 in BC₁) were not dependent upon dosage. Heterozygous individuals displayed the same magnitude of resistance as homozygous plants at twelve times the recommended field dosage. Results suggested that sethoxydim resistance in foxtail millet was controlled by a single, completely dominant, nuclear gene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meiotic behavior of passion fruit somatic hybrids, Passiflora edulis f. flavicarpa Degener + P. amethystina Mikan

Somatic hybrids (SH) obtained by protoplast fusion were investigated for meiotic behavior in order to determine their possible use in breeding programs. Flower buds from four genotypes of SH between P. edulis f. flavicarpa (E) and the wild species P. amethystina (Am) denoted SH (E + Am) # 12, # 13, # 28 and # 35 were collected and the microsporocytes analysed. Meiotic phases showed the presence of abnormalities such as univalents, bivalents and quadrivalents, laggard chromosomes and anaphase bridges. At least 14 bivalents were observed in most of the cells of the hybrid plants. The percentage of cells with quadrivalents ranged from 73.3 in (E + Am) # 13 to 93.3 in (E + Am) # 28 and # 35. Analysis of pollen viability (V) indicated V = 88.23% for # 12, V = 83.86% for # 13, V = 84.20% for # 28, and V = 72.90% for # 35, and fruit development was observed. The high pollen viability, together with the multivalent pairing observed in the four somatic hybrids indicates that these materials can be used for the genetic improvement of yellow passion fruits aiming at introgressions of genes for resistance to diseases and pests. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chromosomal Stability in the Backcross Progenies of Pentaploid Hybrids between Avena sativa L. and A. maroccana Gdgr.

In a backcrossing programme to transfer desirable characters from wild Avena maroccana Gdgr. to cultivated oats, A. sativa L., meiotically stable plants in BC₁F₃ and BC₂F₂ progenies were isolated. The recovery of stable genotypes with 2n = 6×= 42 chromosomes indicated that two backcrosses are enough for such a programme. The cytological observations in various backcross generations are presented and discussed.     

Pollen germination,pollen tube growth and fertilization following self and interspecific pollination of Paspalum species

Summary Crossability between most Paspalum species is very low. This study was undertaken to identify the impediments to hybridization. Accessions of P. intermedium Munro. ex Morong, P. jurgensii Hackel and P. dilatatum Poir were self-pollinated and crossed with one anther. Paspalum intermedium is essentially self-sterile but P. jurgensii and P. dilatatum are highly self-fertile. Following pollination, pollen germination and tube growth were studied by observing the pollinated pistils with fluorescent microscopy. Examination of self-pollinated pistils revealed that the pollen germinated shortly after contacting the stigmas. Germination was over 80% for the P. intermedium and P. dilatatum accessions but only 57% for P. jurgensii. Pollen tubes grew to the micropyle within 45 minutes after pollination in P. dilatatum and 1 hour and 15 minutes in P. jurgensii. However, in the P. intermedium accessions most tubes did not grow beyond the stigma and very few penetrated the style and ovary. Apparently stylar-incompatibility is the reason for the low selfed seed set. In the cross-pollinations, pollen germinated shortly after pollination and germination ranged from 57 to 88% for the different crosses. In all crosses the pollen tubes grew to the micropyle within 30 minutes to 2 hours after pollination indicating that a cross-incompatibility system is not the cause for low crossability among these species. By examining embryo sacs from P. intermedium × P. dilatatum, its reciprocal and P. intermedium × P. urvillei crosses, it was determined that gametes failed to unite in some crosses and this is a major reason for low crossability.     

A new source of cytoplasmic male sterility in sunflower

Summary Interspecific substitutions of the nucleus of Helianthus annuus (2n=34) cv. Saturn into the cytoplasm of H. petiolaris (2n=34) by successive backcrossing, resulted in progenies with indehiscent anthers containing white, rather than normal yellow, pollen. Further backcrossing by cv. Saturn failed to restore pollen shed, suggesting that the male sterility was cytoplasmic. In vivo germination tests of pollen from 23 such plants from eight BC₅ lines, indicated complete pollen sterility for 14 plants, but normal seed set, suggesting that female fertility was not affected. Meiosis in all plants examined was normal.Crosses between seven sources of pollen-fertility restorer, one collection of wild H. annuus, and an existing source of cytoplasmic male sterility, resulted in a high frequency of plants with normal pollen shed in all F₁ progenies. However, no normal pollen shed was evident in F₁ progenies for similar crosses between BC₅ male-steriles and three of the seven restorer sources, nor for the single wild H. annuus evaluated. The foregoing suggests that the backcross substitution lines are a new source of cytoplasmic male sterility. The inheritance of restoration of pollen shed was complex and not fully elucidated. Some data suggested that two independent, complementary, dominant genes were required, but others indicated two to three independent, dominant genes.     

Sexual and somatic hybridization in the genus Lactuca

Summary Various genes for disease resistance identified in wild Lactuca are difficult, even impossible to exploit in lettuce breeding, due to sexual incompatibility between L. sativa and wild Lactuca sp. We adapted two cellular biology techniques to overcome these interspecific barriers: in vitro embryo rescue and protoplast fusion. In vitro rescue of immature embryos was used successfully for sexual hybridization between L. sativa and L. virosa. Vigorous hybrid plants were produced between L. sativa and seven accessions of L. virosa. Protoplast fusion permitted the regeneration of somatic hybrids between L. sativa and either L. tatarica or L. perennis. Hybrids between L. sativa and L. tatarica were backcrossed to L. sativa.