Little is known about the mechanisms controlling interspecific barriers, unlike the well studied intraspecific barrier, self incompatibility (SI),. A unilateral crossing barrier (unilateral incongruity - UI) exists among the Lycopersicon species, in which crossing is impeded or prevented in one direction. Since both UI and SI can give unilateral differences in seed set, suggestions have been made that UI and SI are functionally related. L. pennellii LA716 is self-compatible, unlike the other accessions which are SI, but LA716 still exhibits UI with L. esculentum (esc). We observed the development of pollen tubes in self and cross pollinations of LA716, esc and SI accessions of L. pennellii (pen). Selfed pollen tubes in esc were at the ovary in 24 hours, while pen were 1/2 way down the style and in LA716 the pollen had not germinated. By 48 hours, the pollen tubes in LA716 were in the ovary and growth had halted in pen styles. Crosses with LA716 pollen on esc and pen resulted in pollen tube growth starting within 24 hours continuing to the ovary. Thus, UI is not a SI response and LA716 shows a delayed pollen germination and growth unlike the other Lycopersicon species examined.
Barbara E. Liedl and Martha A. Mutschler
Sierd Zijlstra, Coen Purimahua and Pim Lindhout
Crossing barriers between white- and purple-flowered species were examined. Four accessions of Capsicum annuum and three of C. pubescens were reciprocally crossed with one to four accessions of C. baccatum, C. cardenasii, C. chacoense, C. chinense, C. eximium, C. frutescens, C. galapagoense, and C. praetermissum. Capsicum chacoense is the only white-flowered species that inhibits C. annuum pollen tube growth but allows C. pubescens pollen tube penetration into the egg cell. Capsicum cardenasii and C. eximium exhibit similar crossabilities with C. annuum and C. pubescens: pollen tubes of C. cardenasii and of C. eximium can penetrate the egg cells of C. annuum but not vice versa, and pollen tubes of C. pubescens can penetrate the egg cells of C. cardenasii and of C. eximium but not vice versa.
Qi Zhang, Enda Yu and Andy Medina
attempted to use variability in the genus for crop improvement through interspecific breeding yet overcoming crossing barriers, the male sterility, and incompatibility of the interspecific F 1 and early succeeding generations of distant crosses has been a
Anson E. Thompson, David A. Dierig and Jon P. Rebman
Vernonia galamensis (Cass.) Less. (Asteraceae) constitutes a species complex of six subspecies, one of which contains four varieties. Crossing barriers between the subspecies and varieties are being examined. In the analysis of microspore mother cells, no differences in chromosome number (n=9) were found, and meiosis appeared to be normal within and between subspecies. However, an extended delay in time was observed in all subspecies in which chromosomes remained condensed during the post-meiotic tetrad stage. No apparent effect on pollen formation or pollen tube growth was observed from this unusual phenomenon. Self and reciprocal intraspecific crosses are being made, and pollen tube growth into the ovules assessed by fluorescent microscopy. These techniques are being used to characterize self-incompatibility within subspecies and varieties, and to determine the possible barriers to pollen tube growth and autofertility.
Robert Bernatzky, Richard H. Glaven, Grant R. Hackett and Bruce A. Rivers
Self-incompatibility (SI) is the inability of otherwise fertile gametes to produce viable zygotes upon self-fertilization. The S locus of chromosome 1 in Lycopersicon is thought to be the main controlling factor in SI. However, the significance of other chromosome segments in the control of SI or the effect of a foreign genetic background on the S locus has not been thoroughly explored. In addition, the relationship between SI and wider interspecific crossing barriers remains unclear. Using DNA and protein markers for chromosome 1, we have created a series of backcross lines that contain either 1) the SI locus and flanking chromosome region from a SI species in a SC species background or 2) the same chromosome region from a SC species in a SI species background. The reproductive behavior of these plants will be discussed.
Marjo J. De Jeu, Silvan Kamstra, Anja Kuipers and Evert Jacobsen
The genus Alstroemeria L. is endemic in South America, mainly in Chile and Brazil. Crossing barriers of mainly postfertilization origin hampered widely inter-specific hybridization. Culturing the ovules 2 days after pollination in an hormone-free MS medium with 9% saccharose for 6 weeks and hereafter transfer to a MS medium with 3% saccharose gives germination of the fertilized ovules. In a diallel cross with 5 Chilean and 2 Brazilian species 39 combinations failed, whereas after early ovule culture hybrid plants were obtained in 27 of the incongruous combinations. The rate of success varied between 0.4%–22.5% depending on the species combination. The hybrids were tested in in vitro stage for their true hybridity using isozyme analysis and/or genomic in situ hybridization of chromosomes (GISH). This method can easily be applied in hybrids between Chilean and Brazilian species. Backcrosses were made using the ovule culture again and in the combination (A. aurea × A. inodora) × A. inodora plants were obtained although the pollen fertility was very low (1%–5%). By using species-specific repetitive probes in in situ hybridization (FISH) chromosome specific patterns were obtained enabling us characterizing the backcross hybrids for their chromosome constitution. By this method we can identify our breeding material for special traits linked with identified chromosomes.
Monica J. Norby and Michael J. Havey
Phytophthorainfestans is the casual agent of late blight and is a major threat to potato production worldwide. There are no curative control agents available and resistance genes offer promise in controlling late blight. To date, the primary source of late-blight resistance has been from hexaploid (6x) [4 Endosperm Balance Number (EBN)] Solanum demissum. Mexican diploid (2x) (1EBN) Solanum species possess a wealth of late-blight resistances, but have been neglected due to crossing barriers. Manipulation of EBN and ploidies should allow integration of 2x (1EBN) germplasm into cultivated potato. Synteny between late-blight resistance loci from Solanum species of disparate ploidies and EBNs may facilitate the identification of unique resistance alleles and loci. Isolate MSU96 (US8/A2) of P. infestans revealed a late-blight resistance locus (Rpi1) from 2x(1EBN) S. pinnatisectum (PI 253214) that mapped to chromosome seven (MGG 265:977-985). MSU96 was also avirulent on the late-blight differential R9-Hodgson 2573 (LB3), revealing the presence of the avirulence gene for R9 originating from S. demissum. To test the relationship between Rpi1 and R9, we evaluated a family segregating for R9 and revealed that it does not map to chromosome seven. The independent inheritance of R9 and Rpi1 indicates that Rpi1 is a unique resistance locus. We are conducting a variety of crossing schemes to introgress Rpi1 into cultivated potato.
Martha A. Mutschler, Edward D. Cobb, Barbara E. Liedl and Joseph A. Shapiro
Acylsugar mediates the resistance of Lycopersicon pennellii LA716 to several important insect pests of cultivated tomato, including potato aphid, green peach aphid, leaf miner, fruitworm, armyworm, and silverleaf whitefly. Incorporation of acylsugar-mediated multiple pest resistance could result in a significant reduction in the use of pesticidal sprays in cultivated tomato. Development of a reliable assay for acylsugar production and confirmation of the association between the resistance and acylsugars allowed us to try to breed for the trait by selecting for acylsugar-producing plants. The breeding cycle allows us to progress by one backcross generation per year. The breeding program was faced by several challenges, including interference in gene transfer by interspecific crossing barriers, and the oligogenic nature of the acylsugar-mediated resistance trait. Despite these challenges, the breeding program has produced BC3F2 plants that produce effective levels of acylsugars, are tomato-like in vine appearance, and produce seed-bearing fruit in the field without manual pollination. The current status of the program and future plans will be discussed.
John R. Stommel
Solanum ochranthum Dunal is a nontuber bearing wild relative of the cultivated tomato (Lycopersicon esculentum Mill.), and a potential source of new genes for disease and pest resistance. Because S. ochranthum is sexually isolated from tomato, somatic hybrids between tomato (PI 367942; L. esculentum Mill. var. cerasiforme (Dunal) A. Gray VFNT cherry × L. peruvianum (L.) Mill. backcrossed to VFNT cherry) and S. ochranthum (LA2117) were developed previously to overcome these crossing barriers. Attempts to backcross these hybrids to tomato have been unsuccessful. Pollen fertility and mitotic and meiotic studies in tomato + S. ochranthum somatic hybrids determined the cause of the sterility of the somatic hybrids and identified hybrids with moderate fertility. Chromosome counts of dividing root tip cells delineated tetraploid (2n = 4x = 48) and hexaploid (2n = 6x = 72) genotypes and aneuploidy in these hybrids. Meiotic analysis of developing microspores confirmed the presence of precocious division and laggard chromosomes at anaphase in both hexaploid and tetraploid hybrids. Bridges were observed in hexaploids at anaphase I and II and multivalent configurations were observed at diakinesis. Multivalents and univalents were evident in nearly all cells examined, proving that the two genomes are homoeologous. Aberrant microsporocytes with five to six developing microspores were noted in hexaploid hybrids. The occurrence of homoeologous pairing between chromosomes of both fusion parents is advantageous to effect recombination between these isolated species. However, the negative effects of multivalent formation and univalents likely contributed to observed sterility in these first generation fusion hybrids. Low to moderate levels of pollen fertility (0% to 52%) were found in tetraploid hybrids, while little or no viable pollen (0% to 4%) was observed in hexaploid somatic hybrids.
Paul M. Lyrene
probably attributable in part to the triploid block, which is strong in Vaccinium ( Darrow et al., 1944 ), along with other genetic crossing barriers between sections Batodendron and Cyanococcus . In 1981, we crossed V. arboreum as a pollen parent