Problems in breeding and cytology of sugar cane

In this article a survey is given of interspecific crossing in Saccharum leading to sugar cane forms with chromosome numbers on a level of 2n=about 160.Breeding for immunity in sugar cane, in Java practised since 1893, led in the twenties to sugar cane forms, in which resistance to diseases was attended by a very high sugar yield. These canes, belonging to the so-called third and fourth Saccharum spontaneum nobilisation with chromosome numbers of 2n=about 100 to 120, formed the starting point in 1925 of a step-by-step crossing procedure leading to sugar cane clones third nobilisation new style with chromosome numbers that in comparison with 2n=80 of Saccharum officinarum could be regarded as tetraploids.In 1938, when such sugar cane forms with 2n=about 160 were created in a comparative large number of cases, the author projected a six-year plan for the years 1939–1944 with the purpose of obtaining such plants in very large numbers from numerous cross combinations. This was necessary since the best clones resulting from it had to be compared with the best clones of the third nobilisation old style, themselves being products of a large number of crossing experiments covering many years. This six-year plan was never fulfilled in consequence of the second world war.The fact remains, however, that during the years 1925–1939 in Saccharum polyploidy breeding was executed without the aid of colchicine, resulting in well developed plants with 2n=about 160 chromosomes. Several plants of the first nobilisation¹) n.s. with such chromosome numbers even could be called giants.As result of this investigation it ought to be mentioned also that plants with 180 to 220 chromosomes without exception were poor plants. In these cases the optimum chromosome number must have been surpassed.     

Genetic relationships between attributes in sugarcane clones closely related to Saccharum spontaneum

Summary Saccharum spontaneum is being used in sugarcane breeding programs in attempt to improve characteristics such as ratooning ability and stress tolerance. A population of F₁ (Saccharum officinarum or commercial variety x S. spontaneum) and F₁ x F₁ sugarcane clones was evaluated for sugar yield and a range of yield components in a plant and two ratoon crops. The aim was to determine genetic correlations between attributes in clones with a large component of S. spontaneum, that could be used to help derive appropriate selection indices in such populations.There were close associations between the same attributes measured in different crop-years and the associations between different attributes were generally similar across crop-years. Stalk number and fibre content were positively correlated, as were stalk weight and CCS. The latter two attributes (which are low in S. spontaneum but high in S. officinarum) were negatively correlated with the former two (high in S. spontaneum, low in S. officinarum). Sugar yield was more closely associated with stalk weight and CCS than with stalk number but became more closely associated with stalk number with successive ratoon crops.CCS was positively correlated (r_g=0.55) with cane yield in the plant crop but showed a small negative correlation with cane yield (r_g=–0.20) in the second ratoon crop. CCS (measured in any crop) also had a negative correlation with cane yield in the ratoon crops expressed as a percentage of plant cane yield. This suggests that CCS is negatively correlated with levels of traits contributing to ratooning ability. Intensive selection among such populations for CCS without consideration of ratooning performance may reduce the frequency of favourable specific ratooning characteristics.Abbreviations CCS commercial cane sugar     

Molecular diversity and genome structure in modern sugarcane varieties

Summary RFLP analysis was performed on 40 sugarcane cultivated varieties. Twenty-two maize low copy DNA clones located on different regions of the 10 maize chromosomes were used as probes to survey variability among the sugarcane varieties. A total of 425 fragments, 411 of which were polymorphic, were identified for 22 probe/enzyme combinations. Each variety displayed an average of 7.28 fragments per combination, revealing the complex polyploid origin of modern sugarcane varieties. The average genetic similarity between sugarcane varieties was 0.61. Although cultivated varieties appear closely related to S. officinarum clones, the genes of S. spontaneum seem to constitute the principal component of varietal diversity. A very weak global structuring among the 40 varieties is observed, in agreement with the profuse exchanges of parental materials between sugarcane breeding stations. Traces of linkage disequilibrium can be attributed to the distribution of S. spontaneum chromosomes among sugarcane varieties. The possibility of using modern varieties as a population for detecting associations between molecular markers and agronomic traits is suggested.     

Somaclonal variation in sugarcane through tissue culture and evaluation for quantitative and quality traits

Eight genotypes of Saccharum officinarum were crossed with Saccharum spontaneum and 14 genotypes of S. officinarum were crossed with Erianthus arundinaceus. A total of 39 hybrids were evolved. These 39 hybrids were raised in the field and used as donor clones for in vitro culture studies. Plantlets were regenerated from 1-month-old callus. The grown up plants were transplanted to well prepared field, to study the variations generated for the biometric as well as for biochemical characters. There were significant differences between the donor clones and their sub clones for all the character of interest. The somatic segregation was gradual and wider, showing a range of divergence from the mean towards the end of the scale. Fifty-one sub clones were selected with commercial potential which have 13% fibre, 200 cm stalk length, 10 cm internode length and pure obtainable cane sugar per cent of 10.     

Identification of molecular markers associated with sugar-related traits in a <Emphasis Type="Italic">Saccharum</Emphasis> interspecific cross

The cultivated sugarcane (Saccharum spp. hybrids, 2n = 100–130) is one crop for which interspecific hybridization involving wild germplasm has provided a major breakthrough in its improvement. Few clones were used in the initial hybridization event leading to a narrow genetic base for continued cultivar development. Molecular breeding would facilitate the identification and introgression of novel alleles/genes from the wild germplasm into cultivated sugarcane. We report the identification of molecular markers associated with sugar-related traits using an F₁ population derived from a cross between S. officinarum ‘Louisiana Striped’ × S. spontaneum ‘SES 147B’, the two major progenitor species of cultivated sugarcane. Genetic linkage maps of the S. officinarum and S. spontaneum parents were produced using the AFLP, SRAP and TRAP molecular marker techniques. The mapping population was evaluated for sugar-related traits namely, Brix (B) and pol (P) at the early (E) and late (L) plant growing season in the plant cane (04) and first ratoon (05) crops (04EB, 04LB, 04LP, 05EB and 05EP). For S. officinarum, combined across all the traits, a total of 30 putative QTLs was observed with LOD scores ranging from 2.51 to 7.48. The phenotypic variation (adj. R²) explained by all QTLs per trait ranged from 22.1% (04LP) to 48.4% (04EB). For S. spontaneum, a total of 11 putative QTLs was observed with LOD scores ranging from 2.62 to 4.70 and adj. R² ranging from 9.3% (04LP) to 43.0% (04LB). Nine digenic interactions (iQTL) were observed in S. officinarum whereas only three were observed in S. spontaneum. About half of the QTLs contributed by both progenitor species were associated with effects on the trait that was contrary to expectations based on the phenotype of the parent contributing the allele. Quantitative trait loci and their associated effects were consistent across crop-years and growing seasons with very few QTLs being unique to the early season. When the data were reanalyzed using the non-parametric discriminant analysis (DA) approach, significant marker-trait associations were detected for markers that were either identical to or in the vicinity of markers previously identified using the traditional QTL approach. Discriminant analysis also pointed to previously unidentified markers some of which remained unlinked on the map. These preliminary results suggest that DA could be used as a complementary approach to traditional QTL analysis in a crop like sugarcane for which saturated linkage maps are unavailable or difficult to obtain.     

Cytological studies of 2n male gamete formation in sugarcane, Saccharum L.

The germplasm for modern sugarcane cultivars (Saccharum spp. hybrids)has been derived principally from S. officinarum (2n = 80), and S. spontaneum (2n = 40 to 128). Diploid gamete formation has been significant in developing cultivated sugarcane, but the cytological basis for the processes involved is not clearly understood. This research investigated microsporogenesis in nine clones of Saccharum spp. Hybrids and in S. officinarum and S. spontaneum. Diploid gamete formation occurred in all 11 lines, but was least frequent in S. spontaneum and S. officinarum which produced 0.5% and 0.8%2n gametes, respectively. In the hybrid lines, 2n gametes were formed infrequencies ranging from 0.9% to 4.4%. Cytological evidence was obtained for dyad and triad formation during microsporogenesis. Detailed analysis of chromosome behaviour at meiosis indicated that 2n male gamete formation is probably attributable to the absence of cytokinesis rather than a combination of asynchrony and non-disjunction. The clones were ranked on the basis of the frequencies with which they formed 4 × 1n microspores and the data were analysed using χ ² tests for homogeneity. These established that theSaccharum spp. hybrids could be designated as either ‘high’ or ‘low’ frequency haploid gamete producers. Conversely, the latter group, which formed diploid gametes most frequently (2.2%–4.4%), can be described as high frequency diploid gamete producers. The identification of clones most frequently forming diploid gametes may facilitate the more rapid recovery of desirable sugarcane genotypes because such clones could be selected for preferential use in clonal improvement. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evidence for second division restitution as the basis for 2n + n maternal chromosome transmission in a sugarcane cross

Chromosome doubling in megaspores of sugarcane was first reported over 70 years ago and was seen primarily in interspecific crosses between Saccharum officinarum and Saccharum spontaneum. Since then, there has been robust debate on the mechanism(s) responsible for this doubling with hypotheses ranging from second division restitution (SDR), post-meiotic restitution to fusion of the two innermost cells of the megaspore tetrad. In the present study, we sought to answer this question using molecular marker data from a cross which demonstrated 2n + n chromosome transmission between an S. officinarum clone (IJ76-514; 2n transmission) and a commercial hybrid (Q165 ; n transmission). Molecular marker data from four other bi-parental crosses were used to form a “base-line” for expected segregation ratios (n + n transmission). Comparison of segregation ratios showed that 2n transmission from the S. officinarum clone could be identified by the increased segregation ratios, a characteristic of both SDR and tetrad cell fusion. Simulation was then undertaken based on current understandings of sugarcane meiosis, to determine if the observed segregation ratios matched those simulated for SDR/tetrad cell fusion. Analysis of the simulation confirmed that most and perhaps all progeny appear to have been formed from a mechanism akin to SDR/tetrad cell fusion. The genetic consequences of this are discussed.     

Pedigree,marker recruitment,and genetic diversity of modern sugarcane cultivars in China and the United States

Sugarcane (Saccharum spp) is an important crop for both sugar and biofuel production. However, the sugarcane breeding process has resulted in modern sugarcane cultivars with a narrow genetic basis. To broaden the genetic basis and promote international collaborations in sugarcane cultivar development, we documented the peidgrees of representative sugarcane culativars widely used in China and the United States of America (USA), recruited more than six thousand simple sequence repeat (SSR) markers for sugarcane, and assessed the genetic diversity and relationships beween representative sugarcane cultivars and their potential ancestry accessions. The SSR gentoyping results indicated that both the USA and Chiniese cultivars had low genetic diversity, specifically the Chinese cultivars. The USA sugarcane cultivars experienced high presure of selection for sugar content as they had the closest relationship with S. officinarum, followed by Chinese cultivars, S. robustum, and S. spontaneum. The sugarcane accessions assessed could be divided into five and four groups through cluster and principal component analysis, respectively. S. spontaneum as a potential ancestor contributing to the stress tolerance of sugarcane cultivars was grouped into distinct clusters, and S. officinarum was grouped with sugarcane cultivars in both countries. S. robustum did not seem to contribute to the sugarcane cultivar development in China, but may have contributed to the USA cultivar development. This study not only provided a collection of easy to use SSR markers, but also detailed genetic diversity and relationship among the cultivars in the two counties, which will be referable to promote international collaboration and broaden the genetic basis of sugarcane cultivars.     

Linkage mapping and genome analysis in a Saccharum interspecific cross using AFLP, SRAP and TRAP markers

Framework genetic linkage maps of two progenitor species of cultivated sugarcane, Saccharum officinarum ‘La Striped’ (2n = 80) and S. spontaneum ‘SES 147B’ (2n = 64) were constructed using amplified fragment length polymorphism (AFLP), sequence related amplified polymorphism (SRAP), and target region amplification polymorphism (TRAP) markers. The mapping population was comprised of 100 F₁ progeny derived from the interspecific cross. A total of 344 polymorphic markers were generated from the female (S. officinarum) parent, out of which 247 (72%) were single-dose (segregating in a 1:1 ratio) and 33 (9%) were double-dose (segregating in a 3.3:1 ratio) markers. Sixty-four (19%) markers deviated from Mendelian segregation ratios. In the S. spontaneum genome, out of a total of 306 markers, 221 (72%) were single-dose, 43 (14%) were double-dose, and 42 markers (14%) deviated from Mendelian segregation ratios. Linkage maps with Kosambi map distances were constructed using a LOD score ≥5.0 and a recombination threshold of 0.45. In Saccharum officinarum, 146 markers were linked to form 49 linkage groups (LG) spanning 1732 cM whereas, in S. spontaneum, 121 markers were linked to form 45 LG spanning 1491 cM. The estimated genome size of S. officinarum ‘La Striped’ was 2448 cM whereas that of S. spontaneum ‘SES 147B’ was 3232 cM. Based on the two maps, genome coverage was 69% in S. officinarum and 46% in S. spontaneum. The S. officinarum parent ‘La Striped’ behaved like an auto-allopolyploid whereas S. spontaneum ‘SES 147B’ behaved like a true autopolyploid. Although a large disparity exists between the two genomes, the existence of simple duplex markers, which are heterozygous in both parents and segregate 3:1 in the progeny, indicates that pairing and recombination can occur between the two genomes. The study also revealed that, compared with AFLP, the SRAP and TRAP markers appear less effective at generating a large number of genome-wide markers for linkage mapping in sugarcane. However, SRAP and TRAP markers can be useful for QTL mapping because of their ability to target gene-rich regions of the genome, which is a focus of our future research.     

Pedigree comparison highlights genetic similarities and potential industrial values of sugarcane cultivars

Cultivar pedigrees from two sugarcane origins, 9 Argentine (AR) and 7 American (AM) have been reconstructed, and their genetic similarities (based on coefficient of parentage, COP, estimates) show an average of 0.206 ± 0.054. CP clones that enter the pedigrees of AM cultivars are parents or grandparents of AR cultivars, demonstrating that these genotypes have a strong genetic lineage in common. On average, AR pedigrees are smaller and contain less number of founding species than AM pedigrees. However, the lower height of the former is not explained by the different participation of founding species in the pedigrees. The presence of founding species in AR and AM pedigrees increases with the year of cultivar selection, indicating that more founding species entered the pedigree in recently selected cultivars than in older ones. The ancestry of the 16 cultivars trace back to 3 founding species: Saccharum officinarum, S. barberi, and S. spontaneum, with S. officinarum showing the greatest percentage of participation. As S. sinensis participates in 13 pedigrees and S. robustum in 3, the 5 founding species are present in 2 out of 16 pedigrees. Interestingly, the genus Sorghum enters the pedigree of LCP 85-384 pedigree. Industrial parameters assayed indicate that AR and AM cultivars were preferentially selected for their relatively high sucrose content and sugar yield in accordance with two facts: (i) the genetic background and the sucrose genes of sugarcane hybrids provided by the 68% of participation S. officinarum clones, and (ii) no clones of high fiber S. spontaneum have entered the pedigrees in earlier generations.     

Molecular insights into brown rust resistance and potential epidemic based on the <Emphasis Type="Italic">Bru</Emphasis>1 gene in sugarcane varieties and new elite clones

In most sugarcane cultivation areas, sugarcane brown rust (SBR), caused by Puccinia melanocephala, is an economically important fungal disease that leads to severe yield loss in susceptible cultivars. Bru1, which is the major dominant SBR resistance gene, has been widely used in the prediction of brown rust resistance in sugarcane. In this study, three panels of sugarcane germplasms, the major varieties approved over the past 10 years and new elite clones in the current national regional trial, together with one panel of Saccharum spontaneum, were employed in estimating the possibility of SBR epidemic and to assess the efficiency of 9O20-F4-HaeIII in eliminating false positives. Among the current top five varieties used as sucrose feedstock, accounting for more than 68.9% of the total cultivated area, all were highly resistant to SBR, although only three harboring Bru1. Two major varieties Yuetang60 and Guitang46 without harboring Bru1 were highly susceptible to SBR, together with highly susceptible Funong41, which need prudent promotion. Approximately 60.5% of the 38 new elite clones were Bru1 positive. Considering the susceptibility of Liucheng03-1137, which exhibits a strong promotion momentum, together with Funong41, Guitang46, Yuetang60, and Yunzhe06-47, four were favored by the enterprise due to their superior sucrose content and good stalk yield, despite their high susceptibility to SBR, and additional Yuetang93-159, one current top five varieties with declining resistance, which results in a potential risk for brown rust epidemic. Furthermore, low frequency of the wild germplasm of S. spontaneum from five different countries was Bru1 positive. In addition, a perfect molecular diagnostic result was observed in all modern sugarcane clones using two dominant markers, and HaeIII can prevent the occurrence of false positive results when the 9O20-F4 PCR products of S. spontaneum are digested by RsaI. The prevalent chewing cane Badila without Bru1 is highly resistant to SBR. Our results provide valuable information for the extension of sugarcane varieties and a batch of novel SBR resistance sources with superior comprehensive characters for crossbreeding, and for SBR-resistant gene pyramiding by crossing or through mining and using of new SBR-resistant genes.     

Stability and potential use of RAPD markers in a sugarcane genealogy

Summary A complete ancestral history of the recently developed and closely related South African commercial sugarcane varieties N11 and NCo376, which differ markedly in their response to sugarcane mosaic virus (SCMV), was elucidated from archival records. The genealogy spans seven generations, starting with early intraspecific crosses between varieties of Saccharum officinarum and interspecific crosses between S. officinarum and either S. spontaneum or S. barberi. In total, the genealogy comprises 38 different varieties. Genomic DNA samples from N11 and NCo376 respectively were screened for polymorphisms using the PCR-RAPD technique. Ten polymorphic fragments ranging in molecular size from 317 to 1263bp were identified from a total of 1159 loci amplified with 100 random decamer primers. Two of the 10 polymorphic fragments were shown to be consistently present in N11 (resistant) and absent in NCo376 (susceptible), while 8 showed the reverse occurrence. The primers producing the polymorphisms were used to screen genomic DNA samples from all 19 varieties representing the genealogy. Results have indicated that (1) specific PCR-RAPD generated polymorphic fragments can indeed be identified across the seven generations; (2) certain fragments are sufficiently definitive to be used as markers to trace parentage; (3) the validity of documented crosses and/or the authenticity of germplasm material may be questioned using this technique, and (4) there is the potential to subject the markers to linkage analysis once a full and accurate assessment of the SCMV resistance phenotype is obtained.     

Variation in vegetative parameters related to the nitrogen economy of wild barley,Hordeum spontaneum,in Israel

Summary Hordeum spontaneum, the wild progenitor of cultivated barley, has previously been examined in various studies as a germplasm resource in breeding for grain protein content and related nutritional traits.The nitrogen content and dry weight of leaf and stem (stem plus sheath) at anthesis, and the final grain size and grain protein content were measured in 33 H. spontaneum and two H. vulgare genotypes. H. spontaneum was generally higher in nitrogen content of leaves and stems, but lower in dry weight at anthesis. Consistent with previous reports, the H. spontaneum genotypes were considerably higher in grain protein than the cultivars. There was wide variation between and within populations of H. spontaneum suggesting that for breeding purposes lines combining high vegetative nitrogen content, dry weight and grain protein content can be selected.Incumbent of the Seagram Chair for Plant Science     

Genome size variation in three Saccharum species

Saccharum species are autopolyploids with ploidy level ranging from 5× to 16x, and are considered the most complex genomes among crop plants. In present study, the genome sizes of 28 Saccharum spontaneum accessions, 15 Saccharum officinarum accessions, 28 Saccharum robustum accessions, and 12 Saccharum hybrids spp. were analyzed using flow cytometry. The estimated genome sizes of S. officinarum accessions ranged from 7.50 to 8.55?Gb with an average size of 7.88?Gb. In S. robustum, the estimated genome sizes ranged from 7.65 to 11.78, reflecting the variation of ploidy level. In S. spontaneum, the estimated genome sizes varied widely, with a range from 3.36 to 12.64?Gb, also due to variation of ploidy level. The average monoploid genome size of S. officinarum was 985?Mb, and that of S. spontaneum was 843?Mb. The results also showed that genome sizes were correlated with chromosome numbers, and based which, that the unknown chromosome numbers of some accessions could be predicted. The estimated genome sizes of Saccharum germplasm also helped identify some mislabeled accessions and yielded information critical for sugarcane breeding and genome sequencing programs.     

Resistance to Rhopalosiphum padi (Homoptera: Aphididae) in Hordeum vulgare subsp. spontaneum and in hybrids with H. vulgare subsp. vulgare

Summary Accessions of Hordeum vulgare subsp. spontaneum, the progenitor of cultivated barley, were screened in field and glasshouse trials for resistance to the aphid Rhopalosiphum padi. A few selected lines were furthermore hybridized with a modern barley variety and the resulting populations evaluated. High levels of resistance were found among some of the spontaneum accessions resulting in lower aphid growth rates (maximum reduction 57%). Segregation patterns among siblings in F2 populations were continuous, indicating the presence of several genes with possibly additive effects. The usefulness of H. vulgare subsp. spontaneum for breeding aphid resistant barley is discussed.     

The genomes of Arachis hypogaea. 1. Cytogenetic studies of putative genome donors

Summary Cytological studies of wild diploid Arachis species in the same section of the genus (sect. Arachis) as the cultivated peanut A. hypogaea L. show, with one exception, a karyotype characterized by the presence of 9 pairs of larger chromosomes and one pair of small (A) chromosomes. The exceptional species A. batozocoi Krap. et Greg. has a more uniform karyotype. Interspecific hybrids between diploid species of similar karyotype have moderate to high pollen stainability, those involving A. batizocoi have zero pollen stainability and a very irregular PMC meiosis. Such infertile hybrids are the most likely to produce fertile, stable amphidiploids on doubling the chromosome complement. It is suggested that the cultivated peanut could have originated from such a sterile interspecific hybrid and on morphological and phytogeographic grounds the most likely genome donors are A. cardenasii (nomen nudum) and A. batizocoi of the species within section Arachis, which have been collected up to the present time.Paper number 5560 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, NC 27650     

Karyotypes and chromosome association in aneuploid varieties of Japanese garden iris,Iris ensata Thunb

Summary Karyotypes, chromosome association and pollen fertility of aneuploid varieties (2n=25), Ochibagoromo, Matsusakatsukasa and Isehomare in Iris ensata were analysed and compared with those of eu-diploid varieties (2n=24), Shishinden, Kachô and Asahimaru. The somatic chromosome complement of the aneuploid varieties consisted of 11 pairs and 3 singles of chromosomes and that of the eu-diploid varieties 12 pairs of chromosomes. The singles of chromosomes in the aneuploid varieties had similarity with one another and with a pair of chromosomes in the eu-diploid varieties. The high frequency of normal association was present in the eu-diploid varieties, and this indicated that 12 pairs of chromosomes had full homology between each other. In contrast, the mean chromosome association per cell in an aneuploid variety Ochibagoromo was 4.615_I+10.067_II+0.077_III+0.005_IV, indicating that the chromosome complement of this variety consisted of 11 pairs and 3 singles and that these singles had partial homology among them. The eu-diploid varieties exhibited high pollen fertility due to their regularity of chromosome association; the aneuploid varieties considerably lower fertility, i.e. 28.2% for Ochibagoromo, 31.8% for Isehomare and 43.8% for Matsusakatsukasa. The primary cause for the low fertility of these varieties seemes to be the partial homology among 3 single chromosomes. Finally, the origin and the development of the aneuploid varieties were discussed.     

Relationships among ancestral species of sugarcane revealed with RFLP using single copy maize nuclear probes

Summary DNA restriction fragment length polymorphism (RFLP) analysis was performed on 50 wild and old cultivated sugarcane accessions. Ninety-four maize low copy nuclear DNA sequences of known chromosomal position were screened for hybridization to digested sugarcane genomic DNA blots. Seventy-five (80%) gave very strong hybridization signals and usually yielded many bands and detected profuse polymorphism. Twenty-nine probes and 36 probe/enzyme combinations were selected on the basis of the scorability of the banding profiles. A total of 1110 fragments were separately identified among the 50 genotypes. Multivariate analyses of the data allowed the separation of the three basic species, Saccharum spontaneum, S. robustum and S. officinarum, showed that S. spontaneum had structure which could be related to the geographic origin of the clones and supported current hypotheses on the origin of secondary species S. barberi and S. sinense. The use of more probes did not improve the resolution between the various species examined but identified a few key polymorphisms which were not accounted for by current phylogenetic hypotheses and can guide future analyses. RFLPs in sugarcane will be useful essentially for depicting the genomic constitution of modern varieties of interspecific origin.     

Cytological and geographical characterizations of Hemarthria

Summary Chromosme numbers were determined for 56 Hemarthria altissima (Poir.) Stapf et C. E. Hubb. introduction, forty of these were diploid (2n=18) and 16 were tetraploid (2n=36). These results, combined with previous findings, show that all but three of the tetraploids in the USDA Hemarthria altissima collection originated north of 20 S latitude. All other introductions collected throughout southern Africa were diploid. Meiosis was regular in a hexaploid, 6 tetraploid and 32 diploid introductions but a low frequency of quadrivalents was found in the teraploids. H. uncinata R. Br. and H. uncinata var. spathacea (Domin) Vickery were found to have 2n=36 chromosomes H. compressa (L.f.) R. Br., introduced from Japan, had 2n=54 chromosomes. No major cytological abnormalities exist in these Hemarthria species. Significant variation exists among the ploidy levels for cold tolerance and in vitro organic matter digestion to provide the basis for a forage improvement program.Joint contribution from the Department of agronomy. Institute of food and Agric. Sciences (IFAS), Univ. of Florida. Gainesville, Fl and, Plant Genetics and Germplasm Institute, USDA, Beltsville, MD Published as IFAS Journal Series No. 2849.     

Double-bridge hybrids of Solanum bulbocastanum and cultivars of Solanum tuberosum

Summary Solanum bulbocastanum (2n=2x=24) has valuable characters for potato breeding, but cannot be hybridized directly with S. tuberosum cultivars. Both S. acaule (2n=4x) and S. phureja (2n=2x) were used as bridging species. Triploid S. acaule × S. bulbocastanum were doubled with colchicine and the resulting fertile hexaploid F₁'s crossed with S. phureja. The triple hybrids obtained were tetraploid or nearly so. The two genomes of S. acaule in these triple hybrids probably pair preferentially, which may provoke pairing and possibly crossing over between the chromosomes of S. bulbocastanum and S. phureja.More than 20000 pollinations of the triple hybrids with four potato cultivars had to be made to produce 40 quadruple hybrids. These highly vigorous hybrids varied greatly in many morphological characters, resistance to Phytophthora infestans, fertility and crossability. The chromosome numbers are 48 (24 hybrids), 49 and 46, but some higher ploidy levels (65, 66, 72 chromosomes) were found as well. Their origin is to be sought in the fusion of an unreduced egg cell from triple hybrids (either euploid or hypoploid) and a reduced male gamete from the cultivars. This view is corroborated by their extreme resistance to Phytophthora. Also some 48-chromosome hybrids are highly resistant, which may indicate introgression from S. bulbocastanum.Most quadruple hybrids are readily inter-crossable and crossable as females with cultivars; several also as males. Two could be hybridized with S. bulbocastanum, but the few seeds dit not germinate.Studies of pachytene stage of meiosis revealed the presence of a S. bulbocastanum chromosome in at least one tetraploid hybrid, which is highly resistant to Phytophthora. At metaphase I of meiosis chromosome associations higher than quadrivalents were not found. Except in one hybrid, the frequency of quadrivalents did not exceed one per cell and the average proportion of chromosomes associated as bivalents amounted to 90%.The quadruple hybrids (double-bridge hybrids) appear good starting material for breeding programmes aimed at introducing genes from S. bulbocastanum into S. tuberosum cultivars.