Inheritance of tree and fruit characters in progenies from crosses of sweet cherry (Prunus avium L.) cultivars

Summary Approximately 1000 seedlings from 20 combinations crossed in 1979, 1980 and 1981 (Theiler-Hedtrich 1985a) were tested for several characters: fruit set (yield), fruit size, fruit colour, formation of abscission layer and bleeding after fruit removal from fruit stalks, bacterial canker resistance, flowering and harvesting time. From progeny of crosses with Stella as pollinator, 56% (Vittoria × Stella) and 46% (Schüttler × Stella) of the seedlings were self-compatible, of which 14 were high yielding with good fruit size and quality. From the data recorded it can be concluded:fruit set is a recessive character; only 5 to 20% of very good yielding seedlings were obtained in different progeny, even if the parental plants were both very good croppers.Fruit juice and skin colour was in most progenies ‘black’ even if they were from combinations with ‘white’ varieties, e.g., Merton Glory or Schüttler. Only from the combination Schiittler (‘white’) × Stella (‘black’), 50% of the seedlings were ‘white’; Stella therefore is heterozygous for the character of fruit juice and skin colour.Fruit size is evenly distributed in progeny with respect to the fruit size of their parent plants.Abscission layer formation and non-bleeding is a genetically complex character. In combinations where both parent plants formed fruits with complete abscission layers and which were not bleeding after fruit removal from the stalk, this character was inherited only to 50% (Vittoria × Schüttler) or 85% (Vittoria × Frühe von der Weid) in the progeny. For the genetical control of this character further studies are necessary.Bacterial canker susceptibility was evenly distributed in seedlings from all combinations even if the highly resistant cv. Vittoria was used as one parent plant, thereby not confirming the expected results of a higher proportion of resistant seedlings from combinations with Vittoria.Flowering and harvest time of the seedlings from different combinations was within the range of the parent plants. Only in the combination of Vittoria × Stella (mid-to late-ripening season) one seedling out of 99 was found to form ripe fruits two weeks earlier than the parental plants. From the seedlings tested 40 have been chosen for further evaluation or genetical studies.     

Inheritance of pollen color in hazelnut

In contrast to the yellow color of pollen of European hazelnut (Corylus avellana L.) cultivars, an unusual cream color was observed in some seedlings from self-pollination of the Sicilian cultivar ‘Montebello.’ After elimination of accidental outcrosses, the segregation fit a 3 yellow: 1 cream ratio, indicating the possibility of simple genetic control. Two seedlings with cream-colored pollen were backcrossed to their parent ‘Montebello,’ and the progeny segregated 1 yellow: 1 cream for pollen color. When two seedlings with cream-colored pollen were crossed with each other, all of the resulting seedlings had cream-colored pollen. These segregation ratios indicate that pollen color in hazelnut is controlled by a single locus with yellow dominant to cream color. The symbol pc is proposed for this locus. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pear mutagenesis: In vitro treatment with gamma-rays and field selection for productivity and fruit traits

In vitro shoots of six pear (Pyrus communis L.) cultivars, ‘Conference’, ‘Doyennéd'Hiver’, ‘Passe Crassane’, ‘Bartlett’, ‘AbbéFetel’ and ‘Butirra Precoce Morettini’ were irradiated with gamma rays (3.5 Gy). After three subcultures, microcuttings from the irradiated shoots and from additional non-irradiated microcuttings were rooted to establish plants for survey orchards. All trees were individually observed for variation in fruit traits and for productivity. Trees were selected for improved characters related to production such as early bearing and consistent productivity. Variations observed in fruit appearance concerned degree of russeting, fruit shape and size. The frequencies of the observed variations in fruit traits depended on the cultivar, ranging from 0.81% in ‘Doyennéd'Hiver’ to 3.64% in ‘Passe Crassane’. Of the 97 variants selected, only two showed chimeral behavior. This revised version was published online in July 2006 with corrections to the Cover Date.     

Linked dominant alleles or inter-locus interaction results in a major shift in pomegranate fruit acidity of ‘Ganesh’ × ‘Kabul Yellow’

Summary Inheritance of fruit acidity in pomegranate (Punica granatum L.) was studied in 3 sweet or low acid (‘Ganesh’, ‘Ruby’ and ‘Kabul Yellow’) and 3 sour or high acid (‘Nana’, ‘Daru’ and ‘Double Flower’) varieties and their progenies. The F₁ and F₂ data of ‘Ganesh’ × ‘Nana’ showed that fruit acidity is monogenically controlled and the sour nature is dominant over sweet. Further, whether a genotype produces sweet or sour fruit is determined by a major gene (SS) while a few modifiers with small effects cause fluctuations in the acidity levels within sour and sweet types. All the trees of 3 crosses involving ‘Daru’ produced acidic fruits but those of (‘Ganesh’ × ‘Nana’) × ‘Daru’ reached acidity as high as 71.2 g/l which could be because of cumulative influence of modifying genes derived from the two acidic varieties ‘Nana’ and ‘Daru’. Pollination of functionally sterile ‘Double Flower’ variety with single (normal) flower types revealed that ‘Double Flower’ is a dominant mutant from an acidic fruited genotype (Ss). The segregation pattern in F₁ indicated the possible linkage between genes governing total acidity and flower type. All the F₁ hybrids between ‘Kabul Yellow’ and ‘Ganesh’ (sweet × sweet) were sour fruited with almost 8-fold jump in fruit acidity over the mid-parental value. The steep increase in acidity cannot be convincingly attributed to overdominance which is certainly rare at major gene level. Alternatively, linked dominant alleles or epistatic effect of neighboring loci which readily simulate overdominance (pseudo-overdominance) could have caused a major shift in F₁ fruit acidity.     

Characteristics and inheritance of natural facultative-parthenocarpic fruit-set in ‘Nadja’ tomato under low temperature conditions

‘Nadja’ tomato (Lycopersicon esculentum Mill.) set both seedless and seeded fruits under the fluctuating temperatures prevailing during late-March and early-April in Assiut district, Egypt. Entirely seedless fruits developed under the low temperatures prevailing during Dec. Seedless fruits seemed to develop vegetatively , i.e. with no need for stimulus pollination. All seedless fruits were normally jelled and appeared similar in shape and size to the seeded fruits. Study of F₁, F₂, and backcross generations for the crosses ‘Cal Ace’ × ‘Nadja’ and ‘Peto-86’ × ‘Nadja’ suggested that the expression of the trait was due to single homozygous recessive gene. Yield produced by ‘Nadja’ was higher than the F₁, F₂, and backcross generations in both the early- summer and the late-winter plantings. Potentially useful parthenocarpic segregates were found in F₂ of the two crosses for increased fruit weight of normally shaped and jelled fruits. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of white flower colour and congested growth habit in certain Buddleia progenies

Summary Buddleia fallowiana var. alba (FA) was crossed with B. davidii Nanhoensis Alba (NA) and both were crossed with the B. davidii cultivars Pink Delight (PD) and Royal Red; seedlings from the FA x NA progeny were backcrossed to the two parents. The pattern of inheritance of white or coloured flowers indicates two loci controlling white flower colour. FA is homozygous for a recessive gene, labelled alb-1, and NA is heterozygous for a dominant gene, Alb-2.In most segregating progenies some of the white flowered seedlings had a congested habit of growth and this is tentatively ascribed to two dominant complementary genes. NA is heterozygous for Con-1, which is tightly linked with Alb-2. FA and PD are heterozygous for Con-2.     

Inheritance and Correlation of Self-Compatibility and other Yield Components in the Apricot Hybrid F1 Populations

Summary To provide genetic basis for apricot (Prunus armeniaca Lam.) breeding, inheritance and correlation of yield components including self-compatibility, self-pollinated fruiting rate, fertile flower rate, average fruit weight and fruit number per plant were studied with 5-year-old seedlings of apricot F1 hybrids from ‘Katy’ × (‘Xinshiji’, ‘Katy’ × (‘Hongfeng’ and ‘Katy’ × (‘Taianshuixing’, respectively. Using the criteria that cultivars with self-pollinated fruiting rate ≥6% were self-compatible(SC), we found that the ratios of self-compatible (SC) to self-incompatible (SI) individuals were 27:25, 9:12 and 15:19 in the above three families, respectively, and conformed to the ratio of 1:1 segregation by χ² test, indicating that the S-locus of ‘Katy’ was heterozygous and self-compatibility was dominant to self-incompatibility. Twenty-seven seedlings from the F1 population of ‘Katy’ × ‘Xinshiji’ were chosen for S-allele-specific PCR. As a result, four S-genotypes with the ratio of 10:6:4:7 were obtained, which was linear to the 1:1:1:1 ratio by χ² test. Great differences in self-compatibility degree were observed among seedlings even with the same S-genotype. In the F1 populations, a very extensive segregation in self-pollinated fruiting rate, fertile flower rate and average fruit weight was observed, and average values of these traits were lower than that of mid-parent. Therefore, these traits were confirmed to be quantitative. However, significant differences were found in broad heritability (H _b ²) of following three characters: the H _b ² of self-pollinated fruiting rate (87.1% – 91.4%) was the greatest, with the variation mainly resulted from inheritance; fertile flower rate (36.8% – 49.1%) was the least and its variation was mainly caused by environmental factors. In addition, self-pollinated fruiting rate and fertile flower rate had very significantly positive correlations with single plant yield, so both may play important roles in the determination of single plant yield. In contrast, correlation between yield and average fruit weight was not significant.     

A quantitative trait locus influencing tolerance to Phytophthora root rot in the soybean cultivar ‘Conrad’

‘Conrad’, a soybean cultivar tolerant to Phytophthora root rot (PRR), and ‘OX760-6-1’, a breeding line with low tolerance to PRR, were crossed. F₂ derived recombinant inbred lines were advanced to F₆ to generate a population through single-seed descent. This population was used to identify quantitative trait loci (QTLs) influencing PRR tolerance in ‘Conrad’. A total of 99 simple sequence repeat (SSR), or microsatellite, markers that were polymorphic and clearly segregated in the F₆ mapping population were used for QTL detection. Based on the data of PRR in the field at two planting locations, Woodslee and Weaver, for the years 2000 and 2001, one putative QTL, designated as Qsatt414-596, was detected using MapMaker/QTL. Qsatt414-596 was flanked by two SSR markers from the linkage group MLG J, Satt414 and Satt596. Satt414 and Satt596 were also detected to be significantly (P < 0.005) associated with PRR using the SAS GLM procedure and were estimated to explain 13.7% and 21.5% of the total phenotypic variance, respectively.     

Inheritance of contorted growth in hazelnut

Summary The contorted hazelnut, Corylus avellana L. f. contorta (Bean) Rehder, is an ornamental tree prized for its twisted trunk and branches. Crosses of normal growth habit cultivars and Contorta, as it is commonly called, produced all normal growth habit seedlings. Sib matings of these normal seedlings of Contorta produced offspring in the ratio of 3 normal: 1 contorted, while backcrosses to Contorta segregated 1 normal: 1 contorted. These segregation ratios indicate control of contorted growth by a single recessive gene for which we propose the name twisted and the symbol tw. Progenies segregating simultaneously for growth habit and leaf anthocyanin indicated that the two loci are independent.     

Little heterosis for yield and yield components in hybrids of six cucumber inbreds

Heterosis and inbreeding depression for fruit yield has been reported for pickling cucumber (Cucumis sativus L.). However, cucumber inbreds often perform as well as hybrids, and there is little inbreeding depression. The objectives of this study were to reexamine the amount of heterosis and inbreeding depression for fruit yield and yield components in pickling cucumber, and to determine the relationship between yield components and yield for heterosis. Two pickling cucumber inbreds (M 12, M 20) and inbreds from four open-pollinated monoecious cultivars (‘Addis’, ‘Clinton’, ‘Wisconsin SMR 18’, ‘Tiny Dill’) were hybridized to form four F₁ hybrids (‘Addis’ × M 20, ‘Addis’ × ‘Wis. SMR 18’, ‘Clinton’ × M 12, M 20 × ‘Tiny Dill’). F₁ hybrids were then self-pollinated or backcrossed to generate F₂, BC_1A, and BC_1B progeny. Thirty plants of each generation within each hybrid family were grown in plots 3.1 m long with four replications in each of two seasons. Data were collected from once-over harvest for vegetative, reproductive, yield, and fruit quality traits. Heterosis and inbreeding depression for fruit yield and yield components were not observed in three of the hybrids. Only ‘Addis’ × ‘Wis. SMR 18’ exhibited high-parent heterosis and inbreeding depression for total, marketable, and early fruit weight. For ‘Addis’ × ‘Wis. SMR 18’, heterosis for fruit yield was associated with a decreased correlation between percentage of fruit set and fruit weight, an increased negative correlation between percentage of fruit set and both the number of branches per plant and the percentage of pistillate nodes, and an increased negative correlation between the number of nodes per branch and total fruit weight. Inbreeding depression was associated with a weakening of the strong negative correlations between percentage of fruit set and the number of branches per plant, and between the number of nodes per branch and total fruit weight. Those correlations were associated with high-parent heterosis and inbreeding depression only for one cross, and do not necessarily apply to future crosses in which heterosis may be observed for yield. We did not observe the heterosis or inbreeding depression for yield in cucumber in most of the crosses as was reported by Ghaderi & Lower (1979a; 1979c). This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance and relationships of important agronomic traits in almond

The inheritance of 16 important agronomic traits and its relationship were studied for four years in a population of 167 almond [P. dulcis (Mill.) D.A. Webb] seedlings obtained from a cross between the French selection ‘R1000’ (‘Tardy Nonpareil’ × ‘Tuono’) and the Spanish cultivar ‘Desmayo Largueta’. For some traits (blooming and leafing date) descendants segregated between the value of both progenitors, meanwhile for others the mean of the offspring was lower (bloom density, productivity and ripening date) or higher (in-shell/kernel ratio and double kernels). As expected, kernel bitterness and self-incompatibility behaved as monogenic traits. Some important correlations between traits were detected. The implications of the transmission and the correlation of these traits in the breeding programmes are discussed.     

Effect of alien 5R(5A) chromosome substitution on ear-emergence time and winter hardiness in wheat-rye substitution lines

Ear emergence time and response to vernalization were investigated in 12 alien substitution lines in which a pair of chromosomes 5A of recipient spring wheat cultivars was replaced by a pair of chromosomes 5R of Siberian spring rye ‘Onokhoiskaya’. The recipients were 12 spring cultivars of common wheat, each carrying different Vrn genes. Spring rye ‘Onokhoiskaya’ had the Sp1 (now called Vrn-R1) gene for spring growth habit located on chromosome 5R, but its expression was weaker. The Vrn-R1 gene had no effect on growth habit, ear emergence time and response to vernalization in wheat-rye substitution lines. Ears emerged significantly later in the 5R(5A) alien substitution lines than in the recipient wheat cultivars with the Vrn-A1/Vrn-B1/vrn-D1 or Vrn-A1/vrn-B1/Vrn-D1 genotypes. No difference in ear emergence time was found between most of the 5R(5A) alien substitution lines and the cultivars carrying the recessive vrn-A1 gene. The presence of the Vrn2a and Vrn2b alleles at the Vrn2 (now called Vrn-B1) locus located on wheat chromosome 5B was confirmed.The replacement of chromosome 5A by chromosome 5R in wheat cultivars ‘Rang’ and ‘Mironovskaya Krupnozernaya’, which carries the single dominant gene Vrn-A1, converted them to winter growth habit. In field studies near Novosibirsk the winter hardiness of 5R(5A) wheat–rye substitution lines of ‘Rang’ and ‘Mironovskaya Krupnozernaya’ was increased by 20–47% and 27–34%, respectively, over the recurrent parents.     

Characterization of segregation distortion on chromosome 3 induced in wide hybridization between indica and japonica type rice varieties

We previously surveyed chromosomal regions showing segregation distortion of RFLP markers in the F₂ population from the cross between a japonica type variety ‘Nipponbare’ and an indica type variety ‘Milyang23’, and showed that the most skewed segregation appeared on the short arm of chromosome 3. By comparison with the marker loci where distortion factors were previously identified, this region was assumed to be a gametophytic selection-2 (ga2) gene region. To evaluate this region, two near isogenic lines (NILs) were developed. One NIL had the ‘Nipponbare’ segment of this region on the genetic background of ‘Milyang23’ (NIL9-23), and the other NIL had the ‘Milyang23’ segment on the genetic background of ‘Nipponbare’ (NIL33-18). NIL9-23 and ‘Milyang23’, NIL33-18 and ‘Nipponbare’, and ‘Nipponbare’ and ‘Milyang23’ were respectively crossed to produce F₁ and F₂ populations. The F₁ plants of NIL9-23 × ‘Milyang23’ and NIL33-18 × ‘Nipponbare’ showed high seed fertility and the same pollen fertility as their parental cultivars, indicating that ga2 does not reduce seed and pollen fertility. Segregation ratio of a molecular marker on the ga2 region in the three F₂ populations was investigated to clarify whether segregation distortion occurred on the different genetic backgrounds. Segregation distortion of the ga2 region appeared in the both F₂ populations from the NIL9-23 and ‘Milyang23’ cross (background was ‘Milyang23’ homozygote) and the ‘Nipponbare’ and ‘Milyang23’ cross (background was heterozygote), but did notin the F₂ population from the NIL33-18 and ‘Nipponbare’ cross (background was ‘Nipponbare’ homozygote). This result indicates that ga2 interacts with a ‘Milyang23’ allele(s) on the different chromosomal region(s) to cause skewed segregation of the ga2 region. In addition, segregation ratio was the same between the F₂ populations from NIL9-23 × ‘Milyang23’ and ‘Nipponbare’ × ‘Milyang23’ crosses, suggesting that the both genotypes, ‘Milyang23’ homozygote and heterozygote, of gene(s) located on the different chromosomal region(s) have the same effect on the segregation distortion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbamate-induced flowering in asparagus (Asparagus officinalis L.) seedlings: optimization of treatment and cultivar variation in flowering response and pollen germination

Optimal carbamate treatment conditions were studied for flower induction in seedlings of an asparagus cultivar ‘Mary Washington 500 W’. Flower induction was most accelerated by soaking seeds in 50 mg l⁻¹ carbamate solution for 12 days at 25 ^°C under the fluorescent light. Longer exposure to carbamate over a 12 day period induced a higher percentage of seedlings to flower. A higher percentage of flowering seedlings were male. Flower induction frequency among seven cultivars through carbamate treatment widely ranged from 13 to 67%. ‘Geynlim’, ‘Mary Washington 500 W’ and ‘Welcome’ exhibited a high percentage of flowering seedlings, while ‘Larac’ and ‘Vulkan’ showed low values. Only male flowers were induced in all-male cultivars. Variation in pollen germination was found within all cultivars. ‘Geynlim’, ‘Cito’ and ‘Mary Washington 500 W’ showed high values of average pollen germination. Application of carbamate compound to induce flower production can rapidly produce homogenic cultivars which include both sexes. This is necessary for genetic studies and breeding purposes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification of self-incompatibility alleles in pear cultivars (Pyrus communis L.)

Self and cross-incompatibility determination by means of fruit and seed set experiments or pollen tube growth observations in the style has been frequently reported to be unclear in pear (Pyrus communis L.). Thus,in order to develop a reliable in vivo method to test pollen-pistil incompatibility in pear, pollen tube performance has been studied along the pistil following self and cross-pollinations. Results show that, while pollen tube growth in the style may be an unclear test, ovule observation at the microscope for the presence of pollen tube in the nucellus is a proper method to test incompatibility in this crop. With this analysis we could identify S-alleles of ‘Williams’ (S₁S₂) and ‘Coscia’(S₃S₄), and three of the four possible S-genotypes resulting from the ‘Williams’ × ‘Coscia’ cross, as represented by ‘Butirra Precoz Morettini’ (S₁S₃), ‘Santa Maria Morettini’ (S₂S₃)and ‘Tosca’ (S₁S₄). This result demonstrates that ‘Williams’ and ‘Coscia’ cultivars do not share any allele in common. We also established two new inter-incompatibility groups in pear. Furthermore, the presence of a common allele between ‘Williams’ and ‘Agua de Aranjuez’,and ‘Coscia’ and ‘Agua de Aranjuez’, three apparently unrelated old cultivars, may indicate a narrower genetic base than expected for European pear. This finding together with the fact that 40% of new released cultivars have direct or indirect parental relationship with the cultivars ‘Coscia’ and/or ‘Williams’, anticipates the possibility of new cases of cross-incompatibility for this crop in the future. Both the method described and the determination of the S-genotypes will facilitate the characterisation of self and cross-incompatibility relationships in this species. This revised version was published online in August 2006 with corrections to the Cover Date.     

Yield, yield components and plant architecture in the F3 generation of common bean (Phaseolus vulgaris L.) derived from a cross between the determinate cultivar ‘Prelude’ and an indeterminate landrace

Growth trials were conducted outdoors in the UK to determine the yield, yield components and plant architectural differences between determinate and indeterminate Phaseolus vulgaris bean plants. F3 lines derived from crosses between ‘Prelude’, a determinate cultivar and ‘V8’, an indeterminate landrace, were grown together with the parents and ‘Carioca’, an indeterminate landrace from Brazil. Data were recorded on flowering date, number of nodes on main stem at flowering, plant height at maturity, number of pods/plant, number of seeds/pod, number of seeds/plant, 100-seed weight, seed mass/plant, percentages of diseased and healthy seeds/plant. Determinate F3 lines had significantly lower (P < 0.05) seed mass/plant, fewer pods/plant, fewer seeds/pod, fewer seeds/plant, lower harvest index, shorter stems, earlier date of flowering, fewer nodes at flowering and fewer healthy seeds/plant than indeterminate F3 lines. However, determinate genotypes had significantly larger (P < 0.05) pods to accommodate their larger seeds compared with indeterminate lines. A similar situation was found when the yield and yield components of ‘V8’, ‘Prelude’ and ‘Carioca’ were evaluated; the highest seed mass/plant was given by ‘V8’ and the lowest by ‘Prelude’. Furthermore, some indeterminate genotypes with Type IV growth habit yielded significantly higher (P < 0.05) than the high-yielding parent, ‘V8’, indicating a positive influence on seed yield by crossing different genotypes.     

Significant effect of accidental pollinations on the progeny of low setting Prunus interspecific crosses

The incidence of fortuitous pollination on interspecific hybridizations of the plum rootstock Myrobalan with the apricot cultivars ‘Moniquí’ and ‘Moniquí Borde’ was assessed in this work. Progeny was originated through hand pollination of emasculated flowers of three Myrobalan clones, without bagging, in 1998 and 1999. Fruit set was low and variable among years (1.8–8.0%), but higher than the level of accidental pollination measured with emasculated and non-pollinated flowers (1.2%). Molecular characterization of the progeny was performed with three SSR markers showing that only 28% of the seedlings, obtained by in vitro germination and culture of immature embryos, were hybrids. This represents a lower percentage than expected, and is explained here by the low viability of hybrid embryos and seedlings. The use of molecular markers is discussed in terms of a convenient method for an early identification of putative hybrids in breeding programs with low setting crosses, where the proportion of non-hybrids is magnified.     

Performance of various grapefruit (<Emphasis Type="Italic">Citrus paradisi</Emphasis> Macf.) and pummelo (<Emphasis Type="Italic">C. maxima</Emphasis> Merr.) cultivars under the dry tropic conditions of Mexico

Grapefruit growers in the tropics require information about existing and new citrus cultivars with high productivity potential. The objective of this study was to determine the growth, yield, and fruit quality performance of seven pigmented and four white grapefruit cultivars under the dry tropic conditions of Colima, Mexico. The trees were budded on sour orange (Citrus aurantium L.) rootstock and planted at a distance of 8 × 4 m. ‘Oroblanco’ and ‘Marsh Gardner’ white-fleshed grapefruit cultivars and ‘Chandler’, a pink-fleshed pummelo, were the largest trees with the greatest height (5.0–5.6 m), canopy diameter (6.2–6.3 m), trunk diameter (21.9–23.3 cm), and canopy volume (109–123 m³). Lower height (4.3–4.8 m) and canopy volume (73–96 m³), but with similar canopy diameter to the previously mentioned cultivars, were recorded for the remaining pigmented cultivars. ‘Chandler’ pummelo and four pigmented grapefruit cultivars (‘Shambar’, ‘Río Red’, ‘Ray Ruby’, and ‘Redblush #3’) had yearly productions of 34.8, 34.9, 34.1, 32.7, and 30.6 ton ha⁻¹, respectively. The most productive white grapefruit cultivar was ‘Marsh Gardner’ (30.5 ton ha⁻¹). Grapefruit cultivars having the largest fruit size showed a higher inverse relationship between fruit weight and yield than those with small fruit. Most genotypes had higher values of fruit weight, juice content, and maturity index than those required by the local market. The most promising grapefruit cultivars based on their acceptable growth, yield superior to 30 ton ha⁻¹, and acceptable fruit color were ‘Río Red’, ‘Shambar’, ‘Ray Ruby’, and ‘Redblush #3’.     

Development and application of molecular marker linkage maps in woody fruit crops

Summary Extensive linkage maps, consisting primarily of molecular markers, are being developed for apple, pear, and grape varieties. The intrinsically high heterozygosity of outcrossing perennial species is utilized to produce segregating populations directly from a cross between varieties. Nearly complete linkage maps have been generated for the apple varieties ‘Rome Beauty’ and ‘White Angel’. The map for ‘Rome Beauty’ contains 161 molecular markers, while that for ‘White Angel’ has 251 markers. Maps for the pear varieties, ‘Bartlett’ and NY10353, also are being developed. Linkages conserved between the pear and apple genomes have been identified. In grapes, maps for four varieties are available, the most extensive being those for ‘Cayuga White’ and ‘Aurore’. The apple maps have been used to investigate the genetic basis of morphological and physiological characters. A gene controlling the presence of anthocyanins in the skin of the fruit is located on linkage group 3. Genes controlling early bud break, branching habit, and production of root suckers have also been identified and mapped.     

Single-gene resistance to powdery mildew in zucchini squash (Cucurbita pepo)

Cucurbita pepo (pumpkin, squash,gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). ‘True French’, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straight neck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita,and resistant plants were selected in the F₂ generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to ‘True French’. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlled-environment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0. This revised version was published online in August 2006 with corrections to the Cover Date.