Significant progressive heterobeltiosis in banana crossbreeding

Background Heterobeltiosis is the phenomenon when the hybrid’s performance is superior to its best performing parent. Banana (Musa spp. AAA) breeding is a tedious, time-consuming process, taking up to two decades to develop a consumer acceptable hybrid. Exploiting heterobeltiosis in banana breeding will help to select breeding material with high complementarity, thus increasing banana breeding efficiency. The aim of this study was therefore to determine and document the level of heterobeltiosis of bunch weight and plant stature in the East African highland bananas, in order to identify potential parents that can be used to produce offspring with desired bunch weight and stature after a few crosses. Results This research found significant progressive heterobeltiosis in cross-bred ‘Matooke’ (highland cooking) banana hybrids, also known as NARITAs, when grown together across years with their parents and grandparents in Uganda. Most (all except 4) NARITAs exhibited positive heterobeltiosis for bunch weight, whereas slightly more than half of them had negative heterobeltiosis for stature. The secondary triploid NARITA 17 had the highest heterobeltiosis for bunch weight: 249% versus its ‘Matooke’ grandparent and 136% against its primary tetraploid parent. Broad sense heritability (across three cropping cycles) for yield potential and bunch weight were high (0.84 and 0.76 respectively), while that of plant stature was very low (0.0035). There was a positive significant correlation (P < 0.05) between grandparent heterobeltiosis for bunch weight and genetic distance between parents (r = 0.39, P = 0.036), bunch weight (r = 0.7, P < 0.001), plant stature (r = 0.38, P = 0.033) and yield potential (r = 0.59, P < 0.001). Grandparent heterobeltiosis for plant stature was significantly, but negatively, correlated to the genetic distance between parents (r = − 0.6, P < 0.001). Conclusions Such significant heterobeltiosis exhibited for bunch weight is to our knowledge the largest among main food crops. Since bananas are vegetatively propagated, the effect of heterobeltiosis is easily fixed in the hybrids and will not be lost over time after the release and further commercialization of these hybrids.

Significant progressive heterobeltiosis in banana crossbreeding

Background: Heterobeltiosis is the phenomenon when the hybrid’s performance is superior to its best performing parent. Banana ( Musa spp. AAA) breeding is a tedious, time-consuming process, taking up to two decades to develop a consumer acceptable hybrid. Exploiting heterobeltiosis in banana breeding will help to select breeding material with high complementarity, thus increasing banana breeding efficiency. The aim of this study was therefore to determine and document the level of heterobeltiosis of bunch weight and plant stature in the East African highland bananas, in order to identify potential parents that can be used to produce offspring with desired bunch weight and stature after a few crosses.Results: This research found significant progressive heterobeltiosis in cross-bred ‘Matooke’ (highland cooking) banana hybrids, also known as NARITAs, when grown together across years with their parents and grandparents in Uganda. Most (all except 4) NARITAs exhibited positive heterobeltiosis for bunch weight, whereas slightly more than half of them had negative heterobeltiosis for stature. The secondary triploid NARITA 17 had the highest heterobeltiosis for bunch weight: 249% versus its matooke grandparent and 136% against its primary tetraploid parent. Broad sense heritability (across three cropping cycles) for yield potential and bunch weight were high (0.84 and 0.76 respectively), while that of plant stature was very low (0.0035). There was a positive significant correlation ( P < 0.05) between grandparent heterobeltiosis for bunch weight and genetic distance between parents (r = 0.39, P = 0.036), bunch weight (r = 0.7, P < 0.001), plant stature (r = 0.38, P = 0.033) and yield potential (r = 0.59, P < 0.001). Grandparent heterobeltiosis for plant stature was significantly, but negatively, correlated to the genetic distance between parents (r = -0.6, P < 0.001).Conclusions: Such significant heterobeltiosis exhibited for bunch weight is to our knowledge the largest among main food crops. Since bananas are vegetatively propagated, the effect of heterobeltiosis is easily fixed in the hybrids and will not be lost over time after the release and further commercialization of these hybrids.

Transfer of Downy Mildew Resistance from Wild Basil (Ocimum americanum) to Sweet Basil (O. basilicum)

Sweet basil (Ocimum basilicum) is susceptible to downy mildew caused by the oomycete foliar pathogen Peronospora belbahrii. No resistant varieties of sweet basil are commercially available. Here, we report on the transfer of resistance gene Pb1 from the highly resistant tetraploid wild basil O. americanum var. americanum (PI 500945, 2n = 4x = 48) to the tetraploid susceptible O. basilicum ‘Sweet basil’ (2n = 4x = 48). F1 progeny plants derived from the interspecific hybridization PI 500945 × Sweet basil were resistant, indicating that the gene controlling resistance (Pb1) is dominant, but sterile due to the genetic distance between the parents. Despite their sterility, F1 plants were pollinated with the susceptible parent and 115 first backcross generation to the susceptible parent (BCs1) embryos were rescued in vitro. The emerging BCs1 plants segregated, upon inoculation, 5:1 resistant/susceptible, suggesting that resistance in F1 was controlled by a pair of dominant genes (Pb1A and Pb1A’). Thirty-one partially fertile BCs1 plants were self-pollinated to obtain BCs1-F2 or were backcrossed to Sweet basil to obtain the second backcross generation to the susceptible parent (BCs2). In total, 1 BCs1-F2 and 22 BCs2 progenies were obtained. The BCs1-F2 progeny segregated 35:1 resistant/susceptible, as expected from a tetraploid parent with two dominant resistant genes. The 22 BCs2 progenies segregated 1:1 resistant/susceptible (for a BCs1 parent that carried one dominant gene for resistance) or 5:1 (for a BCs1 parent that carried two dominant genes for resistance) at a ratio of 4:1. The data suggest that a pair of dominant genes (Pb1A and Pb1A’) residing on a two homeologous chromosomes is responsible for resistance of PI 500945 against P. belbahrii.

Production of New Allotetraploid and Autotetraploid Citrus Breeding Parents: Focus on Zipperskin Mandarins

Somatic hybridization through protoplast fusion has proven to be a valuable technique in citrus for producing unique allotetraploid breeding parents that combine elite diploid selections. Many citrus somatic hybrids are now flowering and being used in interploid crosses to generate triploid hybrids that produce seedless fruit, a primary objective of citrus breeding programs. Most of the early somatic hybrids produced for mandarin improvement combined sweet oranges with mandarins, because the performance of sweet oranges in tissue/protoplast culture generally exceeds that of most mandarin selections. However, a high percentage of triploid progeny from interploid crosses using sweet orange + mandarin somatic hybrids as the tetraploid parent produce fruit that are difficult to peel. We report nine new allotetraploid somatic hybrids and five new autotetraploids from somatic fusion experiments involving easy-peel mandarin parents. These tetraploids can be used in interploid crosses to increase the percentage of seedless triploid progeny producing easy-to-peel fruit. Ploidy level of the new tetraploids was determined by flow cytometry and their genetic origin by expressed sequence tag–simple sequence repeat marker analysis.

Microsatellite Marker Development in Rose and its Application in Tetraploid Mapping

Microsatellite or simple sequence repeat (SSR) markers were developed from Rosa wichurana Crépin to combine two previously constructed tetraploid rose (Rosa hybrida L.) genetic maps. To isolate SSR-containing sequences from rose a small-insert genomic library was constructed from diploid Rosa wichurana and screened with several SSR probes. Specific primers were designed for 43 unique SSR regions, of which 30 primer pairs gave rise to clear PCR products. Seventeen SSR primer pairs (57%) produced polymorphism in the tetraploid rose 90-69 mapping family. These markers were incorporated into existing maps of the parents 86-7 and 82-1134, which were constructed primarily with AFLP markers. The current map of the male parent, amphidiploid 86-7, consists of 286 markers assigned to 14 linkage groups and covering 770 cm. The map of the female tetraploid parent, 82-1134, consists of 256 markers assigned to 20 linkage groups and covering 920 cm. Nineteen rose SSR loci were mapped on the 86-7 map and 11 on the 82-1134 map. Several homeologous linkage groups within maps were identified based on SSR markers. In addition, some of the SSR markers provided anchoring points between the two parental maps. SSR markers were also useful for joining small linkage groups. Based on shared SSR markers, consensus orders for four rose linkage groups between parental maps were generated. Microsatellite markers developed in this study will provide valuable tools for many aspects of rose research including future consolidation of diploid and tetraploid rose genetic linkage maps, genetic, phylogenetic and population analyses, cultivar identification, and marker-assisted selection.

A fertile amphiploid between diploid wheat (Triticum tauschii) and crested wheatgrass (Agropyron cristatum)

Alloploidy, one of the most efficient evolutionary mechanisms in nature, has not been extensively exploited in plant breeding programmes. Many genomic combinations remain to be created by plant breeders, to be used directly as new crops or indirectly to widen the genetic basis of crops. The Triticeae tribe, to which wheat belongs, is among the botanical groups in which this strategy has been successfully explored. However, there remain valuable genomic combinations that have not been obtained at the diploid level. The Agropyron complex (wheat-grasses) has recently been the focus of attention for interspecific hybridization, but intergeneric hybrids or amphiploids with wheat have not been reported at the diploid level. Here we report synthesis of a tetraploid amphiploid between Triticum tauschii and Agropyron cristatum by crossing two tetraploid accessions. Using total genome in situ hybridization (GISH) staining on metaphase I pollen mother cells, data on allosyndetic and autosyndetic chromosome pairing have been obtained. These data support the view that the A. cristatum tetraploid parent used in the synthesis of the amphiploid has a segmental alloploidy nature.

Photoautotrophic Micropropagation of Triploid Melon: II. Rooting and Plantlet Growth on Sugar-free Media

Two triploid clones of melon from the same tetraploid parent were grown in vitro with and without sugar, rooted without sugar in media both in a laboratory controlled environment chamber (in vitro) and a greenhouse acclimatization unit (ex vitro), and compared for subsequent nursery growth in the greenhouse unit. The clone `(L-14 c B) × L-14' produced more shoots in both photomixotrophic (with sucrose) or photoautotrophic (sugar-free) conditions. Both genotypes were equally likely to root in sugar-free media, and `(L-14 × B) × L-14' rooted as well from either photoautotrophic and photomixotrophic shoots but `(L-14 × B) × Mainstream' rooted less frequently from photoautotrophic shoots. Seventy-six percent (76%) of the shoots were able to root photoautotrophically in vitro, whereas 47% of the ex vitro shoots were rooted. About 85% of plantlets from all treatments survived after transfer to the nursery. After growth in the greenhouse nursery, the sizes of plants (fresh and dry weight, leaf area) were the same for either clone, from either photoautotrophic or photomixotrophic shoots. Also, after growth in the nursery, plantlets that had been rooted in vitro were larger than those rooted ex vitro. Photoautotrophic rooting demonstrates a concept for integrating micropropagation and plug-type vegetable transplant production.

Mortality of hybrid triploid aspen in Wisconsin and Upper Michigan

In eight field trials in Upper Michigan and Wisconsin, mortality of hybrid triploid aspen (Populustremuloides Michx. × Populustremula L.) was 93% by age 25, which was nearly twice that of native diploid and triploid aspen. Symptoms of decline observed on individuals of the hybrid triploid families included branch dieback, thinning crowns, and eventually tree death. The fungus Lahmiakunzei (Koerb.) (syn. Parkerellapopuli (Funk)) was found associated with a number of families from one specific hybrid triploid cross (T-1-58 × Ta-10). Symptoms of rough, black bark developed on 3% of the trees from this particular cross 10 years after planting and increased to 92% at age 25. Neighboring hybrid diploids of P. tremuloides × P. tremula and native triploids and diploids neither exhibited rough bark symptoms associated with L. kunzei nor showed symptoms of decline like those observed on the hybrid triploids. Mortality of native triploids and diploids in these trials was 41% and 58%, respectively, which was primarily caused by hypoxylon canker. Thinning crowns caused by branch dieback and tree mortality without the presence of L. kunzei has been common on the male tetraploid parent (Ta-10) in clone banks and seed orchards in Minnesota and Wisconsin. This parent was collected from an oceanic region near Ekebo, Sweden, and it, along with its hybrid progeny, appear to be poorly adapted to the climate of Upper Michigan, Minnesota, and Wisconsin, resulting in the high mortality of the hybrid triploids in these trials. The origin of the fungus L. kunzei and the nature of its specificity to T-1-58 × Ta-10 hybrid triploid cross are unknown.

Fruit Characteristics of Hybrid Triploid Melons

Thirteen triploid lines of melon (Cucumis melo L.) were derived from crosses involving five tetraploid and seven diploid lines. Fruit characters were assessed. When allowed to open pollinate in field plots with adjacent diploid pollinators, eight triploid genotypes were sterile or nearly sterile (<1% viable seed). Five triploid genotypes were partially fertile, indicating viable pollen grains were present. Cytological analysis performed on progeny of a partially fertile triploid plant fertilized by open pollination indicated euploid female gametes were common. Triploid hybrids between tetraploid `Miniloup' and several other diploid parents had vegetative and fruit characteristics intermediate to the parents. Most triploid genotypes yielded round fruit in contrast to their diploid parent whose fruit were oval to oblong and the tetraploid parent that had oblate fruit. Sugar levels of some triploid hybrids were as high as diploid parents.

YIELD OF TRIPLOID WATERMELONS IS INFLUENCED BY POLLENIZERS AND INCIDENCE OF MATURE SEEDS BY PARENTS

Mature seeds occur occasionally in triploid watermelon fruit. In one trial, the average number varied from 0.3 to 28.7 seeds per fruit in 30 entries and from 0.5 to 8.6 seeds per fruit in the cultivars within this group. The frequency of mature seed in triploid fruit with the same tetraploid parent ranged from 0.3 to 3.0 and from 1.25 to 5.0 seeds per fruit in triploid fruit having the same diploid parent. Tetra A, with 151 seeds per fruit, produced triploids with 6 seeds per fruit; whereas Tetra B, with 74 seeds per fruit produced triploids with only 1.3 seeds per fruit. Date-of-flowering of diploid watermelon cultivars used as pollenizers for triploids affected maturity date of the triploids. Icebox-types that flower early produced higher early yields of triploid fruit; whereas standard cultivars that flower later produced higher yields late in the season.