Dioecious papayas were introduced shortly after Cook’s 1778 discovery of Hawai’i but were supplanted for commercial uses by the gynodioecious solo papaya brought from the Caribbean in 1911. Growth of a local papaya industry based on hermaphrodite plants was enabled by research allowing prediction of seedling sex segregation and by development of cultivars with high quality, symmetrical fruits free of stamen carpellody, and carpel abortion. The industry expanded into export markets after 1940 by providing an alternative use for land and expertise abandoned by declining sugar plantations, adopting a cultivar capable of tolerating long-distance shipping, developing postharvest technology to overcome fruit fly quarantine restrictions, capitalizing on a growing tourism industry for marketing and air freight logistics, and forming an organization to support industry growth. In recent years, the industry has withstood pest and disease challenges by adopting innovative technologies that have allowed high-quality solo papayas to continue to participate in an increasingly competitive export market.
Rajeswari Srinivasan* and Richard Manshardt
QTL mapping gives an insight into the number, position and effect of loci controlling quantitative traits. Although a few linkage maps already exist for papaya, not many economically important traits have been studied. An investigation was undertaken to map two qualitative traits: 1) fruit flesh color and 2) an isozyme locus, phosphoglucomutase (PGM); as well as two quantitative traits: 1) number of nodes to first flowering and 2) stamen carpellody. An F2 population consisting of 281 plants derived from the parents Kapoho X Saipan Red was used for this study. Field observations suggested that there may be a linkage between PGM locus and one of the major QTLs controlling number of nodes to first flowering. Also, phenotypic data suggested that there may be a linkage between flesh color and carpellody. Marker genotyping was performed on a subset of 84 plants chosen from the phenotypic extremes of the population for node number and carpellody. Using AFLP (Amplified fragment length polymorphism) method, 510 markers were generated with 161 primer pairs. Although papaya has a haploid chromosome number of 9, at LOD score 5.0 and a maximum recombination frequency of 0.25, 25 linkage groups with number of markers ranging from 2 to 109 were generated using the software Mapmaker\EXP. Linkage and QTL maps are being constructed to reveal the molecular markers linked with the traits of interest and the nature of QTLs controlling the quantitative traits.
Richard Manshardt and Dennis Gonsalves
Transgenic papaya line 55-1 with resistance to papaya ringspot virus (PRSV) originated in 1989 by particle bombardment of cultivar Sunset with the coat protein gene (cp) of mild mutant Hawaii PRSV strain HA 5-1. Hemizygous (+/cp) R0 clones of 55-1 displayed resistance to the virulent Hawaii HA strain in greenhouse tests in New York in 1991 and to local strains in a field trial in Hawaii from 1992 to 1994. In the R1 generation produced by crossing the pistillate R0 55-1 with `Sunset', up to 50% of the hemizygous transgenic segregants were susceptible to a local Oahu PRSV strain when inoculated as seedlings but not as mature plants. Similar inoculation experiments in New York showed that hemizygous R1 transgenics were susceptible in differing degrees to PRSV strains from regions other than Hawaii. Homozygous (cp/cp) R2, R3, and R4 populations planted in various locations in Hawaii since 1994 have consistently demonstrated high-level resistance to local strains at all stages of development. When inoculated in New York with eight non-Hawaii PRSV strains, homozygous R3 seedlings were resistant to all but a Thai strain. Transgenic resistance is the result of a complex interaction involving the stage of plant development, transgene dosage, the degree of homology between transgene and challenge virus, and environmental variables. Papaya plants transformed with nontranslatable versions of various cp genes are also highly resistant to PRSV, indicating that the resistance mechanism operates at the RNA level. No loss of resistance due to the appearance of resistance-breaking virus strains or to transgene inactivation has been noted thus far.
Lius Suwenza and Richard Manshardt
Nine transgenic papaya clones, produced previously by microprojectile bombardment, are being characterized for frequency of somaclonal variation. Five clones have proven to be hermaphrodite. Four of these appear to have normal fertility, while the fifth has drastically reduced pollen fertility, averaging about 15% stainability with acetocarmine. Four other clones are pistillate and appear to have normal fertility, with one exception which has been demonstrated to be tetraploid (2n=36 chromosomes). One of twelve plants in a pistillate clone was a somaclonal mutant showing altered leaf and flower morphology. The transgenic clones and their sexual progenies are also being evaluated at the molecular level for expression and segregation of npt, gus, and the coat protein (CP) of papaya ringspot virus (PRV), as well as for PRV resistance.
Charles R Clement and Richard M. Manshardt
The pejibaye (Bactris gasipaes, Palmae) is being evaluated in Hawaii as a source of fresh hearts of palm. Nine open-pollinated progenies from the Benjamin Constant population of the Putumayo landrace are planted at three densities: 1.5 × 2 m (3333 plants/hectare); 1 × 2 m (5000 pl/ha, the commercial density in Costa Rica); 1 × 1.5 m (6666 pl/ha). Harvest started at 15 months after planting and four months later 25% of the plants had been harvested, with 25%, 30% and 21% at 3333, 5000, and 6666 pl/ha, respectively. Mean heart diameters were unaffected by density (mean±SD = 3.2±0.4 cm). Heart lengths were similar (24±5 cm, 23±6 cm, 26±5 cm, respectively), as were heart weights (200±41 g, 187±44 g, 224±42 g, respectively). This relative uniformity was unexpected, as density effected all of these yield components in earlier experiments in Latin America. Potential yields were different (667±136 kg/ha, 835±221 kg/ha, 1491±275 kg/ha, respectively), and are comparable to yields reported from Costa Rica. Actual precocious yields, however, were not different (167 kg/ha, 278 kg/ha, 385 kg/ha, respectively).
Charles R. Clement and Richard M. Manshardt
The pejibaye (Bactris gasipaes, Palmae) is being evaluated in Hawaii as a source of fresh hearts of palm. Nine open-pollinated progenies from the Benjamin Constant population of the Putumayo landrace are planted at three sites in a RCB. The best site started yielding at 15 months after planting, the intermediate at 16 months, the poorest at 18 months. During the first four months of harvest at the best site, 25% of the plants were cut; during three months at the intermediate site, 15% were cut; during the first cut at the poor site, 1% were cut. Progeny harvest percentages ranged from 7 to 53% at the best site, with only three progenies above average (33, 47, 53%). These are considered to be precocious. These three progenies produced average size hearts (172±36, 204±57, 203±44 g/plant, respectively; experimental mean±SD = 205±53 g), but yielded above average at 5000 plants/ha (275, 480, 524 kg/ha, respectively; exp. mean = 272 kg; corrected for % cut). Potential yields of these progenies were near the mean (871±198, 1018±280, 983±197 kg/ha, respectively; exp. mean = 986±381 kg/ha), but their precocity provides early returns to the farmer.
Richard M. Manshardt, Cathy Mello, Sharon D. Lum, and Leanne Ta
Genetically engineered (GE), virus-resistant papaya cultivars in Hawaii are easily identified by a colorimetric assay for the β-glucuronidase (GUS) marker transgene. We used GUS to track pollen movement from a central 1-acre plot of gynodioecious GE `Rainbow' plants into seeds on surrounding border rows of non-GE `Sunrise' papaya. GUS evidence of cross-pollination occurred in 70% of female plants (43% of assayed seeds), compared with only 13% of the predominantly self-pollinating hermaphrodite plants (7% of seeds) segregating in the gynodioecious `Sunrise' border rows. The percentage of GUS+ seeds in border row plants showed a weak negative correlation (r = –0.32) with distance from the nearest GE tree (30 m maximum). In a non-GE papaya field located less than a mile downwind from the `Rainbow' source, no evidence of GUS was found in 1000 assayed seeds. In a separate study, the origin of GUS+ seed discovered in papaya fruits from an organic farm was investigated. Leaf GUS assays revealed that 70% of trees were GE, indicating that the grower had planted GE seed. The impact of pollen drift from GE trees in the same field was determined by screening seed samples from 20 non-GE hermaphrodites for GUS expression. Only three hermaphrodites (15%) showed GUS+ seeds, at low levels ranging from 3% to 6% of contaminated samples. These data indicate that the major source of GE contamination in organic fields is seeds of unverified origin, rather than pollen drift from neighboring GE fields. Organic growers are advised to: 1) plant only seed that is known to be non-GE, preferably obtained by manual self-pollination of selected non-GE hermaphrodites; 2) avoid open-pollinated seed; and 3) grow only hermaphrodite (self-pollinating) trees, removing any female or male plants from production fields.
Charles R. Clement, Richard M. Manshardt, Joseph DeFrank, Catherine G. Cavaletto, and Natalie Y. Nagai
Catherine G. Cavaletto, Natalie Y. Nagai, Charles R. Clement, and Richard M. Manshardt
Fresh pejibaye palm heart samples were evaluated from 1) progenies from the Benjamin Constant population of the Putumayo landrace and 2) progenies from the Yurimaguas population of the Pampa Hermosa landrace. Favorable sensory characteristics included sweetness, tenderness, crispness, and moistness. Negative sensory characteristics found in some samples were astringency and acridity. Sensory scores for astringency and acridity were significantly different (p=.05) between sections of the heart Basal sections appear to be more acrid and astringent than the apical sections. Differences in acridity also exist between plants within a progeny (p=.05). Percent total soluble solids ranged from 3.0 to 6.6, but no obvious pattern was apparent. Samples were also provided to chefs in upscale restaurants where they received favorable comments.
Paula Tennant, Manoel T. Souza Jr., Dennis Gonsalves, Maureen M. Fitch, Richard M. Manshardt, and Jerry L. Slightom
The disease resistance of a transgenic line expressing the coat protein (CP) gene of the mild strain of the papaya ringspot virus (PRSV) from Hawaii was further analyzed against PRSV isolates from Hawaii and other geographical regions. Line 63-1 originated from the same transformation experiment that resulted in line 55-1 from which the transgenic commercial cultivars, `Rainbow' and `SunUp', were derived. Plants of line 63-1 used in this study consisted of a population from a self pollinated R0 bisexual plant. ELISA and PCR tests provided evidence that there are at least two segregating CP loci. To allow for comparison with reactions of the previously reported line 55-1, virus isolates from Hawaii, Brazil, Thailand, and Jamaica were used to challenge seedlings of 63-1. Unlike line 55-1, a significant percentage of inoculated transgenic plants were susceptible to isolates from Hawaii. However, a proportion of plants were resistant to the non-Hawaiian isolates. In contrast, previous work showed that all plants of the hemizygous line 55-1 were susceptible to PRSV isolates from Brazil, Thailand, and Jamaica. Line 63-1, therefore, presents Hawaii with PRSV-resistant transgenic germplasm that could be used as a source of transgenes for resistance to PRSV isolates within and outside of Hawaii.