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Summary of PhD thesis
Submitted June 1998
The University of Queensland Gatton College
The first aim of this study was to determine the usefulness of three approaches for the prediction of sex-type in papaya (Carica papaya L.) seedlings.
Firstly, morphological characters were investigated.
Plant height, plant height at first flowering, stem colour, stem pigmentation, petiole colour, leaf shape, shape of petiole sinus and the number of nodes to first flower were useful for identification of cultivars.
The number of nodes and plant height at first flowering were also useful for distinguishing between female, hermaphrodite and male F1 hybrid plants. However, these characters were not suitable for use in seedlings.
It was found that three morphological characters, viz. stem colour, stem pigmentation and petiole colour in the cultivar Khaeg Dum, from Thailand, were useful for identifying female and hermaphrodite seedlings.
Most crosses examined segregated for sex-type in the ratios expected.
Secondly, a range of isozymes was investigated following the development of a technique for papaya leaves.
Five from fifteen isozyme systems studied (viz. LAP, EST, IDH, ACP, ADH, AMY, ACON, SOD, LDH, PGI, PGM, MDH, PGD, GOT and PER) in the first year of investigations, i.e. isozymes glucosephosphate isomerase (PGI), phosphoglucose mutase (PGM), leucine aminopeptidase (LAP), peroxidase (PER) and esterase (EST) gave variation in banding patterns among ten cultivars of papaya used.
During the second year, four from twenty one isozyme systems studied (ACP, EST, LAP, MDH, PER, PGM, PGI, AO, ME, GPDH, FUM, XDH, AK, APK, G3PDH, HEX, Trehalase, TPI, ALP, H6PDH and SDH), gave banding pattern variations with sex-type.
Among these, PER, LAP and EST were able to distinguish male from female plants in the Australian cultivar Richter and PER and PGI were able to distinguish hermaphrodite from female plants in the Hawaiian cultivar Sunset.
However, none of 21 isozyme systems studied gave pattern variation to distinguish between sex-type amongst F1 hybrid plants.
Thirdly, molecular markers for sex-type determination were investigated.
Two techniques, random amplified polymorphic DNA (RAPD) and DNA amplification fingerprinting (DAF) were compared and the DAF procedure appeared to have distinct advantages.
DAF reactions produced at least five times more bands than equivalent RAPD reactions, permitting more efficient screening. DAF reactions also appeared to be more reliable.
Using bulk segregant analysis, a large number of DAF markers which were present in only male or hermaphrodite pooled DNAs were defined.
Preliminary analyses for linkage associations indicated these markers were closely linked to the sex-determining alleles.
Attempts were made to convert some DAF markers into more convenient SCAR markers, but this proved difficult since DAF bands were difficult to reamplify and clone.
Four sex-specific DAF markers were cloned, but were not converted to sex-specific SCAR markers since the primers apparently amplified DNA from all sexes.
During these analyses, clones homologous to phytochrome B and the del-transposon were identified.
The second aim of this investigation was to investigate whether the fruit quality of yellow-fleshed Australian cultivars could be improved by producing new red or yellow-fleshed papayas with high sugar content by crossing the Australian cultivars with red or orange-fleshed cultivars from Thailand and Hawaii, respectively.
There was a wide range of variation in fruit quality characters amongst F1 fruit from crosses of female plants from the Australian cultivars 2.001 or Richter with hermaphrodite plants from the Hawaiian cultivar Sunset or the Thai cultivar Khaeg Dum.
Three characters were considered important for fruit quality in papaya, viz. the percentage of total soluble solids, fruit weight and flesh thickness.
Four superior F1 fruits (identified as G6-69-1, G6-69A-2, G5-94-2 and R1- 48A-2) were selected on the basis that they produced high sugar content, having more than 13% total soluble solids, had acceptable fruit weight, between 0.5 and 1.5 kg and a flesh thickness of more than 2.2 cm.
These superior F1 fruits also demonstrated high productivity in the field. They were selected to produce the F2 generation, which was examined in the following season.
To produce red-fleshed papaya in the F2 generation, it was necessary to self-pollinate F1 hermaphrodite plants; this generation varied in flesh colour from yellow to red.
There was a wide range of variation in F2 fruit quality characters. All crosses segregated for sex-type and flesh colour as expected.
Fourteen superior F2 fruits (both yellow and red-fleshed) and thirteen superior red-fleshed F2 fruits were selected as new lines with high sugar content, good flavour and medium fruit size.
This demonstrated that the fruit quality of Australian cultivars could be improved by crossing with the orange fleshed Hawaiian and red fleshed Thai cultivars.
These superior F2 fruits are being prepared for propagation by tissue culture using the single nodal cuttings protocol, to ensure they will be available for use by the Australian industry.
Also, approximately 2,000 F3 seedlings from these superior F2 fruits have been planted in a commercial plantation at Yandina for development as inbred lines.
In conclusion, molecular markers showed more promise than morphological characters or isozyme techniques for the prediction of sex-type in papaya seedlings.
Also, crossing Australian papaya cultivars with orange or red-fleshed cultivars shows great promise for improving the quality of Australian papaya fruit.
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