The below mentioned article provides notes on application of molecular markers.
The development of molecular markers is a rapidly expanding field. To date a major effort has been applied to model species such as Arabidopsis, and to major crops such as tomato, rice and maize. There has been some application of isozyme markers to tropical and subtropical fruit crops e.g.. Musa spp., mango, papaya and pineapple.
By comparison there are few notes of the use of DNA markers in tropical and subtropical fruit species. Cultivar identification has been achieved by use of satellite probes in avocado, mango and papaya and by simple sequence repeats and mini-satellite probes in avocado, lychee, mango and papaya.
Magdalita used RAPD analysis to confirm interspecific hybrids between Carica species. There is great potential for the application of genetic markers to tropical, subtropical and indeed all perennial fruit crops.
Firstly, DNA fingerprinting allows accurate identification and characterisation of genotypes and species.
Secondly, DNA markers provide a rapid method of analysis of genetic variation in a population. Analysis of genetic variation between Carica papaya and related species facilitated a breeding program to introgress disease resistance genes from wild relatives into papaya.
Off-types, resulting from genetic variation in in vitro regeneration systems, can be identified by DNA markers in some instances. A RAPD marker has been developed to identify dwarf off-types in banana. Dwarf off-types have been a major problem in micro propagation of elite banana cultivars.
Thirdly, the use of molecular markers can facilitate plant breeding programs. Consequently, conventional breeding and assessment based on morphological markers can be a difficult and slow process with perennial fruit species.
A combination of in vitro techniques and use of molecular markers may help overcome many individual problems. Marker assisted breeding could be particularly useful for gene introgression, breeding for multi-gene resistance, and resistance to a disease not yet in a region or country.
An example of the savings that could be made in terms of time and funds is the potential to select, at the seedling stage, for recessive characters such as red female dioecious papayas. The advantages for fruit species with much longer juvenile phases is obvious.
Currently, the high cost of this research is preventing large scale application in tropical and subtropical fruit species. However, it represents a wide open field of research and the potential gains warrant a large investment of funds.
As with marker technology there is enormous potential in applying recombinant DNA technology to tropical fruit species. Gene transfer offers the potential to add one or two genes to an elite genotype. Most tropical fruit species are highly heterozygous.
They often have difficulties that interfere with traditional plant breeding programs and have long juvenile periods. A good example is papaya industries that are based on dioecious female genotypes.
In Australia, breeding programs over 20 years have produced elite female genotypes after years of concentrated effort, however the genotypes have not been maintained in future seed generations. Plant biotechnology offers the potential to add one characteristic (e.g.. Papaya Ring spot Virus Resistance, PRSV-P) to an elite genotype, which could then be micro propagated in vitro.
Papaya has become the model crop for application of transformation technology to tropical fruit crops. Transgenic papaws resistant to PRSV-P have been produced and are currently being tested in large scale field plantings in Hawaii.
There are also a few preliminary notes on transformation of Musa spp., passion-fruit and pineapple with marker and reporter genes. The only published notes on transformation of a woody perennial tropical fruit species are in mango.
Transformation of other tropical and subtropical fruit species is warranted and could result in considerable benefits. Potential includes reduced juvenility in tropical fruit species (e.g.. mangosteen), disease resistance (e.g. Phytophthora resistance in durian) and insect resistance (e.g.. resistance to fruit fly).
Recent advances in understanding and cloning of promoters provides the potential of tissue specific expression of genes, hence insect resistance could be targeted to meristems, roots, or epidermal tissue. There have been numerous recent notes on transformation of temperate woody perennial fruit species.
This demonstrates that transformation of woody species is achievable. In addition, in the last decade, in vitro regeneration systems, which underpin transformation technology, have been developed for some tropical and subtropical fruit species.