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Marking Males for Sex

It can be hard telling male kiwifruit vines apart from females, but DNA technology is offering orchardists a chance to sex their plants and save time and money in breeding programmes.

Kiwifruit, and all its relatives, have male and female plants which can't be distinguished from each other except by their flowers. Both are needed in orchards, but it is only the females which bear fruit.

The new gene technology now allows the sex of kiwifruit plants to be identified long before they flower by using DNA markers to track the gene involved in determining the sex of the plant.

When breeding new types of kiwifruit, a large number of seedlings are produced and it is usually necessary to plant them out in an orchard and grow them to maturity -- letting them flower -- then select the best female on the basis of fruit characteristics. This takes several years. Half the seedlings will be male with no fruit, and this is wasteful in terms of orchard resources and labour over several years.

The sex marker was developed so that the breeders could distinguish the male seedlings at an early stage, and therefore only plant out females that could be assessed for fruit quality. The sex marker was discovered by a process called bulk segregant analysis, which involved the testing of several hundred random markers with the DNA of sibling male and female plants, to find one which segregated with the sex of the individual.

The molecular marker was then cloned and sequenced -- the piece of DNA that was found in the males only was taken out and the sequence of bases on the DNA was determined. From this information, primers were designed which amplified this piece of DNA in all male plants of the family being studied. It did not amplify in the female plants.

This marker has now been tested in many families, and has been used to identify male seedlings in the breeding orchard. This principle of marking regions of the genome which contain genes for particular characteristics is now being taken further, and it is hoped to be able to identify carrying traits, such as large sweet fruit, different colours or higher levels of Vitamin C.