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Advice on Publications Available.[This article first appeared in Australian Bushfood Magazine and was contributed by the editor. At least one bushfood entrepreneur has replied, indicating his nursery is developing synthetic varieties]
The 'nature or nurture' debate is relevant to crop improvement in much the same way it is argued in human development.
Why do some people appear to be so incredibly capable intellectually and others not so?
Is intelligence the result of parentage or is it a result of the home environment?
If a young person does well academically because his or her parents screen out all distractions that might prevent them studying, can the young person be expected to be a capable performer in the real world?
In a similar manner, the following questions should be asked in relation to new crop plants: if a plant does well in a particular environment, is it because the plant is genetically superior or is it because the environment favours the particular combination of genes the plant possesses?
Will the plant with this combination of genes also be superior in other environments?
The task of the plant breeder is to identify those plants whose combinations of genes are able to perform well in all the environments in which the plant will be grown as a crop.
For a new crop, the range of environments planned for it may be different from the environments in which it grows naturally.
As well, many of the new crops currently being considered for use in Australia are cross pollinated species. This means that any seed produced from a particular plant is the result of the fertilisation of the eggs from that plant with pollen from other plants (of the same species of course). Plants produced in this manner will all be genetically unique.
Hence, a cross-pollinated new crop is naturally a population of genetically different plants. This can often be very beneficial since some plants within the crop will have the appropriate genetic makeup to do well in every kind of environment in which the crop finds itself.
If a superior single plant of a cross pollinated species can be identified, then vegetative propagation of that plant through cuttings to produce a 'clone' can keep it alive and increase its numbers for use as a crop. Such has happened with many fruit trees, ornamentals and other horticultural crops.
However, the disadvantage of clones is that all plants derived from the same original stock plant are genetically identical. There would be none of the variable response to changes in the environment mentioned above: a new disease or sudden stress would affect all plants.
A 'synthetic' variety can be useful because it combines these elements of genetic superiority with genetic variability.
A 'synthetic' variety of a new crop is derived as the seed from random crosses between a limited number, of perhaps six to ten, specially chosen parent plants.
The important step in the production of a 'synthetic' variety is the choice of the six to ten parent plants.
The test used to compare potential parent plants is an evaluation of the 'general combining ability' of all candidate parent plants.
This test is the same as the comparisons a dairy farmer might make when comparing bulls for their ability to produce highly productive milking cows. Each bull being tested for 'general combining ability' is allowed to mate with a number of cows and the resulting female calves are compared for their milk production.
The bull with the best 'general combining ability' is the one who produces the highest yielding offspring, from matings to a number of cows. Such a bull would then be preferred for mating in the future.
Testing for 'general combining ability' occurs in much the same way in a cross pollinating new crop.
Candidate parent plants are chosen on the basis of their appearance: perhaps they seem to be higher yielding and/or superior in terms of quality, etc. The scheme works better if these candidate parent plants can be vegetatively propagated as well.
To test these candidate parent plants for 'general combining ability', they are allowed to be randomly cross pollinated by the other candidate parent plants. If several vegetatively propagated plants are available for every candidate parent plant, plants are arranged so that each plant is surrounded by other candidate parent plants. In this way, a wide range of the other candidate parent plants is used as the source of pollen for each candidate parent plant.
The cross-pollinated seed from each different candidate parent plant is harvested separately (if the candidate parent plants were cloned, the seed from all the members of a clone is combined together).
This seed is then grown in a comparison trial within the environments in which the new crop will be commercially produced.
The comparison trial is used to identify six to ten of the original candidate parent plants which are best able to produce high yielding/good quality offspring when crossed with a random number of other plants.
In other words, the candidate parent plants with the highest 'general combining ability' have been chosen.
The breeder then places these six to ten parent plants in isolation so they can cross with one another to produce the seed of the 'synthetic' variety.
Being able to vegetatively propagate the new crop is a big advantage here because seed production can occur in a nursery which is designed so that each of the six to ten parent plants with high 'general combining ability' is surrounded by all the others.
A commercial advantage in terms of seed production is that seed of a 'synthetic' variety should not be harvested and kept by the farmer too many times. New seed should be used. This is because synthetics, like hybrids, produce offspring seed in the farmer's field which is not exactly the same as the original seed. However, the decline in performance with 'synthetic' varieties is not as rapid as that which occurs with hybrids.
The group of six to ten original parents can be changed and another synthetic produced if some of the parents prove unsuitable or better parents are found, requiring substitution of some parents.
'Synthetic' varieties would appear to offer big advantages for many cross pollinated new crops. It is probable that some new crops currently being produced are not performing as well as they could because they have been established from limited sources of germplasm which are not necessarily well suited to their current environments and purposes.
Any claims made by authors in the Australian New Crops Newsletter are presented by the Editors in good faith. Readers would be wise to critically examine the circumstances associated with any claims to determine the applicability of such claims to their specific set of circumstances. This material can be reproduced, with the provision that the source and the author (or editors, if applicable) are acknowledged and the use is for information or educational purposes. Contact with the original author is probably wise since the material may require updating or amendment if used in other publications. Material sourced from the Australian New Crops Newsletter cannot be used out of context or for commercial purposes not related to its original purpose in the newsletter
Contact: Dr Rob Fletcher, School of Land and Food, The University of Queensland Gatton College, 4345; Telephone: 07 5460 1311 or 07 5460 1301; Facsimile: 07 5460 1112; International facsimile: 61 7 5460 1112; Email:
r.fletcher@mailbox.uq.edu.au[
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GK; latest update 17 October 2001 by: RF