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Yield trial

Hybrid variety

Fig. 6.3 A simplified scheme of breeding hybrid varieties needs a system for targeted pollen transfer. Usually CMS is used; in rare cases cross-pollination relies on self incompatibility. The main reason for breeding hybrid varieties is heterosis, which is the superior performance of hybrids as compared to their inbred parents. Substantial heterosis can be found in many open-pollinated crops. Heterosis in self-pollinated crops is much lower or non-existing.

Hybrid breeding is substantially different from line breeding because selection does not rely on the per se performance of a line but on the performance of its hybrid. In principle, hybrid breeding can be divided into three steps (Fig. 6.3). First inbred lines are developed by successive selfing. In a classic breeding scheme inbreds are developed from different gene pools which had been separated for generations, e.g. by geographical separation. The per se performance of these lines is tested in the field. A number of lines are selected for crossing with a non-related tester line or population (topcross) and the hybrids are grown in the field. Then the best inbred lines from each pool are selected and crossed with each other in a diallel crossing scheme (single cross). The best hybrid combinations are selected and their performance is tested by replicated field trials under different environments. At the end a new hybrid variety is available which can be either the product of a single cross or produced by three- or four-way crosses with three or four parents. When CMS is used for cross-pollination and reproductive organs are harvested, the pollinator must have a gene which restores pollen fertility in the hybrid (see also Chap. 14).

Transgenic lines with superior per se performance can be used as parents for topcross tests. Since the transgenic character is inherited in a dominant way, it is sufficient when one parental component carries the transgene. In that case the transgenic hybrid variety is heterozygous for the transgene. In the case of additive gene action both parents must carry the transgene, however in different genetic backgrounds. Thus it follows that the transgenic character must be introduced into both gene pools by successive backcross breeding (see Sect. 6.3).

X

gametes

IH

RH

R IHH

A

seed parent

I_H_

B pollinator ms male sterile

_ null allele

R male sterility gene

I pollen fertility restorer-Gen

H herbicide resistance gene

ABR_I_HH

B pollinator ms male sterile

_ null allele

R male sterility gene

I pollen fertility restorer-Gen

H herbicide resistance gene

ABR_I_HH

transformation self

BIIHH

F1 hybrids: 100% male fertile, 100% herbicide-resistant

Fig. 6.4 Breeding hybrid varieties with a transgenic male sterility system and pollen restoration (Reynaerts et al. 1993)

The availability of a new male sterility system by genetic transformation offers an interesting alternative for hybrid breeding, even when natural male sterility is lacking (see Chap. 14). This kind of male sterility is called nuclear male sterility (NMS) because the male sterility gene is located in the nuclear genome. Nuclear male sterility is often found in natural populations. However, its practical use is very limited because male sterile plants cannot be maintained as pure lines. After cross-pollination their offspring segregates for male sterile and male fertile plants, which creates a need for laborious phenotypic selection of male sterile plants. By genetic modification genes have been introduced which have a deleterious effect on pollen development. When these genes are transformed together with a selectable marker which allows easy selection of male sterile plants in the offspring male fertile plants can be easily eliminated, e.g. by spraying with herbicides (Fig. 6.4). Pollen restoration can be achieved by a second gene which inhibits the male sterility system within the cell. This system has proved successful in different crops. Plants with a transgenic NMS have been cultivated on a large scale and stable pollen restoration has been demonstrated. This strategy can be applied for all plants. It offers a possibility for hybrid breeding even for crops where CMS systems are not available or their use is limited by negative pleiotropic effects or poor pollen restoration.

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