Cotton bud-yellow 396a: a single recessive photoperiod-temperature sensitive genic male sterile line

1 Current status and problems of heterosis breeding and hybrid seed production

1.1 Hybrid seed production by manual or chemical emasculation and pollination This method is technically simple; any two varieties can serve as parents for each other, and hybridisation is achieved simply by emasculating at flowering and pollinating by hand. Chemical emasculation can also be used for hybrid seed production. Advantages: existing variety resources are easily exploited fully, the matching of hybrids is highly flexible, combinations can be configured freely, and some combinations can use the F2. Disadvantages: high labour intensity, high labour demand, high cost, and the heterosis of the F2 is often reduced compared with the F1.

1.2 Hybrid seed production with a three-line system based on cytoplasmic male sterility This method uses a matched set of three lines: the male sterile line, the restorer line and the maintainer line. No manual emasculation is needed; the hybridisation procedure is completed simply by transferring pollen from the restorer line onto the male sterile line. It saves labour and time, and gives high seed production efficiency and low cost. Disadvantages: this method relies on cytoplasmic male sterility, so the restorer source must have strong restorer factors introduced before it can be converted into a restorer line, and the restorer line restores the male sterile line only incompletely and with large fluctuations, making it very difficult to find strongly heterotic combinations with value for production.

1.3 Hybrid seed production with a two-line method based on single recessive genic male sterility The male sterile line can be used both as the female parent in the hybrid seed production field and as its own maintainer line for propagation, so one line serves two purposes. Advantages: the range of restorer lines to choose from is broad, all good conventional varieties can serve as restorer sources, and the seed production procedure is simple and requires little labour. Disadvantages: the sterility level of the male sterile line is difficult to bring to 100%, and the proportion of sterile plants can only approach 50%.

1.4 Hybrid seed production with a three-line method based on single recessive genic male sterility The complete maintainer line has the general characteristics of polygenic inheritance, and restorer lines are relatively numerous. Advantages: the maintainer line approaches a complete maintaining effect and is relatively stable; the male sterile line has male sterility inherited through interaction between major genes and polygenes and is characterised by complete sterility. The range of restorer lines is broad, making it easy to identify and match strongly heterotic hybrids. Disadvantages: the male sterile line is relatively difficult to propagate itself, and the maintainer line has a relatively narrow region of application.

1.5 Hybrid seed production based on double recessive genic male sterility This is controlled by two pairs of sterility genes, namely ms5ms5ms6ms6, and seed can be produced by the one-line-two-uses method. Advantages: the seed production procedure is simple, and not only the F1 can be used; in the F2 only 1/16 of the plants are sterile, so the F2 can also be used in field production. Disadvantages: during hybrid seed production, 50% or even more of the fertile plants must be removed, which costs labour and effort, and seed production yields are low and costs are high.

2 The photoperiod-temperature sensitive genic male sterile line bud-yellow 396A

2.1 This male sterile line reaches 100% sterility, has excellent quality and agronomic traits and high combining ability, responds sensitively to light and temperature, and its fertility transition can be manipulated artificially. Under high temperature and short daylength it becomes fertile, sets bolls by natural self-pollination and propagates the male sterile line, so no dedicated maintainer line is needed any more. The seed harvested then reverts to male sterile plants under high temperature and long daylength or under low temperature and short daylength, with a 100% proportion of sterile plants, and is used to produce hybrids.

This male sterile line is genically male sterile, has a broad range of restorer lines, and the male sterile gene can easily be transferred into parental varieties with excellent traits; moreover, any hybridisation or artificial mutagenesis and genetic engineering method can be used to transfer it and create new male sterile lines. This male sterile line has a normal, brightly coloured corolla and nectaries, and can attract @#@255@#@ or other insects for pollination, greatly reducing seed production costs. This male sterile line also has an obvious bud-yellow marker: under normal light, temperature and moisture conditions, fertile and sterile plants can be clearly distinguished at the cotyledon stage 5 to 7 days after emergence, and under abnormal light, temperature and moisture conditions they can be accurately distinguished at the latest by the 3 to 5 true-leaf stage. Even if fertile plants appear in the male sterile line or sterile plants appear in the F1, they can all be identified with the naked eye, and the off-type seedlings can be removed to ensure purity.

2.2 Comparison with other forms of hybrid seed production Compared with conventional manual emasculation or chemical emasculation techniques, in this method 100% of the female parent used for seed production is sterile, no emasculation is needed, and manual pollination can also be replaced by bee pollination, so the seed production procedure is simple and seed production costs are reduced. Compared with hybrid seed production using a three-line system based on cytoplasmic male sterility, propagating the male sterile line requires no dedicated maintainer line and no manual pollination; in particular, the restorer line need not be converted, and any cotton variety can be used directly as its restorer source in test crosses, so new cotton varieties that are high quality and widely adapted, stable and high yielding, early maturing and stress resistant can be bred as early, as quickly and as abundantly as possible. Compared with the two-line method based on single recessive genic male sterility, the proportion of sterile plants is raised from 50% to 100%, no fertile plants need to be removed, and the utilisation rate of male sterile line seed is more than doubled, saving labour and time and lowering costs. Compared with the three-line method based on single recessive genic male sterility, propagating the male sterile line is simpler to operate, the region of application is broad, and the natural environment can be used to complete the fertility transition. Compared with double recessive genic male sterility, its sterility is stable, the sterility level is higher, and it can be converted into many kinds of male sterile lines. Therefore, this method is particularly suitable for large-scale mechanised cotton hybrid seed production.

2.3 Application prospects The photoperiod-temperature sensitive genic male sterile line bud-yellow 396A is a major breakthrough in current research on cotton male sterility. Its seed production procedure is simple, and its seed production efficiency and seed production yield are high. It solves many bottleneck problems existing in other hybrid seed production methods, and will not only end the history of hybrid seed production by manual emasculation and pollination but also improve cotton quality, and is of great significance to the industrialisation of hybrid cotton seed.

China's annual cotton planting area is about 5 million hectares. If hybrid seed production by the photoperiod-temperature sensitive genic male sterility method is used and the F1 is applied in the field, calculated at the lowest increase of 20%, more than 2 million t of lint cotton could be added, increasing income by more than 30 billion yuan, equivalent to the total output of the current 1 million hectares of cotton fields, equal to recreating one quarter of China's existing cotton fields, with enormous economic, social and ecological benefits.