The Partnership no. 18

Science

Science “The use of Double Haploids makes it possible to continuously improve the fixed parental lines that generate the hybrids.” 22 | The Partnership Xana Verweij lines might flower at different times meaning that the female line is ready for pollination while the male has already or still hasn’t flowered. This can result in suboptimal or minimal seed production. The breeder therefore has to make sure the two parent lines are properly synchronised. Therefore, seed production of F1-hybrids is more challenging and time-consuming compared to OP varieties. For some crops, such as lettuce, it is not viable, and may not even be possible, to develop hybrids. Growers of these crops therefore still use true-to-type varieties. Biotechnology in development of hybrids As mentioned above, a breeder of a hybrid is bringing multiple traits together because the genes of two parents are being combined. More genetic variations therefore means more properties to select on and to take into account, making the process more wide-ranging, more complex and therefore more time-consuming. Lindeman: “One parent will have the one trait we want and the other parent will have the other. We have to do this for dozens of traits in a single variety. And as mentioned before, we are not just talking about a single resistance, but several different ones and colour, shape, flavour and abiotic factors such as resilience to salt, drought or wet conditions." How do we tackle tasks on this scale and with this complexity in a quick and convenient way? Unlike breeders in the early 20th century, these days we have techniques at our disposal for things like introgression of traits or producing uniform parent lines more quickly. We can already use marker technology to select for the presence of specific properties in seedlings, which allows breeders to process larger populations without having to grow the plants on to the adult stage. This saves a significant amount of time in the breeding process and makes our work much more targeted and more efficient. Straight from gamete to embryo In many crops, the use of doubled haploids, or DHs, also speeds up the process because the lines made using this technique are immediately completely uniform. In this technique, a pollen cell or egg cell, which only contains one copy of each chromosome, is used to produce a plant with two copies of the same chromosome. Because the copies are identical, any variation in the line has been eliminated and it can be used immediately as a potential parent line of a hybrid variety. “This technique also simplifies the fixing of complex traits where many genes are involved,” explains Manager Research & Applications Xana Verweij. “This makes it possible to continuously improve the fixed parental lines that generate the hybrids. In this technique, we put the gametes under stress, for example under the influence of heat. The cell's survival mechanism kicks in and the cell can therefore suddenly do something completely different from what it is used to doing, such as develop into an embryo and duplicate its chromosome number. To do this, we have to apply the right stress factors at exactly the right time. We are undertaking research to identify which stress factors induce double haploids in various Enza Zaden crops." Reflections The combination of the marker technology and DHs therefore enables us to efficiently combine a large number of traits in a hybrid variety, which ultimately results in a hybrid that performs better and produces higher quality seed. But the quality comes with a higher price tag than a self-propagated, true-to-type strain. In addition, growers are unable to produce hybrid seed themselves and are therefore in a sense dependent on seed companies. Lindeman: "Also, it is often said that replacing local true-to-type varieties by hybrids will be at the expense of genetic diversity. That is true if no additional measures are taken. But seed companies have an interest in this diversity, and public authorities Doubled haploids is a technique in which a pollen cell or egg cell is used to produce a plant with two copies of the same chromosome. This uniform line can then be used as a potential parent line of a hybrid variety. The picture shows an anther (male reproductive organ) encasing pollen cells from which a haploid plant develops. Haploid plant Choice for organic growers are also aware of how important it is. Action is therefore being taken across the globe to collect and preserve this diversity in gene banks." The advantages of hybrid varieties are considerable for growers. They therefore switch to hybrids whenever they have the opportunity and can invest in them. Future and challenges And what of the future? Of course, it remains to be seen how the process of developing hybrid varieties can be made increasingly more efficient and quicker, for example by getting plants to flower earlier, including outside their normal flowering period. Plants would then be generative more often throughout the year. "It would also be useful to get a better understanding of the heterosis effect and to be able to track and predict it with genetic markers,” Lindeman adds. Anther Organic growers across the globe are increasingly switching to modern hybrid varieties, although open-pollinated organic varieties are still grown on a large scale. “We give organic growers a choice,” says Marcel van Diemen, Senior Breeder at Vitalis. "For example, we have noticed that certain true-to-type varieties - such as the red Uchiki Kuri pumpkin - are still very popular with organic growers because of their outstanding resilience, even alongside the hybrid variety Orange Summer. And not just because of the seed price, but also because it is a very reliable variety that grows well under certain difficult conditions. In addition, biodynamic growers prefer true-to-type varieties because they want to be able to make further selections themselves from the varieties they grow. That’s much more difficult with hybrid varieties produced from male sterile plants." The Partnership | 23