| Competition between diploid, tetraploid and aneuploid
rye in "tetra-rye" populations in practical farming A. HAGBERG and S. ELLERSTROM Swedish Seed Association, Svalof, Sweden A number of problems arise when a new autotetraploid population is released for use in practical farming. Three of them are dealt with in a recent paper on tetraploid rye by Hagberg and Ellerstrom (in press), e.g. a) the frequency of aneuploids and their effect on the yield of the population, b) the effect of vicinism between diploid and tetraploid populations grown close to each other, c) the effect of diploid admixtures in tetraploid populations. From nine lots of the same variety of tetraploid rye grown in different parts of southern and central Sweden, samples were taken to check the frequency of diploids, triploids and aneuploids. By sieving, the samples were divided into four grain size fractions. No diploids or triploids were found. Thus, their frequency in the populations must be less than 0.1%. Hypotetraploids were found mainly among the small seeds while hypertetraploids were more evenly distributed among the fractions as can be seen from Table 1. It is possible, by sieving and rejecting the small seeded fraction, to decrease the frequency of hypotetraploids to some extent. The nine lots investigated had an average frequency of aneuploids of 14.7% while the same material 10 generations earlier had 22.7%. However, there was a considerable variation between the samples from the nine lots, the extreme values being 6.8 and 20.4%. When 4x and 2x rye are crossed, triploid embryos are formed but most of them (about 97%) abort. Thus, there is an effective sterility barrier between the two levels of ploidy. By growing 2x and 4x rye close to each other there is a risk of sterility arising and causing decreased yield in both crops. Table 2 summarizes the data obtained from experiments during the years 1952-1955, growing plots of 4x rye at different distances from a large field of 2x rye. As is evident from these results, 100 metre is a "safe" distance, since there is no decrease in yield. The fertility might be slightly decreased but it is compensated by a larger seed size. In practical farming it will be impossible to avoid admixtures of 2x into 4x rye populations. In the "seed certification regulations" there has to be 4 maximum limit for the frequency of diploid admixtures that can be tolerated by a tetraploid population. To be able to give the information required for these regulations, large plots of mixtures between 2x and 4x rye have been investigated during the years 1952-1956. The main results are summarized in Table 3. It is evident from these data that the tetraploid rye is self cleaning even if the population contains a rather high frequency of diploids. There is an equilibrium at about 40%. Above this frequency the diploids will dominate and tetraploids will disappear, whereas below this "threshold value" the diploids will disappear gradually and the tetraploid population is self cleaning. The "threshold value" is somewhat influenced by environmental factors. This "threshold value" varies widely in different crops. Red clover, for instance, has a "threshold value" of about 3% and it is very difficult to keep the admixtures of diploids in a tetraploid population below this value. Due to the high "threshold value" in rye this problem is not severe in this crop. As soon as a tetraploid population of rye is established by the plant breeder, there is practically no risk that this population reverts to diploidy if the population is large enough. |