II.14 Heterosis for grain size in six-row x two-row F1 hybrids.
P.T. Gymer, Rothwell Plant Breeders Limited, Rothwell, Lincoln, United Kingdom.
Our attempts at Rothwell to develop F1 hybrid barley began by using the BTT known as 18-17, which was the female parent of the first hybrid launched in Arizona (Thompson, 1970). Although 18-17 is not adapted to Northern Europe, hybrids with European two-rows as male parents gave encouraging results. In particular, these hybrids have consistently shown considerable heterosis for grain size, as compared with their two-row parents in trials throughout Europe. Average improvement of a large number of such hybrids in trials at Rothwell was 24%, 19%, 29% and 16% in 1972, 1973, 1974, and 1975 respectively. No such heterosis was observed when six-row European varieties were used as the male parents. 18-17 itself is a pedicelled-lateral six-row.
In 1975 we grew a small space-planted trial under glass, using a sessile-lateral six-row variety, a two-row and a deficiens variety, all high yielding lines selected from the extensive breeding and trials programme at Rothwell.
The trial plots were 25 x 25 cms, containing 25 plants each. Owing to limited seed availability, replicate numbers of the various entries were not identical, but the replicates were randomised throughout the trial. Accordingly, statistical analysis was made by means of t-tests, not analysis of variance.
The trial contained the three inbred parents, plus three Fl's, six-row x deficiens, six-row x two-row and the reciprocal two-row x six row. Since the F1 seed was produced by hand-crossing, it was smaller than the parental seed. For this reason, a small quantity of small seed of two of the parents was produced by clipping the florets just after anthesis.
Each plot was harvested complete, and the number of ears, thousand grain weight and total yield were measured; from these data the mean grain numbers per ear and per plot were calculated.
It was evident that these experimental conditions were favourable to the growth of every seedling; the numbers of ears produced by de-awned and normal parent seed were almost identical
6-row: normal seed - 74.8 ears per plot; de-awned seed - 74.25 ears per plot.
Deficiens: normal seed - 142.25 ears per plot; de-awned seed - 138.0 ears per plot.
The reciprocal hybrids six-row x two-row and two-row x six-row were likewise very similar, and the data have been combined, as have the data from normal seed and de-awned seed of the parents.
From Table 1, it can be seen that the two F1 hybrids behaved very similarly; their tillering was intermediate between the parents, their grain number per ear slightly more than their two-row or deficiens parent (significantly so for the 6-row x 2-row hybrid), and their thousand grain weight was enormously above that of either parent. The grain yield per plot of the best F1 was 20% more than of the best inbred, and significantly more than the yield of its deficiens parent. Heterosis for grain size was thus more than sufficient to compensate for the lesser tillering relative to the two-row and deficiens parents.
Table 1 Yield components of hybrids and parents in glasshouse microtrial
This heterosis seems to be a universal phenomenon in six-row x two-row Fl's and has been reported by several workers (e.g. Hagberg, 1953; Carleton and Foote, 1968; Stolen, 1974; Tseng and Poehlman, 1974). The middle two of these publication attribute the large grain size to compensation for the reduced total grain number. However, careful examination of their own data, as well as that obtained at Rothwell, reveals (i) that in some trials compensation cannot be detected at all, the hybrids having grain numbers equal to those of their parents, (ii) that compensation, when it does occur, is linear within the ranges studied - if it were not, extrapolation to very low grain numbers would give a ridiculously high grain size, and (iii) that compensation generally accounts for an increase in grain size over the two-row parent of between 5 and 8%. In the glasshouse trial reported here, its contribution was calculated to be about 7%, leaving between 25 and 30% heterosis unaccounted for.
Since six-rows and two-rows differ chiefly at the Vv and Ii loci on chromosomes 2 and 4, genes at or near these might be responsible for the heterosis for grain size. However, the Ii locus can be ruled out, as the heterosis occurs whether the six-row parent has sessile or pedicelled lateral florets (i.e. II versus ii). The effect of the Vv locus and adjacent segment should be detectable from the grain sizes of F2 plants. From preliminary studies on two 18-17 x 2-row hybrids at Rothwell, it appears that most of the heterosis is still present in homozygous VV F2 plants, between 1 and 2% being attributable to heterozygosity at the Vv locus, this being perhaps the effect of the larger lateral florets.
We conclude that the heterosis for grain size is due to the different genetic backgrounds of six-row and two-row barley, chiefly involving genes not closely linked with the Vv or Ii loci. Riggs and Hayter (1975), using sophisticated statistical analysis considered such heterosis to be due to non-allelic interaction.
Such heterosis ought to be fixable.
It is hoped to investigate this subject further in 1976 by studying F2's of the hybrids from the 1975 glasshouse trial.
References:
Carleton, A.E. and W.H. Foote, 1968. Heterosis for grain yield and leaf area and their components in two- x six-rowed barley crosses. Crop Sci. 8: 554-557.
Hagberg, A., 1953. Heterosis in barley. Hereditas 39: 325-348.
Riggs, T.J. and A.M. Hayter, 1975. A study of the inheritance and interrelationships of some agronomically important characters in spring barley. Theor. and Appl. Genetics 46: 257-264.
Stølen, O., 1974. Problems related to the development of hybrid barley. Ph.D. thesis, University of Wisconsin.
Thompson, R.J., 1970. Barley as a cross-pollinated crop. Barley Genetics II: 319-322. Proc. 2nd Int. Barley Genetics Symposium, Pullman, Washington, 1969. Washington State University Press, Pullman.
Tseng, S.-T., and J.M, Poehlman, 1974, Hybrid performance among six-rowed x two-rowed winter barleys (Hordeum vulgare L. and Hordeum distichum L.) Theor. and Appl. Genetics 44: 294-303.
