| Transfer of v1 gene from common wheat
to emmer wheat K. TSUNEWAKI Laboratory of Genetics, Kyoto University, Kyoto, Japan The polyploid series of wheat offers an excellent opportunity to investigate the transfer of major genes between species which differ in chromosome numbers. The effects of allopolyploidy on the phenotypic expression of those genes can be studied by such transfer, which provides a new means to elucidate the genetic role of polyploidy in wheat evolution. A virescent seedling found by NEATBY (1933) in F6 of a cross, Garnet x Double Cross, was revealed to be under control of a simple, recessive mendelian factor, that was later designated by v1 and located on chromosome 3B ( SEARS 1954 ). In 1959 the author obtained from Dr. E. R. SEARS a virescent strain of a common wheat variety, Chinese Spring, that possesses v1 gene. An emmer wheat, T. carthlicum var. stramineum was crossed to this v1-carrying Chinese Spring, and the F1 hybrids were successively backcrossed to the emmer parent. In the first backcross generation plants with 14 pairs of chromosomes were cytologically selected for further backcrosses. Germination and segregation of virescent plants in the selfed progenies of seven B1 and a single B2 plant are shown in Table 1. Two facts are evident from this table. In the selfed progenies of V1v1 heterozygotes virescent plants were much fewer than expected from the 3 : 1 ratio of greens vs. virescents. At the same time, their germination rate was significantly lower than that of their normal sib lines. This can not be attributed to a maternal effect, since the germination rate of backcrossed seeds, which were set on V1v1 and V1V1 plants, were almost the same, i.e., 91 and 93 per cent, respectively. Nor, can it be due to a genetic disharmony caused by combining genes of emmer and common wheats, because seeds of normal sib lines germinated well, in spite of having almost the same gene combinations. If the germination rate of a normal seed population were 91 per cent ( =ger mination rate of V1V1 ) and its decrease in the selfed progenies of V1v1 to 75 per cent were attributable only to seed lethality of v1v1 the seed population raised by self-pollination of the heterozygote should have consisted of 78 per cent greens and 22 per cent virescents; these figures fit well the 3 : 1 ratio of greens vs. virescents. In fact, occasionally obtained virescent segregants of emmer wheat were very weak. From the results of this investigation, it is concluded that v1v1 homozygotes of emmer wheat are mostly seed lethal. Apparently, the gene v1 upsets the physiology of embryos more drastically on the tetraploid than on the hexaploid level. SEARS (1954) reported that plants of common wheat monosomic for chromosome 3D and heterozygous for v1 bore 15 to 20 per cent shriveled seeds, and all their virescent offspring were disomic. From this he assumed seed lethality for mono-3D v1v1. Since emmer wheat homozygous for v1 seems to be equifunctional to nulli-3D v1v1, though the latter has the remaining six chromosome pairs of D genome, SEARS' result confirms to a certain extent the present finding that v1v1 emmer is seed lethal. A large number of genes become duplicated as a consequence of polyploidy and one of the duplicates may mutate without impairing any preexisting characters. Common wheat, in fact, has many duplicated genes which are homoeologous in nature, including the present V triplicates. The results of the present investigation demonstrate that a hexaploid is more tolerant of a deleterious gene than a tetraploid, and consequently has a greater potentiality for maintaining an apparently deleterious gene. |