II.31 A tertiary trisomic balanced for both msg2 and sex2.
J. F. Scheuring and R. T. Ramage, Department of Plant Sciences. University of Arizona, Tucson, Arizona 85721, U.S.A. "R"
The balanced tertiary trisomic line 63-j-18-17 was used to produce the female parent of the first commercial hybrid barley. It is a six-row line with an extra T27d chromosome that is balanced for the male sterile allele msg2 located on chromosome 2. While increasing seed of 63-j-18-17, a head-row was found that contained only trisomic plants instead of the expected about 70 percent male sterile diploid and 30 percent trisomic plants. Selfed seed from plants in the row produced only trisomic plants. A study was initiated to determine the breeding behavior of this "true-breeding" trisomic line.
Trisomic plants in the "true-breeding" line were emasculated and pollinated by a two-row BTT 27d msg2 and by the cultivar 'Betzes'. Also, a trisomic plant from the "true-breeding" line was used to pollinate a six-row, male sterile diploid. The crossed seed and selfed seed of the "true-breeding" trisomic parents were grown.
The selfed seed of the "true-breeding" trisomic produced only trisomic plants. The number of flowers and the number of selfed seed produced by 10 of the trisomic plants were counted. A total of 2699 flowers set 909 seed (34 percent seed set).
Crossed seed from the two-row BTT 27d msg2 parent produced 27 male sterile diploid and 10 trisomic plants. Seed set on the trisomic plants was determined to be 72 percent. The male sterile diploid F1 plants were pollinated by a six-row, male fertile diploid and essentially 100 percent seed set was obtained.
Crossed seed from the 'Betzes' parent produced 29 male fertile diploid and 7 trisomic plants. Seed set was determined to be 64 percent on the trisomic and 71 percent on the diploid plants.
Crossed seed from the six-row, male sterile diploid female that was pollinated by the "true-breeding" trisomic produced only male sterile diploid plants. These male sterile F1 plants were pollinated by a six-row, male fertile diploid and essentially 100 percent seed set was obtained.
The number and kinds of plants obtained from the crosses could be explained by assuming that the "true-breeding" trisomic was also balanced for a zygotic lethal that prevented seed development after pollination.
When the trisomic was selfed, all diploid seed would be homozygous for the recessive male sterile and zygotic lethal alleles and all trisomic seed would carry the corresponding dominant alleles. The 66 percent empty flowers and 34 percent seed set found on trisomic plants approximates the percentages of diploids and trisomic plants expected from selfing a BTT. F1 trisomic plants from crosses with either the two-row BTT or 'Betzes' would be heterozygous for the zygotic lethal and would be expected to produce around 18 percent zygotic lethals. Trisomic plant from the two crosses average 68 percent seed set (30 percent empty flowers). The difference between the observed 32 percent empty flowers and the expected 18 percent due to zygotic lethality is probably a result of sterility caused by the trisomic condition. Male sterile diploid F1's from crosses using the "true-breeding" trisomic as either male or female would be heterozygous for the zygotic lethal. These plants should have, and did have, essentially 100 percent seed set when pollinated by normal plants. The male fertile diploid F1 plants from crosses of the "true-breeding" trisomic as female with a 'Betzes' male should be heterozygous for the zygotic lethal. These plants exhibited about 3 normal seed: 1 empty flower on their spikes.
The nature of the zygotic lethality was studied on diploid plants produced by the "true-breeding" trisomic female x two-row BTT 27d msg2 male crosses. These plants were male sterile and heterozygous for zygotic lethality. They were pollinated by the "true-breeding trisomic. From this backcross, it was expected that one-half of the crossed seed would be zygotic lethals. All seed grew in an apparently normal manner for the first 15 days after pollination. Then, about one-half of the seed stopped growing and began to shrivel. By 6 weeks after pollination the shriveled seed were so reduced in size that they were barely visible within the lemma and palea. The character was designated a shrunken endosperm mutant that expresses xenia. It was assigned the symbol sex.
As the "true-breeding" trisomic is balanced for both the msg2 and the sex alleles and produces only "true-breeding" trisomic progeny, the dominant Sex allele must be located very close to the break-point of the T27d chromosome. If the sex locus was carried on chromosome 2, it would be very tightly linked with the msg2 locus and less tightly linked with the two-row locus, V. If it was located on chromosome 7, it would segregate independently of the msg2 and V loci Linkage information from two sources was obtained.
Diploid male sterile plants from the "true-breeding" trisomic x the two-row BTT 27d msg2 cross should have been heterozygous at both the Sex and V loci. These plants were pollinated by a normal six-row line and the crossed progeny grown and classified. The crossed progeny consisted of 38 Vv SexSex, 36 Vv Sexsex, 20 vv SexSex and 31 vv Sexsex plants. The X2 value for independence calculated from these numbers is 0.120 indicating that the V locus and the sex locus are inherited independently.
Diploid male fertile plants from the "True-breeding" trisomic x 'Betzes' cross should have been heterozygous at the V locus, the msg2 locus and the sex locus. The F2 was grown and classified. It consisted of 5 Msg2_VV SexSex : 19 Msg2_VV Sexsex : 15 Msg2_Vv SexSex : 25 Msg2_Vv Sexsex : 2 Msg2_vv Sex Sex : 5 Msg2_vvSexses : 1 msg2msg2VV_ _ : 9 msg2msg2Vv_ _ : 18 msg2msg2 vv_ _ plants. Based on these numbers, the X2 value for independence between Msg2 and V is 33.202 and between V and sex is 1.941. This indicates that msg2 and V are linked and that V is inherited independently of Sex. The recombination value for the msg2 and V region was calculated to be .19 + .043, close to the .20 to .25 values usually reported for this region.
As the sex locus is very tightly linked with the break-point of chromosome T27d (and thus with the msg2 locus) and is inherited independently of the V locus, it must be carried on chromosome 7. The only other reported shrunken endosperm mutant that expresses xenia, se6, was assigned to chromosome 6 (Jarvi, A.J., 1970. Ph.D. Thesis. Montana State University). The symbol se6 has been changed to sexl (Eslick, R,F. and E.A. Hockett, BGN 6:115, 140). As the sex mutant in the "true breeding" trisomic is carried on chromosome 7, it should be non-allelic to sexl which is carried on chromosome 6. Therefore it has been assigned to symbol sex2.