II.28 Cytological and breeding behavior of a large fragment of chromosome 6.
R. T. Ramage, Michele Paluska and R. F. Eslick. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721 and Department of Plant and Soil Science, Montana State University, Bozeman, Montana 59715, USA.
In the process of obtaining spontaneous mutants from fields of Betzes barley, we collected an "erectoides" mutant. Selfed seed of the mutant produced approximately 60% normal and 40% erectoides plants. All of the normal plants produced progenies containing only normal plants. The erectoides plants produced progenies containing approximately 60% normal and 40% erectoides plants.
Root-tips of the erectoides plants were examined cytologically and found to contain an extra chromosome. The extra chromosome is a fragment consisting of the long arm plus most of the short arm of chromosome 6, i.e., in the fragment, the satellite, the nucleolar organizing region and a small portion of the short arm are deleted. Figure l shows the pair of normal chromosomes 6 and the pair of fragment chromosomes contained in a tetrasomic plant.
Meiosis of fragment trisomics was observed and at Metaphase I, 336 (16%) cells had a straight chain of 3, 1268 (60%) had a U-shaped trivalent and 504 (24%) of the cells had a bivalent plus univalent. Root-tips of the progeny of fragment trisomics were examined cytologically and 58 diploids, 38 fragment trisomics, 2 primary trisomics for 6, 4 tetrasomics (2 normal + 2 fragment 6's) and 1 plant with 21 normal chromosomes plus 3 fragments were observed. The fragment trisomics were almost normal in fertility and vigor and differed from normal Betzes mainly in having shorter rachis internodes giving the spike an "erectoides" appearance. The tetrasomics were very weak and highly sterile.
It is of interest to try to correlate meiotic behavior of the fragment trisomics with their kinds of selfed progeny. If it is assumed that: (1) U-shaped trivalents produce equal numbers of gametophytes with a normal haploid complement and with an extra fragment 6; (2) straight chain trivalents produce equal numbers of gametophytes with a normal haploid complement, with an extra fragment 6, with an extra normal 6 and with the fragment 6 alone (the straight chain trivalents are expected to separate at random at Anaphase I resulting in products containing either a normal 6 on one side plus a normal and a fragment 6 on the other or two normal 6's on one side plus a fragment 6 on the other); (3) bivalent plus univalent configurations produce only gametophytes with a normal haploid complement (the univalent is expected to be fragment and be lost during subsequent stages of meiosis; and (4) the products of meiosis in megasporocytes are the same as of microsporocytes; then, eggs will be expected in a ratio of 58 with a normal haploid complement : 34 with an extra fragment 6 : 4 with an extra normal 6 : 4 with a fragment 6 alone.
In the male, pollen grains with two normal 6's and with the fragment 6 alone probably will effect fertilization in a very low frequency, if at all, due to slower development of gametophytes containing an extra normal chromosome and to lethality due to the deficiency of the nucleolar organizing region. Assuming that these two types of pollen never effect fertilization, the frequencies of sperm containing a normal haploid complement and containing an extra fragment 6 that would combine with the calculated kinds of eggs to produce the observed progeny can be calculated. The ratio of the two types of pollen most likely to have produced the 102 plants is 89 with a normal haploid complement : 11 carrying an extra fragment of 6, i.e., about 11% pollen transmission of the fragment.
The assumed kinds of eggs and sperm would be expected to produce a population of 55 diploids, 39 fragment trisomics, 4 primary trisomics and 4 tetrasomics. The actual population observed was 58, 38, 2 and 4. Also, about 4 percent gametophyte abortion would be expected if gametophytes containing the fragment 6 alone abort. The observed percentage of abortion in the female of several plants ranged from 5 to 7. Pollen abortion was more difficult to evaluate as some pollen grains were intermediate in size and in stainability. Presumably, these were carrying an extra chromosome. Pollen abortion was estimated to be around 10 percent.
We found these data to be most interesting because of the closeness of fit of the predicted population to what is expected based on commonly assumed meiotic and gametophytic behavior. Also, apparently gametophytes that are deficient for the nucleolar organizing region (and the satellite) fail to develop in either male or female.
