II. 25. A novel compensating partial tetrasomic diploid plant of barley.

R. J. Singh and T. Tsuchiya, Department of Agronomy, Colorado State University, Fort Collins, Colorado 80523, U.S.A.

One green plant with chromosome number 13 + 1 acro 3L^S + telo 3S was found in the F2 population of 14 + 1 acro 3L^S x yst2 (yellow streak 2). The Giemsa banding technique revealed that one normal chromosome 3 was substituted by an acrocentric chromosome 3L^S and a telo 3S. Genetically this plant was diploid except a small segment of the short arm was in trisomic condition. Similar type of plant was obtained by Tsuchiya (1972) for chromosome 1 where telo 1L + telo 1S compensated for a complete chromosome 1 (2n = 13 + 1 telo1L + 1 telo1S).

The origin of a plant with 2n = 13 + 1 acro 3L^S + 1 telo 3S in the progenies of 14 + 1 acro 3L^S has been assumed as follows: The misdivision of a normal chromosome 3 may have occurred at anaphase I in one of the florets of a F1 plant and by chance seven chromosomes moved to one pole and 6 + 1 acro 3L^S + 1 telo 3S (product of misdivision) moved to the other pole. Thus the gametes of this floret constituted 7 chromosomes and 6 + 1 acro 3L^S + l telo 3S. Since gametes with 6 + 1 acro 3L^S + 1 telo 3S is genetically almost balanced and survived, it gave a seed with 13 + 1 acro 3L^S + 1 telo 3S chromosome after fertilization with a 7 chromosome gamete.

It is of interest to know which chromosome misdivided. The acrotrisomic analysis showed that the gene for yellow streak 2 (yst2) is located on the short arm. The genotypic constitution of acrotrisomic F1 plant was Yst2 Yst2 yst2. The chromosome carrying recessive gene yst2 came from the male parent and a normal and an acrocentric chromosome with a dominant allele Yst2 came from the female parent. Suppose misdivision occurred in a normal chromosome which carried recessive allele yst2, then telocentric chromosome 3S will carry yst2 and normal and acrocentric chromosomes will have dominant allele Yst2. The selfing of this plant will give all green plants. On the other hand, if misdivision occurred in a chromosome with dominant allele Yst2, all the diploid plants are expected to be recessive homozygotes (yst2, yst2). However, all of 172 plants analyzed from the selfing of 13 + 1 acro 3L^S + 1 telo 3S plant were green. This indicates that misdivision occurred in the chromosome which came from the male parent.

It is expected to obtain plants with various chromosome constitution in the selfed progenies of plants with 2n = 13 + 1 acro 3L^S + 1 telo 3S. Chromosome number of 172 plants were analyzed in somatic metaphase. The frequencies of plants with chromsome number 2x (46): 13 + 1 acro 3L^S + 1 telo 3S (80): 12 + 2 acro 3L^S + 2 telo 3S (39) fit the expected ratio of 1:2:1 (x² = 0.74). With the occurrence of four Triplo 3S plants, telotrisomics for both arm of chromosome 3 were established now. One plant each was found for 2n = 14 + 1 acro 3L^S, 13 + 1 acro 3L^S + 2 telo 3S, and 13 + 2 acro 3L^S +l telo 3S. Thirty-three plants were studied in the progeny of cross between 13 + 1 acro 3L^S + 1 telo 3S and 2x. Eighteen plants were 2x and 15 were 13 + 1 acro 3L^S + 1 telo 3S, which fits the expected 1:1 ratio (x² = 0.28).

Morphologically no visible difference was recorded among plants with 2x, 13 + 1 acro 3L^S + 1 telo 3S, 12 + 2 acro 3L^S + 2 telo 3S chromosomes, although plants with 2n = 12 + 2 acro 3L^S + 2 telo 3S is a tetrasomic for a short segment of the telo 3S.

Meiosis was studied in the original 13 + 1 acro 3L^S+ 1 telo 3S plant. General trend of chromosome association was .3S-3S.3L-3L^S . This pairing behavior was readily recognized at metaphase I due to the differences in the length of telo 3S and acro 3L^S. Two hundred sporocytes were analyzed at metaphase I and 79.5% sporocytes showed 1 III + 6 II chromosome configurations while remaining (20.5%) cells had 6 II + 1 II het + 1 I configuration. In trivalent configuration, acro 3L^S was more tightly associated with the long arm of chromosome 3 by forming an interstitial chiasma (54.5%) than telo 3S where its chiasma was terminalized already. In 10% of the sporocytes, acro 3L^S and telo 3S were associated with their respective arms by an interstial chiasma. In 6 II + 1 II het + 1 I configuration, only telo 3S was the univalent.

Chromosome association at metaphase I in plants with chromosome number 12 + 2 acro 3L^S + 2 telo 3S was mostly 8 II (76.9%), with some sporocytes showing 7 II + 2 I. Generally normal 12 chromosomes formed 6 closed bivalents while two acro 3L^S and two telo 3S formed open bivalents, respectively, which were easily identified. Chromsome separation at anaphase I was mostly (90%) 8-8. With this meiotic behavior it is expected that plants with 2n = 12 + 2 acro 3L^S + 2 telo 3S will breed true.

Reference:

Tsuchiya, T. 1973. A barley strain with 2n = 12 + 4 telocentric chromosomes (2 pairs). Cereal Res. Comm. 1(3):23-24.

BGN 11 toc
BGN Main Index