II. 2. Isochromosome for the short arm of barley chromosome 6.
G. Künzel, Zentralinstitut fair Genetik and Kulturpflanzenforschung der Akademie der Wissenschaften der DDR, 4325 Gatersleben, German Democratic Republic. "R"
A trisomic barley stock has been discovered which represents what may be called a case of isotertiary compensating trisomy following the terminology proposed by Khush (1973). In such a trisomic, one missing chromosome is compensated for by one isochromosome and one tertiary (translocated) chromosome.
The two-rowed spring barley stock arose from the progeny of a cross between male sterile plants (msg6 backcrossed into variety 'Trumpf') and the translocation line T 52 of variety 'Frigga' with the reciprocal translocation involving chromosomes 5 and 6. The gene msg6 is cytologically located on the short arm of chromosome 6 (Künzel 1982), very closely linked to the centromere, and gave a recombination value of 0.2% (95% confidence interval 0 to 1.6) in relation to the respective break point of T 52 (Künzel and Scholz 1982).
A fraction consisting of small kernels from semisterile (translocation heterozygous) F3 plants of the mentioned cross was cytologically screened for trisomics. Among 310 analysed seed 31 proved to be trisomic. Progeny tests with some of the trisomic plants resulted in a stock segregating in male sterile diploids and fertile trisomics in a ratio resembling the relationships expected for a balanced tertiary trisomic (BTT, Table 1). However, a more thorough analysis of mitotic chromosomes, including seed samples of more than 15 trisomic plants harvested in 1982 and 1983, respectively, revealed a stable trisomic condition for all of the parental plants studied inconsistent with the expectations for the presence of a BTT.
The trisomic progeny of each plant (in total more than one hundred trisomic seedlings analysed) showed a karyotype consisting of eleven non-satellited chromosomes, two normal chromosomes 7, one normal chromosome 6, and one short symmetrical chromosome with each arm bearing a satellite of chromosome 6 looking like an isochromosome for the short arm of chromosome 6. Analysis of the pairing configurations at meiosis I revealed a chain of five chromosomes in the majority of pollen mother cells; one end of the chain was found to be attached to the nucleolus at diakinesis. In about one-third of the meiocytes, pairing configurations involving a lower number of chromosomes and one ring-shaped univalent were observed indicating the presence of an isochromosome.
The findings support the conclusion that the trisomic barley stock lacks one normal chromosome 6 which is replaced by one isochromosome of the satellite bearing short arm of chromosome 6 and one translocated chromosome consisting of the long arm of chromosome 6 fused with one arm or a segment of chromosome 5. The balanced breeding behaviour of the trisomic stock (with all of the diploid segregants male sterile) indicates that at least one msg locus on the isochromosome carries a dominant allele. Therefore, it is supposed that the trisomic stock originated from a BTT having the translocated chromosome with the msg6 locus of T 52 as an extra chromosome. Spontaneous breaks within the centromeres of both a normal chromosome 6 and the translocated extra chromosome followed by a reciprocal fusion of the respective chromosome arms may have produced the balanced isotertiary compensating trisomic constitution.
However, with respect to the precise segment composition of both compensating chromosomes some uncertainty remains since the break positions within the translocated chromosomes 56 and 65 of the parental translocation line T 52 are not exactly known. Both translocated chromosomes can not be distinguished from normal chromosomes 5 and 6 in mitotic analyses. However, intercrosses between different translocation lines involving chromosomes 5 and 6 gave clear-cut indications that the breaks should be in either both the short or both the long arms in T 52. An interchange between the short arms includes the possibility that the short symmetrical chromosome with two satellites of chromosome 6 does not represent in isochromosome in the strict sense but contains one arm with a small centromeric segment of the short arm of chromosome 5, undetectable in mitosis.
The trisomic plants of the present stock show an elongated basal rachis internode of the spike, slightly reduced seed set and tillering capacity but are rather viable under field conditions even in competition with the segregating diploids.
The described stock is easy to maintain and might serve as a good source of secondary trisomics for the short arm of chromosome 6, which are expected to be also balanced as to msg6. A search for such secondary trisomics will be continued.
References:
Khush, G.S. 1973. Cytogenetics of Aneuploids. p. 301, New York and
London: Academic Press.
Künzel, G. 1982. Differences between genetic and physical centromere distances in the case of two genes for male sterility in barley. Theor. Appl. Genet. 64:25-29.
Künzel, G. and F. Scholz. 1982. Untersuchungen zur Koppelung zwischen induzierten Translokationen and Genen fur mannliche Sterilitat bei Gerste. Kulturpflanze 30:155-165.
