| The frequency of monosomic and monotelosomic or mono-isosomic plants in
the progeny of a plant with an isochromosome or a telocentric and its homologue
(2n=42; 20II+HetII) will be expected to vary for different
chromosomes. This frequency will depend on the extent of asynapsis between
the telocentric or isochromosome and its homologous chromosome during meiosis.
The plants with an isochromosome (2n=42; 20II+HetII=20II+iso+1I)
would be expected to produce a higher proportion of monosomic plants than
those with a telocentric (2n=42; 20II+HetII=20II+telo+1I).
The frequency of pairing with a homologue is higher for a telocentric than
for an isochromosome, because the isochromosome often pairs with itself
and the homologue remains unpaired. Also, the expected relative transmission
rates of a univalent, telocentric and isochromosome through the male gametes
would be as follows: univalent > telocentric > isochromosome. These considerations
and the nonselective female transmission will explain the expected higher
frequency of monosomics and mono-isosomics from plants with an isochromosome
(2n=42; 20II+HetII=+iso+1I) than from plants
with a telocentric (2n=42; +HetII=20II+telo+1'). These monotelosomic or mono-isosomic. stocks (in variety A) can be used as recurrent female parents in the substitution of individual chromosomes from variety B into the genetic background of the variety A. The use of these stocks will save time by avoiding the need for selfing after each backcross and will minimize the risk of univalent shift. After the monosomic, telosomic and isosomic stocks have been developed in the recipient variety, these can be tested against the standard Chinese Spring stocks of telosomics and isosomics. This procedure should prove useful for workers planning to initiate development of a monosomic set in a variety into which they later intend to substitute individual chromosomes from another variety. Literature Cited MAAN, S. S,, and K. A. LUCKEN 1966. Development and use of an aneuploid set of Chinese Spring wheat in T. timopheevi ZHUK. cytoplasm. Can. Jour. Genet. Cytol. 8 (in press). PERSON, C. 1956. Some aspects of monosomic wheat breeding. Can. Jour. Bot. 34: 60-70. SEARS, E. R. 1953. Nullisomic analysis in wheat. Amer. Nat. 87: 245-252. UNRAU, J., PERSON, C., and J. KUSPIRA 1956. Chromosome substitution in hexaploid wheat. Can Jour. Bot. 34: 629-640. (Received Oct. 25, 1966) |
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