IV.11. Desynaptic genes.
J. M. Hernandez-Soriano and R. T. Ramage. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.
Since our report of last year (BGN 3:91), 8 mutants have been assigned to 6 new loci and 8 new mutants have been collected. The present status of the desynaptic mutant collection is shown in Table 1. Twenty-three mutants have been assigned to 14 loci. The mutants des,,x and des,,y are not allelic to each other and probably occupy two new loci; the 3 crosses needed for confirmation are being made. The 8 new mutants are of spontaneous origin in the cultivar 'Klages', C.I. 15487 and represent 8 partially sterile plants collected in a seed increase field. They were assigned the symbols des,,z, des,,aa, des,,ab, des,,ac, des,,ad, des,,ae, des,,af and des,,ag. All 8 mutants were found to be allelic and may represent only one mutational event as they were all collected from the same field. Allele tests indicate that the Klages mutants are allelic to des4. Crosses to confirm allelism of the Klages mutants and des4 are being made.
Table 1. Desynaptic mutants in barley.
As the desynaptic collection grows, problems with allele tests are being encountered. Many of the F1's involving different loci show a low degree of desynapsis instead of a normal phenotype. The degree of desynapsis in the F1 is lower than in either parent, but, the F1 between two highly desynaptic mutants is sometimes more desynaptic than some of the other mutants in the collection. For example, the F1 of des7j (d = 6.1) and des11r (d = 5.7) shows an average degree of desynapsis of 1.8 ranging from 7 ring bivalents (d = O) to 3 ring bivalents + 3 rod bivalents + 2 univalents (d = 5). This degree of desynapsis is greater than that exhibited by des3c (d = 0.9) or des9n (d = 0.9) and almost as much as by des1O (d = 2.0).
It has been surprising to find 14 loci among the first 23 mutants examined (and probably 16 among the first 25) because of the method by which most of the mutants have been collected. All but 3 of the mutants are of spontaneous origin and were collected by selecting partially sterile plants from either commercial or seed increase fields. Usually, mutants occupying several loci were collected from the same field. Based on our collecting experience, the collection of the 8 desynaptic plants of Klages and finding only one locus was unusual. Desynaptic genes are quite common in the Plant Kingdom; Katayama (1964) reported that desynapsis (or asynapsis) has been found in 20 families, 50 genera and 70 species of plants. Based on our work with barley, many loci must be involved in maintaining normal meiotic pairing conditions.
In general, all of the desynaptic mutants in the present collection behave in a similar manner. The chromosomes are paired in pachytene and undergo desynapsis in diplotene or early diakinesis. The mutants differ in the amount of unpairing shown at Metaphase I. The amount of unpairing is expressed as "degree of desynapsis" which is calculated by the formula d = r + i where r is the number of rod bivalents and i is the number of univalents in a Metaphase I cell. The degree of desynapsis is an expression of the average number of unpaired bivalent arms per Metaphase I cell (Hernandez-Soriano, 1973).
Of the mutants in the present collection, only des13t expresses deviant behavior. There are two types of meiotic cells in this mutant. About 58% of the microspore mother cells have normal early prophases and exhibit a low degree of desynapsis (d - 0.8) at Metaphase I. The other 42% of the microspore mother cells have chromosomes that appear "sticky" at Metaphase I. The stickiness is accompanied by very elongated ring bivalents and by extensive chromosome fragmentation. We decided to include des13t in the desynaptic collection because of the desynaptic behavior of some of the meiotic cells and because of the possibility of desynapsis and chromosome stickiness being related. In Collinsia, Mehra and Rai (1972) reported that a single recessive gene for desynapsis is allelic to another single recessive gene that conditions chromosome stickiness.
Limited amounts of seed of the desynaptic mutants are available upon request from R. T. Ramage, Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.
References:
1. Hernandez-Soriano, J. M., 1973. M. S. Thesis. University of Arizona, Tucson, Arizona.
2. Katayama, T., 1964. Further review on the heritable asynapsis in plants. La Kromosomo 57-59: 1934-1942.
3. Mehra, R. C. and K. S. Rai, 1972. Cytogenetic studies of meiotic abnormalities in collinsia tinctoria. II. Desynapsis. Can. J. Genet. Cytol. 14: 637-644.
