BGN 10: The uses of trisomics in identifying inverted chromosomes (paracentric) BARLEY GENETICS NEWSLETTER, VOL. 10, III. GENETIC AND CYTOLOGICAL TECHNIQUES
Tsuchiya, pp. 83-86

III. 2. The uses of trisomics in identifying inverted chromosomes (paracentric).

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

METHOD 1. Analysis of trisomic F1 hybrids between trisomics and inversion homozygotes

A. Cross trisomics with inversion homozygotes
B. Analyze meiosis of trisomic F1 hybrids

Trisomic (AAA)    x    Inversion homo. (aa)
         F1             AAa

Here A represents the normal chromosome, and a the inverted chromosome.

In a few cells, at anaphase I, especially at early anaphase I, the trivalent or relic (trace) of trivalent could be recognized. In the critical lines shown above bridge/fragment figures shown in Figure 1a will be found in some sporocytes at meiotic anaphase I. For details on the configurations to be found at AI - TI and/or AII - TII readers refer to McClintock (1938).

Figure 1. Chromosome configurations at meiotic anaphase I in the F1 hybrids between primary trisomics and paracentric inversion homozygote. a, critical combination showing trivalents with bridge/fragment. b, noncritical combination showing trivalent without bridge/fragment.

In noncritical lines the bridge/fragment in the trivalent (Figure lb) will never be found. Of course, fairly large numbers of sporocytes should be studied: the number of sporocytes to be analyzed depends upon the frequency of crossing over. It is not quite sure, but the crossover values in some regions might be higher in trisomic than in diploid because of the trisome condition (see for detail, Swanson's Cytology and Cytogenetics, p. 264-267). It is desirable to study meiosis of parent trisomics in order to make sure that there is no inversion in the parents.

METHOD II. Analysis of back cross hybrids (BC1)

Trisomic portion could be used for the analysis of meiosis in the same way as in Method I.

As shown above, the segregation ratio for presence and absence of fragment, bridge/fragment or bridge at AI-TI/AII-TII would be 2:1 in diploid portion. The diploid portion would be much easier to handle (analyze), because of easiness of collecting materials and of analyzing the figures. In the diploid portion just analyze the presenc or absence of fragment, bridge/fragment, or bridge at AI-TI/AII - TII and compare the ratio in all seven trisomic types.

In the noncritical combinations the ratio for presence and absence of fragment/bridge figures would be 1:1, while in the critical line the ratio for presence and absence = 2 (Aa) : 1 (aa) as shown before.

As a whole, the analysis of diploid portion in back cross F1 (BC1) would be the simplest way of analysis and should be most accurate. If some 75 to 100 diploid plants from one cross combination are analyzed it would be sufficient enough to find the difference between 1 Aa : 1 aa and 2 Aa : 1 aa (Hanson, 1959).

In case of anather back cross, AAa x AA, the ratio should be different from AAa x aa mentioned above.

The following descriptions and tables show all expected cases mentioned above.

(A) In critical combinations:

(B) In noncritical combinations:

The situation is exactly the same as diploids. The trivalent has nothing to do with inverted chromosome. The trivalent is completely free from bridge/ fragment. In both trisomic and diploid portion of BC1 hybrids the ratio of inversion-hetero and -homo is 1:1 as shown in the following table.

METHOD III. Genetical analysis without cytology.

The procedure is exactly the same as Method II. The only difference is that there is no need to study cytological behavior in meiosis in Method III. Also only diploid plants are studied in this method.

In the table shown above, the segregating ratios are 2Aa: laa in AAa x aa and 2AA : 1 Aa in AAa x AA in critical combination. Aa is heterozygote for an inversion and should show some extent of semi-sterility, while AA (normal homo) and aa (inversion homo) should show (almost) complete fertility.

In the noncritical combinations the segregating ratio would be 1:1 for fertile and semisterile plants.

Method III could be used to obtain at least a supplemental information for the cytological analysis, or may be used in the preliminary experiment in order to find the information roughly about the inverted chromosomes. So that more detailed cytological analysis can be made for only the suspected critical combination.

References:
Hanson, D.W. 1959. Minimum family size for the planning of genetic experiments. Agron. J. 51:711-715.

McClintock, B. 1938. The fusion of broken ends of sister half chromatids following chromatid breakage at meiotic anaphases. Missouri Agric. Expt. Sta. Res. Bull. 290:48pp.

Swanson, C.P. 1963. Cytology and cytogenetics. Prentice Hall. 596 pp.

BGN 10 toc
BGN Main Index