Estimation of spontaneous mutability of the gene Trithorax-like in Drosphila melanogaster.

 

Ivanov, Yu.N.  Institute of Cytology and Genetics, Siberian Division of Russian Academy of Sciences, Novosibirsk, 630090, Russia. FAX: (3832)33 12 78. E-mail: iyn@bionet.nsc.ru.

        It may be interesting to the reader acquainted with our recent work on isolation of Trithorax-like (Trl, 3L, 70F1-2) gene mutations and measurement of their incidence rate (Ivanov, 2000) to know what subsequent specification these not quite usual results have been given.

            As before, we estimated the incidence of Trl* mutations induced by the preset dose of g-irradiation, and from this, knowing the multiplicity of increase of the mutation frequency at the given dose, found the spontaneous incidence of Trl* mutations.  The schedule of crosses for detection and isolation of Trl* mutations remains the same (Ivanov, 2000).  In a repeated experiment we used a dose of 7000 R and estimated the frequency of induced Trl* mutations separately in ova and spermia, i.e., in the former case irradiated were females and in the latter males.  For estimation of mutability of the Trl gene in ova and spermia, 594 and 603 detecting crosses were carried out, respectively (totally 1197).  Due to the high death rate of cultures because of mould, only 444 ova and 372 spermia (totally 816 gametes) turned out to have been studied.  As a result, four Trl* mutations were isolated, and, although all of them occurred in ova, the hypothesis of equal mutability in ova and spermia is not rejected, because the gamete sample was too small.  All the four Trl* mutations were isolated from D/+, but not Sb/+, compounds (where + designates an irradiated wild-type chromosome), which is in accordance with the conclusion of a previous experiment: the chromosome bearing D favors isolation of Trl* mutation and that bearing Sb, on the contrary, diminishes the chances of their isolation.  According to the data of the two experiments, the chances of isolation of Trl* mutations from D/+ and Sb/+ compounds turned out to be 14/1579 and 2/1598, respectively.  It is supposed that Sb/Trl* compounds die more often than D/Trl* due to the dominant lethality of Trl* mutations.

            The mutability of gene Trl induced by a dose of g-irradiation 7000 R is

 

;   .

The spontaneous incidence of mutations is lower than that induced at this dose by  times, i.e. amounts to

;   .

Now, we have two measurements of spontaneous mutability of the Trl gene: (Ivanov, 2000) and , the difference between them being not significant: .  Therefore, in order to obtain a more accurate value of spontaneous mutability of gene Trl, one may combine these measurements with weight coefficients , by the formula

.

As a result, we obtain: . In this way, we estimate the spontaneous incidence of Trl* mutations at  with a relative error of about 50%.

            Acknowledgments:  The author thanks Dr. Victor G. Kolpakov for his consistent help in translation of manuscripts into English and correction of the text, and also Dr. Valentine S. Pykhalov for this help in mathematical treatment of the data.

            Reference:  Ivanov, Yu.N., 2000, Dros. Inf. Serv. 83: 117-122.