The analysis of genetic recombination on the polaron hybrid DNA model

By H. L. K. Whitehouse and P. J. Hastings

Genet. Res. (1965), 6, 27–92

On p. 58, Table 7, in the column headed ypc, the entries relating to genotypes nos. 17 and 18, 19 and 21, 20 and 22, and 27 and 28 should be transposed within each pair, so that the column reads: 17, ⅛; 18, —; 19, ⅛*; 20, ⅛; 21, —; 22,—; 23, *; 24, ; 25, *; 26, ; 27, ⅛*; 28, —; 29, ; 30, ; 31, *; 32, . Total (wild types), *.

The inheritance of three protein fractions of third instar larval lymph in Drosophila melanogaster as detected by starch gel electrophoresis is described. Each of the three is governed by a single autosomal gene. In two, the dominant allele determines presence, and the recessive absence of the respective fraction. The third protein fraction is governed by a single pair of co-dominant alleles.

Comparison of the starch-gel pattern to that obtained on cellulose acetate and on polyacrylamide gel fails to show definite sub-fractionation of those protein bands of which the inheritance has been studied.

Difference in bristle-making abilities in extreme scute and in wild-type Drosophila were investigated using flies mosaic for the two kinds of tissues. It was found that both wild-type and scute tissues were autonomous with respect to bristle differentiation. From the experimental evidence it was postulated that the differences in bristle-making abilities between the two genotypes might be due to differences in tissue competence rather than differences in pre-pattern or in level of bristle-making substances.

Starting from a genetically variable stock homo- or hemi-zygous for Bar two selection lines were set up, one selected for decreased sensitivity to the effect of larval temperature on eye size (‘canalization line’), the other for increased sensitivity (‘anti-canalization line’). In each generation a sample of larvae was grown at 25°C. throughout life and another sample at 25°C. for the first 48 hours, followed by 18°C. until emergence. In the canalization lines a selection was made of individuals (five males and five females out of 100) least affected by the temperature treatment and the anti-canalization line for a similar number most affected by the treatment. These ten males and females were allowed to mate at random and from the eggs produced random samples were then treated in the next generation in a similar manner in the two temperatures. Precise counts of facet numbers on the right eyes were made at generations 4 and 9 and it was clear that selection had been effective both in decreasing and increasing temperature sensitivity.

The whole canalization line can be regarded as a population which has been subjected to disruptive selection for two different criteria: one (small eyes) being regarded as adaptive to the low-temperature regine or habitat; the other (large eyes) being regarded as adaptive to the high-temperature habitat. A similar type of analysis can be applied to the anti-canalization line in which, however, the selective values of the phenotype are regarded as reversed in value in the two habitats. The experimental procedure employed involved not only random mating between individuals selected according to these two criteria but also random allocation of the offspring of this panmictic population to the two habitats of the next generation. This corresponds to a natural population in which there is disruptive selection exerted by two different habitats but no habitat preferences exhibited by members of the population. Under these circumstances, in our experiments, disruptive selection produced only rather slight increases in phenotypic variance, which were rather larger in the anti-canalization than in the canalization lines. Possible reasons for the difference between this result and those reported by Thoday and Gibson are discussed, and it is suggested that an important reason may be the slowness of the response to selection in the first few generations in our lines.

The effects of tight linkage on the total response due to pairs of identical additive loci, segregating in a population initially in linkage equilibrium, have been studied both algebraically and by means of computer simulation. Particular attention has been given to the effects of finite population size on the probabilities of (a) the elimination from the population of the gamete carrying both ‘plus’ alleles; (b) the joint preservation of the two types of repulsion gametes; (c) the recovery of the desired combination of plus alleles through crossing-over; and (d) the fixation of the gamete in the population following its recovery.

The study is restricted to situations in which linkage is known to have an appreciable effect on total selection response, i.e. to the case of genes of large effect initially at low frequency. A comparison of regimes with the same expected response under free recombination has shown the probability of (a) to be high, and the probability of (b) to be very nearly the same for all regimes tested. Provided that the recovery of the gamete carrying both plus alleles is an unlikely event at any given point in time, the probability of the fixation of the gamete, once reconstituted, is expected to be independent of population size for genes of large effect. In this context, approximate algebraic expressions have been derived for the probability of effective recovery of the required gamete, and for the mean waiting time involved.

Polarized segregation was studied at the asco and tan spore loci of Neurospora crassa and at the gray and hyaline loci of Sordaria fimicola. Different alleles, re-isolates and temperatures of incubation of crosses were studied with the asco locus and different temperatures of incubation with the h and g loci. The mutants were crossed with the corresponding wild-types by methods giving a considerable degree of synchrony in the development of perithecia; segregation patterns were scored from perithecia at widely differing stages of ascal maturity. Significant polarized segregation was not generally observed when little or no dehiscence had occurred from a cross but usually became readily apparent as dehiscence proceeded. This indicated that the differences in frequency between the ascal classes that had previously been interpreted as showing polarized segregation at meiosis were actually due to a post-meiotic differential bursting of asci. This bursting was studied separately from the usually concurrent pigmentation of immature asci by comparing results from asci mounted in water with those from asci in the 2 M sucrose solution which was used throughout these experiments: the differential bursting produced by mounting asci in water gave similar polarized segregation results to those from natural dehiscence. A hypothesis to account for differential bursting in physiological terms was advanced and its predictions discussed. It seems likely that cases of apparent polarized segregation described in other Ascomycetes are also due to post-meiotic phenomena.

Seventeen non-motile strains of Chlamydomonas reinhardii isolated by Dr Lewin and thirty-six newly isolated strains have been examined in the electron microscope for structural abnormalities of the flagella. Fourteen of them have straight, paralysed flagella in which the central fibres are replaced by an irregular core of disorganized material. The fourteen mutations map at four unlinked loci. Some of them are leaky; light and electron microscope observations on leakiness are described. In the former case leakiness is measured by the proportion of motile cells, and in the latter by the proportion of intact centre fibres seen in transverse sections. The degree of leakiness is to some extent characteristic of particular loci.

A partial suppressor of some of these mutations has been isolated which acts on all the mutant alleles at two loci and to a much lesser extent on four out of five alleles at a third locus.

A number of Hfr strains have been studied as to the action of radiations on the transmission of their genetic material to female cells. They form two classes, one in which irradiation produces only lesions that prevent transfer, and one in which new modes of chromosome transfer seem to be induced. One group of strains, derived from a common F+ male, appears to give rise to the same induced mode of transfer, while no correlation could be found in the other Hfr's studied.

A close examination of reports on circling mutants of the mouse suggested that the commonly held view that the abnormalities of the inner ear are responsible for the waltzing syndrome was probably without foundation. It was thought that a study of kinky (Fuki/+) mice might resolve the doubt, because this mutant has the widest range of abnormalities of the inner ear and behaviour. The results showed that correlation between the two types of abnormalities, although high, was far from complete. This is interpreted as signifying that they are related collaterally rather than lineally. It is argued that they both originate in some primary abnormality of the central nervous system, and circumstantial evidence is presented in support of this argument. It is further suggested that the mode of gene action is essentially similar in all circling mutants, that is the abnormalities of the inner ear are consequent on some early abnormality of the neural tube and the ganglia in the region of the otic vesicle.

1. The rate of hydrolysis by sheep plasma of some carboxylic and phosphate esters has been determined for a random flock, and for a flock previously selected for its ability to hydrolyse di-(2-chloroethyl) aryl phosphates.

2. A discontinuous variation in hydrolysis rate was found with all substrates tested and, using combinations of substrates, six types of plasma could be distinguished, each type having a different pattern of esterase activity.

3. The most useful substrates for distinguishing between phenotypes were 1-naphthyl acetate and 4-ethoxycarbonylcoumarin-7-yl acetate. Three rates of hydrolysis were possible for each of these esters, and the highest rate for one was invariably combined with the lowest rate for the other, although the converse did not apply.

4. To explain these results, and those of Lee (1964), it has been postulated that the quantitative production of esterase hydrolysing 1-naphthyl acetate is governed by the presence of an allele, termed Esa, at a particular gene locus. Similarly, the production of esterase hydrolysing 4-ethoxycarbonylcoumarin-7-yl acetate is determined by allele Esb, and where neither substrate is attacked the presence of a third allele, Esc, is proposed.

5. The hydrolysis rates of haloxon, 1-naphthyl butyrate and 4-nitrophenyl butyrate varied in the same way as that of 1-naphthyl acetate, whereas the hydrolysis of indophenyl acetate followed the same pattern as that of 4-ethoxycarbonylcoumarin-7-yl acetate. The variation in hydrolysis rate of Coroxon could be explained by assuming that Esa and Esb are equal in this respect.

6. A mating experiment produced results which were in accordance with the genetic hypothesis, but were too few in number to provide confirmation.

7. The genetic marking of six types of sheep is possible, utilizing the variation in plasma A-esterase activity.

1. Lines of Drosophila melanogaster, which differ greatly in body size and/or development time, have been created by selecting for either large or small body size or longer or shorter development time on different chemically defined axenic media. The ten lines studied here have been selected either on the optimum medium or on media deficient in either protein, ribonucleic acid or choline.

2. The lines show a variable degree of correlation between adult size and the duration of the larval period. Selection for one trait may involve a positively correlated change in the other or have no such effect. There is a fairly regular association between the composition of the diet and the presence or absence of evidence for genetic correlation.

3. The biochemical composition of the various lines, and also the unselected stock from which they were derived, has been compared at successive stages from egg to adult in animals grown on the optimum axenic diet. Records of wet weight, protein, RNA and DNA content per individual have revealed differences in composition which can be releated to characteristic differences in the response to selection.

4. Comparisons between the individual content of the various constituents at the critical size, i.e. at the time in early third instar when larvae can complete development even if no longer allowed to feed, and in newly emerged adults indicate a high positive correlation between DNA content and the time to reach the critical size and also the time to pupation. Protein and RNA content, on the other hand, may vary within wide limits without alteration of the development time. The association between DNA content and development time which underlies the correlation between body size and development time, is related to the rate of DNA synthesis in early larval life, such that the rate of synthesis tends to be negatively correlated with the absolute DNA content at the critical size and in the adult.

5. Since the protein/DNA ratio is comparatively unchanged in lines with correlated changes in body size and development time, but varies widely in lines in which development time is unaltered, the contrasts may reflect, in adults at least, differences in respectively cell number or cell size.

6. An earlier hypothesis of the origin of correlated and uncorrelated changes in body size and development time has been modified. It appears that the absolute critical size is positively correlated with the size of the adult, whether or not changes in development time have occurred. The intrinsic differences in growth operate from very early stages of development. This has been demonstrated in the differences in RNA/DNA ratio in early larval life and also in a high correlation between the time to complete development in the egg and the time to reach critical size after hatching from the egg.

7. There is also evidence, in the relative deviations from unselected of the various estimated constituents, of partial independence of growth between early and late larval life, and this may reflect differences in the growth of the imaginal discs.

8. The data are discussed in relation to earlier experimental evidence relating to the growth of Drosophila. The point is made that judicious selection for appropriate parameters of growth will often create differences great enough to reveal developmental interrelations which would otherwise be difficult to detect.