The interpretation of gene conversion data from ordered eight-spored asci

By J. R. S. Fincham, W. G. Hill and E. C. R. Reeve

Genetical Research, Vol. 35, No. 2, 1980

On p. 181 the phrases ‘wild to mutant’ and ‘mutant to wild’ in lines 8–9 in the third paragraph should be interchanged: p and r are correction frequencies from mutant to wild and q and s are correction frequencies from wild to mutant, as is apparent in the Table immediately following.

On p. 188 the calculated ratios p/q and r/s may appear to be at variance with the values given in Table 2. This is due to the latter having been rounded to two significant figures. The more accurate values for the + × h3 cross are as follows: p = 0·043, q = 0·161, r = 0·157, s = 0·426 (firstsolution) and p′ = 0·026, q′ = 0·493, r′ = 0·243 and s′ = 0·128 (second solution).

On p. 191 (foot of page) some primes are misplaced. The correct formulae for r, q and s (as given on p. 182) are:

On p. 194, line 7, G = 2B + C should read G = 2B + D.

The selection component analysis developed by Christiansen & Frydenberg (1973, 1976) is a method to study the components of natural selection and is based on an analysis of population samples which include mothers and their progeny (mother–offspring combinations). Because only one progeny per mother–offspring combination is analysed, gametic selection and the reproductive components of selection in males are poorly characterized. We investigated the information which can be gained by analysing all progeny in each combination and showed that analysis of entire litters enables detection of gametic selection. Testing and estimation procedures are derived for this purpose. Sexual selection in males and mate preferences remain poorly characterized because the only information about the reproductive components in males is contained in the sample of male gametes and is insufficient to reconstruct the breeding structure of the male population. The format and interpretation of a selection component analysis is presented which takes these results into account.

The Clc2 gene of the mouse codes for the ubiquitously expressed chloride channel ClC-2, a member of a family of at least seven voltage gated chloride channels, some of which are implicated in hereditary diseases. Using a mouse interspecies back-cross panel, we have mapped Clc2 to Chr 16, proximal to the somatostatin gene Smst, extending a region of documented conserved synteny to human Chr 3q.

Hair follicle initiation and development was examined in tabby mutants and compared with a previous study of this sequence in normal mice. Initiation commenced at about the 17th embryonic day and continued for 1 and possibly 2 days after birth. Although central primary follicle initiation was of shorter than normal duration, it was more rapid, with the result that the central primaries achieved a greater maximum density, and their total numbers were greater in tabby than in normal mice.

In general structural terms, follicle group development proceeded normally except that no secondary follicles appeared, and for this reason, the mature grouping pattern of follicles was comparatively simple. Central primary follicles, the first to be initiated, were evenly spaced in the skin but the later association of lateral primaries with them was unusually close, and the resulting groups of primary follicles were relatively more compact in the mutants than they were in normal mice.

No simple explanation in terms of the altered timing of follicle initiation or the reduced complexity of the follicle group could be given for the abnormalities of the tabby coat.

Selection for high and low numbers of secondary vibrissae in tabby mice produced some correlated changes in the maincoat follicle population, but there was no apparent alteration to the time of onset of initiation. Evidently selection did not simply modify the effects of the Ta gene.

1. Ovalbumin was shown, by starch gel electrophoresis, to exist in two genetically different forms, A and B. It was suggested that these are determined by two alleles at one locus, named Ov.

2. Ovalbumin of each genetic type is electrophoretically heterogeneous. Dephos-phorylation of each type with human prostatic phosphatase and calf intestinal phosphatase removed some of the heterogeneity, but some remained.

3. The genetic difference was shown not to reside in the fragment released from ovalbumin by the proteolytic enzyme subtilisin.

4. The genetic difference was evident in the ovalbumin present in the fluid contained in right oviduct cysts.

An adenyl cyclase deletion mutant (cya) of E. coli failed to exhibit a heat-shock response even after 30 min at 42 °C. Under these conditions, heat-shock protein synthesis was induced by 10 min in the wild-type strain. These results suggest that synthesis of heat-shock proteins in E. coli requires the cya gene. This hypothesis is supported by the finding that a presumptive cyclic AMP receptor protein (CRP) binding site exists within the promotor region of the E. coli htp R gene. In spite of the absence of heat-shock protein synthesis, when treated at 50 °C, the cya mutant is relatively more heat resistant than wild type. Furthermore, when heat shocked at 42 °C prior to exposure at 50 °C, the cya mutant developed thermotolerance. These results suggest that heat-shock protein synthesis is not essential for development of thermotolerance in E. coli.

1. A population of Paramecium aurelia, syngen 9, in Blackford Pond, Edinburgh, has been sampled over a period of 6 years and the types of immobilization antigen present identified.

2. Three antigen-determining alleles at the G locus and six alleles at the X locus were found.

3. No significant changes in relative numbers of the various alleles were apparent over the period studied, nor was there any difference between opposite ends of the pond.

4. An appreciable proportion of G-type heterozygotes was found, but the numbers were significantly less than would be expected by random mating alone. Autogamy is presumed to be responsible for this deficit.

5. Whether there is any natural selection in favour of heterozygotes over homozygotes in unknown. A possible method for obtaining information on this subject is discussed.

The distribution of visits to a particular gene frequency in a finite population of size N with non-overlapping generations is derived. It is shown, by using well-known results from the theory of finite Markov chains, that all such distributions are geometric, with parameters dependent only on the set of bij's, where bij is the mean number of visits to frequency j/2N, given initial frequency i/2N. The variance of such a distribution does not agree with the value suggested by the diffusion method. An improved approximation is derived.

Genetic systems involving developmental inactivation of entire chromosomes occur in two widely different groups of organisms: mammals and coccids (Homoptera: Insecta). The two groups show several similarities and some interesting contrasts with respect to this unusual cytogenetic phenomenon. Although mammalian X chromosomes and coccid paternal sets are components of different genetic systems, comparisons between them nevertheless suggest approaches that might prove to be of value. Further, the occurrence of facultative heterochromatization in these two wholly unrelated taxa must mean that this type of heterochromatization represents a fundamental capacity of chromosomes.

A significant excess of female offspring resulted when pregnant females of the D. simulans species were left to lay their eggs on food containing quinacrine. The same result occurred in a subsequent experiment in which male parents were injected with quinacrine. This was the first time that a chemical compound showing an affinity with DNA in vitro altered the sex ratio in a consistent manner. This effect had been predicted based on the unique quinacrine fluorescent staining pattern in D. simulans, in which only the Y stains intensely in mitotic chromosomes. It seems that treatment acts on spermatids or spermatozoa causing decreased functioning of the Y-bearing ones, resulting in an excess of female offspring. Interestingly, the species D. mauritiana and D. melanogaster, although very closely related to D. simulans, do not have its staining pattern and as predicted did not respond to treatment; therefore, the important parameter appears to be related to the staining pattern of mitotic chromosomes and not to the phylogenetic relationship.

The exact test for neutrality based on the Ewens sampling distribution described previously (Slatkin, 1994) is not correct. The problem is that the test as described is based on the probability of the ordered configuration of numbers of alleles, while it should be based on the probability of the unordered configuration. The correctly implemented exact test leads to results that are similar to those from the homozygosity test proposed by Watterson (1977) for relatively small sample sizes but can still differ substantially for larger sample sizes. Programs to perform the exact test are available from the author.

The effectiveness of selection for increased pupa weight in Tribolium was compared for three different selection systems. In all three systems the same number of breeding individuals was used each generation. Population L was a large random mating population with 24 males and 48 females selected each generation. The C4 and C8 populations were each divided into 6 subpopulations (lines) consisting of 4 males and 8 fem ales. Each of the three populations was replicated. In C4, selection for pupa weight was within lines for three generations, followed by a generation of among-line selection when the best two out of six lines were selected. These lines were then crossed to produce 6 new subpopulations, and the cycle was repeated. The C8 population was handled in exactly the same manner except that seven generations of selection within lines were practised before each generation of among-line selection. Selection response for the 42-generation period was significantly greater in the L population than in either subdivided population. No consistent differences among the selection systems were apparent when evaluating short-term response for the first 12 generations of the experiment. The results were interpreted as indicating that the influence of multiple-peak epistasis was not of major importance for this trait in determining ultimate response to selection when starting from a base population of previously unselected lines and utilizing a within- and among-line selection regime.

A method has been devised for the selection of auxotrophic mutants in higher plants. The method depends upon the incorporation of BUdR into the DNA of non-auxotrophic cells and upon its lack of incorporation into the DNA of auxotrophic cells. A wide range of auxotrophic types were recovered.

Penetrance and expressivity of the Drosophila mutant Abnormal-abdomen (A53g) may be reduced by adding inhibitors of protein synthesis, of RNA synthesis, and of oxidative phosphorylation to the defined, sterile medium on which larvae are developing. When grown on regular diet, the mutant flies contain a higher concentration of total protein than do wild-type flies. The metabolic inhibitors which reduce the expressivity of the mutation also lower protein concentration in these mutant flies. The reduction of protein synthesis is directly correlated with the reduction of expression of the mutant genotype. The regulation of the morphological abdominal abnormalities by the complex A53g genotype is discussed in relation to the control, by this same genetic system, of the mechanisms of protein synthesis.

Epistatic models are examined for conditions which result in a negative covariance between a sire's purebred and test-cross progenies. It is found that heterozygote superiority is not a necessary condition for its occurrence. The frequencies of the same allele in the selected population and in the tester should be divergent. The implications of using the covariance in prediction are discussed. The important result to note is that selection in the crossbred may not be optimum even with a negative covariance.