Yeast cells contain many copies of mitochondrial (mit) genomes. The question we tried to answer was how mit− mutations occurring in one genome as a result of mutagenic treatment might yield homoplasmic mutant cells. Three processes were considered. First, that these cells originate by segregation of mutant and standard alleles during cell division. Secondly, that they originate through intracellular selection, for which cell division is not required. Thirdly, that recombination involving the mutant and standard alleles is non-reciprocal and unidirectional mit+ → mit− so that the mutant allele is spread into the entire population of mitochondrial genomes within a cell, thus making it homoplasmic mit−. The results indicate that the first process, although efficiently producing homoplasmic cells from heteroplasmic zygotes (for review see Birky, 1978), seems to play only a minor, if any, role in producing homoplasmic mutant progenies from mutagenized cells. The most important is the second process, that is, intracellular selection occurring in cells which have one or a few genomes carrying mit− mutations, while the remaining genomes are irreversibly damaged. The third process, unidirectional mit+ → mit− conversion, does not seem to play any part.

Three new t-haplotypes, tp 4, tp 12 and tp 14, were isolated from M. musculus male mice captured in Central and East Bohemia, Czechoslovakia, about 400 km from the zone of hybridization between M. musculus and M. domesticus species. Complementation tests have shown that all three new t-haplotypes belong to tw 73 group. When compared with 5 t-haplotypes from M. domesticus they displayed the same pattern of BamHI restriction fragments with H-2 class I genes, and they also shared the t-specific 5·2 kb TaqI fragment of the alpha globin pseudogene. However, they differed from M. domesticus t-haplotypes at the D17Leh443 locus since they all displayed a 10·5 kb MspI fragment, labelled by the Tu443 probe, not found in wild type-chromosomes or in M. domesticus t-haplotypes. A hypothesis is proposed that t-haplotypes in M. domesticus originated by a single successful introgression from a parental species during speciation.

1. A new hereditary defect affecting the hair coat of Swiss albino mice is described. The defect was found to be inherited as a recessive character and is designated as bare (ba).

2. The gene affected the vibrissae as well as pelage hairs. The lack of vibrissae and other sensory hairs at birth, helped to differentiate them easily from the normal animals. The first hairs on the body appeared at 13–14 days of age. The hairs were thin and tiny and remained there till about the thirtieth day. The hairs in the second hair cycle appeared at about 45 days, and again disappeared within 10 days. The animals were entirely naked when they were 6 months old.

3. Gross as well as microscopic examination of hairs in the first hair cycle did not show the four different normal types of hairs. The hairs were comparatively very small, thin, and the internal structure did not show any regularity in the arrangement of the air cells.

4. No difference was found in the number of hair follicles in the bare and the normal Swiss mice.

5. Histology revealed the presence of keratinized globular masses instead of straight hair. This abnormality persisted in all the hair cycles.

6. The bare mice were compared with the other mutants and placed in the Alopecia-Naked group where the abnormality was in the keratinization of the hair.

The I factor is a LINE-like transposable element responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. Inducer strains of this species contain several I factors whereas reactive strains do not. I factors are stable in inducer strains, but transpose at high frequency in the germ-line of females, known as SF females, produced by crossing reactive females and inducer males. Various abnormalities occur in SF females, most of which result from this high rate of transposition. We report here that recombination is increased in the germ-line of these females. This is a new characteristic of the I-R system of hybrid dysgenesis that might also be associated with transposition of the I factor.

The th2-haplotype is transmitted at low frequencies (< 0·30) by + / th2 males in normal matings. In the studies described here, the transmission frequency of the th2-haplotype from Rb7 / th2 males was determined for normal and delayed matings and in vitro inseminations. The data show the transmission frequency from the two in vivo inseminations to be less than 0·30 and to be statistically equivalent. However, the in vitro transmission frequency (0·80) is significantly greater than either of the in vivo frequencies. The results show that the environment in which fertilization occurs affects the transmission frequency of this specific t-haplotype significantly.

The pattern of chromosomal variation is investigated in house mice from the Danish hybrid zone between the translocation-prone Mus musculus domesticus and the chromosomally conservative M. m. musculus. The cytogenetic analysis confirmed the non-introgression of three pairs of Robertsonian (Rb) fusions from M. m. domesticus into the M. m. musculus genome. The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal clines which increased in steepness the closer they were to the centre of the hybrid zone as defined by allozymes. Analysis of alternate hypotheses suggests that chromosomal differentiation of the Danish domesticus occurred after contact was established with musculus. The staggering of the clines would reflect the order of arrival of theRb fusions into the hybrid zone. Several models with different processes of underdominance of the chromosomal heterozygotes are discussed to account for the difference in width between clines. A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome provides the best fit for the observed pattern. Selection against Rb fusions with little effect on the recombination of linked allozyme markers supportsthe view that no reduction in gene flow due to chromosomal heterozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musculus genome.

Mice which are either homozygous or heterozygous for the CatFraser mutation have ocular cataracts accompanied by selective reduction of the γ-crystallins, a homologous family of proteins present in the lens and encoded by a family of tightly linked genes. We measured the concentrations of four different mRNAs, each encoding a different γ-crystallin, in the lenses of homozygous CatFraser mice and in normal controls at various stages of development by preparing Northern blots from lens RNA, probing with RNAs complementary to each of the four messages and densitometry of the bands thus generated. The results show that, for each of these messages, the ontogenetic patterns observed in normal mice are retained in the mutant, but at much lower concentrations.

Wild-type Caenorhabditis elegans fails to reach adulthood if L1 larvae are incubated in the presence of 30 mM or greater concentrations of caffeine. Eleven mutants have been isolated in which caffeine has a less pronounced effect on development. The mutations are recessive, define two genes, and have been mapped. The mechanism(s) of resistance is unknown.

The 6-phosphogluconate dehydrogenase (6-PGD) E.C. no. 1.1.1.44, zymogram phenotypes of wheat–rye addition lines and triticale were determined. Genes involved in the production of 6-PGD were located on the long arms of C and F chromosomes of Imperial Rye and the long arms of II and IV chromosomes of King II rye. The results indicate that this enzyme is a dimer.

The insertion site numbers of the transposable elements (TEs) copia, mdgl, 412 and gypsy were determined in various natural populations of Drosophila melanogaster and D. simulans by in situ hybridization. We showed that, while all elements except gypsy had many insertion sites scattered over the chromosomes in D. melanogaster, only the 412 element in D. simulans presented a high number of insertions, and this number was lower than in D. melanogaster. This low 412 site number per genome in D. simulans was associated with a lower proportion of insertions on the X chromosome in comparison with D. melanogaster, as determined in diploid genomes (0·090 for D. simulans against 0·137 for D. melanogaster) and in haploid genomes (0·102 against 0·146), each value being, moreover, lower than the value of 0·20 expected on the hypothesis of no selection against insertional mutations. These results suggest that selection is a major mechanism explaining 412 copy number regulation in Drosophila, and is stronger in D. simulans than in D. melanogaster.

The genetic and phenotypic regressions and correlations between ovulation rate and body weight were examined in a random bred strain (Q) of laboratory mice during the course of three experiments. These experiments were (1) a sib analysis; (2) selection for natural and induced primiparous ovulation rate; and (3) replicated selection for 6-week weight. The following results were obtained:

(a) The genetic correlations between body weight and natural and induced ovulation rate were positive, and approximately equal to 0·4 and 0·6 respectively.

(b) The genetic regressions of natural and of induced ovulation rate on body weight were approximately 0·4 and 0·9 eggs per gram respectively.

(c) The genetic regressions of body weight on natural and on induced ovulation rate were approximately 0·5 and 0·25 g per egg respectively.

(d) The phenotypic correlation between natural ovulation rate and body weight was approximately 0·4 and the corresponding regression of ovulation rate on body weight approximately 0·4 eggs per gram.

(e) The phenotypic correlation between induced ovulation rate and body weight declined from 0·4 at 6 weeks of age to zero at the time of scoring, the corresponding regressions of ovulation rate on body weight declining from 0·1 eggs per gram to zero.

It was concluded that natural ovulation rate itself, and both its components (FSH activity and ovarian sensitivity) are positively genetically correlated with body weight. Furthermore, the observation that large mice shed at least as many eggs as small ones in response to the same dose of PMS showed that the response was more closely related to the absolute dose than to the resultant concentration.

A multi-step Chloramphenicol (CM)-resistant derivative of an RC-stringent strain of Escherichia coli auxotrophic for threonine and leucine was resistant also to Aureomycin (AM) and Puromycin (PM). All three antibiotics released the repression of RNA synthesis due to amino acid starvation in the CM-sensitive parent strain, their relative activities being about 1:10:100 for AM: CM: PM. High doses of AM and CM failed to induce RNA synthesis. The CM-resistant strain required greater concentrations of each antibiotic than the sensitive strain to induce the same level of RNA synthesis, and appeared to be about one hundred times, ten times and five times more resistant to CM, AM and PM, respectively, than the sensitive strain.

Interspecific aneuploids were made in the genus Cucurbita by pollinating the autoallotroploid (diploid C. moschata – haploid C. palmata) with pollen from diploid C. moschata. Seventy-eight, or 0·36%, of the potential ovules showed some degree of continued development after pollination. The high frequency of continued ovule and embryo development indicated a preferential distribution or loss of C. palmata univalents during meiosis in the autoallotriploid and that megagametes aneuploid for more than one C. palmata chromosome were capable of initiating continued ovule development after fertilization. Lethal effects of interspecific aneuploidy were expressed by complete embryo abortion to sterility of mature plants. Nine plants developed to maturity and seven were cytologically identified. Four were trisomic and two were monosomic for single C. palmata chromosomes, i.e. they were 2n + 1 and 2n + 19 (n = 20) respectively. One plant was 2n and phenotypically identical to the C. moschata parent. One 2n + 1 plant was fertile and the single C. palmata chromosome was transferred in the succeeding generation.

In a population of Drosophila melanogaster started from an inbred wild-type strain the recessive second chromosome lethal studied had shown overdominance which after many generations was lost. In the present study the persistence of this lethal was tested in three series each of five populations. The genetic backgrounds of the different series of populations were obtained from (a) the inbred strain, (b) the above original population after the overdominance had been lost, and (c) a population started from the same inbred strain and where another lethal had shown overdominance which subsequently had been lost. The lethal was overdominant in the (a) background but detrimental to the heterozygous carriers on the other backgrounds. The detrimental effect of the lethal was stronger in the (b) background than in the (c) background. The varying behaviour of the lethal is possibly due to different adapted background genotypes and/or different degrees of heterozygosity of the gene pools.

The fixation probability of a mutant in a subdivided population with spatially varying environments is investigated using a finite island model. This probability is different from that in a panmictic population if selection is intermediate to strong and migration is weak. An approximation is used to compute the fixation probability when migration among subpopulations is very weak. By numerically solving the two-dimensional partial differential equation for the fixation probability in the two subpopulation case, the approximation was shown to give fairly accurate values. With this approximation, we show in the case of two subpopulations that the fixation probability in subdivided populations is greater than that in panmictic populations mostly. The increase is most pronounced when the mutant is selected for in one subpopulation and is selected against in the other subpopulation. Also it is shown that when there are two types of environments, further subdivision of subpopulations does not cause much change of the fixation probability in the no dominance case unless the product of the selection coefficient and the local population size is less than one. With dominance, the effect of subdivision becomes more complex.