Genetics of inbred Drosophila melanogaster. XXIII. Diversity in cytoplasmic elements affecting recombination.

From an inbred line of Drosophila melanogaster, in which both marker genes in the X-chromosome and an autosomal inversion had been induced, we introduced the genome into cytoplasms of various sources. Two or eight sister-lines were established from three sets of cytoplasms. Tests of recombination in the distal y--rb region in isogenic females revealed similarity in early broods within sets of sister-lines, but differences between different sources of cytoplasms. Later broods showed increasing recombination and also greater variability. There was a clear excess of recombination in offspring to inversion-heterozygous females. In the set with eight sister-lines there were two groups of four, which during the first 4.5 years had been propagated with 14 and 28 days generation interval respectively. These two groups differed in the inter-chromosomal effect in early broods. The possibility is discussed that there are more than one set of extra-chromosomal elements that affect recombination in the distal region of the X-chromosome. It is concluded that selective forces operate on the extra-chromosomal elements in other ways than in the process with which they were traced in the tests. The consequences of diversities in extra-chromosomal elements are discussed.

Genetics of inbred Drosophila melanogaster. XXIV. Effects on the strength of the M-type.

Several branches with genome from either of two inbred lines of Drosophila melanogaster, introduced into various sources of cytoplasms, were used in tests of the strength of the M-type. In contrast to the conventional tests of dysgenesis, we examined primary non-disjunction and chromosome loss in successive broods from M/P hybrid females. In broods from the 2nd week of egg-laying, there were increased frequencies of X0-males and a strong depression of fertility. There were no indications of cytoplasmic influence on the M-type. In a third line with the same cytoplasm as in a couple of the above mentioned ones, there was a lower effect on chromosome loss and on fertility. Crosses between different genotypes, but with the same source of cytoplasm, suggested a delayed genic influence in maternal inheritance. Tests of recombination in the X-chromosome indicated a gradual depression from the mid part to the distal end. There seemed to be no genic effect on the depression of recombination. It is suggested that there are pleiotropic effects of the P-factors and that the response to the various processes could be under both genic and cytoplasmic control in the M-type females.

Genetics of inbred Drosophila melanogaster. XXV. Simultaneous tests of I-inductivity and M-type.

From a branch of an inbred line of Drosophila melanogaster we introduced the genome into cytoplasms from various sources. Among a pair of sister-lines, which at first showed R-type in the I-R system, one line switched to I-type. From a branch of this I-type line, for several generations kept at 18 degrees C, we established sub-lines derived from four single females. They were propagated at 25 degrees C with 14 or 28-day generation interval, respectively. It was presumed that 14-day generation interval gave strong, and 28 days interval gave weak R-elements. Tests of the strength of the I-inductivity disclosed no relation to the assumed strength of the R-elements. By the same test technique, primary non-disjunction and chromosome loss, the M-type was studied in the eight sub-lines. There appeared diversities in the M-type, assumed to be related to chromosomal mutations. Both primary non-disjunction and chromosome loss were affected by the P-M reaction.

Genetics of inbred Drosophila melanogaster. XXI. Re-reversion in the strength of R-elements.

In tests of the I-R hybrid dysgenesis system in Drosophila melanogaster we have in former tests examined the transition from strong to weak R-type by modifying the breeding protocol as regards the generation length, achieved by collecting parental flies from early or late egg-laying. To examine if independent occurrence of weak R-elements showed similar or different response at re-reversion of the generation interval, four sublines were tested. It was found that transition from long to short generation interval eventually resulted in a stepwise change to strong R-type. The change towards strong R-type occurred at a slightly slower rate than we earlier found in the opposite direction at transfer towards weak R-type. From lines bred at 14 days generation interval at 25 degrees C we kept, as a safeguard, branches at 18 degrees C. After 27 generations, tests of the strength of the R-reactivity revealed that various cytoplasms showed a depressed R-reactivity with no heterogeneity between sister lines. Return of sub-branches from 18 degrees to 25 degrees C for a couple of short generations (14 days) or a single long generation (28 days) showed results indicating heteroplasmy for weak and strong R-type elements in the branches kept at 18 degrees C. The vulnerability of the R-elements is a useful model for tests of extrachromosomal heredity, which may shed light on the problems involved in identifying other extrachromosomal systems.

Genetics of inbred Drosophila melanogaster. XXII. Cytoplasmic influence on recombination.

To examine the possible influence of various sources of cytoplasms on recombination in the major part of the X-chromosome in Drosophila melanogaster, tests were performed in intra- and inter-line crosses. Two to six sister lines of various types were included. It was shown that cytoplasms of various sources could have different effects in the distal y-cx region but not in the mid (cx-v) or the sub-proximal (v-f) regions in both intra- and inter-line crosses. There were no diversities between sister-lines that had been separated for over 100 generations. Tests of two sets of sister-lines with the same source of cytoplasm in which different genomes had been introduced confirmed the earlier finding that there is an interaction of chromosomal and cytoplasmic influences on recombination. It is shown that changes in one extrachromosomal element (R-elements) had no demonstrable consequences on the effect on recombination. The necessity to consider cytoplasmic-bound variability is stressed.

X-ray-induced recessive lethal mutations in adult and foetal female mice.

Tests of X-ray-induced recessive lethal mutations in adult and foetal mouse females were performed. The tests were based on family analysis which provided a possibility of making a distinction between pre-existing recessive lethal mutations and newly arisen (spontaneous + induced) ones. The way the tests were carried out provided material for the estimation of the frequency of spontaneous lethal mutations per genome. The two different estimates from this material, 0.72 and 0.91%, respectively, are similar and fall within the range previously suggested by Lüning [5]. The difference, though statistically nonsignificant, between data from irradiated mature and maturing oocytes in adult females and material from the unirradiated part, indicates a mutation rate of 13 x 10(-5) per rad per gamete according to one estimate and 8-12 x 10(-5) according to the other in comparison with 9 x 10(-5) from irradiated spermatogonia [6]. In the limited data obtained after foetal irradiation, there is no indication of oogonia and developing oocytes being more sensitive to irradiation than oocytes in adult females fertilized within 6 weeks of treatment.

Isolation of a spermatozoid-releasing and -attracting substance from female gametophytes of Laminaria digitata.

SEVERAL dioecious marine brown algae have been valuable in studying chemical communication during sexual reproduction. For example, female gametes of Ectocarpus siliculosus, Cutleria multifida and Fucus serratus secrete olefinic hydrocarbons into the seawater to attract male gametes(1-3). A more complex system is found in the order Laminariales, which includes the large kelps. Mature female gametophytes in members of this group secrete highly volatile material with spermatozoid-attracting activity, which also induces explosive discharge of antheridia(4). We report here a study of this volatile material. It is now possible to attribute spermatozoid-releasing and -attracting activity to one specific compound.

X-ray-induced recessive lethal mutations in the mouse.

A test of X-ray-induced recessive lethal mutations in mouse spermatogonia (500 rad) was carried out. The test was based on familial analysis, which allowed division on the P pairs into those with lethal heterozygous members and in others assumed to be lethal-free. The F1 males from the latter group, in back-crosses to their daughters, gave an excessive rate of intra-uterine death in comparison with lethal-free males. The excessive death is assumed to reflect the rate of new (induced + spontaneous) recessive lethals or rather lethal equivalents. Three ways of estimating the rate of new recessive lethal mutations gave a mean of 5.5% per genome. From the previous tests we can assume that 1% are spontaneous mutations. Thus the data indicate that the mutation rate per rad per gamete is 9 X 10(-5). This value is identical with a previous estimate. The results are discussed in relation to population tests performed in the early 60's. It is concluded that the lack of observable deterioration in the populations after several consecutive generations of exposure is in accord with the estimates in the present analysis which are more than an order of magnitude lower than assumed at the start of the population tests. It is also stressed that species with different DNA contents show similarities in point estimates of doubling dosages.