John B. Gibson

Effects of disruptive selection. VI. A second chromosome polymorphism.

S of Papers read at the HUNDRED AND THIRTY-SEVENTH MEETING of the Society held on 10th and 11th NOVEMBER 1961, at UNIVERSITY COLLEGE, LONDON STUDIES ON HETEROGENEOUS CLONES (HETEROCLONES) IN STREPTOMYCES COELJCOLOR G. SERMONTI Ist,tuto Superiore di Sanita, Rome

Regular responses to selection 2. Recombination and accelerated response

1. It has been shown that the lines dp 1, dp 2, vg 4 and vg 6 of Thoday & Boam (1961) each have two high sternopleural chaeta number genes or ‘effective factors’ between h and eyg in chromosome III. Their line dp 6 does not contain these two genes. 2. Lines derived from ancestors of dp 2 and vg 4 before the latter produced their accelerated responses have third chromosomes affecting chaeta number as if they had only one or other of these genes. 3. Of the three stocks from which all the lines derived, one, Inbred Oregon, lacks these genes. The second, vg/vg , has third chromosomes similar in effect to Oregon. The third, dp/dp , was heterogeneous, having a class of third chromosomes similar in effect to those of Oregon and a class similar to those having one high gene. 4. It is suggested that the history of the accelerated response in dp 1, dp 2 and vg 4 was as follows. Initially most of these third chromosomes were − − at the two loci, but a minority (derived from the dp/dp stock) were + − and − + (where + indicates the allele increasing chaeta number. Selection would reduce the frequency of − −, and hence increase the proportion of + −/− + heterozygotes and the probability of recombination to produce + +. Origin and multiplication of + + would account for the accelerated response.

Modes of variation in alcohol dehydrogenase in Drosophila melanogaster

Neben zwei elektrophoretisch verschiedenen Varianten derDrosophila-Alkoholdehydrogenase kommen innerhalb dieser Varianten weitere Variationen vor, die sich in ihrer spezifischen Aktivität und Temperaturempfindlichkeit unterscheiden. Es wird daraus geschlossen, dass der Grad des Polymorphismus von Populationen grösser sei, als aufgrund elektrophoretischer Untersuchungen geschätzt wird.

Social mobility and IQ components

This study examines the relationship between verbal, performance and total IQ scores and social class of 193 male householders living in a Cambridge suburb. The correlation coefficients between IQ scores and present occupational status were significantly higher than the correlations between IQ scores and social class of origin, suggesting that intragenerational social mobility is positively related to IQ. Parent–offspring data were available for 85 father–son pairs. Analysis of the IQ differences between fathers and sons in relation to their social class differences provided further evidence for selective migration related to both IQ components and total IQ scores. In this sample there was a simple relationship between the extent of social mobility and the degree of difference between the fathers and sons IQ scores.

Stabilising selection in constant and fluctuating environments

SummarySelection in both constant and fluctuating temperature environments for a constant bristle number in Drosophila melanogaster resulted in the maintenance of that number and in a decrease of both genetic and environmental variance. Although mean bristle number in the control lines increased over the early-generations there was a significant net decrease. Phenotypic variance decreased significantly, especially in later generations. In both control lines and in the line selected at 20–29° C. there was a significant decrease in asymmetry of bristle number. There was also a decrease in environmental variance in these three lines. Transplant experiments showed that when the lines were cultured in alien environments phenotypic variance tended to increase in the fluctuating and decrease in the constant temperature environment. However at generation 39, the phenotypic variance of lines selected in the fluctuating environment increased when samples of the lines were cultured in the constant temperature environment. The results indicate that stabilising selection can be effective in both fluctuating and constant temperature environments.

The alcohol dehydrogenase polymorphism in natural populations of Drosophila melanogaster : restriction map variation in the region of the Adh locus in populations from two hemispheres

Restriction endonuclease variation in the 12 kb region surrounding the Adh locus was measured in seven Australian and six Chinese populations of Drosophila melanogaster. There is a higher level of nucleotide-substitution variation in the Australian populations than in the Chinese, which is possibly a reflection of their origins. None of the restriction site polymorphisms, nor any of the insertions, showed a significant association with latitude. A 0.2 kb deletion varied with latitude in the Chinese populations. In accordance with previous studies, a majority of the insertions were located in a region 1.5–3.5 kb 3′ from the Adh coding region, and a majority of the deletions were at a site 3 kb 5′ to the Adh coding region. Two of the insertions shared homologies with known mobile elements. Overall, the data suggest that restriction endonuclease variation in the Adh region is not related to the cline in Adhs frequencies.

A Tirant insertion in the alcohol dehydrogenase locus of Drosophila melanogaster

An alcohol dehydrogenase allele AdhAAS44 isolated from an Australian natural population of Drosophila melanogaster is associated with low activity in adult flies. A 5.0 kb insertion in the first intron between the distal and proximal promoters has been partially sequenced and shown to be homologous to the retrotransposon Tirant. The insertion is the major change in the transcriptional unit of AdhAAS44, but there is also a single nucleotide change in the adult Adh enhancer. The main phenotypic effect associated with the Tirant insertion is a reduction in alcohol dehydrogenase activity and transcript level in adult flies. There is a very much smaller reduction in 3rd instar larvae. The data indicate that Tirant affects transcription from the distal promoter. The possible mechanisms for this differential effect on transcription are discussed.

Expression of the GPDH-4 isozyme of sn -glycerol-3-phosphate dehydrogenase in three Drosophila species

A fourth recently discovered isozyme of sn-glycerol-3-phosphate dehydrogenase (GPDH-4) in Drosophila melanogaster is shown to be a translational product of the Gpdh transcript which contains exons 1 through 7. This transcript was also found in two other Drosophila species, D. busckii and D. virilis. In contrast to D. melanogaster and D. busckii, the Gpdh transcript containing exons 1–6 is absent in D. virilis adults. The reason for this difference between D. virilis and the two other species is intriguing but remains elusive. We have ruled out the possibility that a replacement of an amino acid residue in exon 7 played any role in generating this interspecific variation.

Effects of disruptive selection X. Selective migration

SummaryTwo populations of Drosophila melanogaster were exposed to disruptive selection for sternopleural bristle number. There was 25 per cent, gene flow between the halves of the populations selected for high bristle number and the halves selected for low bristle number. In one population the migrant males were selected in the same direction as the flies in the recipient half (D+) and in the other population the migrants were selected in the opposite direction to the flies in the recipient half (D−). Both populations rapidly showed divergence between their halves and for the first three generations the divergences were very similar. The D+ population eventually attained a greater divergence than the D− population. The D− population maintained a cryptic sex-linked polymorphism. The results are compared to those of Millicent and Thoday (1961) and discussed in relation to the effects of selective migration and disruptive selection. It is concluded that divergence under sufficiently intense disruptive selection is possible even when the migration is much less favourable to divergence than it would be under random mating.

EFFECTS OF DISRUPTIVE SELECTION. IX. LOW SELECTION INTENSITY.

Received 27.1X.63