Regular responses to selection 2. Recombination and accelerated response

Abstract

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.

Effects of temperature on selection for scutellar Bristles

“…Disruption of the stabilised genome would reveal cryptic variability which could lead to rapid responses to selection if the negative interactions had been relatively large. Accelerated responses to selection might frequently occur in this way in characters previously subjected to stabilising selection rather than from the formation of extreme genotypes by intrachromosomal recombination such as was demonstrated by Thoday and Boam (1961) in their sternopleural bristle selection lines (see Thoday et al, 1964). Fraser et al (1965) describe a number of scutellar selection lines at 25° C. in which rapid responses, similar to that in the present 20° C. line, occurred for a few generations.…”

Effectiveness of Regular Cycles of Intermittent Artificial Selection for A Quantitative Character in Drosophila Melanogaster

“…Clustering of loci affecting the trait under selection could thus have a considerable effect on the results of intermittent selection. That recombination turning repulsion into coupling linkages can be of great importance in selection response is clearly shown by the part played by the linked major genes responsible for much of the response in some of the sternopleural bristle number lines of Thoday and Boam (1961), as reported by Thoday, Gibson, and Spickett (1964). However, Robertson (1966) reported that suppression of all crossing over on chromosomes 2 and 3 of Drosophila melanogaster led to the mean of five replicate lines showing a response for sternopleural bristle number about 60% of that obtained for the mean of five replicate lines with normal recombination allowed.…”

Maximum likelihood estimation of linkage between a marker gene and a quantitative locus

“…Mather (1957, 1960) investigated the distribution of polygenic activity affecting chaeta number and viability characters along chromosome III of Drosophila melanogaster by using appropriate mutant loci on the same chromosome as genetic markers. In a series of fundamental studies, Thoday and his colleagues showed how major mutants can be used to identify, map and evaluate individual loci affecting quantitative traits (Thoday, 1961(Thoday, , 1979Spickett and Thoday, 1966;Thoday et a!., 1964). Since then this method has been applied in basic studies of biometrical or quantitative genetics of various organisms such as in Drosophila (Williams, 1968(Williams, , 1977(Williams, , 1978(Williams, and 1980Davies and Workman, 1971;Davies, 1971;Dominguez and Rubio, 1986;Shrimpton and Robertson, 1988a, b), in wheat (Patterson et a!., 1968;Law, 1966), in maize (Edwards et al, 1987) and in tomato (Tanksley and Rick, 1980;Weller, 1987;Weller et al, 1988).…”