THE GENETIC STRUCTURE OF NATURAL POPULATIONS OF DROSOPHILA MELANOGASTER. XI. GENETIC VARIABILITY IN A LOCAL POPULATION

Abstract

Six hundred and ninety-one second chromosomes were extracted from a Raleigh, North Carolina population, and the following experimental results were obtained: (1) Salivary gland chromosomes of all lines were observed and the number of inversion-carrying chromosomes was 130, among which 76 carried In(2R)NS, 36 carried In(2L)t, 4 carried In(2L)t and In(2R)NS, and 14 carried different kinds of rare inversions. (2) Viabilities of homozygotes and heterozygotes were examined. The frequency of lethal-carrying chromosomes was 275/691 (or 0.398):70/130 (or 0.538) in inversion-carrying chromosomes and 205/561 (or 0.365) in inversion-free chromosomes. The former is significantly higher than the latter. The average homozygote viability was 0.4342 including lethal lines and 0.7163 excluding those, the average heterozygote viability being 1.0000. The detrimental load to lethal load ratio (D:L ratio) was 0.334/0.501 = 0.67. The average viability of lethal heterozygotes was less than that of lethal-free heterozygotes, significantly in inversion-free individuals but not significantly so in inversion-carrying individuals. Inversion heterozygotes seem to have slightly better viability than the inversion-free heterozygotes on the average, but not significantly so. (3) The average degree of dominance of viability polygenes was estimated to be 0.293 ± 0.071 for all heterozygotes whose component chromosomes had better viabilities than 0.6 of the average heterozygote viability, 0.177 ± 0.077 for inversion-free heterozygotes and 0.489 ± 0.082 for inversion heterozygotes. (4) Mutation rates of viability polygenes and lethal genes were estimated on the basis of genetic loads and average degrees of dominance of lethal genes and viability polygenes. Estimates were very close to those obtained by direct estimation. (5) Possible overdominance and epistasis were detected, but the magnitude must be very small. (6) The effective size of the population was estimated to be much greater than 10,000 by using the allelism rate of lethal-carrying chromosomes (0.0040) and their frequency.—On the basis of these findings and the comparison with the predicted result (Mukai and Maruyama 1971), the mechanisms of the maintenance of genetic variability in the population are discussed.

An experiment on recombination load inDrosophila melanogaster

“…In an extensive study of components of genetic variation in viability, Mukai & Yamaguchi (1974) report finding a significant difference between the viability of homozygotes for chromosomes extracted from females and from males in D. melanogaster. They also examined the viabilities of chromosomal heterozygotes and found that the progeny of F x M crosses (where F denotes chromosomes derived from wild females and M denotes male-derived chromosomes) were significantly superior to those of F xF, which had, however, a similar mean viability to the progeny of the M xM crosses.…”

The origin and maintenance of supergenes contributing to ecological adaptation in Atlantic herring

“…If an inversion overlaps with co-adapted or beneficial allelic combinations, selection is likely to promote its maintenance and spread 11 . However, suppressed recombination in heterozygotes can result in impaired purifying selection and consequent accumulation of deleterious mutations in the inversion region, which theoretical and empirical data have demonstrated to ultimately result in the degradation of the inversion through the process of Müller's rachet 6,7,[19][20][21][22][23] . Interestingly, recent literature on vertebrate species supports the hypothesis that inversions can also evolve without pronounced accumulation of mutation load [24][25][26][27][28] .…”

The effects of inversion polymorphisms on patterns of neutral genetic diversity

“…While it is often considered that population subdivision in organisms like Drosophila is likely to have only minor effects on genetic diversity, given the generally low levels of F ST among populations (Singh and Rhomberg 1987;Schaeffer 2002;Lack et al 2015Lack et al , 2016, there is evidence from studies of allelism of recessive lethals that local deme sizes are in reality somewhat restricted in size, with limited migration among them (Wright et al 1942;Mukai and Yamaguchi 1974;Ives and Band 1986); this conclusion has recently been confirmed by a resequencing study of a single US population over time (Lange et al 2022). It is known that population subdivision with a large number of demes increases the amount of linkage disequilibrium among neutral loci when genomes are sampled from the same deme (Wakeley and Lessard 2003), because population subdivision increases local homozygosity, thereby reducing the effectiveness of recombination.…”