Chapter 28 



MUTATIONAL LOADS AND THEIR 

 CONSEQUENCES TO POPULATIONS 



FIGURE 28-1, 



Chromosomal 

 complement of 

 D. pseudoobscura. 



d: 



ROSOPHILA PSEUDOOBSCURA IS 



a fruit fly commonly found 

 in northern Mexico and west- 

 ern United States. When collected in the 

 wild almost all individuals are alike pheno- 

 typically, except for the sex differences, ap- 

 pearing "wild-type" or normal. We cannot 

 accept such a phenotypic uniformity as evi- 

 dence for genotypic uniformity, however, 

 since we already know of the possibility that 

 a wild-type population may appear uniform 

 yet contain concealed genetic variability in 

 the form of isoalleles or recessive mutants 

 having a point mutational origin. In fact, 

 we have already indicated the existence in 

 natural populations of Drosophila of isoalleles 

 (Chapter 9), of pericentric inversions and re- 

 ciprocal translocations (Chapter 21), and of 

 paracentric inversions (Chapter 27). What 

 we would like to have is an estimate of the 

 total amount of genetic variability present in 

 a natural population. This can be studied 

 using D. pseudoobscura} 



D. pseudoobscura has five pairs of chromo- 

 somes, composed of the usual X and Y sex 

 chromosomes, three pairs of large rod- 

 shaped autosomes (II, III, IV), and a dotlike 

 pair of autosomes (V) (Figure 28-1), There 

 are a number of laboratory strains of this 

 species whose chromosomes are marked by 

 various mutants of point and rearrangement 

 type. It is possible, therefore, to perform a 

 series of crosses between laboratory strains 



^ Based upon work of Th. Dobzhansky and col- 

 laborators. 



239 



and flies collected in the wild which yield 

 information on the presence of mutants in 

 the wild-type flies. In practice, autosomes 

 II, III, and IV of wild-type flies were indi- 

 vidually made homozygous to detect the 

 presence of recessive mutants (cf. Figure 9- 

 4) that are lethal (causing death of all indi- 

 viduals before adulthood), or semilethal 

 (causing more than 90 and less than 100 per 

 cent mortality before adulthood), or subvital 

 (causing significantly less than normal but 

 greater than 10 per cent survival to adult- 

 hood), or sterile to females {female sterile) 

 or sterile to males {male sterile). 



The results of such a study are summarized 

 in Figure 28-2. About 25% of all auto- 

 somes tested this way carried a recessive 

 lethal or semilethal mutant. Recessive sub- 

 vital mutants were found in about 40% of 

 III chromosomes tested, and in more than 

 90% of tested II's and IV's, while mutants 

 causing sterility were present in 4-14% of 

 tested chromosomes. The natural popula- 

 tion clearly carries a tremendous load of 

 detrimental mutants. 



How is this load of mutants distributed 

 in the fly population? Consider first one 

 pair of the autosomes tested. Each member 

 has a 25% chance of carrying a lethal or 

 semilethal, and a 75% chance of being free 

 of such a mutant. The chance that both 

 members of a pair of chromosomes will carry 

 a lethal or semilethal is (0.25)^, or 6.25%. 

 You cannot tell from the data presented 

 whether the lethals and semilethals found in 



