Genetics of Natural Populations. Vii. The Allelism of Lethals in the Third Chromosome of Drosophila Pseudoobscura

Genetics of Natural Populations. Viii. Concealed Variability in the Second and the Fourth Chromosomes of Drosophila Pseudoobscura and Its Bearing on the Problem of Heterosis

“…The main constituents of this variability are recessive mutant genes which are carried in heterozygous condition and which are therefore concealed from direct observation and are detectable only by means of special genetic techniques. In Drosophila pseudoobscura a considerable proportion of wild third chromosomes contain recessive lethals and semilethals, viability and development rate modifiers, and other changes (STURTEVANT 1937;DOBZHANSKY and QUEAL 1938;DOBZHANSKY 1939;DOBZHANSKY and WRIGHT 1941;WRIGHT, DOBZHANSKY, and HOVANITZ 1942). Data to be reported in the present article show that a similar situation obtains in the second and fourth chromosomes of this species.…”

“…Most of the wild flies used in the present study were collected in 1940 and 1941 in three localities, Andreas, Pinon and Keen, on Mount San Jacinto, California. A description of these localities may be found in WRIGHT, DOBZHANSKY, and HOVANITZ (1942). Population samples were taken throughout the season when the flies are reasonably abundant in each locality.…”

“…A summary of the data on the frequencies of different types of concealed variants in the large autosomes of D . pseudoobscura is presented in table IO. In this table, the figures for the second and fourth chromosomes refer to the San Jacinto populations described in the present article, The figures for the lethals, semilethals, and visibles in the third chromosomes refer to San Jacinto populations studied by WRIGHT, DOBZHANSKY, and HOVANITZ (1942) and may safely be compared with the corresponding figures for other chromosomes. The viability modifiers in the third chromosome have not been examined with the same thoroughness as in the two other autosomes.…”

Genetics of Natural Populations Ix. Temporal Changes in the Composition of Populations of Drosophila Pseudoobscura

“…Inhabitants of large portions of the distribution area can be characterized by the presence or absence or by relative frequencies of certain types of Y chromosome (DOBZHANSKY 1935(DOBZHANSKY , 1937a(DOBZHANSKY , 1939a, of gene arrangements in the third (DOBZHANSKY andSTURTEVANT 1938, DOBZHANSKY 1939a, andunpublished data) and in the X chromosomes (STURTEVANT and DOBZHANSKY 1936 and unpublished data), and of various modifying genes (summary in DOBZHANSKY 193 7b). Microgeographic races differ in the incidence of gene arrangements in the third and X chromosomes (DOBZHANSKY and QUEAL 1938, KOLLER 1939, DOBZHANSKY 1939b) and of concealed recessive lethals (DOBZHANSKY and WRIGHT I 941, WRIGHT, DOR-ZHANSKY, and HOVANITZ 1942). Differences between microgeographic races are smaller than but similar in kind to those between macrogeographic races; it is reasonable to suppose that the latter are compounded from the former.…”

The relevance of chromatin architecture to genome rearrangements in Drosophila

“…Berdan et al [17] used simulations to show that established inversion polymorphisms which accumulate deleterious recessive mutations can lead to reinforcement of a balanced polymorphism. Seven to 20% of D. pseudoobscura inversions have at least one lethal recessive mutation [18][19][20][21], however, only 0.07% of the lethal mutations are allelic [18]. This would limit the efficacy of natural selection to maintain a balanced polymorphism through selection against homokaryotypes expressing deleterious recessive alleles, although the frequency and allelism are not sufficient to explain this balanced polymorphism (S.W.…”