Release of Genetic Variability Through Recombination. Iii. Drosophila Prosaltans

Abstract

HIS is the third in a series of articles on release of genetic variability through T recombination in three species of Drosophila. The previous articles have dealt with Drosophila pseudoobscura (SPASSKY et al. 1958) and D. persimilis (SPIESS 1959). The present article is concerned with D. prosaltans. The ecological peculiarities of this species are pertinent. It is native exclusively in the tropics, while the other two species live mainly in the temperate zone. D. pseudoobscura is widely distributed, very common, ubiquitous, and ecologically versatile; D. persimilis occurs in a much less extensive geographic area, and although it builds very dense populations in favorable habitats it is specialized to live chiefly in cool and humid locations; D. prosaltans is a rare form, which reaches considerable population densities only ephemerally in few scattered neighborhoods (DOBZHANSKY and PAVAN 1950). The spontaneous mutation rates for autosomal lethals are, at similar temperatures and in homologous chromosomes, about twice as high in D. persimilis and D. prosaltans as they are in D. pseudoobscura SPASSKY 1952, 1954). The genetic loads carried in the populations (accumulated recessive lethal, semilethal, and subvital gene complexes in the chromosomes) seem nevertheless to be higher in D. pseudoobscura than in D. persimilis or in D. prosaltans SPAS-SKY 1953, 1954). In accordance with this, the loss of fitness produced by inbreeding and homozygosis for naturally occurring gene complexes is greater in D. pseudoobscura than in D. persimilis or in D. prosaltans. It is tempting to speculate that the adaptive norm of D. pseudoobscura depends upon balanced heterozygosis to a greater extent than is the case in D. persimilis and in D. prosaltans. In other words, the genetic architecture of D. pseudoobscura approaches that postulated by the "balance" hypothesis, while the genetic architectures of the other two species incline relatively more towards the situation envisaged by the "classical" hypothesis ( DOBZHANSKY 1955). Material and technique in Drosophila prosaltansThe experiments have followed the same plan as those with D. pseudoobscura and D. persimilis (SPASSKY et al. 1958, SPIESS 1959. Ten second chromosomes, permitting normal or subnormal viability in homozygotes, were extracted from The work reported in this article has been carried out under Contract Number AT-(30-1)-

GENETIC LOADS AFFECTING FECUNDITY IN NATURAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA

“…The mean percentage of non-Bare flies in the cultures of all 256 second-chromosome homozygotes turns out to be 22.64 + 0.79%, and eliminating the lethals and semilethals, 28.93 * 0.42%. These are the usual values (DOBZHANSKY and SPASSKY 1953;. Even the difference between the viabilities of the quasinormal homozygotes and the heterozygotes is highly significant (t = 6.90, p < 0.001).…”

“…For the study of the fecundity, the females were aged for five days, and crossed mostly to their brothers. Although some of the males homozygous for wild second chromosomes are sterile (DOBZHANSKY and SPASSKY 1953), such males are known to copulate normally with the females, so that the latter had the proper physiological stimulus for oviposition. Moreover, about one half of the tested females from the Chiricahua Mountains population, and most of the females from the Lincoln locality, were outcrossed to fertile heterozygous males; these females showed, on the average, the same fecundity, namely 301.0 f 15.6 eggs, as the females crossed to their homozygous brothers-310.7 * 16.7 eggs.…”

“…A total of 256 males from these samples (or sons of wild females) were crossed individually to females from a laboratory strain with suitable mutant markers on the second chromosome. The experimental procedure is described in DOBZHANSKY, HOLZ and SPASSKY (1942), DOBZHANSKY (1946), andSPASSKY (1953). Briefly, females and males heterozygous for the same wild second chromosome and a chromosome with a dominant mutant marker (Bare) were intercrossed (2 or 3 pairs per bottle); the offspring consisted of non-Bare flies (homozygous for a wild second chromosome) and Bare heterozygotes, theoretically in the proportions of 33.3 : 66.7%.…”

“…The components that reduce the fertility have received relatively little attention. Sterility loads have been found by DOBZHANSKY, HOLZ and SPASSKY (1942), DOBZHANSKY and SPASSKY (1953) and DOBZHANSKY, SPAS-…”

The population genetics of speciation: the evolution of hybrid incompatibilities.

“…if, on the other hand, complementary interactions are very rare, an enormous number of substitutions may be required before speciation is likely. We know only that complementary lethals and steriles can be found in nature (DOBZHANSKY 1946;DOBZHANSKY et al 1959;KFUMBAS 1960) and that, at least in Drosophila mlanogaster, they are very rare. TEMIN et al (1969) found that =0.005 of all second and third chromosome combinations harbor complementary lethals.…”