Genetic loads affecting fertility in natural populations of Drosophila pseudoobscura.

A study of the genetic loads affecting fecundity of females of Drosophila It has been shown that females homozygous-for second chromosomes derived from natural populations deposit on the average fewer eggs than do females carrying two second chromosomes derived from different wild progeni-tors. The present report extends the study to the egg-to-adult viability of the progenies of homozygous and heterozygous females, and thus permits estimation of the reduction of the fertility caused by the manifestation of these normally concealed genetic loads. Samples of Drosophila pseudoobscura were collected in Borrego Valley and Palm Canyon (California) by MR. DAVID RICHMOND, and near Tucson (Arizona) by DR. WILLIAM HEED, in November 1966. A total of I46 males (or sons of wild females) from these samples (92 from California and 54 from Arizona) were crossed individually to females from the laboratory strain with a dominant mutant marker (Bare) on the second chromosome. The analyzer stock was outcrossed (by MR. SPASSKY) for several generations to flies of Arizona origin. Homozygotes and heterozygotes for wild-type second chromosomes were obtained by a standard technique described by DO~ZHANSKY, HOLZ and SPASSKY (194.2), MARINKOVIC (1967) and others. Briefly, after backcrossing i,ndividual males from the F, generation to females from the Delta/Bare balanced strain, females and males heterozygous for the same wild chromosome and a chromosome with the mutant marker Bare were intercrossed (two pairs per bottle, three replications per each combination). In the following generation, the expected propomrtions of the wild-type homozygotes and Bare heterozygotes should be 33.3 : 66.7. Wild-type heterozygotes were obtained in a similar way, by crossing males and females From each of the three replicate cultures, a sample of 40 to 60 eggs was taken and transferred with a small piece of medium onto the surface of Spassky’s medium, in a half-pint bottle. After 16 and 23 days, the numbers of emerged flies were counted, and the percentage survival of the progenies of homozygous or heterozygous mothers was estimated for all 118 second-chromosome homozygotes, and for 82 heterozygous combinations (62 nonlethal and 20 lethal/nonlethal) . Some of the homozygous females whose eggs showed extremely low survival were dissected to observe abnormalities in their reproductive organs. Attempts have also been made to determine at which stages of embryogenesis development ceased (eggs older than 48 hours have been used). After dissolving the chorion in a 3.5% hypochloric acid, a morphological examination of the eggs was made.