Opposite Heterotic Effects on Male Weights of Reciprocal Species Hybrids

Reciprocal species hybrids have long interested the geneticist as these hybrids represent complex interactions both of genomes and also of genomes with different cytoplasms. Heterosis, the property of an F1 hybrid falling outside the range delimited by its parents in regard to one or more characters (Darlington and Mather, 1949), has sometimes been observed in species hybrids (Gordon and Smith, 1938). The present report concerns the finding of opposite heterotic effects on weights of males obtained from reciprocal crosses between D. pseudoobscura and D. persimilis. The strains of. flies used in these experiments were obtained from Prof. Th. Dobzhansky's laboratory at The Rockefeller University, through the courtesy of Mr. B. Spassky. These consisted of: (a) three wild-type strains of D. pseudoobscura, one each from Mara (British Columbia), Mather (California), and Chichicastenango (Guatemala), and (b) three wild-type strains of D. persimilis one each from Quesnell (British Columbia), Orick (California), and Porcupine Flats (California). The wild-type strains of each species were crossed amongst themselves (AS' x B2; BS' x C2; CS' x A2) to establish three stock cultures of each species. For our experiments 15 virgin females were gathered from each stock culture of a given species. The total of 45 females were placed in a food bottle with 45 males, similarly gathered from either the same or from the other species. Two intraspecific and two interspecific crosses were made. Five replicas of each type of cross were made. The flies for each replica were obtained from a different generation of the stock cultures. Each group of 45 pairs of flies was transferred every 48 hours to a freshly yeasted, standard half-pint bottle containing 3/4 inch of Spassky's cream of wheat-molasses medium. All experimental and stock flies were maintained at 25 ? 2 C. Enough transfers of each replica were made so that 100 male and 100 female progeny could be collected from each group of 45 pairs. Newly hatched flies were collected daily and the sexes separated. After aging the flies for 7 to 14 days, batches of 100 were etherized and immediately weighed on an analytical pan balance. Table 1 gives the grand average wet-weight (in milli-