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Environment-dependent heterosis in Drosophila melanogaster A. DOMÍNGUEZ J. ALBORNOZ Deparlamento de Genetica, Universidad de Oviedo, 33071 Oviedo, Spain Summary Heterosis for viability, rate of development and fecundity were measured in optimal or stress environments (development at high larval density for preadult traits and both crowded develop- ment and low temperature for egg laying) using diallel crosses among 5 inbred lines of D. melanogaster from different geographic origins. Some cases of significant heterosis for viability and rate of development were found, but the results did not permit any general conclusion to be drawn about the effect of environment for these traits. Every pair of lines displayed heterosis for fecundity both under optimal and crowded development conditions, while only 2 pairs of lines showed significant heterosis for fecundity at low temperature. Contrary to what is usually found for most traits, heterosis for fecundity was greater in the optimal environment. Hybrids were more affected by environmental stress than their inbred parents, but the error variance within environ- ment was lower for the hybrids. This suggests that the greater homeostasis of hybrids to minor changes in environment can not be extended to major changes in the environment in which different sets of genes may be implicated. Key words : Drosophila melanogaster, heterosis, genotype x environment interaction. Résumé Hétérosis dépendante du milieu chez Drosophila melanogaster A partir d’un diallèle entre 5 lignées de D. melanogaster de différentes origines géographi- ques, on a mesuré l’hétérosis sur la viabilité, la vitesse de développement et la fécondité dans un milieu optimal ou de stress (haute densité larvaire pour la viabilité et la vitesse de développement et développement à haute densité larvaire et basse température pour la ponte d’oeufs). On a trouvé quelques cas d’hétérosis sur la viabilité et la vitesse de développement, mais les résultats ne permettent pas de conclure sur une tendance générale de la variation de l’hétérosis en fonction du milieu pour ces caractères. Toutes les paires de lignées présentent une hétérosis sur la fécondité, à la fois en milieu optimal et en milieu à haute densité larvaire, alors que seules 2 paires de lignées présentent une hétérosis significative pour la fécondité à basse température. Contrairement à ce qu’on trouve habituellement pour la plupart des caractères, l’hétérosis sur la fécondité est plus importante dans le milieu le plus favorable. Les hybrides sont plus affectés par un stress environnemental que leurs parents consanguins, tandis que la variance d’erreur intra-environne- ment est plus faible chez les hybrides. Ce fait suggère que la plus grande homéostasie des hybrides face à des modifications mineures de milieu peut ne pas se maintenir lorsque se produisent des modifications majeures du milieu, dans lesquelles différents ensembles de gènes peuvent être impliqués. Mots clés : Drosophila melanogaster, hétérosis, interaction génotype x milieu. I. Introduction The magnitude of heterosis is conditioned very much by the environment (for a review see B ARLOW , 1981). For most traits, heterosis appears to be greater in subopti- mal environments. This is in accordance with the hypothesis of L ERNER (1970) that hybrids are likely to be more homeostatic than homozygotes in the presence of environmental variation. As a consequence of greater homeostasis, hybrid superiority would be more pronounced in suboptimal environments. Drosophila hybrids were shown to display lesser variation than parental lines within a given environment for a variety of traits : survival, size and developmental time (R OBERTSON & REEVE, 1952) ; fecundity (R OBERTSON & REEVE, 1955) ; wing and thorax length and percentage emergence (T ANTAWY , 1957). A number of studies in Drosophila have also shown greater heterosis under extreme environmental conditions than under optimal ones. Most of these studies dealt with viability (D OBZHANSKY et R l. , 1955 ; D OBZHANSKY & L EVENE , 1955 ; PARSONS, 1959 ; F ONTDEVILA , 1970 ; YOUNG, 1971 ; T ACHIDA & M UKAI , 1985) and with longevity (PARSONS, 1966 ; C LARE & L UCKINBILL , 1985). Nevertheless, SANG (1964) found clear differential effects of departures from optimal nutritional conditions on the performance (survival, weight and developmental rate) of various genotypes, and the crosses were not better « buffered » in this respect. This paper reports a study of hybrid vigour in Drosophila melanogaster over some optimal and suboptimal environmental conditions. Three fitness traits, viability, deve- lopmental time and fecundity were measured in the same lines and hybrids. With this information it was possible to test the homeostasis of the hybrids for different traits within and across environments. II. Materials and methods Five inbred lines of D. melanogaster were used : Teverga-5 (Spain), Crkwenica (Czechoslovakia), Israel (Israel), Kreta-75 (Greece) and Hampton Hill (Great Britain). The last four lines came from the Ume5 Drosophila Stock Center. The culture medium used throughout the experiments was composed of 12 g of agar, 100 g of sugar, 100 g of baker’s yeast and 5 ml of propionic acid per litre of water. For oviposition scores, 4 g/i of charcoal were added to the medium, and a spot of live yeast was put on the surface. Attention was paid to 3 traits : viability, rate of development and fecundity under optimal and one or two suboptimal environmental conditions. Both viability and developmental time were scored under 2 environmental conditions : « optimum den- sity (30 eggs per vial, 24 °C) and « high density (300 eggs per vial, 24 °C). Fecundity was scored under 3 environmental conditions : « optimal conditions » (females developed under the defined optimal developmental conditions were allowed to oviposit at 24 °C), « crowded development » (females developed under the defined high density environment were allowed to oviposit at 24 &dquo;C) and « low temperature » (females from the optimum density conditions were allowed to oviposit at 17 °C). Figure 1 shows a description of the experimental procedure. A 5 x 5 diallel cross, including reciprocals, was performed. Forty males and 40 females were mated to produce each of the 25 crosses. Then females were allowed to oviposit for 16 hours. Random samples of eggs were placed in glass vials (25 mm x 115 mm) containing 4.5 ml of culture medium, 30 (optimum density, 5 replicates) or 300 (high density, 2 repli- cates) per vial, and allowed to develop at 24 ± 1 &dquo;C. The number of replicates was different for the 2 density treatments because the treatment itself implies more indivi- duals to be measured under crowded development than under optimal conditions. The number of adults which emerged from these cultures was counted each day. Viability was then scored as the proportion of eggs that became imagos in each vial. Rate of development was scored as the reciprocal of the mean time of development in days in each vial. Females emerging from optimum density and high density cultures in the 2 or 3 days of maximum emergence were selected for oviposition experiments. Two females were placed into each vial together with 2 young males. Females from optimum density were allowed to oviposit at 24 ± 1 &dquo;C (optimal conditions) or at 17 ± 1 °C (low temperature). Females from high density cultures were placed at 24 ± 1 °C (crowded development). Five replicates were set up for each cross and environmental condition. Fecundity was scored as the average daily egg laying per female in the fourth and fifth days of age. Rate of development was preferred to developmental time because it had more satisfactory statistical properties. Error variances of developmental time changed with [...]... in hybrids between inbred lines of Drosophila melanogaster in relation to the level of homozygosity Genetics, 42, 535-543 ANTAWY T A.O., M A.M., A E.L., 1973 Studies on natural populations of Drosophila OURAD FFIFI XIV Gene-environment interaction in Drosophila melanogaster Egypt J Genet Cytol., 2, 244-257 YOUNG S.S.Y 1971 The effects of some physical and biotic environments on heterosis of direct... of inbred lines and crosses of Drosophila melanogaster Genet Res , 5, 50-67 NEDECOR S G.W., C W.G., 1967 Statistical methods 703 pp., The Iowa State University OCHRAN Press, Ames ACHIDA T H., M T., 1985 The genetic structure of natural populations of Drosophila melanoUKAI gaster XIX Genotype-environment interaction in viability Genetics, 111, 43-55 ANTAWY T A.O., 1957 Heterosis and genetic variance in. .. Gene-environment interaction in Drosophila melanogaster Egypt J Genet Cytol., 2, 244-257 YOUNG S.S.Y 1971 The effects of some physical and biotic environments on heterosis of direct and associate genotypes in Drosophila melanogaster Genetics, 67, 569-578 . E.C.R., 1955. Studies in quantitative inheritance. VIII. Further analysis of heterosis in crosses between inbred lines of Drosophila melanogaster. Z. Indukt. Abstamm. Vererblehre,. as in stability to minor environmental changes, although the lines were from very different origins. Therefore, the heterosis we have found can not be explained in terms. (variances between replicates within genotype and treatment) of inbred lines and hybrids are compared in table 1. Error variances of inbred lines and hybrids for viability