Heterosis, also known as hybrid vigor, is the superior performance of a heterozygous hybrid relative to its homozygous parents. Despite the scientific curiosity of this phenotypic phenomenon and its significance for food production in agriculture, its genetic basis is insufficiently understood. Studying heterosis in yeast can potentially yield insights into its genetic basis, can allow one to test the different hypotheses that have been proposed to explain the phenomenon and allows better understanding of how to take advantage of this phenomenon to enhance food production. We therefore crossed 16 parental yeast strains to form 120 yeast hybrids, and measured their growth rates under five environmental conditions. A considerable amount of dominant genetic variation was found in growth performance, and heterosis was measured in 35% of the hybrid-condition combinations. Despite previous reports of correlations between heterosis and measures of sequence divergence between parents, we detected no such relationship. We used several analyses to examine which genetic model might explain heterosis. We found that dominance complementation of recessive alleles, overdominant interactions within loci and epistatic interactions among loci each contribute to heterosis. We concluded that in yeast heterosis is a complex phenotype created by the combined contribution of different genetic interactions.

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The existence at Shīmbār in the Zagros Mountains of ancient sculptures and inscriptions was first reported by A. H. (later Sir Henry) Layard, in his classic study, ‘A description of the province of Khuzistan’. The research of succeeding years has made surprisingly few additions to this account of the archaeological sites of the province, which though nowadays less well known than some of Layard's other archaeological work, is none the less remarkably careful and complete. In a travel narrative published many years later, his Early adventures in Persia, Susiana and Babylonia, Layard gives another short account of his journey to Shīmbār, which helps to fill in the details of his visit. Shīmbār is an enclosed valley with high mountain walls, situated about 35 miles north-east of the oilfields centre of Masjid-i Sulaimān, in the province of Khuzistan. The name is said by the Bakhtiāri to be the equivalent in their Luri dialect of the Persian Shῑrῑn Bahār ‘Sweet Spring’. The valley is celebrated amongst the tribes as a resting-place on their spring migration to the upland pastures. Its regular inhabitants are mostly members of the Mauri clan.

The new inscription of Aśoka from Kandahar, published by Émile Benveniste and André Dupont-Sommer, forms, together with the one stemming from Pul-i Darunteh (Lamghān) published by W. B. Henning, a special class of Aśoka inscriptions. Both these inscriptions are bilingual, but they do not belong to the common type of bilingual documents, in which the text of each language is separately inscribed on a different part of the stone's surface. Here the two languages are mixed, each short section in one language is followed by one in the other language. The two languages involved, both written in Aramaic characters, are Aramaic and Middle Indian (Prākrit).

REVIEWS social hierarchy under Muhammad 'All. Other useful articles are contributed by Jacob Landau on the Jews, J. N. D. Anderson on law reform and Afaf Loutfi el-Sayed on the As for the essays on political history, they cover a wide spectrum and are all of the highest quality. H. S. Deighton's ' The impact of Egypt on Britain : a study of public opinion ' traces the formation of the Egyptian stereotype in Britain. Abdel Hamid el-Batrik discusses ' Egyptian-Yemeni relations (1819-1840) and their implications for British policy in the Red Sea '. The origins of the Nationalist Party are studied by Arthur Goldschmidt, Jr., and those of the Liberal Constitutional Party by Mahmud Zayid. Elie Kedourie has an incisive and stimulating article on the genesis of the Egyptian constitution of 1923 and P. J. Vatikiotis analyses some political consequences of the 1952 revolution. Altogether, this symposium offers a rich and varied fare, at once tasty and nourishing.