NEW WORLD JUGLANDACEAE. II. HICKORY NUT OILS, PHENETIC SIMILARITIES, AND EVOLUTIONARY IMPLICATIONS IN THE GENUS CARYA

Gas chromatographic analyses of the nut oils of 155 samples, representing 15 taxa of Carya, confirm the basic uniformity of fatty-acid composition within the walnut family. All taxa contain small amounts of the saturated fatty acids-palmitic and stearic-and variable amounts of the unsaturated oleic and linoleic acids, and at least trace amounts of linolenic. Quantitative differences in the relative amounts of the five fatty acids support the morphologic recognition of two sections within the genus. Members of the section Carya are characterized by higher linoleic and lower oleic acid percentages than are those of section Apocarya. It is also true, however, that tetraploids of section Carya tend to have higher linoleic and lower oleic acid percentages than diploids within the same section. Numerical analyses of the oil data reveal close similarities between certain members of the more "primitive" section Apocarya and the diploid members of section Carya. The highly heterogeneous assemblage within section Apocarya could be split into three groups on the basis of oil data. It is equally obvious, however, that the American taxa of the genus Carya have had a long and reticulate phylogeny and that recognition of additional categories above the species level would not result in natural assemblages. SPECIALISTS on long-lived perennials have relied mainly on the analysis of natural populations for their systematic treatments. To a large degree these treatments were necessarily derived from the more accessible morphological and anatomical features. In 1961 we initiated a gas chromatographic survey of the oils of the North American species of Carya with the hope of adding a new dimension to the evolutionary study of amenable woody plants. Our preliminary analyses (Stone, 1964b) confinned the qualitative similarity of the oils previously reported for the cultivated pecan (C. illinoinensis, Hilditch and Williams, 1964) and bitternut (C. cordiformis, Riebsomer, Larson, and Bishman, 1940) and offered suggestive evidence on the genetic control of interspecific differences in the relative amounts of the five fatty acids (Stone, Adrouny, and Adrouny, 1965). The purpose of this paper is to present the completed oil survey and to examine these re' Received for publication 8 January 1969. This research was supported by National Science Foundation grants GB-2313, GB-5233X, and the Department of Defense's Joint Services Electronics Program under grant Af-AFOSR-766-67. The authors express their appreciation to the many scientists, especially Dr. Delzie Demaree, who supplied fresh samples for analysis; to Salpi Adrouny, Sonia Adrouny, and John Wilhelm for their services in chromatographing the nut oils; and to Dr. B. L. Turner for offering helpful comments on the manuscript. sults in light of possible evolutionary implications. M\ATERIALS AND METHODS-Mature fruiting specimens were collected on the trees in the fall of the year. Since storage conditions are known to affect the fatty-acid composition (Stone et al., 1965), only those nuts that were quick frozen or analyzed soon after collecting are included in the averages. Perhaps the greatest source of error is due to the largely uncontrollable influences of different enviroinments. The only check made on seasonal variability is the comparison between a 1.961 and a 1962 collection of Carya ovalis (Stone 960) from St. Francis County, Ark. In this particular case there was less than a 3 % difference between the oleic-acid content and less than 1 % difference in linoleic-acid composition. M\iost certainly, however, phenotypic variation is much more of a problem than these two samples would suggest. Since there was no way of controlling the environmental conditions influencing tree growth and fruit production, the environmental error was "neutralized" by sampling trees from a variety of ecological and geographical sites. The procedures employed in the extraction of oils, isolation of fatty acids, preparation of methyl esters, operation of the gas chromatograph, and calculation of the percentages of fatty acids have been detailed elsewhere (Stone et al., 1965). One or more nuts were sampled from 15.5 trees