Dynamics of gene flow in an experimental population of Cucumis melo ( Cucurbitaceae).

Gene flow in an experimental garden of Cucumis melo (Cucurbitaceae), the cultivated muskmelon, was measured by placing plants carrying a dominant gene (green cotyledons) in the center of an 18 x 18-m field of recessive plants (yellow cotyledons). At the end of the growing season, all fruits were collected, and seeds planted for analysis of the progeny. The 123 fruits yielded 41,875 seedlings whose genotypes were recorded. Gene flow decreased with increasing distance from the central plot, while the variance increased. The decrease in frequency of the green gene did not follow a leptokurtic pattern over the distance measured, and there was a marked asymmetric and patchy pattern of gene flow in the field. The directionality contributed to a bimodal pattern of gene frequencies among fruits at the edges of the field, some fruits with no green seedlings, some with many. Bumblebee movements were recorded during 4,296 flower visits in an identically arrayed melon field. Bees usually moved to very nearby plants and visited only a few flowers on any one plant. This suggests that pollen carryover from the central patch out was quite high in the test garden. These findings show that local gene flow patterns in plant populations may be complex, and do not always follow regular and symmetrical models. The complex arrangement of progeny genotypes, when acted upon by local selective forces, may contribute to small-scale differences often seen in plant populations. RECENT PROGRESS on the ecological genetics of plant populations has concentrated on demographic processes and gene flow dynamics. Several studies have shown that the structure of plant populations often includes small scale differences in genotypes (Schaal and Levin, 1976-; Raven, 1976); gene flow dynamics apparently contributes to this pattern. With mechanisms for measuring the morphological and biochemical variation in nature now widely available, the role of gene flow in maintaining and creating the variation needs clarification. For plants, gene flow is mediated by seed and pollen dispersal. Studies of seed movement and germination (Harper, 1977, chapters 2-5) and of pollen movement (Levin and Kerster, 1974) generally show restricted patterns that vary with the type of disperser and the 1 Received for publication 11 July 1981; revision accepted 30 June 1982. I thank Jan Roueche for dedicated and expert help in maintaining the plots and analyzing the data. Helga Cohen and Judy Mishkin played major roles in collecting and analyzing data. Seed stocks for this work were supplied by Dr. Perry Nugent of the U.S. Department of Agriculture Vegetable Breeding Laboratory, Charleston, S.C. This work was funded by N.S.F. Grants DEB 78-24992 and 7927022, and funds from the Southern Regional Educational Board. Experimental work was done at the Clemson University Sandhill Experimental Station, Pontiac, S.C. and the University of South Carolina Department of Biology; I thank both institutions for use of facilities. J. Mishkin, J. R. Powell, R. G. Harrison, K. K. Kidd and this journal's reviewers made helpful suggestions, for which I am grateful. morphology of the seed and pollen grain. Much work on estimating pollen flow is based on detailed observations of pollinator movements, which assumes a close correlation between these movements and pollen transferred among flowers. This procedure has contributed much to the theory of plant microevolution, such as the role of density-dependent population structure in altering gene flow patterns (Levin and Kerster, 1969). However the correlation is difficult to generalize among vectors (Primack and Silander, 1975; Levin, 1981), and may not always be accurate. Actual gene flow has been measured in several agronomic studies where interest has been in minimum distance needed to isolate cultivars. In these studies, crop strains with genetic markers are interplanted in geometric arrays, natural pollination occurs, then the F1 is searched for the marker phenotypes (e.g., Crane and Mather, 1943; Bateman, 1947a; Griffiths, 1950). Few such studies on actual gene flow have been done in natural populations because the existence of large monomorphic patches within a population of contrasting phenotypes is rarely found in nature. Consequently, studies in experimental gardens are of particular interest in understanding microevolutionary processes in plant populations. This study uses the technique of growing plants in a test garden that are homozygous for different alleles at one locus that controls a morphological trait. The microdistribution of